Actual vs Nominal Plywood Thicknesses - Woodworking Talk - Woodworkers Forum
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post #1 of 14 Old 04-04-2019, 02:43 PM Thread Starter
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Actual vs Nominal Plywood Thicknesses

Hello all,

First time poster here. Im ramping up to make a bathroom vanity, simple shaker style. But Im struggling with how I handle the difference between actual wood thickness and nominal thickness.

My background is mechanical engineering and Ive fully designed the cabinet in Solidworks (3D CAD) and plan to make some flat drawings and start making my first cabinet. But when I lookup 3/4 plywood at a home improvement store like Home Depot, I find that 3/4 plywood is actually 23/32 nominal (seems being under by 1/32 is normal because its sanded flat) but from there, the actual thickness is even less still, Home Depot says 3/4 birch is actually 0.703, so much closer to being 3/64 undersized.

It seems to me while 1/32 isnt impossible, working in 1/16 as your smallest increment is probably most realistic since a pencil line can be nearly 1/32 thick. Im wondering how this is handled? My dad who does a lot of woodworking, but mainly does things like jewelry boxes, says that youll eat up that gap easily with glue and imperfections and just plan as if it was 3/4. But when I look at how thick 3/64 is on my calipers, it seems like an awful lot to eat up.

How is this normally handled in cabinet making, or is there plywood I can buy that is actually 3/4 that I should be using instead of what I find at Home Depot? Seems crazy to try and measure and add/subtract 0.047" from my measurements to take into account this difference in thickness.

Thank you in advance for any help you can give
Jason
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post #2 of 14 Old 04-04-2019, 03:08 PM
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Welcome to the forum, Jason! Add your location to your profile so it shows in the side panel. That often helps us to help you. You can add your name to your signature line and it will show in each post.

I rarely work with sheet goods - I'm more like your dad - but if you're cutting dado slots then you'll have to adjust your dado stack to fit the sheets you buy, assuming you're using a table saw.

There are plenty of folks here who do a lot of sheet goods and I'm sure they'll chime in with best practices for how they do that.

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post #3 of 14 Old 04-04-2019, 03:42 PM
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buy the wood, see how thick it is, cut dados/grooves/etc to match the thickness of the existing wood.
btw, next week they'll have a new batch and it's 50-50 whether it is the same thickness.


see if there is a plywood supplier in your area - "furniture grade" (a very abused term) plywood has more plies and generally tighter control on quality/thickness.



as for planning dimensions, use outside or inside dimensions as appropriate. i.e. if you're building 24 inch wide cabinets, you make them 24" wide and let the interior dimension be what it is. .... if you're using prefab drawer bodies, measure to the insides so the drawers&hardware 'fit'


if you intend to do lots of woodworking, buy a thickness planer and rough sawn stock - then you can control the final thickness....
lumber from the big box stores, not so much.
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post #4 of 14 Old 04-04-2019, 04:38 PM
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I find thickness varies across the sheet. Factory edges are thinner. Also, the veneer on the factory edge is thinner than in the middle.
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post #5 of 14 Old 04-04-2019, 09:23 PM
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Plywood today is gauged in metric measurements. It's when you measure in inches it seems odd sizes.

I worked with a lot of professional cabinet makers that only measured in 1/16" increments. If they wanted to measure todays plywood which would be a 32nd less they would write it 3/4"- and if they worked with 3/4" wood that came 1/32" more they would write 3/4"+. I used to really catch it from them if they had to use one of my cutting tickets where I would write 25/32".
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post #6 of 14 Old 04-06-2019, 02:07 PM
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Quote:
Originally Posted by Jsunnb View Post
Hello all,
snip

How is this normally handled in cabinet making, or is there plywood I can buy that is actually 3/4” that I should be using instead of what I find at Home Depot? Seems crazy to try and measure and add/subtract 0.047" from my measurements to take into account this difference in thickness.

Thank you in advance for any help you can give
Jason
This is woodworking, wood moves and changes sizes, mills and manufactures don't always finish material to the same dimensions, plans are very handy as a guide, but there is also a bit of cutting to fit involved, a lot of times we would be better off trading our computers for a story stick.

Wise men speak because they have something to say; fools because they have to say something -Plato

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post #7 of 14 Old 04-06-2019, 03:17 PM
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Quote:
Originally Posted by FrankC View Post
This is woodworking, wood moves and changes sizes, mills and manufactures don't always finish material to the same dimensions, plans are very handy as a guide, but there is also a bit of cutting to fit involved, a lot of times we would be better off trading our computers for a story stick.
I would trust a story stick farther than I could throw my computer. (Which most of the time I want to)
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post #8 of 14 Old 04-06-2019, 03:38 PM
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FWIW the plywood at the big box stores isn't as good as the plywood that you will find at lumberyards. You'll get what you pay for. Don't expect to find any wood that will have measurement tolerances that you are used to dealing with in engineering. Use the outside dimensions of your cabinet and cut the other parts to fit. Cut most parts initially oversize and sneak up on the final dimensions.
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post #9 of 14 Old 04-06-2019, 03:52 PM
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I guess there is a reason why some learn machining and others woodworking and the two are not mutually inclusive.

One hot spring day while visiting a machinist friend of mine he was proudly showing me the precise fit of the door he had installed on his new garden shed.

I commented that maybe he had made it a bit tight, should maybe have left more clearance between the door and the frame.

His reply was not the way he was taught, sloppy work was not acceptable in his world.

He called me a few weeks later, it had been raining for a few days and his door was stuck shut.

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post #10 of 14 Old 04-06-2019, 06:50 PM
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Quote:
Originally Posted by FrankC View Post
I guess there is a reason why some learn machining and others woodworking and the two are not mutually inclusive.

One hot spring day while visiting a machinist friend of mine he was proudly showing me the precise fit of the door he had installed on his new garden shed.

I commented that maybe he had made it a bit tight, should maybe have left more clearance between the door and the frame.

His reply was not the way he was taught, sloppy work was not acceptable in his world.

He called me a few weeks later, it had been raining for a few days and his door was stuck shut.
You know, i actually had the exact opposite experience, in that i went from woodworking to machining. Turns out, 1/32" in machining is a hell of a lot more than 1/32" in woodworking...

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post #11 of 14 Old 04-07-2019, 11:50 PM
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Sometimes that happens, nominal lumber comes in undersized. It's a good idea to not count on it being perfectly to size upon purchase.



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post #12 of 14 Old 05-20-2019, 09:40 PM
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I think a lot of our stuff is imported/exported, and is metric. .708 is 18mm exactly. Funny how its never over size.
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post #13 of 14 Old 05-21-2019, 11:05 AM
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I think a lot of our stuff is imported/exported, and is metric. .708 is 18mm exactly. Funny how its never over size.
Quite true. Im working on a project with nominal 3/4 plywood that turns out to be imported from Chile of all places. Its bang on 18 mm thick. Interesting though how melamine and MDF, which are also sized in metric, are available in 19 mm which is pretty darn close to 3/4.
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post #14 of 14 Old 05-21-2019, 01:21 PM
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I have an "undersized plywood" router bit set. They seem to work well with the nominal plywood sizes, but not with the MDF or particle board sizes.

For 1/4" slots I just run the regular saw blade through once and then a second time a little out of register to enlarge it to fit the 1/4" sheet thickness. This is my standard for cabinet backs.
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