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Accounting for undersized plywood in a router table

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Forum topic by huyz posted 03-08-2016 08:00 AM 734 views 0 times favorited 11 replies Add to Favorites Watch
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huyz

49 posts in 326 days


03-08-2016 08:00 AM

Topic tags/keywords: undersized plywood stumpy nubs stumpy nubs router table router table baltic birch question

This is my 3rd project, definitely lots of learning along the way so far..

I’m building Stumpy Nubs’ router table, which requires plywood in 3/4”, 1/2” and 1/4”. I’m almost done with it, but learned the hard way that plywood is almost never the thickness advertised, and it’s almost never flat (I’m using baltic birch and it still has slight bowing).

Anyway, after putting most panels together I thought I just made terribly bad measurements. After pondering the exact amount the off-cuts were made I realized most if it is due to the undersized plywood. The plans and cutlist are designed for plywood that matches its advertised size.

The only real way I see to account for this is to REALLY understand the entire project before cutting anything, which parts are face to edge, when they stack, etc. It was a big enough task doing this to spec, I’d probably go nuts actually calculating out the difference based on actual ply thickness.

What do you all usually do to account for undersized plywood when it’s a multi-part/complex project like this?


11 replies so far

View rwe2156's profile

rwe2156

2198 posts in 946 days


#1 posted 03-08-2016 01:00 PM

It irks me, too, because there is no such thing a 3/4” plywood anywhere but they continue to draw plans as if there was.

What I do is go through the whole plan and adjust the measurements. Good rule of thumb is increase inside panels 1/16 thinking 3/4 ply is (supposedly) actually 23/32.

I’ve tried converting everything to metric and figure on 19mm thick plywood. That can be a mind bender.

-- Everything is a prototype thats why its one of a kind!!

View TechTeacher04's profile

TechTeacher04

326 posts in 997 days


#2 posted 03-08-2016 01:12 PM

If you are referring to jointery I cut the dado or groove to a nominal size thinner than the ply and then cut a corresponding rabbet on the ply to equalize the difference. You can also get router bits that are sized for the undersized ply, but I am cheap and use the standard sizes.

View bonesbr549's profile

bonesbr549

1176 posts in 2532 days


#3 posted 03-08-2016 01:18 PM



If you are referring to jointery I cut the dado or groove to a nominal size thinner than the ply and then cut a corresponding rabbet on the ply to equalize the difference. You can also get router bits that are sized for the undersized ply, but I am cheap and use the standard sizes.

- TechTeacher04

What he said.

I long ago even bought a plywood bit that was undersized for 3/4 ply and that did not help, as nominal thickness varied.

My solution is if it’s 3/4 (nominal) ply, I cut a 1/2 or 5/8” dado, and mill the mating ply down to the exact thickness. Solves all that crap, and gives me a better joint.

Have a good one.

-- Sooner or later Liberals run out of other people's money.

View MadMark's profile

MadMark

978 posts in 918 days


#4 posted 03-08-2016 05:39 PM

You can get full .750” 3/4 ply at Lowes. Alternately you can get undersized bits at Rockler.
M

-- Madmark - Madmark2150@yahoo.com Wiretreefarm.com

View greatview's profile

greatview

110 posts in 2623 days


#5 posted 03-08-2016 05:57 PM

I use MDO for many projects that will be painted or left bare. I just measured some 1/2” MDO and it is actually 0.508. Plus you get the bonus of a very flat and smooth surface that is also waterproof. (Most highway signs are made from MDO.) It comes in both single sided and double sided.

-- Tom, New London, NH

View pintodeluxe's profile

pintodeluxe

4858 posts in 2279 days


#6 posted 03-08-2016 06:35 PM

I started with those undersized router bits for plywood dados, but they either fit or they don’t. Now I prefer an exact width dado jig for use with a handheld router, or else a dado blade. Either of these methods offers good adjustability for snug fitting joints.

I keep a scrap of plywood with several sizes of dados cut with multiple methods. Then I take my project stock and test the fit in the various slots. Usually one will fit perfectly.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View xeddog's profile

xeddog

117 posts in 2473 days


#7 posted 03-08-2016 06:57 PM

Whenever possible I cut dadoes on my table saw with a good dado set. I get good fits when I take the time to add appropriate shims to fit my test piece. If the pieces are to unwieldy, or just not practical to use a table saw, I use a router and a jig that allows me to cut a dado that EXACTLY fits the actual part that goes into that dado.

Wayne

View huyz's profile

huyz

49 posts in 326 days


#8 posted 03-08-2016 07:35 PM

Thanks guys.

Yeah I got my dadoes to size perfectly with a dado stack and shims. The issue was accounting for panels coming together that are now thinner/shorter than expected. Most parts of this project won’t matter/just looks ugly I guess except for where the sled part meets the router table top, might have to shim that a bit.

View clin's profile

clin

514 posts in 461 days


#9 posted 03-08-2016 11:56 PM

When I cut things up, I try to make all cuts relative to one measurement.

For example, I just built some base cabinets. So for a 24” wide cabinet, I set the fence to 24”, but then put a spacer block on it made of two pieces of the same plywood the sides are made of. Then using my crosscut sled, when I cut the bottom panel, and top spreaders, they come out exactly 24” less the two side thicknesses.

Note: I happen to be using butt joints and pocket screws, no dados.

For the face frame rails, I then adding another spacer block to account for the width of the face frame (which I made two ply thickness also). So at this point, I have 4 plywood thickness on the block.

The point is to not move the fence and add appropriate blocks to the fence (which you need for safe cross cuts anyway).

I try do do this sort of thing as much as possible. generally works out well. But plywood thickness can still vary within a single sheet. So it’s never going to be perfect.

-- Clin

View SuperCubber's profile

SuperCubber

871 posts in 1750 days


#10 posted 03-09-2016 01:28 AM

My advice is to take cut lists with a grain of salt. If you must use the cut list, make one cut at a time, and compare the measurement of your next cut to what the cut list says.

I’m sure you’re aware after this incident, but pretty much all cut lists I’ve seen work like Stumpy’s, in that they go off a 3/4” measurement, as opposed to 23/32.

-- Joe | Spartanburg, SC | "To give anything less than your best is to sacrafice the gift." - Steve Prefontaine

View rwe2156's profile

rwe2156

2198 posts in 946 days


#11 posted 03-09-2016 12:59 PM



You can get full .750” 3/4 ply at Lowes. Alternately you can get undersized bits at Rockler.
M

- MadMark

I’m gonna go check that next time I’m there. If its imported I’ll guarantee its 19mm.

I’ll believe it when I see it cause I haven’t seen a 3/4” think piece of plywood in 30 years.

-- Everything is a prototype thats why its one of a kind!!

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