Cutting Tools for Hole Machining

Chapter 4: Design of Drills Cutting Tool Technology

Dr. Rasheed Amirah 57

CHAPTER 4 Cutting Tools for Hole Machining

Design of Drills

Drilling is one of the most widely used methods of making holes.

The cutting tool in this case is a drill (Figure 4.1).

Fig. 4.1: Elements of a twist drill.

Drill is a multiple-cutting edges tool used for production a hole in a solid workpiece. It may also be used for enlarging and existing holes.

During drilling rotary motion about the axis of the tool and straight line feed motion along the tool axis are required (both of these motions are imparted to the drill).

The drill is clamped in the spindle which rotates it and feeds it downward into the workpiece clamped stationary on the table.

On the other hand, in turning machine it is convenient to rotate the workpiece and the hole tool is not allowed to rotate, but fed axially.

Drill is a more complex tool than a SPT. The machining process goes in complex conditions:

1. Flow of chip and the cutting fluid; 2. Friction due to the flow of the chip and rubbing of the tool with the

generated hole;



3. Cutting speed varies from max. to zero along the cutting edge; 4. The rake and clearance (relief) angles vary along the cutting edge of

a twist drill.

These factors make the chip formation process in drilling and drill more severe (complex).

4.1. Principle Elements and Parts of the Drill:

Twist drills are the most commonly used in normal circumstances in making a hole. So the main elements and parts of the twist drills as shown in (Figure 4.1) are:

Body – the portion of the drill extending from the shank or neck to the outer corners of the cutting lips.

Point - The cutting end of a drill, made up of the ends of the lands and the web. In form it resembles a cone.

Point angle – The angle included between the cutting lips projected upon a plane parallel to the drill axis and parallel to the two cutting lips.

Cutting edges or lips – the lines of intersection of the faces in the drill flute and the relief faces in the flanks, produced by grinding, extending from the chisel edge to the periphery.



Chisel edge – The edge at the end of the web that connects the cutting lips.

Chisel edge angle – The angle included between the chisel edge and the cutting lip.

Face – The surface on which the chip impinges and along which it flows as it is separated from the workpiece.

Lip relief surfaces (flanks) – The surfaces of the tool facing the workpiece.

Web – The central portion of the body that joints the lands. The extreme end of the web forms the chisel edge on a two-flute drill.

Web thickness – The thickness of the web at the point, unless another location is indicated.

Land – The peripheral portion of the body between adjacent flutes.

Land width- The distance between the leading edge and the heel of the land.



Drill diameter – The diameter over the margins of the drill measured at the point.

Flutes – Helical or straight grooves cut or formed in the body of the drill to provide cutting lips, to permit chip removal, and to allow cutting fluid to reach the cutting lips.

Margin – The cylindrical portion of the land which is not cut away to provide clearance.

Clearance – The space provided to eliminate undesirable contact between the drill and the workpiece.

Clearance diameter – The diameter over the cutaway portion of the drill lands.

Helix angle – The angle made by the leading edge of the land with a plane containing the axis of the drill.

Heel – The trailing edge of the land.

Neck – The section of reduced diameter between the body and the shank.



Shank – The part of the drill by which it is held and driven.

Tang – The flattened end of a taper shank intended to fit into a driving slot in a socket.

4.3. Principal Elements in the Constructions of Drills:

according to their constructions, all existing types of drills can be classified into the following main groups:

1. Twist drills, 2. Straight-flute drills, 3. Flat drills, 4. Deep-hole drills, 5. Trepanning drills, 6. Gun drills, 7. Tapered drills, 8. Centre drills and 9. Special-purpose combination drilling tools.

Each of the enumerated types can be further divided into many design versions.

In the construction of the drill the elements to be considered for the design are:

1. drill diameter D;

2. point angle 2φ;

3. flute helix angle ω;

4. elements of the drill point geometry γ, α. and δ (rake, relief and cutting angles);

5. web thickness (core diameter) d;

6. land width b;

7. margin width f;

8. back taper; 9. shape of the lip and flute profile;

10. flute length lf;

11. And overall length Lo.



4.3.1. Drill diameter D:

The diameter of the drill should always be slightly smaller than the diameter of the hole to be drilled, since drills always cut oversize.

To reduce friction between the drill and the machining surface, the diameter of the drill over the margins at the body of the drill (finishing section) is slightly tapered back towards the shank.

The back taper is to provide longitudinal clearance.

The recommended back taper can be selected from the tables 4.1 and 4.2).

Table 4.1: recommended value of the back taper.

Drill diameter, mm 1 to 6 Over 6 to 18 Over 18

Back taper, mm/ 100 mm of length 0.03 to 0.07 0.04 to 0.08 0.05 to 0.10

Table 4.2: HSS twit drill standards.

№ Drill dia.

mm Helix

angle, ω

Web thickness,

W

Back taper in 100 mm

Cylindrical land (Margin)

Length

Width, f

Height, h

Flute, Lf

Over all, Lo

1 0.25-0.35 18o 0.3D 0.015

Refer

Table

16

Refer

Table

16

2 0.40-0.45 19o 0.3D 0.015

3 0.5-0.70 20o 0.3D 0.02 0.2 0.1

4 0.75-0.95 21o 0.3D 0.02 0.3 0.1

5 1.0-1.9 22o 0.27D 0.03 0.4 0.1

6 2.0-2.9 23o 0.27D 0.04 0.6 0.15

7 3.0-3.4 24o 0.27D 0.05 - -

8 3.5-4.4 25o 0.27D 0.06 - -

9 4.5-6.4 26o 0.27D 0.07 0.7 0.2

10 6.5-8.4 27o 0.25D 0.08 - -

11 8.5-9.9 28o 0.20D 0.09 0.8 0.3

12 10-18 30o 0.16D 0.10 1.0 0.5

13 Over 18 30o 0.13D 0.10 0.07D 0.03D

NOTES: - Web thickness increases 1.4 to 1.8/100 mm towards shank. - The helix angles are for drilling steel with σt up to 70 kgf/mm2. Refer table 13 for other

workpiece materials. ___________________________________________________________________________



4.3.2. Point angle 2φ:

Productivity and durability of drill in many respects depend on the value of the drill point angle 2φ.

So as so this angle (2φ) of drill has significant effect on the:

Cutting force components; On the length of the cutting edge (lip); On the elements of the chip section.

increasing of the drill point angle (2φ) lead to:

Decreasing the active length of the cutting edge and increasing the thickness of the cut-off layer,

- in this case the forces which have effect on the cutting edge length increase, causes increasing of the wear of the drill.

The section of the cut-off (removing) layer remains constant, but its (cut-off layer) deformation decreases, total cutting force component, which determines the torque, decreases.

Increasing the total axial cutting force of drill, because the position of the plane (N-N, Figure 3.7), perpendicular to the cutting edge, changes relative to the axis of the drill, in this case part of the forces, which effect on the cutting edge, mutually is balanced.

Fig. 4.7

Decreasing the rake angle, causes worsens of penetration of this edge into the workpiece material, which leads to increasing the axial forces, because of that, the danger of the appearance of the drill buckling increases.

A smoother change in the rake angles along the main cutting



edge, therefore the drill cutting ability improves and the chip disposal facilitates.

Experiences show, that with decreasing the point angle (2φ) from 140°

up to 90°, axial cutting force decreases to 40-50%, and torque increases about 25-30%.

The standard drill point angle is (118°). It is used for general purpose drilling of wide variety materials, including mild steel, cast irons, and many alloy steels.

Point angles smaller than 118° are preferred for many cast irons, copper, fiber aluminum alloys, die castings, and abrasive materials.

Point angles greater than 118º are used for hard steels and other difficult materials.

According to the experimental data and manufacturing experience, recommended values of the point angle 2φ for machining different workpieces materials are given in table 4.3 and 4.4.

4.4.3. Flute helix angle ω:

Flute helix angle (ω) of drill effects on the:

Strength of drill; Rigidity of drill; Chip disposal.

Increasing this angle leads to:

1. Increasing the rake angle γ; 2. Facilitate the cutting process; 3. Improving the chip disposal; 4. Increasing the torsion rigidity of the drill; 5. But in the same time the rigidity in the axial direction decreased.



Table 4.3: The recommended values of rake angle γ and point angle 2φ of drills tipped with cemented carbides.

Table 4.4: Recommended Drill-Point Geometry For Various Materials.

Material

Hardness, BHN (Rockwell values in

parentheses)

Point Angle, deg.

LP relief angle, deg.

Chisel Edge angle, deg.

Helix Angle, deg.

Point Grind*

Free-machining plain carbon steels, plain carbon steels, free-machining alloy steels, alloy steels, nitriding steels, armor plate, tool steels, cast steels.

100-225 225-325 325-425 RC45-52

118 118

118-135 118-135

12-15 10-12 8-10 7-9

125-135 125-135 125-135 125-135

24-32 24-32 24-32 24-32

S S C C

Ultra-high- strength steel. 175-425 RC45-52

118 8-10 125-135 24-32 C

Gray, ductile and malleable irons.

110-225 225-400

118 118

8-12 10-12

125-135 125-135

24-32 24-32

S S

Ferritic, austenitic, martensitic, and precipitation- hardening stainless steels.

135-200 200-325

118 118

10-12 7-10

125-135 125-135

24-32 24-32

S C

325-425 RC48-52

118-135 7-10 120-130 24-32 C

Titanium alloys. 110-440 118 7-10 125-135 24-32 C

High-temperature alloys. 140-400 118-135 9-12 125-135 24-32 C

Tungsten alloys. 180-320

Molybdenum, columbium, and tantalum alloys.

170-290 118 7-10 125-135 24-32 S

Nickel alloys. 80-360 118 8-12 125-135 24-32 C

Nitinol alloys. 210-360 RC48-52

118 7-10 125-135 24-32 C

Aluminum alloys. 30-150 500kg

90-140 12-15 125-135 24-48 S

Magnesium alloys. 40-90 500kg

70-118 12-15 120-135 10-30 S

Copper alloys. RE20-100 118 12-15 125-135 10-30 S

Zinc alloys 80-100 118 12-15 120-135 24-32 S

Workpiece material γ, degree 2φ, degree

Structural, carbon and alloy steel 0-4 116-118

Tool steel -3 116-118

Manganese hard steel 0 116-118

Cast steel -3 116-118

Thermal-treated steel -3 130-135

cast iron ≤ 200 6 116-118

Cast iron > 200 0 116-118

Bronze, brass and aluminum 4-6 116-18

Babbitt 4-6 140

plastics 0-2 60-100



NOTE: Use stub-length drills whenever possible on high-strength materials. * S – standard; C – crankshaft.

____________________________________________________________________________________

Influence of the Flute helix angle (ω) on reduction of the torque and axial cutting forces, affects so much when this angle (ω) increases up to 25º-35°.

- With Further increasing of the helix angle (ω), the cutting forces actually do not decrease, but weakening the strength of the cutting edge in the periphery of drill.

- To avoid this, the face of the drill should be sharpened at an angle less of ω.

International organization for standardization ISO recommends

three types of drills:

1. Type H: for machining brittle materials (cast iron, bronze, brass) (ω=10º -16°);

2. Type N: for machining wide types of materials, forming segmented chips (ω = 25º - 35°);

3. Type W: for machining ductile materials (aluminum, copper, duralumin and others) (ω = 35º - 45°).

Table 4.4 lists the recommended values of flute helix angle ω in accordance with the diameter of general-purpose twist drills.

Table 4.5 lists the recommended values of the point angle 2φ and flute helix angle ω for drilling various materials.

3.4.4. Core diameter or web thickness (W) and chisel edge of twist drills:

The web thickness W (core diameter) (Figure 4.8) is an important element of twist drills design.

If the web is too thin, the rigidity of the drill will be insufficient to withstand a high drilling torque.

On the other hand, with a thinner web the axial thrust is reduced and drilling is easier since the chisel edge is shorter.



The web thickness depends on the tool material and the diameter of the drill as given in table 4.2, 4.6:

Fig. 4.8: web thickness, web taper and back taper.

In twist drills with milled flutes, the web thickness increases by (1.4 to 1.8 mm per 100 mm) toward the shank.

- This raises the strength and rigidity of the drill.

Table 4.5: Recommended Values of ω and 2φ for Drilling Various Work Metals with Drills of Tool Steels*

Workpiece material Flute helix angle, ω

Point angle, 2φ

Workpiece material

Flute helix angle, ω

Point angle, 2φ

Steel, σt up to 50 kgf/mm2

35° 116° marble - 80°-90°

Steel, σt from 50 up to 70 kgf/mm2

30° 1160-118° Copper castings

and brass 25°-30° 130°

Steel, σt from 70 To 100 kgf/mm2

25° 120° Bronze,

HB 100 and harder 15°-20° 135°

Steel, σt from 100 to 140 kgf/mm2

20° 125° Soft bronze,

HB <100 8°-12° 125°

Stainless steel 25° 120° Aluminum alloys 35°- 45° 130°-140°

Gray cast iron 25° -30° 116°-120° Plastics, ebonite, laminate fabric base

8°-12° 60°-100° Copper

35° - 45°

125°

* The data of this table refer to drills of a diameter beginning with 10 mm and up. Other angles should be

used for smaller drills. These data are for drills of high-speed, carbon and alloy tool steels, but not for those of, or tipped with, cemented carbides.

__________________________________________________________________________________

Table 4.6: Web thickness of twist drill.

Tool material Drill dia., D, mm Web thickness, mm

Carbon steel and HSS 6 to10 (0.20 to 0.25) D

Over 10 (O.13 to 0.16) D

Carbide tipped 6 to 10 mm (0.27 to 0.30) D

over 10 (0.20 to 0.26) D



NOTE: Carbide-tipped drills are made with webs comparatively thick because the drill body is weakened by the slot for the tip.

4.4.5. The land width b:

It is selected from considerations of drill strength and the width of the flute, from the condition that the flutes provide sufficient room to accommodate the chips and to eject them from the lips during operation.

Usually, the land width is taken equal to the flute width, i.e. to one fourth of the drill circumference in two-flute drills.

The flute width should be increased slightly, however, in drills having higher helix angles ω.

In drawings of drills, the land width b is indicated perpendicular to a helical flute.

Table 4.7 shows the proportion of the land width (b) with respect to the drill diameter (D) for high-speed steel drills:

Table 4.7: The values of the land width b with respect to the drill diameter D.

Drill diameter, mm Land width b, mm

3 to 8 0.62D

8 to 20 0.59D

over 20 0.58D

3.4.6. Margin width f:

Margin is a narrow cylindrical strip on the leading edge of the land which is ground to the diameter of the drill.

The width of the margin (f) and the height (h) are given by:

f = (0.06 – 0.07) D …4.1

h = (0.02 – 0.03) D …4.2

Or can be selected from table 4.2.



3.4.7. Shape of the lips (cutting edge) and flute cross section:

The lip, or cutting edge, of a twist drill is the line formed by the intersection of the face and lip relief surface, and in most cases is straight.

- However, investigations have shown that a more constant rake angle (γ) can be obtained with a curvilinear (convex) lip.

The shape of the flute cross section is not specified in drawings of twist drills but another element is indicated:

The tooth profile of the fluting cutter (for drills with milled flutes)

Or the profile of the rolled drill blank (for twisted drills).

The profile of the fluting cutter (its shape is shown in Figure 4.9) can be determined by two methods:

By the graphical method in which the profile of the cutter is constructed from the given flute profile,

And analytically method, with the curves making up the profile being calculated by analytical formulas.

All the elements of the profile of the fluting cutter (radii Ro and Rf

and width B) can be determined by a simplified analytical method.

The approximate formula for determining radius Ro of the cutter profile for a drill of diameter D is:

Ro = CR Cr Cfc D …4.3

Fig. 4.9: Profile of the flute milting cutter.



The coefficients CR can be found as follows (Coefficient CR depends upon angles 2φ and ω):

( )√

Coefficient Cr, taking into consideration the variation in web thickness (core diameter), is:

(

)

Where: d - is the core diameter, or web thickness, of the drill.

The coefficient (Cfc) taking into account the influence of the fluting cutter diameter is:

( √

)

Where: Dfc- is the diameter of the fluting cutter.

The radius to which the top of the cutter profile is rounded is:

Rf = Cf D …4.7

Where: Cf is a coefficient determined by the formula:

Cf = 0.015 ω0.75

…4.8

The width B of the cutter is:

Since angle ψ1 is usually small (equal to 10°), width B can be assumed with some approximation that:



Strictly speaking, the constructed or calculated cutter profile is suitable only for a single diameter of drill and for definite values of the helix angle, point angle and web thickness.

But this would require a great many cutters for fluting a size range of drills.

In practice a single cutter is used to flute drills within a certain diameter range.

This leads to insignificant inaccuracies, which can be ignored.

The body of a drill can be made not only by milling flutes, but also by hot forge rolling and twisting.

The main difference between milled and rolled drills is that, the flutes latter are produced by rolling round stock (Figure 4.10), and then twisting the blanks with straight flutes. This procedure saves high-speed steel.

Fig. 4.10: Principle of hot forge rolling and twisting of drill blanks: (a) rolling the flutes: (b)

twisting the blank

Several techniques are used for rolling drill blanks (hot forge rolling; cross, or transverse rolling; longitudinal helical rolling, etc.).

3.4.8. The overall and flute length:



The overall length (Lo) and the flute length (lf) affect the rigidity of the drill.

A. Flute length (lf):

A longer fluted length permits for larger number of sharpening along the length of the lip.

The flute length should be shorter for carbide-tipped drills (15 - 45 mm) than for high-speed steel (HSS) drills because it is sharpened a limited number of times.

B. Overall length (Lo):

The overall length of the drill is made to the standards keeping in view with the rigidity of the drill for the given diameter.

Consequently, where there is no need for a long drill (as, for instance, in centering), short length or stub drills should be used to reduce the danger of breakage.

Table 4.2, 4.8 shows the values of the overall length (Lo) and the flute length (lf).

4.4.9. The type (shape) of the shank:

Depends upon the method used for holding the drill.

Small drills (up to 10 or 12 mm in diameter) have a straight

shank and are held in chucks.

Larger drills generally have tapered shanks.

- The shanks of taper shank drills have a Morse taper.

- To fine the number of the shank Morse taper, the friction torque between the shank and taper sleeve is equated to the three times maximum moment of force resisting cutting to get the taper mean diameter, from which the taper number is

obtained.

The tang and other dimensions are made to the standard. That means:

3Mrc = Mfr …4.11

Where: Mrc - maximum moment of force resisting cutting;



Mfr - friction torque moment.

The moment of forces resisting cutting (the torque) can be determined by the following equation:

Where: Cm – coefficient. From table 9; qm, ym – exponents. From table 9; D- Drill diameter, mm; S- Feed, mm/rev.;

( )

The friction torque between the shank and taper sleeve is:

( )

( )

Where: μ = 0.096 is the coefficient of friction between steel and steel;

Fx – axial thrust force, N; D1, d2 – see figure 4.11; θ =1o26’16” is half the angle of taper (the angle of the taper is

0.05020; sinθ = 0.0251), Figure 4.11; Δθ = 5’ is the tolerance limit of the taper.

Table 4.8: Overall length (Lo) and the flute length (lf) for twist drills.

D, mm

Standard Extra length

shank D, mm

Standerd Extra length

shank Lf, mm

Lo, mm

Lf, mm

Lo, mm

Lf, mm

Lo, mm

Lf, mm

Lo, mm

2.7 3

33 67 66 100

Str.

9.6 10.6

87 150 121 184 Str.

3.2 36 73 69 106 10.7 11.8

94 161 128 195 Str.

3.5 39 78 73 112 12.0 13.2

101 172 134 205 MT1

3.8 4.0 4.2

43 84 78 119 13.3 14

108 189 140 221 MT1

4.5 47 91 82 126 14.2 15.0

114 212 144 242 MT2

4.8 5.0

52 97 87 132 15.2 16.0

120 218 149 247 MT2



5.3

5.5 6.0

57 105 91 139 16.2 17.0

125 223 154 252 MT2

6.2 6.5

63 114 97 148 18.25

19 135 233 158 256 MT3

6.8 7.5

69 123 102 156 19.2 20.0

140 238 166 264 MT3

7.6 8.5

75 131 109 165 20.2 21.0

145 243 171 269 MT3

8.6 9.5

81 141 115 175 21.2 22.3

150 248 176 274 MT3

22.5 23.0

155 253 180 278 MT3

Table 4.9: Coefficients CF and Exponents of the Formulas for Calculating the Cutting Forces and torque in drilling, boring and coring.

Metal Being Machined

Machining operation

Cutting tool

material

Coefficients CF and Exponents

Torque Mrc Axial thrust force Fx

Cm qm xm ym CF qF xF yF

Structural and Carbon Steel σb

=75 kgf/mm2

Drilling

HSS

0.0345 2.0 - 0.8 68 1.0 - 0.7

Boring and Coring

0.09 1.0 0.9 0.8 67 - 1.2 0.65

High Temper-re Steel

HB 141

Drilling 0.041 2.0 - 0.7 143 1.0 - 0.7

Boring and Coring

0.106 1.0 0.9 0.8 140 - 1.2 0.65

Cast Iron HB 190

Drilling Cemented

carbide

0.012 2.2 - 0.8 42 1.2 - 0.75

Boring and Coring

0.196 0.85 0.8 0.7 46 - 1.0 0.4

Drilling

HSS

0.021 2.0 - 0.8 42.7 1.0 - 0.8

Boring and Coring

0.085 - .075 0.8 23.5 - 1.2 0.4

Malleable cast iron

HB 150

Drilling 0.021 2.0 - 0.8 43.3 1.0 - 0.8

Cemented carbide

0.01 2.2 - 0.8 32.8 1.2 - 0.75

Boring and Coring

0.17 0.875 0.8 0.7 38 - 1.0 0.4

Copper Heterogeneous

Alloys , HB 120

Drilling

HSS

0.012 2.0 - 0.8 31.5 1.0 - 0.8

Boring and Coring

0.031 0.85 - 0.8 17.2 - 1.0 0.4

Alum-m and Silumin

Drilling 0.005 2.0 - 0.8 9.8 1.0 - 0.7



Fig. 4.11: Diagram of forces acting on drill taper shank.

The shank taper mean diameter is:

Or

( )

After determining (dm), from table 10 select the nearest lager taper, i.e. Morse taper and all other dimensions.

4.4.10. Formulas for Calculating the Axial Thrust and Torque in Drilling

In general the axial or total thrust force is:

And the moment of resisting cutting (the torque) is:

Where: CF and CM - Coefficients characterizing the work material and the machining conditions, table 4.9;

D - Drill diameter, mm; S - Feed, mm/rev, table 4.11;

qF, qM, yF and yM - exponents of the drill diameter and feed, table 4.9;



FMK and MMK - ( ) general correction factors taking

into account the changes in the machining conditions. See table 4.12.

Table.4.9 lists the values of CF, CM, yF, yM, qF and qM for drilling various metals having definite values of (σb) or (HB) with drills having a standard point geometry (shapes ST, DT and DTM, see table 4.13), using a cutting fluid (in drilling steel) or dry dri lling (for cast iron). In all other cases, the listed values of CF and CM are

to be multiplied by correction factors . This is taken into account, in the formulas for Fx and M, by

the general correction factors FMK and

MMK .

Table 4.10: Basic dimensions of external Mores taper with tongue.

Designation of taper

diameter

Morse tapers

0 1 2 3 4 5 6

Angles of taper

1:19.212 =0.05205

1:20.047 =0.04988

1:20.020 =0.04995

1:19.922 =0.05020

1:19.254 =0.05194

1:19.002 =0.05263

1:19.180 =0.05214

dm

D1 d2

d3max l3max l4max

a bh13

c emax

R R1 v

9.045 9.2 6.1 6.0

56.5 59.5 3.0 3.9 6.5

10.5 4.0 1.0

0.06

12.065 12.2 9.0 8.7

62.0 65.5 3.5 5.2 8.5

13.5 5.0 1.2

1.06

17.780 18.0 14.0 13.5 75.0 80.0 5.0 6.3

10.0 16.0 6.0 1.6

0.065

23.825 24.1 19.1 18.5 94.0 99.0 5.0 7.9

13.0 20.0 7.0 2.0

0.065

31.267 31.6 25.2 24.5

117.5 124.0

6.5 11.9 16.0 24.0 8.0 2.5

0.07

44.399 44.7 36.5 35.7

149.5 156.0

6.5 15.9 19.0 29.0 10.0 3.0

0.07

63.348 63.8 52.4 51.0

210.0 218.0

8.0 19.0 27.0 40.0 13.0 4.0

0.07



Notes: 1- Dimensions are in mm. 2- Drills up to 0.5 mm in dia. are fabricated without margins.

3- Dimensions D1 and d2 are theoretical which result from diameter dm and rated dimensions (a and l3), respectively 9positions of the datum plane).

Table 4.11: Feed value S, mm/rev. for drilling steel, cast iron, copper and aluminum alloys using drill made of HSS.

Drill Diameter D, mm

steel gray and Malleable cast iron, copper and aluminum alloys

HB < 160 HB 160-240 HB 240-300 HB >300 HB<= 170 HB > 170

2-4 4-6 6-8

8-10 10-12 12-16 16-20 20-25 25-30 30-40 40-50

0.09-0.13 0.13-0.19 0.19-0.26 0.26-0.32 0.32-0.36 0.36-0.43 0.43-0.49 0.49-0.58 0.58-0.62 0.62-0.78 0.78-0.89

0.08-0.10 0.10-0.15 0.15-0.20 0.20-0.25 0.25-0.28 0.28-0.33 0.33-0.38 0.38-0.43 0.43-0.48 0.48-0.58 0.58-0.66

0.06-0.07 0.07-0.11 0.11-0.14 0.14-0.17 0.17-0.20 0.20-0.23 0.23-0.27 0.27-0.32 0.32-0.35 0.35-0.42 0.42-0.48

0.04-0.06 0.06-0.09 0.09-0.12 0.12-0.15 0.15-0.17 0.17-0.20 0.20-0.23 0.23-0.26 0.26-0.29 0.29-0.35 0.35-0.40

0.12-0.18 0.18-0.27 0.27-0.36 0.36-0.45 0.45-0.55 0.55-0.66 0.66-0.76 0.76-0.89 0.89-0.96 0.96-1.19 1.19-1.36

0.09-0.12 0.12-0.18 0.18-0.24 0.24-0.31 0.31-0.35 0.35-0.41 0.41-0.47 0.47-0.54 0.54-0.60 0.60-0.71 0.71-0.81

Table 4.12: Correction factor K for Steel and Cast Iron, taking into account the effect of the metal being machined on the cutting Forces.

Not: nominator value of n for cemented carbide, denominator value of n for HSS.

Table 4.13: Types of Drill Points.

Drill diameter, mm

Type of point Designation Sketch Materials drilled

0.25 to 12** Ordinary (standard) S

Steel, cast steel, cast iron

Metal Being Machined

Formula For Calculation

Value Exponent n

Force Fz When Machining With

SPT

Torque Mt and Axial Force Fx for Drilling, Boring

and Core Drilling

Fz for Milling

Structural and Carbon Steel σb,

kgf/mm2

≤ 60 ≥ 60

n

bK

75

0.75/0.35 0.75/0.75

0.75/0.75 0.75/0.75

0.3/0.3 0.3/0.3

Gray Cast Iron n

HBK

190 0.4/0.55 0.6/0.6 1.0/0.55

Cast Iron n

HBK

150 0.4/0.55 0.6/0.6 1.0/0.55



Standard with thinned web

ST

Cast steel with σb up to 50 kgf/mm2

Double- angle point with thinned web

DT

Cast steel (σb > 50 kgf/mm2) and cast iron; with a foundry skin

12 to 80

Double- angle point with thinned web and

margin relief DTM

Steel and cast steel (σb > 50 kgf/mm2) and cast iron; foundry skin removed

Double- angle, thinned web and

notched point DT-2

Cast iron with foundry skin removed

** An ordinary (standard) point is used for drills over 12 mm in size if the same drill is used for various work materials in operation on a foundry skin or with the skin removed, as is often the case in small-lot production.

3.5. Design Features of Various Types of Drills

At the present time, all the main types of drills have been standardized:

a. Carbon and high-speed steel drills with straight and tapered shanks and of various lengths (short, standard, long, etc.);

b. Carbide-tipped drills with straight and tapered shanks;

c. Centre drills; etc.

The main constructional dimensions are specified in these standards. A brief description follows of carbide-tipped drills and certain nonstandard designs which have found application in industry.

3.5.1. Cemented Carbide-Tipped Drills:

These drills:

1. used for drilling cast iron, hardened steel, plastics, glass, nonferrous metals, marble, granite and other nonferrous materials;

2. rarely use for drilling the workpieces of steels, because of the instability of work (possibility of breakdown, crumbling-off and an



insignificant increase in the productivity with their usage);

3. Are especially efficient for operation at high speeds and small-feeds;

There are several kinds of drills tipped with cemented-carbide cutting elements.

Drills of diameter up to 3 mm are usually made as one piece (whole made of cemented carbides).

Drills of (3 up to 12 mm) diameter are made with cutting part (cutting and finishing parts) of cemented carbide butt welded with a steel shank (Figure 4.13),

Fig. 4.13. Cemented Carbide-Tipped Drills.

In order to increase the rigidity and strength of the cemented carbide-tipped drills (for increasing their successful work), it is necessary to increase their web thickness (core diameter) to (0.25 diameter drills-

0.25D) compared with the high-speed steel drills.

Drills of small diameters (used for drilling the holes in the hard materials) their web thickness (core diameter) can be increased to (0.32-0.35

diameters of drill “0.32-0.35D”) with the simultaneous decreasing the length of the cutting part (cutting and finishing parts) of the drill.

The drill flute helix angle is selected equal to 20°.

Drills used for machining the holes, which their length up to 3-4 times of drill diameter, flute helix angle (ω) must be increased to 45° – 60°.

To create a more rigid and stronger datum surface for cemented carbide-tipped drills, the face of the drill, under the tip and at its



length, performed flat with an angle of (7° -10°), and with chisel edge length equal to (0,1-0,15 drill diameters).

3.5.1.1. “U” cemented carbide tipped drill with indexable insert

Figure 14 shows cemented carbide tipped “U” drill with indexable insert.

This type of drill are:

manufactured with minimum diameter of 17.5 and above;

With straight flutes with special holes for supplying the cutting fluid to the cutting zone.

They can be used conveniently

For finishing rough machined holes,

For machining holes of different metals, including carbon and alloy steels. Thus, when machining the holes of carbon steels with ultimate strength not more than 800 MPa (about 80 kgf/mm2) the cutting speed of the drill must be (100 to 140 m/mm).

Fastening (setting–up) and accurate fixation of the tip on the drill body produced by screw (2) with a cone head.

After the cutting edge becomes blunt, the insert (1) (see Figure 4.14) is indexed to bring a new edge of the insert in operation. After all the edges of the insert are worn out, the insert can be

replaced quickly without removing the “U” drill from machine. Thus re-sharpening is completely eliminated.



Fig. 4.14: “U” drill with indexable insert.

Indexable insert face has chip-breaking grooves to get small length chip, which easily disposed away from the tool grooves.

As one of the inserts overlaps the center, “U” drill can be also used for central cutting after starting a hole with a center drill.

3.6. Spade (or flat) Drill.

It is similar to brazed drills in that they employ a steel body.

It is available in diameters from 12 to 75 mm for (L/D, overhang to diameter) ratios from 2 to 10.

Many point geometries which can be ground on a twist drill are not available on spade and indexable drills.

Spade drills consist of a body and a removable (throw-away) cutting blade or bit, which is precisely located and clamped in a special slot at the end of a steel drill body (Figure 4.15).

The blade may be clamped with either one or two screws, with the two screw system usually being more stable.

Spade drills can be used at high penetration rates and are comparatively rigid.

In general, spade drills are not used for finishing operations

requiring tolerances better than 0.08 mm on the diameter unless special care is used to set the blade in the drill body.

Spade drills increase the variety of possible cutting edge materials as compared to conventional twist drills; the blade may be made of solid HSS, HSS-Co, WC, cermets, or ceramic, or may be (PCD,



polycrystalline diamond) or (PCBN, polycrystalline cubic boron nitride) tipped.

This drill offers the economical benefit of throwaway insert, and often eliminates geometrical variation due to regrinding.

Fig. 4.15: Spade (flat) indexable drill.

3.7. Subland and Step Drills.

Subland Tool – Higher Initial Cos

Fig. 4.16: Subland drill.



Subland drills are special tools for drilling multi-diameter holes.

Each diameter has its own flute and land as shown in (Figure 4.16), this results in a complex flute geometry, which is necessary for what is effectively two or more tools sharing a common axis and

core.

Figure 4.17 shows how a subland tool is manufactured:

1. Operations performed between centers.

2. Concentricity guaranteed.

3. Minor diameter never has to be re-established

Fig. 4.17

In a conventional multistep drill (Figure 4.18), the smaller diameter ends at the larger diameter’s cutting lips, and both share common flutes, lands, and margins, it is thus a modified standard drill.

Step Tool – Lower Initial Cost

Fig. 4.18: step drill.



The advantages of the Subland drill over the step drill are the preservation of the geometry for all diameters after regrinding and therefore the lager number of regrinds possible.

Subland vs Step Drills

Investigation Procedure

1. Two-diameter tool, with both diameters on the same set of flutes,

2. The small diameter’s entire geometry eventually must be reground,

3. Intersection between diameters is a week point.

1. Two-diameter tool, with individual flutes for each diameter,

2. Only the cut edges need to be resharpened, 3. The intersection between diameters

produces a sharp edge.

Maintenance – Step Tools:

In step tool construction, because you have both diameters on the same set of flutes and lands, you will have to create an undercut at the intersection of both diameters.

It gets weak due to undercutting at the intersection.

This may cause the minor diameter to break when subjected to minor strain.

After many grinds, if it doesn’t break, there is no small diameter left.

Maintenance – Subland Tools

Because you have individual flutes for each diameter. All you have to do is end grind each diameter maintaining the same step length dimension. You will never have to grind the tool "diameter".

Only areas to sharpen

Sharpening required here

And eventually here



Performance:

Concentricity plays a very important part in:

1. Number of pieces per grind. 2. Accuracy of the cavity.

3.8. Deep-hole drills:

By deep drilling is understood drilling holes at the depth, which exceeds the diameter of drill 5-6 times and more, such drills use for continuous 80 mm) drillings.

To the deep drilling the following requirements present:

- Straightness of the hole axis, - Hole concentricity with respect to the external surface of

workpiece and cutting tool, - Hole cylindricity, - Working accuracy, - Obtaining the necessary surface roughness, - Ease chip disposal from the hole.



The drilling conditions are sharply changed in deep holes.

Chip ejection and heat disposal are deteriorated;

Drill rigidity is substantially reduced, etc.

An ordinary twist drill is unsuitable for deep-hole drilling.

Two methods of drilling deep holes are employed:

1) Ordinary drilling, in which all removed allowance is converted into chips (generally used for holes up to 70 mm in diameter);

2) Trepanning method, in which an annular recess is cut in the work so that a core is left in the central part of the hole.

This method is used only for large-diameter holes, because a trepanning head for small holes is insufficiently strong.

Deep-hole drills can be classified as:

1. Multiple-flute drills with a web, i.e. drills, having two main lips; 2. Single-flute drills.

Figure 4.19 shows three types of two-flute drills.

Drill shown in figure 4.19, a, provided with through helical holes in the lands of the drill for feeding the cutting fluid to the drill point.

Fig. 4.19: Two-flute deep-hole drills:

(a) oil-holt twist drill; (b) tour-margin twist drill with internal chip ejection; (c) two-flute twist drill with external chip ejection



The two-flute assembled twist drill shown in Figure 4.19, b, has four

margins (instead of two) which form channels in the lands for the cutting fluid.

- Chips are ejected along the flutes and then through inclined holes into a central hole from where they pass through the driving tube at the end of which the drill is held.

- Cutting fluid at a pressure of 10 to 20 atm is fed into the annular space between the driving tube and the drilled hole.

- Drilling is done in a special machine equipped with a pumping station for feeding the cutting fluid.

The two-flute twist drill with external chip ejection, shown in Figure 4.19, c, consists of a short cutting head fastened to an oil-feeding bar.

- The cutting fluid is delivered through the central hole of the bar in drilling and distributed by smaller holes to the lips.

- Chips should be produced in the form of tightly curled helices in order to ensure quiet efficient operation of the drill (this concerns all deep-hole drills).

- The provision of chip breaker grooves on the lips enables chips of this type to be obtained.

- Disadvantage of this drill is that it cannot ensure a sufficiently straight hole, especially if it has been incorrectly sharpened and the lips are of different length.

Deep holes can also be drilled by using high flute helix angle (figure 4.

20).

- These high-helix drills are intended for drilling holes of a depth over 10 diameters in cast iron, steel, light alloys and wood.



Fig. 4.20: High helix drills.

The main advantage of multiple-flute drills, in comparison with

single-flute designs, is the higher production capacity.

At the same time, all constructions of multiple-flute drills have a common disadvantage: they must have a web and, consequently, a chisel edge.

Even an ideally sharpened multiple-flute drill with a web may deflect to the side from the true axis of the workpiece.

Owing to the web, such drills operate with vibration and therefore cannot produce holes, with a high class of surface finish.

These disadvantages of multiple-flute drills can be eliminated to some extent by using single-flute drills.

The simplest construction of single-flute (single-lip) drill, the so-called half-round drill, is shown in Figure 4.21.

It consists of a cylindrical shank on which the front end is cut away to the center and ground to the required angles. To avoid jamming of the drill in the hole, the face is from 0.2 to 0.5 mm above the center, depending upon the drill diameter.



Fig. 4.21: half-round drill.

Have a single cutting lip, perpendicular to the hole axis and extending 0.5 to 0.8 mm beyond the center. The auxiliary lip can be cut away at an angle of 10°. This drill operates with guidance in a previously started hole. The front end of the drill has a cylindrical bearing surface by means of which it is guided in the hole being drilled. The geometry of these drills is unfavorable. The cutting angle equals 90° and the relief angle is from 8° to 10°. To reduce the friction between the bearing surface and the hole walls, a flat is provided at an angle of 30° as well as a back taper of 0.03 to 0.05 mm per 100 mm of length.

4.9. Gun drills:

They are used to obtain accurate holes with a straight axis.

These drills are also of the single-flute type with a one-sided cutting arrangement.

Such drills (Figure 4.22, a) consist of two main parts:

Cutting element of high-speed steel or cemented carbide

And a drive tube of carbon steel.

The drive tube is of the shape shown in section A-A.

- Cutting fluid, forced at high pressure through the internal passage in the drive, fulfills two purposes:

It carries heat away from the cutting element And flushes the chips from the cut through the single

external flute.



Fig. 4.22: Gun drill: (a) Construction; (b) diagram of the acting cutting forces.

The cutting element, has an offset point providing an outside and an inside cutting lip.

- During operation the drill is subject to one sided torsion, compression and buckling.

- For this reason, the drive tube should be as rigid as possible and the flute therefore is of minimum cross section.

- On the other hand, considerations of free chip ejection require that this flute be of maximum feasible cross section.

- A flute angle of ψ=100° to 120° has been established to be the most expedient in practice and is recommended.



4.10. Trepanning drills:

In drilling holes of large diameter (D>80 mm) it is advisable to resort to trepanning in which an annular recess is cut out, leaving a core that is removed at the end of the operation.

Trepanning drills (Figure 4.23) consist of a head in which cutting blades or bits are secured and a drive tube.

- Cutting edges are provided on the blades at the end face, and projecting beyond the outside and inside diameters of the head. Upon rotation the blades cut an annular recess.

- Each trapezoidal stocking blade is followed by a flat finishing blade. The head has margins (or cylindrically ground wear pads of a plastic or wood) to provide guidance in the machined hole.

Fig. 4.23: Trepanning drill.

3.11. Twist drill problems

1. Over size holes are caused due to unequal cutting by two lips having unequal lip length or unequal lip angles.

- The problem can be overcome ensuring symmetry in lip angles and lengths.

LIP ANGLES UNEQUAL LIP LENGTHS UNEQUAL



2. Undersize holes are encountered while drilling elastic materials like rubber, and light metals with high coefficient of thermal expansion.

- Contraction results from cooling of workpiece after removal of drill.

- The problem can solve by flooding the workpiece with coolant to keep it cool or using oversize drill.

3. Excessive speed causes faster wear at corners.

4. Chipping of cutting edges can be countered by reducing lip clearance angle or feed or both.

5. Unequal chips from two lips also indicate unequal cutting due to unequal lip angles or lengths.

6. Hard spot in material can be drilled conveniently by reducing speed and using turpentine as a coolant.

7. Disposal of discontinuous chips in deep holes can be facilitated by compressed air blast and use of a magnetized stick for chip removal. While drilling deep holes, drill should be withdrawn periodically (after drilling a length equal to two to three times drill diameter) to remove swarf.

8. Jamming. The depth of drilled hole should not exceed its flute length. For when the flute end submerges into the drilled hole, passage for the chips gets blocked, chips clog, and jamm the drill.

9. Break through digging can be contained by reducing the feed as drill point emerges after drilling through the workpiece.

10. Drill does not cut when the cutting lips are not provided with clearance angles.

- The problem can be solved by grinding 12° clearance angle.

4.12 Improvement of Drill Point:

Web of standard conical point drill becomes chisel-shaped after grinding clearance on flanks.



Instead of standard cone and chisel edge, drill point can be ground to a variety of shapes according to the material being drilled and other requirements.

Fig. 4.24: types of drill points.

Following are some of the drill points commonly used in industry:

Oliver Point:

- The web is ground conical to make the drill self-centering. - The flanks are ground helicoid.

Spiral Point:

Grinding the flank surface as a three dimensional spiral or spiro-helicoid reduces negative rake in the web portion, increases chip space, and provides self-centering action.

Notched or Crank-Shaft Point:

Used widely for small, thick webbed drills (used for drilling deep oil holes). The web is ground as shown to reduce drilling thrust. This point is found particularly convenient for drilling hard materials.



Four Facet Prismatic Point:

- The point is ground with four faces which provide primary and secondary clearances.

- This splits up chisel edge portion into two portions eliminating dead chisel effect, thus providing self-centering action, better finish, and accuracy.

Radial Lip Point:

- Distributes the cutting edge wear uniformly all along the lip. - Drilling stresses are distributed better than chisel point and there is

no burr at the exit of drill. - Secondary reliefs increase the penetration rate immensely. - A wide variety of materials can be drilled accurately, with good finish

by using this point.

Special Points:

- In deep-hole drilling with a large diameter drill, a wide chip is formed which is difficult to dispose out through the flutes.

- Such a chip also increases friction and impedes cutting fluid delivery to the drill lips .

- The width of the chip can be reduced by providing special

chip-breaker grooves or notches either, on the face (Figure 25, a) or on the lip relief surface (Figure 4.25, b).

- The depth of the grooves should be approximately (0.05D) and their width approximately (0.07D) , where (D) is the drill diameter.

- Such grooves divide a wide chip into several narrow ones . They improve the cutting conditions by reducing the forces acting during drilling and heat generation.

- Care should be taken in re-sharpening the lips of the same length and symmetrically positioned, as otherwise the drill will cut oversize and may depart to one side (from the true axis of the hole).



Fig. 4.25: Chip breaker grooves on twist drills: (a) on faces; (b) on lip relief surfaces.

4.13 Drill Point Grinding

Drills are sharpened by grinding the relief surfaces at the point.

Drill point grinders can be divided into three groups:

1) Those that grind a conical surface;

2) Those grinding a helical surface;

3) Those grinding flat surfaces.

Two methods of grinding drill points on machines of the first type are illustrated in Figure 4.26, a and b.

They differ only in the location of the axis of the imaginary cone in reference to the drill.

The drill holder which locates the drill during the sharpening operation is designed so that the apex of the cone along whose the surface of the point is ground is located at a definite distance from the drill axis.

This distance equals (1.16 D) for the first method (Figure 4.26, a) (and 1.9 D) for the second (Figure 4.26, b).

Moreover, the axis of the grinding cone is offset from the drill axis "by an amount (K) equal to from 1/13 to 1/10 of the drill diameter.

The different locations of the imaginary cone axis in relation to the drill axis lead to different kinds of variation of the relief angle along the lips.

It is better if the relief angle is greater at the periphery than at the center of the drill.



This requirement has made the second method more popular because it grinds a drill point with a relief angle sharply increasing toward the center of the drill.

This increase is much less on drill points ground by the first method.

Fig. 4.26: Drill sharpening methods:

(a) ant (b) by grinding conical surfaces; (c) by grinding helical surfaces

Helically ground points (Figure 4.26, c) are extensively used.

In the grinders producing this type of point the drill is held in a chuck and rotated slowly about its axis (CC).

In addition to the main rotation about its axis (AA), the grinding wheel has two supplementary motions:

- Planetary rotation about axis BB (the wheel spindle axis is offset in reference to the axis of a sleeve which has independent rotation),

- and axial reciprocation produced by a cam.

The supplementary (planetary) rotation of the grinding wheel moves its active surface along the drill lips.

All the relative motions of the drill and grinding wheel are interconnected in such a manner that the lip relief surfaces are ground on a helical surface.



This method of grinding drill points enables a greater increase in the relief angle (by 25%) to be obtained toward the center of the drill.

- This is a distinct advantage over the more widely used first and second methods.

Flat drill point grinding (Figure 4.27) is suitable for small drills (up to 3 mm in diameter) and is seldom used for larger drills.

Large drills are usually ground by a two-plane method.

- The first plane provides the required relief angle behind the lip and the second plane, ground at a much larger angle, eliminates interference of the heel on the land with the work surface during drilling.

Fig. 4.27: Flat drill point grinding.

Double plane

Plane I

Plane II

Plane

Cutting Tools for Hole Machining

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