Drilling cutting regimes for hole drilling

Selecting a cutting mode for drilling

While drilling, the drill bit performs two movements: the main movement is rotary and the feed movement is progressive. Main movement determines cutting speed of furthest (peripheral) point of the drill. Depth of cut for drilling in solid material t = mm, for reaming t = mm (D. drill diameter; d. diameter of the reamed hole). Feed rate per revolution S. The cutting depth (mm/rev) in drilling is equal to the movement of the drill bit or workpiece in the direction of the rotary axis per revolution. (n. number of revolutions per minute of the drill bit or machine spindle). To increase productivity when drilling, work at the highest possible speed and feed rate. However, both are limited by the strength of the machine’s mechanisms,

By cutting conditions are meant such a combination of cutting speed and feed rates that should maintain high durability of drills, provide maximum productivity of the machine and the required accuracy and quality of machining. Knowing the drill bit diameter, the material from which the drill bit is made, and the type of metal to be cut, you can select the cutting conditions from the tables below. Cutting speeds for drilling are given in the table.

Example. Select cutting speed and RPM from table. 1 When drilling a through hole in carbon constructional steel, in = 650 MPa, by 12 mm drill D. steel R18 (operation with cooling, drilling machine 2118). According to the table, we find out that the drilling machine tool 2118 has a feed s = 0.2 mm/rev, Knowing the magnitude of the feed and the diameter of the drill, from the table we find the cutting speed of 29.5 m/min and speed 781 rpm. From the kinematic diagram, we choose the closest suitable spindle speed (735 rpm) with an automatic feed rate of 0.2 rpm.

I. Adjust the machine to the chosen spindle speed. To do this, use a special crank to move the arm with the motor “toward you” in order to loosen the belt tension. Then, without removing the protective cover, transfer the belt to the third pulley stage on the spindle, and then to the third pulley stage of the electric motor. The belt can only be moved from one stage to the next once the electric motor has come to a complete standstill.

Then move the crank of the motor arm “away” and tension the belt. The belt tension should not be too tight or too weak.

II. The machine is set up for the selected feed rate. The mechanical feed is provided by the feed box, which is driven from the spindle pulley through a gearbox by means of a belt transmission.

The 2118 drilling machine has a mechanical feed rate of 0.2 mm/rev by setting the crank to the center position and then fixing the stop screw on the side disc.

III. Cutting tools are cooled. During operation, the drill bit becomes very hot, causing the cutting edges to become dull. In order to increase the stability of the drill bit, a cooling agent is used, which is pumped from a reservoir to the drill bit.

When drilling, the coolant is applied continuously for the entire duration of the drilling operation and is mainly used for the cutting edges and the chips that are removed from the drill bit.

The following coolants are recommended when drilling various metals:

Material to be cut Coolant
Structural or tool steel Emulsion, mixed oils, aqueous soda solution
Cast iron Dry or emulsion
Brass Dry or emulsion
Copper Emulsion or turf oil
Aluminum Emulsion, kerosene

The drill bit performs two movements during drilling: the main movement is rotary and the feed movement is progressive. Main movement determines the cutting speed of the furthest (peripheral) point of the drill. Depth of cut for drilling in solid material t = mm, for reaming t = mm (D. drill bit diameter, d. reamed hole diameter). Feed rate per revolution S. (mm/rev) during drilling is equal to the movement of the drill bit or workpiece in the direction of the rotation axis per revolution. (n. number of revolutions per minute of the drill bit or machine spindle). In order to increase productivity when drilling work is to be carried out at the highest possible speed and with the highest possible feed rate. But both are limited by the strength of the machine tool mechanisms,

By cutting mode is meant such a combination of cutting speed and feed rate values, which should maintain high durability of drills, provide maximum productivity of the machine and the required accuracy and quality of machining. Knowing diameter of drill bit, drill bit material and quality of metal, it is possible to choose cutting conditions according to the following tables. The cutting speeds for drilling are shown in the table.

Example. Select cutting speed and RPM according to Tab. 1 When drilling a through hole in carbon construction steel with D = 650 MPa, through hole D. 12 mm of steel P18 (work with cooling, drilling machine 2118). According to the table, we know that the drilling machine 2118 has a feed rate s = 0.2 mm/rev, Knowing the feed rate and drill diameter, we find the cutting speed of 29.5 m/min and the rotational speed of 781 rpm from the table. According to the cinematic diagram, we choose the nearest suitable spindle speed (735 rpm) with an automatic feed rate of 0.2 rpm.

I. The machine is tuned for the chosen spindle speed. To do this, use a special handle to move the arm with the electric motor “toward you” in order to loosen the belt tension. Then, without removing the protective cover, transfer the belt to the third pulley stage on the spindle, and then to the third pulley stage of the electric motor. The belt can only be switched from one stage to the other once the motor has come to a complete stop.

Then the motor arm handle is moved “away” and the belt is tensioned. The belt tension must not be too tight or too weak.

II. Machine set up for selected feed rate. The mechanical feed is provided by the feed box, which is driven by a belt drive from the spindle pulley via a gearbox.

On the 2118 drilling machine, the mechanical feed rate of 0.2 mm/rev is set to the middle position of the crank, and then the stop screw is fastened to the side plate.

III. Cutting tools are cooled. Drill bit heats up strongly during operation and causes blunting of cutting edges. In order to increase the service life of the drill bit a coolant is used which is pumped from a reservoir to the drill bit.

When drilling, the coolant is applied continuously over the entire length of the drilling operation, and is mainly directed towards the cutting edges and the chips that are removed from them.

The following coolants are recommended when drilling various metals:

How to calculate and tabulate the cutting data for drilling

The most common form of mechanical hole machining is drilling. Reaming, countersinking, and reaming are also equated with.

Cutting regime is a complex of elements determining the cutting process conditions.

Drilling is accompanied by the same physical phenomena: heat release, shrinkage of chips, capping, etc. д. At the same time, the drilling process has its own characteristics. Thus, chip formation occurs under more severe conditions than in turning. Drilling makes it difficult to remove chips and supply coolant. In addition, the angle and speed of cutting are variable over the length of the blade. This creates unequal working conditions for the various points of the blade.

The elements of cutting conditions at drilling refer to. depth of cutting, feed, a cutting tool durability period, cutting speed, spindle speed, force and power of cutting.

When calculating the cutting conditions it is possible, neglecting the hardness of the machining system, to imagine that it is simultaneous boring by several cutters, so the principle of calculation will be similar to the turning operations.

Depth of cutting is determined as follows: when drilling in solid material

while drilling

at reaming

where d-diameter of earlier drilled hole, mm.

Feed rate S amount of drill bit movement along the axis one revolution. A distinction is made between the feed rate per tooth Sz, feed per revolution So and feed rate per minute S m, mm/min, which are in the following relation:

where:. cutting tool rotational speed, min.1 ;

Number of teeth of the cutting tool.

Cutting speed V peripheral speed of the point of the blade farthest from the drill axis.is determined by formula

cutting tool rotational speed, min.1 ;

Cutting speed is a variable value that varies for different points on the blade. At the center of the drill speed is zero.

The machine (main) time for drilling and reaming is calculated by the formula:

, min

where. length of working stroke.

a) drilling; b) countersinking; c) reaming

Use a vertical drilling machine to drill with a twist drill of diameter D and depth l.

it is necessary: choose a cutting tool, set the cutting mode, determine the main time, determine the cutting power.

Determination of cutting conditions for drilling operation

The purpose of the work is to learn how to determine the cutting conditions for the drilling operation, develop a card for setting up.

Determining cutting conditions begins with selecting a machine tool, cutting tools, fixture for fixing the workpiece.

Cutting conditions for drilling, reaming, countersinking are determined in the following sequence.

Deep Hole Drilling with Carbide Cutting Tools. How to drill deep and straight!

1 Determine cutting depth t, mm. The depth of cut during drilling t = 0,5D, during reaming, countersinking t = 0,5(D-d), where d and D are the diameters of hole before and after processing, mm.

2 The feed rate S for drilling is determined according to table 11[2].

If the hole depth is within 3D£ l £ 5D, the correction factor Kls = 0.9 is used; if the depth of hole is 5D l £ 7D. KIs = 0,8.

For medium machine rigidity, a correction factor for feed rate Kjs = 0.75.

The feed rate for countersinking is determined by table 12.

When countersinking blind holes the feed rate must not exceed 0.3. 0,6 mm/turn.

3 Corrected by S according to the machine data sheet (appendix A).

4 The cutting speed V, m/min is defined:

Average durability T of steel drill bit is 15 min (drill diameter up to 5 mm), 25 min (diameter 6-10 mm), 45 min (diameter 11-20 mm), 50 min (diameter 21-30 mm), 70 min (diameter 41-50 mm);

When machining gray cast iron, the durability is 20 min (drill diameter up to 5 mm), 35 min (diameter 6. 10 mm), 60 min (diameter 11-20 mm), 75 min (diameter 21. 30 mm), 105 min (diameter 31-40 mm), 140 min (diameter 41-50 min).

Average durability of tzenkers when machining steel and gray cast iron is 30 min (diameter 11-20 mm), 40 min (diameter 21-30 mm), 50 min (diameter 31-40 mm), 60 min (diameter 41-50 mm).

The coefficient Cv and degree indices are given in table 13.

Correction factor for cutting speed Kv is calculated by formula:

where Kmv. coefficient that takes into account the properties of the machined material, determined by the formula:

where sv and HP are actual parameters, which characterize the processed material;

Kg. coefficient, which characterizes the steel group (for carbon steel (C £ 0.6%). Kg = 1; chrome steel. Kg = 1.1; chromium-nickel steel. Kg = 0.9);

nv. degree index (for carbon steel (C£0,6%) sv£ 550 MPa nv =. 0,9; dc 550 MPa nv = 0,9; chromium steel nv = 1.05; chrome-nickel steel. nv = 0.9; when machining gray cast iron nv = 1,3);

Кцѵ. coefficient that takes into account the influence of the tool material (Кцѵ = 1,0);

КІѵ. the coefficient that takes into account the depth of drilling: (КІѵ = 1,0 (at hole depth 3D), КІѵ = 0,85 (at hole depth 4D), КІѵ = 0,75 (at hole depth 5D);

КІѵ = 1,0 (for reaming and countersinking).

In case of single drill bit sharpening, it is necessary to decrease the calculated cutting speed by coefficient Csv = 0.75.

5 Spindle speed n, min1. is determined by the formula:

6 Corrects n rate according to the machine certificate (Appendix A).

7 The actual cutting speed is determined:

where pst. spindle speed corrected according to the machine certificate.

8 Determine the torque M, Nm, and axial force Ro, N, according to the formulas

The value of coefficients Cm and Cp and degree indices are given in table 14.

Determination of cutting conditions for drilling

Based on given diameter of the hole and material to be machined determine tool material, drill bit diameter and its basic dimensions.

2 Cutting depth t, mm (figure 36) [1, c. 381]

Figure 36. Cutting pattern in drilling

When drilling holes without limiting factors select maximum allowable feed S (Figure 37) on drill strength [1, tab. 35, с. 381].

Figure 37. Main and feed movement

where T. dwell time, min [1, table. 40, с. 384];

coefficient and degree indexes values are chosen according to reference books [1, tab. 38, с. 383];

where. coefficient, taking into account the quality of processed material [1, tab. 1-4, с. 358-360];

5 Rotational speed of the tool, rpm [2, p. 226]

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5.1 Determination of the actual speed, rpm

ne. actual rotational speed of the tool (we choose the nearest smaller number from the number of spindle rotational speeds according to the machine’s data sheet) (Appendix C).

5.2 Actual cutting speed, m/min [2, s. 169]

where the values of the coefficient and degree indices are given in the reference literature [1, tab. 42, с. 385];

where. coefficient, considering quality of processed material [1, tab. 9-10, с. 362-363].

where the values of coefficient and degree indices are given in the reference books [1, tab. 42, с. 385].

It is necessary to compare the obtained power with the machine capacity (see Fig. Appendix B) and make a conclusion about the possibility of using the machine tool of this model for machining the workpiece.

9 Determining the Morse taper number of the shank

Mean diameter of cone of the shank [2, p. 192], mm

where D и d2. shank dimensions (table B.1);

Where Мkr. moment of resistance of cutting forces, N-m;

= 1 o 26’16”. half of the taper angle (taper equals 0.05020; sin = 0251);

Having defined the value dsr (mm), according to GOST 25557-2006 the nearest larger cone is chosen (Morse cone number) and its main dimensions are given (see. table B.1). The design of a typical twist drill is shown in Figure 38.

Figure 38. Spiral drill bit with tapered shank

10 Geometric and design parameters of the drill working part

It is necessary to determine the form of sharpening and dimensions of the drill blade elements, the angle of inclination of the helical groove. the double angle in plan 2, the angle of inclination of the transverse edge. the back angle [1, tab. 47-49, с. 228-229].

The center hole is made according to the form B (GOST 14034-74).

Drill core thickness dc affects the rigidity and vibration resistance of the drill in operation, and consequently, its durability, and is chosen depending on the drill diameter D (Table B.2).

The reverse taper of the drill bit (reduction in diameter toward the shank) is used to reduce friction of the drill bit ribbons against the walls of the hole to be machined, but it should not be too large, t.к. in this case the intensity of tool wear increases. The size of the reverse taper depends on the drill diameter D (table B.3).

Width of the flute f0 and the height of the back of the back K is chosen according to the diameter of the drill D (table B.4).

(3.12)

15 Determination of geometrical elements of the cutter profile for milling of the drill bit groove (figure 39)

Figure 39. Profile of a groove cutter

Determination of geometrical elements of mill profile for milling a drill groove is carried out if necessary by graphic or analytical method. Let us use the simplified analytical method.

Calculation example for a twist drill

drilling, cutting, hole

Task: calculate and construct a drill with conical shank made of high-speed steel for drilling a hole with diameter of D = 24 mm and the depth l = 18 mm, material to be machined. steel 50, V = 750 MPa. Vertical drilling machine 2H135.

Material of drill bit cutting part. Р6М5 (GOST 19265-73).

Material of the shank. steel 40Х (GOST 4543-71).

Based on the given diameter of the hole, we clarify according to GOST 10903-77, whether there is a twist drill of the specified size [1, tab. 45, с. 225]: D = 24 mm.

Main dimensions: L = 271 mm; l = 160 mm.

= 9,8; q = 0,4; y = 0,5; m = 0,2 [1, tab. 38, с. 383];

where [1, tab. 1- 4, с. 358-360];

= 1 [1, table. 6, с. 361];

= 1 [1, tab. 41, с. 385].

5.1 Determination of the actual rotational speed

where = 0.0345 q = 2; y = 0,8 [1, tab. 42, с. 385];

, [1, tab. 9-10, с. 362-363].

where = 68; q = 1; y = 0.7 [1, tab. 42, с. 385].

Power of the machine model 2H135 according to the passport is 4,5 kW (cm. appendix B). Consequently, the selected modes of cutting meet the passport data of the machine.

9 Determination of number of Morse taper of shank

According to GOST 25557-2006, the nearest larger cone is selected, t.е. Morse taper with foot (cm. Table B.1). Main dimensions: D = 23.825 mm; D1 = 24,1 mm; d2 = 19.1 mm; l3 = 94 mm; a = 5 mm; еmax = 20 mm.

10 Geometric and structural parameters of the working part of the drill [1, tab. 47-49, с. 228-229]:

Shape of sharpening. DP (double with undercutting of the cross edge).

Parameters of the drill bit blades: b = 4.5 mm; a = 2,5 mm; l = 5 mm; h = 2.5 mm; k = 3.6 mm; l1 = 2.5 mm.

Center hole is made according to the form B (GOST 14034-74).

11 Thickness of drill core (cm. table B.2)

12 Reverse taper of the drill bit (cm. table B.3)

Width of ribbon f0 = 1.6 mm; the height of the back of the back K = 0.7 mm.

= 0.58-24 = 14 mm. (3.29)

15 Determination of geometric elements of the cutter profile for milling drill grooves

В = R0 Rk = 12.05 4.6 = 16.65 mm. (3.35)

The design of the twist drill is shown in Figure 40.

Figure 40. Spiral drill bit according to GOST 10903-77

Countersinks are used to enlarge the diameters of cylindrical holes, in order to increase their accuracy and surface finish, to obtain holes of a given profile or to prepare them for further reaming. The kinematics of countersinking, like drilling, is reduced to the rotation of the countersink around its axis and the progressive movement of the feed along the tool axis. Countersinks allow to get holes with a tolerance of 11-12th quality and provide the roughness parameter Rz = 2040 microns.

Countersinks, like drills, have a working part 1 (Figure 41). However, a countersinker has the working part with three or more helical teeth instead of two. They are formed by helical grooves with an angle of 10° to 30°. Screw teeth are connected to form the core 7, which has a larger cross-sectional area than that of drills. This gives the countersinks a higher rigidity, which allows them to compensate for the bent hole axis during drilling.

Figure 41. Cylindrical countersink

Countersinks, as well as drills, have a reverse taper, defined by the reduction of the nominal diameter by 100 mm of the length of the working part. Working length of standard countersinks allow for repeated re-sharpening and reconditioning of worn blades of cutting teeth.

Ribbon 11 along the edge of the helical groove in the calibrating part 2 is necessary for directing it during operation, gives the final size of the hole and surface finish.

On the cutting part 3 which is adjacent to the working face of the countersinker there are cutting edges 8 of all its cutting teeth. The front surfaces 9 of the countersink teeth are the helical surfaces of the countersink grooves. Back surfaces 10 of the teeth are inclined end surfaces.

Shank 5 with a foot 6 and a neck 4 serve for attaching the countersinker to the machine tool. Shanks come in tapered and cylindrical shapes. The most common countersinks are those with a tapered shank.

Countersinks are divided into the following main groups according to the type of treatment:

Figure 42. Types of countersinks

According to the method of attachment, countersinkers are divided into tail and attachment. They can be solid or prefabricated, made of tool steels and carbide.

Cylindrical countersinks for reaming holes Most common in industry. The one-piece cylindrical countersink is shown in Figure 41.

As it was mentioned above, countersinks with the conic shank are most often used, but countersinks with the cylindrical shank for the quick-change chuck are also used. The advantage of this design is the quick installation and removal of the tool. The tapered shank gives better tool centering.

drilling, cutting, hole

In order to save tool materials countersinks of big diameters are made as one-piece and collapsible (figure 43).

Figure 43. Cylindrical countersinks

One-piece countersinks (figure 43, а) are designed with helical grooves. Number of teeth of such countersinks. 4, their diameter D 32-80 mm.

A prefabricated countersink, which allows for diameter adjustment, is shown in Figure 43, б. The teeth can be made of high-speed steel or steel 45 with brazed carbide inserts. Number of teeth z = 4.

A slip-on countersinker with tungsten carbide plates soldered directly to the body is shown in Figure 43, в. Countersinks of this type are made with four teeth, 34 80 mm in diameter.

To provide the alignment of a cylindrical recess with a prefabricated hole, countersinks for cylindrical recesses are equipped with a countersink trunnion guide (see Single-piece tool. figure 42, б). The countersink is made in one piece with a countersink or a removable countersink. Countersinks with a removable trunnion are easier to sharpen, because the front teeth are sharpened with the trunnion removed. But in countersinks with a trunnion made in one piece with the body the trunnion guide is worn out during re-sharpening, so after several sharpening operations the countersink is not needed.

The interchangeable guide part expands the field of application of the countersinker, as it allows to install pins of different diameters and to work on different surfaces.

Countersinks for tapered countersinks (cf. figure 42, в) are designed for tapered bores of small depths. They are designed with straight teeth with a flat front surface. The countersink has a tooth count ranging from 6 to 12, depending on the size. Also such countersinks are called countersinks.

Countersinks (see figure 42). figure 42, г) are designed for machining the face surfaces of bosses, different countersinks, etc.п. These countersinks have teeth on the front face only and vary in number from 4 to 6. The teeth of end countersinks are often made of carbide, especially when machining cast iron.

Example of calculation of the cutting mode (drilling)

In a lever made of steel 45 (σc = 750MPa) is necessary to drill a through hole Ø 20HI2 (Fig. 2.2).

Starting point. Forged forging without a hole, weight 2.5 kg.

Vertical drilling machine tool model 2HI35, Working part of the drill bit is made of steel Р6М5.

Fig.2.4- Lever Data sheet of the machine tool 2H135: maximum diameter of machining hole of steel. 35 mm; engine power. 4,5 kW; machine efficiency. 0,8.

Spindle speed (min.1 ): 31,5; 45; 63; 90; 125; 180; 250; 355; 500; 710; 10004 1440.

Feed rate (mm/rev): 0.1; 0.14; 0.2; 0.28; 0.4; 0.56; 0.8; 1.12; 1.6.

Maximum axial cutting force allowed by the machine mechanism 1500 kg (1500N).

The depth of cut for drilling in solid material is equal to half of the drill bit diameter t = D/2.

We choose the drill bit 20 mm of normal sharpening with jumper sharpening (Normal-NP). Drilling is carried out with cooling.

Choice of feed rate (table 1.11) (for 240300 NV), drilling diameter 1625 mm). Feed rate table. 0,230,32 mm/rev. We take the average value of feed, equal to 0,27 mm/rev.

According to the datasheet of the machine the feed is corrected up to Sst = 0,28 mm/rev.

Checking the maximum axial cutting force is not required because the drilling diameter is less than the maximum 35 mm.

The cutting speed is determined by the empirical relationship.

By substituting in the calculation formula we get:

V = (9,8 · 20 0,4 ) / (60 0,2 · 0,28 0,5 ) = (9,8 · 3,31) / (2,26 · 0,529) · 0.8 = 21.7 m / min.

Kp = Kmr = (σin / 750) n = (750/750) 0.75 = 1.0. Mkr = 10 ? 0.0345 x 20 2.0 x 0.28 = 10 x 0.0345 x 400 x 0.30 = 49.68 N x m Ne = (49.68 · 355) / 9750 = 1.8 kW.

Required power 1.8 kW and less than the power developed on the spindle 4.5 · 0.8 = 3.6 kW.

The main technological time for machining the hole:

In a lever of steel 45 (σc = 750MPa) is necessary to drill a through hole Ø 20HI2 (fig. 2.2).

Initial workpiece. Forged forging without hole, weight 2.5 kg.

Vertical drilling machine model 2HI35, Working part of drill made of steel Р6М5.

Fig.2.4 Lever Passport data of the machine 2H135: Maximum diameter of the machined hole of steel. 35 mm; motor power. 4,5 kW; machine efficiency. 0,8.

Spindle speed (min.1 ): 31,5; 45; 63; 90; 125; 180; 250; 355; 500; 710; 10004 1440.

Feed (in mm/turn): 0,1; 0,14; 0,2; 0,28; 0,4; 0,56; 0,8; 1,12; 1,6.

Maximum axial cutting force allowed by the machine mechanism is 1500kg (1500N).

The depth of cut for drilling in solid material is equal to half of drill diameter t = D/2.

We choose the drill bit 20 mm of normal sharpening with jumper sharpening (Normal-NP). Drilling is carried out with cooling.

Choice of feed (table 1.11) (for 240300 NV), drilling diameter 1625 mm). Feed rate table. 0,230,32 mm/rev. We take the average value of feed, equal to 0,27 mm/rev.

According to the machine certificate the feed is corrected up to Sst = 0,28 mm/rev.

Checking by the maximum axial cutting force is not required, since the drilling diameter is less than the maximum 35 mm.

The cutting speed is determined by the empirical dependence.

Substitution of this formula into calculation formula gives us:

V = (9,8 · 20 0,4 ) / (60 0,2 · 0,28 0,5 ) = (9,8 · 3,31) / (2,26 · 0,529) · 0,8 = 21,7 m/min.

Cp = Cmr = (σin / 750) n = (750/750) 0.75 = 1.0. Mcr = 10 ? 0.0345 x 20 2.0 x 0.28 = 10 x 0.0345 x 400 x 0.30 = 49.68 N x m Ne = (49.68 · 355) / 9750 = 1,8 kW.

Required power of 1.8 kW and less than the power developed on the spindle 4.5 · 0,8 = 3,6kW.

Calculation of the cutting action when drilling

Calculate cutting conditions while drilling according to figure 2.1 (8.2, a [1]) and table. 2.1 (8.2, B-20[1]) machining with cooling.

Choice of tool material and cutting part geometry

Choice of tool material grade, drill design and geometry

In the manufacture of drills, high speed steels are mainly used. According to table. П.1.1 [1] according to the material to be machined we choose Р6М5. Spiral drills of high-speed steel ([4], tab. 40 pp. For CNC machines with tapered shank (tapered shank ensures quick change of worn tool), GOST 12121-77.

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Shape of sharpening. biplanar (tab. П.4.5 [1]).

Drill sizes are set depending on the processed material: drill length L = 197 mm,

Working part length l=116 mm ([4], tab. 42 page.147),

Let’s choose the Morse taper relative to the drill bit diameter ([4] tab.42, pp. 150)

The geometric parameters of the drill bit are determined by the following criteria :

1) depending on the material of the workpiece [No. gr., s in ] ;

2) depending on the material of the cutting part of the drill bit [Р6М5] ;

2.2 Choice of cutting depth t

When drilling, depth of cut t=0,5D (1)

2.3 Feed rate selection s

The feed during drilling, without limiting factors, is chosen as the maximum allowable by the strength of the drill [1,P. 4.6]. For drill diameter 10 mm and material Steel 5 (170HB), maximum feed Sî = 0,25 mm/rev.

After selecting the feed rate, its value is corrected by the machine.

According to the machine tool passport 2S132PMF2 [1Table. П.7.9] we take the closest feed s=0,25 mm/rev.

2.4 Calculation of cutting speed V

The cutting speed, m/min, in drilling is determined by the formula:

where Cv is a constant for a particular group of machined material;

q, m, y. coefficient and exponents of degree;

Kv. correction factor for cutting speed

The value of the coefficient C V and degree indexes х, у и m are given in table. П.4.7. С V =9,8; q=0,4; y=0,5; m=0,2.

The values of the durability period Т. in Tab. П.4.9[1].

Recommended durability period Т=25 min

General correction factor K V is the product of coefficients of influence of workpiece material KMV, tool material KIV, coefficient of drilling depth KIV.

By formula (1.2) and the values from (tab. П.3.15, [1]) for drilling we calculate КМV

Calculation of spindle turns п is determined by formula (1.2) where D. drill diameter, mm:

Let’s define more exactly number of revolutions (Tab. П.7.8, [1]), we will take n=1800 rpm.

Let’s calculate the specified cutting speed.

2.5 Calculation of torque Мkr and axial force Р

Calculation of torque, N-m, and axial force, N, while drilling is made by formulas

The values of the coefficients CM и Sr and degree indexes q. y are given in (tab. П.4.11, [1]).

The coefficient Kr, which takes into account the machining conditions, depends only on the material of the machined workpiece Kr = KMr (tab.П.3.22,[1]).

Maximum axial cutting force allowed by the machine’s feed mechanism, Qst=24500 N [1 tab. П 7.5]

The greatest force allowed by the machine’s feed mechanism is compared with the axial component of the cutting force. If this condition is not satisfied, it is necessary to reduce feed.

2.6 Calculation of cutting power N cut

Power of cutting, kW, is calculated by formula

The power at the machine tool spindle is compared with the power of the cutting process. if this condition is not satisfied, it is necessary to reduce the cutting speed.

Power at the spindle of the machine, kW, we calculate by the formula

2.7 Calculation of machine time Тм

where L. total length of the tool passage, mm;

п. number of spindle revolutions per minute;

where l. length of the machined surface, mm;

At double sharpening of drill bit l1=0,35D=0,3510=3,5 mm [4];

l2 tool output (overrun) value, mm.

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Cutting conditions for drilling.

holes are made with a certain accuracy of size, shape, location and roughness according to the technical requirements of the working drawing.

Methods of machining holes on a lathe.

Drilling is the process of obtaining blind and through cylindrical holes in solid material with an accuracy of H12-H13 qualifications and a surface roughness of Rz 20. Rz80 (roughing).

Reboring. increasing the diameter of the drilled hole.

When machining holes on a lathe the main motion is rotation of the workpiece, the feed motion is movement of the drill along the hole.

Drill bits are divided into two groups: twist drill bits and special drill bits.

Elements of the twist drill bit.

Drill shank: tapered (for diameter from 6 to 80mm) and cylindrical for small diameters up to 20mm.

Material: high-speed steel P6M5, as well as drill bits equipped with hard alloy plates VK8.

Angle at apex. 2. For general purpose drills 116 0. 118 0. Inspection of sharpening is carried out with a template.

Drill bits mounting: with conical shank in the tailstock quill (adapter sleeves); with cylindrical shank in the drill chuck.

Drill bit installation: firmly fixed without noticeable runout; perpendicular to the workpiece face; no bumps on the face; coincidence of quill axis with spindle axis.

Preparing for drilling: trim the face cleanly; make a small tapered depression in the center of the face.

Cooling: steel. emulsion; base metals. with cooling or dry; cast iron. without cooling.

Reboring of holes. Holes with diameters greater than 30 mm are drilled with two drills. Diameter of the first drill bit is half of the hole diameter.

Cutting conditions for drilling.

Depth of cut during drilling. half of the drill bit diameter t = (mm), where D is the drill bit diameter;

at reaming. t = (mm).

2.Feed rate S = (mm/rev). movement of the drill per one revolution.

3.cutting speed v is determined according to the formula

where D. drill diameter; n. number of spindle revolutions per min.; = 3,14. constant number. The rotational speed of the cutting tool is determined by the formula:

n = min.1

Requirements for machining holes:

holes are made with certain accuracy of size, shape, location and roughness according to the technical requirements of the working drawing.

Methods of machining holes on a lathe.

Drilling is a process of getting blind and through cylindrical holes in solid material with accuracy of H12-H13 grades and surface roughness of Rz 20. Rz80 (roughing).

Reaming. increasing the diameter of the drilled hole.

When turning holes on a lathe the main motion is to rotate the workpiece, the feed motion is to move the drill along the hole.

The drills are divided into 2 groups: twist drill bits and special drill bits.

Elements of twist drill bit.

Shank: conical (for diameters from 6mm to 80mm) and cylindrical for small diameters up to 20mm.

Material: high-speed steel R6M5, as well as drills are equipped with hard alloy plates VK8.

Corner at apex. 2. For general purpose drills 116 0. 118 0. Checking the sharpening with a drilling template.

Drill bits installation: with conical shank in the tailstock quill (adapter sleeves); with cylindrical shank in the drill chuck.

Drill bit positioning: firmly fixed without any noticeable run-out; perpendicular to the workpiece face; no bumps on the face; coincidence of pin axis with spindle axis.

Preparation for drilling: trim the face cleanly; make a small tapered depression in the center of the face.

Cooling: steel. emulsion; base metals. with cooling or dry; cast iron. without cooling.

Hole boring. Drill holes larger than 30 mm in diameter are made with two drill bits. Diameter of the first drill bit is half the hole diameter.

Cutting conditions for drilling.

Depth of cut during drilling. half of drill diameter t = (mm), where D. drill diameter;

at reaming. t = (mm).

2.Feed rate S = (mm/rev). movement of drill bit per one revolution.

3.Cutting speed v determined according to the formula

where D. drill diameter; n. number of spindle revolutions per min.; = 3,14. constant number. The rotational speed of the cutting tool is determined by the formula:

n = min.1

Hole drilling

Holes in solid material are created by drilling. Drilling and reaming on lathes is mainly used as a preprocessing method.

Drilling is carried out with a rotating workpiece, less often with a rotating drill fitted in the machine spindle

Hole drilling produces hole size accuracy up to the 12th quality class and roughness up to the 3rd-4th quality class. Boring enlarges the diameter of the previously drilled hole and can increase the accuracy of the hole by about the same degree of accuracy under certain conditions.

The metal turning under consideration, is performed on lathes, and mainly spiral drills are used as cutting tools.

Spiral drill bit is a two-tooth cutting tool consisting of a working part, a neck and a shank. The working part involves a cutting part and a guide part.

Drill setting on the machine

Drilling on a lathe is performed by a non-rotating drill, which is fixed in the tailstock quill.

Drills with a tapered shank are installed directly into the quill hole if their dimensions are the same, or by means of adapter sleeve 2. dressed on the drill bit shank 1.

Drill bits with a cylindrical shank are fixed on the machine by means of drill chucks, one of the designs of which is shown in Fig. 55, а. In the inclined holes of the housing 3 cams 4 in the form of cylindrical rods with bevels for fixing the drill bit and a threaded part on the outer surface are installed. Inside the sleeve 5 is fixed nut with tapered thread, which connects to the thread cams. If the key 2 rotates the coupling, the cams, moving in inclined holes will be compressed, ensuring fixation and centering of the drill. Body 8 has a tapered hole on the back side, with which it is firmly seated on the shank 1. Such chucks are produced in three sizes: PS-6, PS-9, PS-16 (figures designate the largest diameter of the fastened drill bit).

If a frequent change of the tools installed in the tailstock is required, it is convenient to use quick-change chucks (Fig. 55, б). The cartridge consists of the shell 2 with a conic shank 6 and two bores in which the balls 3 are free-folded. Adapter sleeve 1 with conical Morse hole is installed in the housing. On the outer surface of the sleeve two radius grooves are made, in which balls are sunk when the chuck is in working position. On the housing loosely mounted clutch 4, the longitudinal position of which is limited by spring rings 7 and 9 and spring loaded ball 5, locking the clutch in operating condition. The hole 5 is provided for air outlet during installation of the adapter sleeve into the chuck.

The action of the chuck is as follows, The required drill bit is inserted into the adapter sleeve and together with it is set into the chuck. The clutch is shifted to the right. Then when the sleeve is moved to the left it pushes the balls into the recesses of the sleeve and secures it. To change the tool, just move the sleeve to the right, and the sleeve with a drill bit is freely withdrawn from the chuck.

For drilling with mechanical feed is sometimes used a simple device in the form of a sleeve with a rectangular shoulder, which it is fixed in the tool holder slide.

In deep hole drilling it is necessary to frequently remove the drill bit from the hole in order to clean the chips. In this case, to significantly reduce the time of withdrawal of the drill and return it to its original position is possible by using a fairly simple chuck (Fig. 55, c). It consists of a housing 2 with a tapered shank, a drill bit holder 1 with a screwed-in handle 3. There is an oblong groove in the housing with a series of cross grooves. To retract the drill, simply pull the knob out of the groove and swing the drill to the right. Return of the drill bit to the working position is carried out in reverse order.

Preparing for drilling

Important conditions for high-quality turning a hole with a drill: firmly clamping the workpiece without noticeable runout, perpendicularity of its end face to the axis of rotation, the lack of bumps and convexity on the face, the coincidence of the axis with the axis of the spindle, giving the original direction drills.

The workpiece, installed in the lathe chuck, if necessary, aligned and firmly fixed. Its face is cleanly trimmed before drilling. To give the initial direction to the drill, especially with its long length, it is recommended to make a small cone recess in the center of the face. It is carried out with a stop cutter (Fig. 56, a) or a short rigid drill (Fig. 56, b). The angle of the center depression is made by 20-30° less than the angle at the top of the working drill. Under this condition, the bridge of the drill bit in the initial moment will not participate in cutting (Fig. 56, b), thus greatly reducing the danger of the drill drifting to the side.

To increase the rigidity of long drill bits, it is recommended to support them at the beginning of drilling with the reverse side of the cutter, secured in the toolholder slightly above the axis of the centers.

Before drilling a deep hole, the workpiece should first be overdrilled with a short drill of the same diameter to a depth approximately equal to the diameter of the hole. The pilot drill then cannot deflect to the side after it has received its original heading.

Correct drill setting is just as important. Its shank and quill bore should be wiped dry and the dents on the shank removed with a file. Insert the drill bit into the quill with a jolt.

Drilling techniques

Usually applies the following method of drilling on a lathe After preparatory work include the spindle and manually turn the hand wheel tailstock lead to the end face rotating workpiece. Avoid hammering when drilling, otherwise the drill bit might break. In the beginning, the drill bit is fed forward slowly, but when it penetrates the metal to a depth slightly greater than the length of the cutting part, the feed can be increased. The drill bit feed must be smooth and without jerks.

Particular caution must be exercised when the drill bit comes out and through holes. At this point the cutting edges are under unequal strain and can burst out. Therefore the feed rate should be lowered sharply at the exit.

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Before turning off the spindle, the drill bit must be withdrawn from the hole, otherwise it may jam in the hole due to the elastic deformation of the metal.

When drilling, cuttings hardly come out of the holes, therefore, the drill bit should be periodically cleaned with a wire brush.

The depth of the deep hole is checked against the millimeter scale of the quill, against the handwheel limb of the tailstock, or, in their absence, against the chalk line that is applied to the drill bit.

It is recommended to cool the drill bit to increase durability. Steels are drilled with emulsion, non-ferrous metals with cooling or dry, cast iron without cooling. The coolant jet is directed onto the drill bit near the face of the workpiece and is switched on at the same time as the start of cutting.

Manual feed of the drill bit, especially in large diameter holes, is too difficult. That is why some models of modern lathes have a device for mechanically moving the tailstock. It consists of a locking device. consisting of two angle bars that are fastened to the cross slide and tailstock plate respectively. The tailstock must be removed from the machine bed before starting the mechanical feed.

Countersinking

Large diameter holes are difficult to drill because of the high feed forces. That is why holes over 30 mm in diameter are drilled with two drill bits. The diameter of the first of them is taken equal to about ½ of the hole diameter. That way the bridge of the second drill bit does not participate in cutting, greatly reducing the feed force and reducing the likelihood of drift of the drill bit to the side. Reaming techniques are the same as for drilling.

Cutting conditions for drilling and reaming

The cutting depth t of drilling is characterised by the drill bit size and is equal to ½ the drill bit diameter. For boring, it is determined by the half-difference of hole diameters after and before machining.

The feed rate S for drilling and reaming corresponds to the axial movement of the drill bit per revolution of the workpiece and is expressed in mm/rev.

The cutting speed v for a non-rotating drill bit is equal to the circumferential rotation speed of the machined hole surface in m/min.

The drill feed is most often applied manually on lathes. When working with a mechanical feed for holes of diameter from 5 to 30 mm in steel workpieces, it can be selected within the range of 0.1-0.4 mm/rev. Larger feeds within specified limits are accepted for drills with larger diameter. When drilling cast iron the feed can be increased by about 1.5 times; the same is true for reaming holes.

Cutting speed for high-speed drills in steel and cast iron workpieces is selected within the range of 20-40 m/min; for drills equipped with tungsten carbide plates it can be increased by 2-3 times. Large cutting speeds are used for drills with smaller diameters.

Features of deep hole drilling

When processing deep holes, operating conditions for twist drill drills worsen dramatically: difficult exit chips and coolant supply to the cutting edges, reduced rigidity and the risk of pulling the drill sideways. In such cases, it is recommended to use drills for deep drilling, the design of which provides for the possibility of partial or complete elimination of the above drawbacks.

Cutting edge cooling and chip exit from deep holes are improved when using helical drills with channels for supplying coolant under pressure (Fig. 58, а). However, these drills, having insufficient rigidity, do not ensure strict straightness of the hole axis, and are used only for machining holes of low accuracy.

To improve the direction of the drill bit in the hole and the cooling conditions for cutting edges, quadrilateral helical drills are used (Fig. 58, б). Such drills have a slightly increased thickness of the core, and on the backs of each tooth two guide strips are made. The additional grooves 1 thus formed allow the fluid to freely approach the cutting edges without encountering red-hot chips on its way. When such drills are used, the accuracy of processing of holes is somewhat increased, but the drawbacks inherent to conventional twist drills (low rigidity, the presence of a bridge) remain.

Deep, high-precision holes are machined with cannon drills and gun drills. Characteristic feature of their design. presence of one tooth and large guiding surface.

Cannon drill bit (fig. Fig. 58, c) is a round rod with a cylindrical shank 3. To form cutting edge 1 and space for chip exit, working part 2 of the drill is cut off along the radius, and to reduce friction against the hole wall a small inverse conicity is created on the guiding part. The disadvantages of these drills are: difficult exit of chips from the hole and insufficiently effective cooling of the cutting edge.

The gun drill bit (Fig. 58, d) is usually made of a tube of high-speed steel. Along its entire length, with the exception of shank 3, an angular chisel groove is fluted. A sickle-shaped channel is created inside the drill bit through which the coolant is fed. Pressurized jet of fluid fed under high pressure not only intensively cools the cutting edge, but also washes chips out of the hole. Due to the broken shape of the cutting edge 1, wide chips are separated and a centering cone is formed at the bottom of the hole, which improves the direction of the drill during cutting.

To give gun drills and gun drills their original direction, the hole is pre-drilled with a short spiral drill bit.

Drilling, reaming, countersinking

Drilling is the most common method of producing holes in solid material.

Cutting conditions for drilling

Spiral drill bits made of tool steels, high-speed steels, and hard alloys are used for drilling holes.

For drills from high speed steels cutting speed v=25-35 m/min, for drills from tool steels v=12-18 m/min, for carbide drills v=50-70 m/min. At the same time, large values of cutting speed are taken with increasing drill diameter and decreasing feed.

Standard drill bits have an apex angle of 118 degrees, but for machining harder materials (and deeper holes) it is recommended to use drill bits with an apex angle of 135 degrees.

Drills with conical shanks are installed directly into a tapered hole of the tailstock quill, and if cone dimensions do not match, then adapter sleeves are used. To fix drill bits with cylindrical shanks (up to 16 mm in diameter) drill jaws chucks are used. which are installed in the tailstock quill. The drill bit is fixed by cams 6 which can be brought together or apart by moving in the housing slots 2. On the ends of the jaws there are bars that are in meshing with the thread on the inner surface of the ring 4. From the key 5, through the bevel gear, the sleeve 3 with the ring 4 is driven in rotation, along the thread of which the cams 6 move up or down and simultaneously in the radial direction. For mounting in the tailstock quill, chucks are equipped with tapered shanks 1.

Cooling while drilling

To reduce friction between the tool and the hole wall, drilling is carried out using coolant, especially in steel and aluminum workpieces. Cast iron, brass and bronze workpieces can be drilled without cooling. Cooling during drilling lowers the temperature of the drill bit heated by the heat of cutting and friction against the hole walls, reduces the friction of the drill bit against these walls and, finally, contributes to chip removal. Use of coolant helps to increase cutting speed by 1,4-1,5 times.

Emulsion solution (for structural steels), compound oils (for alloy steels), emulsion solution and kerosene (for cast iron and aluminium alloys) are used as coolant. If the machine does not have a coolant, then a mixture of machine oil and kerosene is used as a coolant.

Tool protection while drilling

To keep the tool safe when drilling one should work with maximum permissible cutting speed and minimum allowable feeds. In feed drilling, when the drill bit comes out of the workpiece, the feed must be reduced sharply to prevent drill bit breakage.

Special care must be taken when the depth of the hole to be drilled is greater than the length of the drill bit’s operating section. If the entire helical groove of the drill bit ends up in the hole, the chips generated during drilling will not have an exit, will fill the grooves and the drill bit will break. In such cases, it is necessary to occasionally withdraw the drill from the hole and remove chips from both the hole and the drill grooves.

An improperly sharpened drill bit results in a tapered hole with a very rough surface. In addition, a drill bit that is not sufficiently sharpened (blunt) will cause burrs to form at the exit end of the hole. Unequal length of cutting edges and their asymmetrical sharpening, eccentric location of the bridge and different width of the ribbons cause the drill bit to jam in the hole, which increases the friction forces and leads to tool breakage.

Increased drilling efficiency

To improve the efficiency of helical drills, the following methods are used:

  • cross-edge undercutting,
  • Angle change at the apex,
  • band sharpening,
  • double sharpening,
  • predrilling of holes, etc.

Accuracy and surface roughness achieved by drilling

drilling holes with a hole diameter that is slightly larger than the diameter of the drill bit. This is due to the fact that the drill leads away from the axis of the hole, even with minor errors made in the sharpening of the drill bit and its installation on the machine, as well as the uneven hardness of the machined material.

Reboring of holes

When drilling large diameter holes, the feed pressure can be too high, which is very tiring for the operator. Sometimes the power of the machine may not be enough when working with such drills. In such cases, the formation of the holes is made with two drills of different diameters in succession, the ratio of which should be such that the diameter of the first drill is greater than the length of the transverse edge of the second drill. That way the lateral edge of the second drill bit is not involved in cutting and the amount of feed force required is greatly reduced, along with an important reduction in drill drift away from the workpiece axis.

In practice, it is usual to take the diameter of the first drill bit equal to about half of the second one, which provides favorable conditions for drill bit wear and equal distribution of feed force during the work of both drills.

Reaming makes it possible to obtain more accurate holes and reduces the drift of the drill from the workpiece axis. Cutting conditions for hole reaming are the same as for drilling.

The countersink is a more productive tool than the twist drill for enlarging the diameter of holes made by casting or punching.

Countersinks are made of high-speed steel, less often for difficult cutting conditions, they are equipped with carbide plates.

Countersinks with a tapered shank are used for bores of 10 to 40 mm in diameter. In appearance they are somewhat similar to twist drills, but have three helical grooves and therefore three cutting edges, which increases the rigidity of their construction, allows to increase the cutting conditions in comparison to reaming, and consequently the productivity.

Bit countersinks in one piece and tungsten carbide plates are used for bores of 32 to 80 mm in diameter. These countersinks have four helical grooves and therefore four cutting edges. They are held in the headstock of the machine by a mandrel where they are centered by the tapered bore. For large bores with diameters from 50 to 100 mm, countersinkers are made with inserted blades.

To prevent the countersinker from turning while working, two dowels (keys) are made on the mandrel, which enter into the appropriate grooves of the countersinker.

A countersinker, which removes a small allowance and is guided by three (or four) tapes, produces a hole diameter that is more accurate than a drill. The lack of sideways movement of the countersink away from the axis of the hole being worked ensures straightness of the latter better than when working with a drill. To reduce countersinking retraction, particularly with deep cast or tapped holes, the countersink should be bored to a depth approximately half the length of the countersink before countersinking.

The countersink is stronger than the drill, so the feeds (per revolution of workpiece) for countersinking can be greater than for drilling. A countersink, on the other hand, has a greater number of cutting edges than a drill, so the chip thickness of each edge is smaller than the chip thickness of a drill. Thanks to this, the surface of the hole worked with the countersinker is cleaner. This allows the use of countersinks not only for roughing, but also for semi-finishing holes after a drill, roughing tool or roughing cutter before reaming, and even for finishing holes.

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