Knowledge

How the geometry of a drill influences tool life and hole tolerance

The drill geometry is the number and position of the drill cutting edges, chip flutes and the angle used. 

Depending on the type of hole to be created, a distinction is made between the following:

Twist drill

Drilling into solid material is performed with a twist drill. A cylindrical hollow body is generated by machining the material. For drilling in a diameter range up to 20 mm and a hole depth up to 100 mm, the twist drill is the most frequently used tool. 

Tap

During tapping, a solid hole is created first, then a thread is cut using taps. This step can be performed manually or using the machine. The resulting internal thread is standardised so that it matches a corresponding external thread.

Stepped drill

A stepped drill generates a stepped hole so that it is possible to allow a connection (e.g. a screw head) to sink into the material. Subsequent machining such as reaming or countersinking is not required.

Geometry of a twist drill

Different geometries affect the dimensional accuracy of the hole and the tool life of the drill.

In order to understand which aspect of a drill geometry influences which factor for tool life or drill tolerances, the geometry of a twist drill can be considered in detail, by way of example. Drills that have indexable inserts instead of cutting edges are subject to the same challenge of striking the perfect balance between chip clearance, cutting speed and feed rates.

  1. Point angle
  2. Main cutting edges
  3. Main flank
  4. Secondary cutting edge
  5. Chip flute
  6. Guide land
  7. Side rake angle

Diameter of the twist drill

The diameter of a twist drill tapers in the area of the chip flutes from the drill tip to the shank. The taper measures 0.02 to 0.08 mm on a chip flute length of 100 mm and reduces friction in the bore hole. This also facilitates chip flow.

Point angle – centring of the twist drill

The point angle is located on the head of the twist drill. The angle is measured between both cutting edges on the point.

The smaller the point angle, the easier the centring in the material. On curved surfaces, the risk of slipping is thus also lower. Small point angles are better for machining poor heat-conducting, short chipping materials; the long main cutting edges enable good heat dissipation over the tool. However, if the point angle is too small, the bore hole or chip flute can become blocked due to chip compression. A small point angle also increases cutting edge wear.

A large point angle is used for good heat-conducting or long-chipping materials, as this results in excellent chip flow and a low cutting force. However, a large point angle leads to wandering of the drill more easily and a larger hole.

Most twist drills have a point angle of 118 degrees. Ninety degrees are used for hard plastics subject to wear, 130 degrees for soft and tough materials and 140 degrees for long-chipping light metals.

Main cutting edges of the twist drill - responsible for machining performance

Two main cutting edges are always present on the twist drill, which are connected by a chisel edge. The main cutting edges take care of the actual drilling process. Compared to short cutting edges, long cutting edges deliver a better machining performance. 

Chisel edge of the twist drill - the shorter the better

The chisel edge is located in the centre of the drill tip and has no cutting effect. It applies pressure and friction to the workpiece and is therefore a hindrance in the drilling process. The length of the chisel edge can be reduced using an appropriate grinding process. This thinning or cross-grinding results in a significant reduction in frictional forces and thus a reduction in the required feed force. At the same time, the drill tip is centred better in the workpiece. 

Flute profile (spiral groove) of the twist drill - responsible for tool life

The twist drill has two opposing spiral chip flutes which allow for clearance of chips and the supply of coolant. They are usually ground, milled or rolled into the blank. Wide flute profiles are flatter and allow for larger drill core diameters.

Poor chip clearance means greater heat generation, which in turn can lead to annealing and finally to breakage of the drill.
A bad chip blockage can cause radial movements of the drill and influence the hole quality, tool life and reliability of the drill. This can also lead to drill and insert breaks. The wider the flute profile, the better the chip clearance.

Core - stability of the twist drill

The core thickness is the deciding factor for the stability of the twist drill. Twist drills with a large (thick) core diameter offer greater stability and are therefore suitable for higher torques and harder materials. 

Guide lands and secondary cutting edges of the twist drill - responsible for the radial run-out accuracy and quality of the bore hole wall

  1. Main cutting edge
  2. Flank
  3. Secondary cutting edge

Guide lands are the result of relief grinding along the chip flutes. Depending on the drill diameter, these are 0.1 to 5 mm wide and support the guidance of the drill in the bore hole. The quality of the bore hole wall is heavily dependent on its composition. 

The secondary cutting edge forms the transition from guide land to chip flute. It loosens and cuts chips that have jammed on the material.

The length of the guide lands and secondary cutting edges is heavily dependent on the helix angle.

Helix angle of the twist drill - determines the application based on the material

The helix angle is formed by the flute direction and drill axis. It determines the size of the rake angle on the main cutting edges and thus the process of chip formation.

Larger helix angles deliver effective chip clearance for soft, long-chipping materials. By contrast, smaller helix angles are used for hard, short-chipping materials.

Twist drills with a very small helix angle (10° - 19°) feature a drawn-out helix. In contrast, twist drills with a large helix angle (27° - 45°) feature a compressed, short helix. Twist drills with a normal helix have a helix angle of 19° - 40°. 

Which drill for which material - three types

In the DIN manual for drills and countersinks, the division of application groups into the three N, H and W types is defined under DIN1836: 

  1. Type N: Normal helixes for materials with normal hardness such as general construction steels, non-ferrous metals and cast iron. Not suitable for soft materials
  2. Type H: Drawn-out helixes for hard, short-chipping, brittle and tough materials such as steel, hard plastic, plexiglass or laminates
  3. Type W: For soft, long-chipping and tough materials such aluminium, copper or soft plastics.

Cutting speed & wear

If the correct cutting conditions are selected, wear is even across the tool. Uneven wear can occur if the cutting speed is too high, the feed is too high or the material is too hard. The drill must then be reground on the flank until the wear on the main cutting edge, chisel edge and guide land has been completely rectified. If the wear on the guide land is not rectified, the drill will jam.