Collets are consumables, not tooling
A collet grips over a limited elastic range, typically one millimetre of collapse in total, and its accuracy depends on being compressed within that range. Clamp a 6 mm tool in a 6 to 7 mm collet and the compression is minimal, so the grip is light; clamp it in a 5 to 6 mm collet and the compression is at the limit, so the collet takes a set.
Once a collet has taken a set it no longer returns to its free diameter and its runout becomes inconsistent. It will still grip, which is exactly why worn collets stay in service long past the point where they are causing tool breakage.
| Type | Range per collet | Runout | Typical use |
|---|---|---|---|
| ER (DIN 6499) | 1 mm | 0.008 mm standard, 0.005 mm precision | General purpose, widest availability |
| DA | 1 mm | 0.015 mm | Light duty, drills and taps |
| TG | 0.5 mm | 0.008 mm | Higher grip force for milling |
| Precision double-angle | 0.5 mm | 0.003 mm | Small tools, finishing, high speed |
Nut torque and why it matters
A collet is compressed by the nut, so the torque applied to the nut sets the gripping force. Under-torque and the tool slips or pulls out; over-torque and the collet is deformed permanently and the nut threads are stretched.
Use the manufacturer's torque figure with a proper wrench. A nut tightened by feel is the most common cause of both tool pull-out and premature collet failure, and neither looks like a torque problem when it happens.
Where runout comes from in a collet assembly
The runout at the cutting edge is the sum of the holder taper error, the collet bore error, the collet's seating in the nut, and the tool shank itself. Each is small; together they are the reason a holder advertised at 0.003 mm measures 0.015 mm at the tool.
This is why we quote assembled runout when the tool, holder and collet come from us, and why replacing a collet is often a more effective fix for poor finish than changing the cutting parameters.
Miniature tools need miniature holders
Below about 3 mm diameter the tool shank is thin enough that the holder dominates the result. A drill or end mill at 1 mm will break in a holder that would be perfectly acceptable for a 10 mm tool, because the runout scales with the length of the slender shank rather than staying constant.
For micro work, a precision collet or a shrink-fit holder with a matched small bore is the difference between a stable process and a breakage rate that has nothing to do with the cutting parameters.
Frequently asked questions
When should a collet be replaced?
Treat collets as consumables. Replace them when runout measured at the tool exceeds your limit, when the collet no longer holds the tool against light hand pressure, or on a fixed interval in production. A worn collet still grips, which is why it is usually left in service too long.
Can I use any nominal size in a collet?
No. Each collet collapses about one millimetre, so a tool should sit within the collet's stated range with the collet compressed in the middle of that range. Clamping at the extreme reduces grip and permanently sets the collet.
How tight should the collet nut be?
Use the holder manufacturer's torque figure with a proper wrench. Under-torque causes tool pull-out, and over-torque deforms the collet and stretches the nut threads. Tightening by feel is the most common cause of both failures.
What is the difference between ER and TG collets?
ER collets are the general-purpose standard with the widest availability. TG collets have a smaller collapse range and a higher grip force, which suits milling where pull-out is a risk.
Do you supply balanced collet nuts?
Yes. A standard nut is a significant unbalanced mass at high speed, so balanced nuts are supplied for high-rpm work, and the assembly is balanced with the collet and tool fitted.