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Thread: Undersized Plywood router bits

  1. #1
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    Undersized Plywood router bits

    I'll keep it short and sweet - am I better served picking up a set of undersized plywood bits (7/32", 11/32", 15/32" and 23/32") or the equivalent metric offerings (6mm, 8mm, 12mm, 18mm)? Just doing the math seems that the metric ones are closest to the actual dimension. Please advise me. I don't want to get into a brand debate, just advice on proper sizes.

  2. #2
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    To me, the problem is that there is no undersized "standard" for the plywood itself, metric or imperial.

  3. #3
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    The borgs seem to be busy selling imperial (or at least its labeled as such and seems to measure up to the quoted thickness), but everything I've gotten there has huge voids and few laminate layers. I started buying from a dedicated plywood supply house and everything they sell is routinely double the number of (thinner) laminates, zero voids, but is all metric. The greatly increased quality from the plywood supply for a trivial price difference keeps me going back there. Seems the metric bits would be a better match, can anyone speak to this experience?

    Quote Originally Posted by Nick Decker View Post
    To me, the problem is that there is no undersized "standard" for the plywood itself, metric or imperial.

  4. #4
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    I bought a set about 10 years ago, but never used them because the plywood that I get never seems to be close enough to these bit sizes. I made a jig like the one in post #22 of this link https://www.routerforums.com/general...ize-bit-3.html. You use a straight flush cutting bit with no bearing and guide the router base against the upper rails using the same under size straight bit every time that you used when first making the jig to trim the lower pieces. The jig can then be closed around the board to be installed in the dado and locked at that width. Then the jig is placed in position on the board to receive the dado and clamped in place. You then cut the dado by running the router against one side of the jig and then return against the other side of the jig. A perfect fitting dado no matter what the thickness of the board that will be installed into it with no math required in the setup

    Charley.

  5. #5
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    Quote Originally Posted by Charles Lent View Post
    I bought a set about 10 years ago, but never used them because the plywood that I get never seems to be close enough to these bit sizes.
    This was my experience as well. I have standardized on an undersized bit and a jig that matches the material with two passes as Charley describes.

    Dado Jig Refresh (10).jpg
    Last edited by glenn bradley; 08-13-2018 at 11:10 AM.
    "A hen is only an egg's way of making another egg".


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  6. #6
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    This is the only way to go. As has been said the thickness varies from supplier to supplier and even from the same supplier. The only ply I have found that is true sized in imperial is that with MDF core.
    George

    Making sawdust regularly, occasionally a project is completed.

  7. #7
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    Thanks Glenn and Charley, I'll have a look at this jig.


    Quote Originally Posted by glenn bradley View Post
    This was my experience as well. I have standardized on an undersized bit and a jig that matches the material with two passes as Charley describes.

    Dado Jig Refresh (10).jpg

  8. #8
    I measured thicknesses of 3 plywoods sheets I recently bought at HD.

    The varied from 18mm to just under 19mm and even different places on the same sheet were as much as .5mm off.

    1/4" sheets ran 5mm, not 6, but were pretty uniform.

  9. #9
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    I have a set from Whiteside and they have been convenient to have over time. But as others have said, the variability in sheet goods thickness is sometimes great...even on the same darn sheet, and especially for "cheap imports".
    --

    The most expensive tool is the one you buy "cheaply" and often...

  10. #10
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    Even really nice B.B. ply has some variation, I’ve found .005-.010” not uncommon, which seems petty until you plan around a measured size.

    Most often I am using it as a substrate, so I will veneer it then plane it to an imperial thickness.

    I take no issue with metric but most of my tooling is imperial so the odd bit if metric throws a wrench in the works.
    Bumbling forward into the unknown.

  11. #11
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    I think this gives you a reason to buy both sets. I see absolutely don't see why any self respecting woodworker
    would not have both

  12. #12
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    One expense at a time there chap.

    Quote Originally Posted by lowell holmes View Post
    I think this gives you a reason to buy both sets. I see absolutely don't see why any self respecting woodworker
    would not have both

  13. #13
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    Re: both

    Whiteside makes this conversion if you wish to switch from 1/4" to 5.5 mm and back & want to stick close to some kind of budget.

    I just bought the 5.5 mm "Shaker stile and rail set" instead of the conversion since I use a lot of 5.5 mm underlayment for jigs, fixtures & cabinet backs.
    My granddad always said, :As one door closes, another opens".
    Wonderful man, terrible cabinet maker...

  14. #14
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    Quote Originally Posted by glenn bradley View Post
    This was my experience as well. I have standardized on an undersized bit and a jig that matches the material with two passes as Charley describes.

    Dado Jig Refresh (10).jpg
    I made a jig like that and use a 1/2 X 1/2 top bearing bit. The bearing runs against the jig's jaws. Set the width of the slot using the piece that's going to fit there, route and perfect fit. The downside is that jig won't cut a dado less than 1/2" wide.
    Last edited by Curt Harms; 08-14-2018 at 7:00 AM.

  15. #15
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    Plywood in North America has been made in metric thicknesses for decades, as the NA standard is metric.

    The issue is that although metric bits would be the exact nominal size, the sanding allowance on panels would result in variations anyway..............Rod.

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