The twist drill is the most common type of drill. The shank of the drill is held by the machine tool, which in turn imparts a rotary motion. This shank may be straight or tapered. The body of the drill is typically made up of two spiral grooves known as flutes, which are defined by a helix angle that is generally about 30 degree  but can vary depending on the material properties of the workpiece. The point of the drill generally form a 118° angle and includes a 10 degree clearance angle and chisel edge .The chisel edge is flat with web thickness of approximately 0.015 x drill diameter. This edge can cause problems in hole location owing to its ability to "walk" on a surface before engaging the workpiece. In the case of brittle materials, drill point angles of less than 118 are used, while ductile materials use larger points angles and smaller clearance angles.

Main technical conditions of twist drill

Dimensions

Drill diameter

The values indicated in the relevant dimensional standards apply in respect of the diameter of twist drills.

Test point: on the lands at the corners

Testing equipment: micrometer

Tapering of diameter

The diameter of twist drills usually reduces from the drill tip towards the shank in the area of the flutes.

Test values: The taper on diameter amounts to 0.02 to 0.08mm over a length of 100mm

Test point: at the outside diameter on the land.

Testing equipment: micrometer and indicating measuring instruments.

Straight shank

Tolerance for shank diameter f11, tolerance for roundness and parallelism 0.02mm for the shank length.

Concentricity tolorance

The concentricity tolerance of the twist drill is calculated from the equation.

Tr=0.03+0.01*L/d

in which "L" is the total length and "d" the diameter of the drill (all dimensions in mm)

Length

The tolerance of lenght for the total length corresponds for the degree o accuracy "very coarse" according to DIN 7168 part 1. The flute lengths giver in the relevant dimensional stands are minimum dimensions.

Point angle

Test value: 118°or 135º

Test point: at the cutting edges

testing equipment: universal bevel protractor indicating measuring instruments.

Materials and hardness

materials: M2, M35, M42, W9Mo3Cr4V, W4Mo3Cr4V

hardness: HSS  HRC63-66; HSS-E HRC64-68

Test point: on outside diameter on the land or adjacent relieved land

Test equipment: hardness tester

Marking

Twist drills with diameter 3mm and upwards shall be marked with

Diameter; material; name or mark of manufacturer

Twist drill with straight shank

figure 1

Twist drill with taper shank

figure 2

General dimensions of Morse taper shanks

Cutting portion

figure 4

In the context of cutting technology, land width b is the body clearance land width which is to be by b see DIN 6581.

Web thickness K

Test values: the web thickness according to the figure 5 showed as below shall not be less than the minimum value indicated in the figure 6.

figure 5	figure 6

Margin width b

Test values: the land width as in figure 7 shall lie within the limitting values indicated in the figure 8.

Test point: 5mm behind the corner

Testing equipment: slide gauge

figure 7	figure 8

Angle on twist drills

A   helix angle

Recommended test value: recommended ranges depending on the tool types N, H and W according to DIN 1836 and the diameter of the drill included in figure 9.

Test point: at the corner

Testing equipment: according to VDI Guideline 3331 part 1, section margin width b.

figure 9	figure 10

B point angle

Test value: usual executin for tool types N and H: 118, for tool type w:130

test point: at the cutting, see figure 11

testing equipment: according to VDI Guideline 3331 part 1, section margin width b.

figure 11

(1)Drills are worn off irregularly. It should be sharpened prior to developing into excessive wear.

(2)Resharpening
a  Grind the correct point angle to suit your application-(figure12)
b  Check that both cutting lips have the same angle. On a 130 degree point, each lip should be 65 degree toward the
axis. The point must be on center, i.e., the chisel edge must produce cutting lips of equal length(figure12).
Grind primary relief and Secondary clearance,(figure9)
c   Grind web thinning(figure12)
　

Web thinning

A   Without thinning
Suitable for drill of general purpose. Thanks to thin web thickness, web thinning is not need.
This without web thinning type is applied to design of drills for mild steel, alloy stee|, cast iron, stainless steel, titanium, inconel, etc. and conventional cutting conditions.

B  Type c thinning (DIN1412 FORM C, SPLIT POINT)

Because Split point enables good centering when drilling and breaks .the chips, chip removals is easy-
Suitable for dri11 design in high hardened tough materials, i.e, heat treated steel, titanium alloy, stainless steel,i ncoroy inconel, nimonic, etc.

C  Type R thinning (HELICAL THINNING)

Helical thinning ensure to frequent chip breaking and removal. The different direction force of cutting edges and helical thinning pa 同 s enables that chips curl, break and remove through the flutes. ln addition helical thinning makes the chip room up to center, remove the chisel and enables good centering.

D  Type A thinning (DtN1412FORM A)
A type thinning makes thin chisel, good chip removal and favorable centering.
This type is the easiest type to grind the thinning. ln narrow web and wide fluted drills, keeping of the rigidity and smooth chip removal are possible-

E  Type B thinning(DIN1412FORM B)
ln case of work materials with low cutting resistance and good chip removal, ie, cast iron, aluminium, plastic etc, B type thinning is suitable.
Especially when drills for high hardended steels are designed, this type is applied to decrease rake angle and avoid chipping of cutting lips.

A	B	C	D	E

figure 13