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I'm making a panel and have imperial router bits. I can only get ply in metric. If I buy 6mm ply, will it create a resistance fit with a slot cut using a 1/4" bit. Not sure if the 0.35mm difference will be problematic or not.

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  • Welcome to WSE. 6mm = 0.23622047 inches. The plywood is going to be loose in the slot. It might be helpful to know more about what you are building so viewers can offer suggestions on what to do.
    – Ashlar
    Commented Dec 15, 2022 at 0:54
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    Is your ply micrometer tested at 6mm? But I digress. Just run a 3/16” bit twice to get the exact fit you want. Commented Dec 15, 2022 at 1:46
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    Just to state the obvious: Be sure that whatever bit you buy has the proper size shank for your router. The 0.02" difference in diameter noted by @Ashlar between 6mm and 1/4" is annoying if your dado doesn't fit. It's potentially painful to deadly if you try tightening a 6mm shank into a 1/4" collet then spin the bit up to 10K RPM or more.
    – FreeMan
    Commented Dec 16, 2022 at 16:40

1 Answer 1

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You can't guarantee your router bit will leave a fairly exact 1/4" slot with a single pass1.

And you certainly have no guarantee the plywood is 6mm exactly, and/or is consistently 6mm — perhaps the majority of modern plywood is not consistent.

Loose fit? No biggie
Regardless of this, let's say you do end up with a slightly sloppy fit2, does this actually matter for your application? It doesn't in a few applications including cupboard backs, drawer bottoms and in frame-and-panel construction.

For the first two, if you do need or want the plywood firmly anchored this can be achieved easily:

  • using glue blocks;
  • brad-nailing the perimeter;
  • using framer's triangles or similar at the back/underneath;
  • glueing the ply into the slot with a gap-filling adhesive (epoxy being the most common).

For the third:

  • tacking the ply in with dabs of glue (pretty much any glue will do here);
  • using foam spacer blocks or balls in the slots removes play and silences any tendency for the panel to rattle.

Want a more exact fit?
As @AloysiusDefenestrate points out in the Comments, running a narrower router bit twice can give you the exact fit you want.


1 Unless you have measured the bit with a calipers to confirm the size it's probably safest to assume it isn't exactly 1/4". With non-premium router bits it's likely there is some variation bit to bit.

2 Bear in mind the reverse might be a possibility!

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  • Another option for firming up a slightly loose slot would be, after applying wood glue in the slot, mix in a little sawdust for additional binding material. Then insert your ply, and allow to dry.
    – Jim Fell
    Commented Dec 16, 2022 at 20:25
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    @JimFell, yes, technically that would work (and it's a trick I've used myself for filling minor joinery gaps). Not sure I'd want to mess with it on long slots (especially if numerous) and anyway using epoxy as the adhesive would accomplish this goal in a single step.
    – Graphus
    Commented Dec 17, 2022 at 8:15

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