A 6 mm router bit shank and a 1/4 inch shank are different sizes. One quarter inch is 6.35 mm. That 0.35 mm difference is small on a ruler and large enough to matter in a precision clamping connection. Match the bit shank to a collet explicitly specified for it.
The same problem appears with 12 mm and 1/2 inch. Half an inch is 12.7 mm. A measurement that looks close is not a compatibility statement. Do not force a bit into a smaller collet or try to compensate for a loose fit by tightening harder.
Identify the router before the bit
Find the router's model number and manual. Check which collets and any manufacturer-approved reducing sleeves it supports. A collet from another router may have the same nominal bore while using a different taper, nut or length. Those external dimensions are part of the system.
Trend's 6 mm T4 collet, for example, is identified by both its bore size and the intended router family. That is the level of identification to look for when ordering a replacement. Keep the model reference with the collet so an unlabeled spare does not return to the drawer as a mystery part.
Read markings on the bit packaging and shank. If markings are missing, measure the clean straight shank with a suitable caliper and ask the manufacturer to identify it when uncertainty remains. Measurement can reveal an obvious mismatch; it cannot certify an unknown or damaged cutter for use.
Separate three different sizes
The cutting diameter determines the shape or width of the cut. The shank diameter determines the collet fit. The overall cutter length affects reach, clearance and safe use. A large cutting diameter does not imply a half-inch shank, and a long bit should not be used to solve a reach problem without checking the tool's limitations.
The table converts common nominal shank sizes. It deliberately does not group similar values into interchangeable pairs. Convert dimensions with 25.4 mm per inch, then select the exact supported size. Keep metric and imperial bits in labeled storage positions if you own both systems.
Inspect and install according to the manual
Disconnect power before changing the bit. Inspect the collet, nut and shank for damage, corrosion and contamination. Follow the router and cutter instructions for insertion depth, withdrawal from the bottom if required, tightening and speed limits. Generic advice about how far to insert a bit cannot replace a manufacturer's minimum insertion mark or model-specific procedure.
Do not make improvised shims from tape or sheet metal. Use only reducing components approved for that router and application. If a collet no longer grips correctly, stop using it and obtain the correct replacement rather than experimenting under power.
Record compatibility once
Make a small inventory with router model, approved collet part number, bore size and matching bit storage row. Photograph the relevant label for your own records. When borrowing a bit, check its shank before setting up the cut.
After installation, check guard, base opening, workholding and cutter clearance with the tool disconnected, following the manual. A correct shank fit answers only one part of the setup. The cutting diameter, material, speed and permitted cutting method still need to match the job.
Once the bit and collet are confirmed compatible, a shelf project still needs a separate check of dado width against the actual plywood.
| Marked shank | Exact metric size | Compatibility check |
|---|---|---|
| 6 mm | 6.000 mm | 6 mm system only |
| 1/4 in | 6.350 mm | 1/4 inch system only |
| 8 mm | 8.000 mm | 8 mm system only |
| 12 mm | 12.000 mm | 12 mm system only |
| 1/2 in | 12.700 mm | 1/2 inch system only |
Related workshop guides
- Fit a shelf dado to the plywood you actually bought
- How to sand inside curves without changing their shape
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