Tolerance of bored holes in Baltic Birch

I am planning a small project made with 12mm Baltic Birch plywood. I intend on installing threaded inserts that will be used to attach adjustment feet that use a 1/4" machine screw. So, how accurate is the diameter of bored holes using a 1/8" bit to drill holes in plywood? Should I bore them a little oversized?

I would get some scrap plywood and try some different hole sizes and pick the one that works for the insert.

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Also, use a caliper to measure the actual width of the bit’s cutting diameter. Be careful not do damage the flute(s).

I always measure the diameter of my bits, since having your machine calibrated to exactly 100% accuracy can be upended if the bit is slightly smaller/larger that what is entered into Easel.

For instance, my Amana Tool 51377-Z CNC SC Spiral O Single Flute (1/4") typically do not measure 0.25". They are “nominally” 0.25" and have a ± allowance from the manufacturer. I have purchased 5 of these bits recently and all of them have measure 0.228" on the nose. I enter each bit as exactly what I measure to ensure the accuracy of the milling operation is as good as it can be for the machine.

For threaded inserts meant for wood, this may not be a huge issue, but it is never a bad idea to be as accurate as possible.


Brandon R. Parker


Thanks for your insight. I had not thought of the possibility that the router bit may be undersized. I will check that first and correct it in the setup if needed.

Your machine should have no problem with this task. You’re overthinking it. Do a test, adjust, go for it. You’re not boring holes in aluminum for NASA.

To measure an endmill at that level of precision, you really need an optical comparator.

I learned that the original issue was the bit being used was not actually 1/8", but was a smaller metric size. The correct 1/8" bit now creates a much cleaner hole and the tolerance was closer to the design specs, but it still made the hole undersized just enough to make it hard to insert the the brass insert. I adjusted the radial stock to leave with a -0.005 in Fusion and the inserts now go in with just the right amount of effort.