Drill twist introduction
May 22, 2021
Table of Contents
Definition
Drill twist is usually used to remove material to create holes. It’s designed with a cone-like internal structure and narrow at the top of the web with gradually increasing thickness to the shank. 1
Geometry characteristic
The major characteristic are listed below
- Spiral (rate of twist)
- Point angle
- Lip angle
- Function depth
Spiral (rate of twist)
It can control material removal rate.
| Type | Feed rate | Spindle speed | Usage or common phenomenon |
|---|---|---|---|
| Fast spiral | High | Low | Remove large volume of chips |
| Low(elongated flute) | Traditional usage | Traditional usage | Gall on bits or otherwise clog the hole, such as aluminum or copper |
Point angle
Harder materials require a larger point angle, and softer materials require a sharper angle. The correct point angle for the hardness of the material influences wandering, chatter, hole shape, burr formation, and wear rate.
| Point angle | Chisel edge | Chip property |
|---|---|---|
| > 118 | Concave | Improve chip breaking |
| 118 | Straight(Standard) | - |
| < 118 | Convex | Prevent burr |
Small point of angle will reduce edge strength and make it prone to chipping and fracturing.
Lip angle
It determines the amount of support provided to the cutting edge. A greater lip angle will cause the bit to cut more aggressively and leads to binding, wear and catastrophic failure. A flat bit is sensitive to small changes in lip angle due to small supporting area.
Functional length
It determines how deep a hole can be drilled and also bit stiffness and hole accuracy. Longer bit can driller deeper but less stiffer, leading to location inaccuracy and axis wandering.
Common lengths are listed in below table:
| Type | Length | Extras |
|---|---|---|
| Stub-length or Screw-Machine-length | Short | - |
| Jobber-length | Medium | Very common |
| Taper-length or Long-Series | Long | - |
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Others
- The diameter-to-length ratio is usually between 1:1 and 1:10.
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