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Actual vs Nominal Plywood

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Forum topic by Jsunnb posted 04-04-2019 06:58 PM 825 views 0 times favorited 24 replies Add to Favorites Watch
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Jsunnb

5 posts in 109 days


04-04-2019 06:58 PM

Topic tags/keywords: plywood nominal thickness actual thickness cabinet making

Hello all,

First time poster here. I’m ramping up to make a bathroom vanity, simple shaker style. But I’m struggling with how I handle the difference between actual wood thickness and nominal thickness.
My background is mechanical engineering and I’ve 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 it’s 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” isn’t impossible, working in 1/16 as your smallest increment is probably most realistic since a pencil line can be nearly 1/32 thick. I’m wondering how this is handled? My dad who does a lot of woodworking, but mainly does things like jewelry boxes, says that you’ll 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?

Thank you in advance for any help you can give
Jason


24 replies so far

View jmartel's profile

jmartel

8495 posts in 2572 days


#1 posted 04-04-2019 07:06 PM

Buy some real plywood from a lumber dealer if you want true 3/4”. Plus it’s not crappy plywood as well.

Also, that plywood at Home depot is actually 18mm, and marketed as 3/4” since that’s the closest equivalent in US units.

-- The quality of one's woodworking is directly related to the amount of flannel worn.

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Jsunnb

5 posts in 109 days


#2 posted 04-04-2019 07:24 PM

Awesome! That explains a lot then. Thank you! I’ll see what I can find.

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bondogaposis

5456 posts in 2773 days


#3 posted 04-04-2019 07:49 PM

This is pet peeve of mine, there is no reason to have actual and nominal sizes in plywood. If they can make particle board and MDF in 3/4” they can make plywood as well. The US is a big enough market, plywood manufacturers could make it as they used to. It makes no sense in cabinet grade especially.

-- Bondo Gaposis

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Jared_S

206 posts in 381 days


#4 posted 04-04-2019 08:14 PM

Buy plywood, measure actual thickness and adjust your drawing/cuts to suit.

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Jsunnb

5 posts in 109 days


#5 posted 04-04-2019 08:32 PM



Buy plywood, measure actual thickness and adjust your drawing/cuts to suit.

- Jared_S

The main reason I don’t want to go this route if its not necessary is that I’d be trying to adjust measurements by 0.047” which is less than a 1/16”, I just feel like the tools I have would never support that kind of accuracy. But if no local lumbar yards have true 3/4” plywood, I’ll have to give it my best shot with the 18mm (0.706”) plywood.

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Jared_S

206 posts in 381 days


#6 posted 04-04-2019 09:17 PM

Buy plywood, measure actual thickness and adjust your drawing/cuts to suit.

- Jared_S

The main reason I don t want to go this route if its not necessary is that I d be trying to adjust measurements by 0.047” which is less than a 1/16”, I just feel like the tools I have would never support that kind of accuracy. But if no local lumbar yards have true 3/4” plywood, I ll have to give it my best shot with the 18mm (0.706”) plywood.

- Jsunnb

Good luck, i haven’t found it in American made ply from reputable suppliers.

Mdf all day long at at .750”, cabinet ply .710 to 725 and anything in between. Half inch will be .480” give or take.

Cutting to 1/64 is about as good as you need on a plywood box. Faceframe, door and drawer components are more critical.

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Lee

135 posts in 1300 days


#7 posted 04-04-2019 11:22 PM

+1 about the only time you need to be that critical is with dado’s and grooves. just adjust the dado stack to the plywood you have, make some test cuts on scrap wood for a good fit.

-- Colombia Custom Woodworking

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YesHaveSome

155 posts in 680 days


#8 posted 04-04-2019 11:31 PM

Just draw it full size and then use relative dimensioning once you start making cuts and putting it together. Even if it was exactly 3/4”, if you just start cutting out all of the pieces based on your plans alone and try to put it together you’ll probably run into issues.

-- But where does the meat go?

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Jsunnb

5 posts in 109 days


#9 posted 04-04-2019 11:33 PM



Just draw it full size and then use relative dimensioning once you start making cuts and putting it together. Even if it was exactly 3/4”, if you just start cutting out all of the pieces based on your plans alone and try to put it together you ll probably run into issues.

- YesHaveSome

Hmm, that might be the best way to go. Thank you for the suggestion!

View MrRon's profile

MrRon

5572 posts in 3665 days


#10 posted 04-05-2019 01:15 AM

I make all my drawings using Autocad. I work to a thousands tolerance, even in wood. I will be off, but it is only by a few thousands. I’ve worked to thousands all my life, so it is not a problem for me. When I draw a plan, I take thicknesses of all the wood I use into account as if they were metal. I know others will not do as I do, but I am very comfortable with my way of doing. I build large scale trains and the details involved require precision. For furniture, it isn’t necessary.

View TungOil's profile

TungOil

1274 posts in 917 days


#11 posted 04-05-2019 01:52 AM

As mentioned above the ply is 18mm….so invest in a metric tape measure and build to mm dimensions instead. Many of us have switched especially for cabinetry. All the Blum hinges and slides a metric as well.

Also, do yourself a huge favor and find an industrial plywood supplier and purchase pre-finished birch ply for your cabinet project. You will thank me later. Cover the exposed ends with a raised panel.

-- The optimist says "the glass is half full". The pessimist says "the glass is half empty". The engineer says "the glass is twice as big as it needs to be"

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Kelster58

755 posts in 962 days


#12 posted 04-05-2019 02:28 AM

I’m with the draw it 3/4” and when cutting grooves, dados, and rabbits adjust your dado set to fit the plywood you are using. If quality matters I would not consider using plywood from a box store. Find cabinet grade plywood from a specialty lumber store. Home Depot and Lowes plywood is not in any shape, way, or form worth your time and effort. I drive 75 miles to a specialty lumber store for cabinet grade plywood. Cabinet grade plywood has 7 layers, typically the inner layers are poplar, very few if any voids, and has finished layers on both sides that are thick enough to take sanding and finishing. The plywood I purchase is USA made, 3/4” x 96-1/2” x 48-1/2”, and is right around $100.00 per sheet for red Oak. Well worth the money…..........

-- K. Stone “Tell me and I forget, teach me and I may remember, involve me and I learn.” ― Benjamin Franklin

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Jsunnb

5 posts in 109 days


#13 posted 04-05-2019 02:35 AM

I think my biggest take away from this thread is to go to a lumber yard. That is my Saturday plan. It’s only about 20 miles away, so not too far. And second is to measure as I go.

I’ve been truly impressed with the responses I’ve been getting from everyone, excited to be a member of this forum, tons of knowledge to learn from!

View jonah's profile

jonah

2075 posts in 3720 days


#14 posted 04-05-2019 03:35 AM

You really don’t have to adjust much of anything to account for the difference in thickness. Sure, you cut your dados and rabbets differently, but you’d do those to the size of the actual material anyway, so there’s no point stressing over it. The missing thickness just means your final piece is an eighth inch smaller than you wanted it to be in your drawing.

As much work as possible in projects should be done with relative dimensioning – with the actual materials to be used – rather than going by a plan.

View therealSteveN's profile

therealSteveN

3116 posts in 996 days


#15 posted 04-05-2019 03:43 AM

Can you state clearly where working with pieces of 2 different sizes is causing you issues? What is it you are trying to do, that this discrepancy you see not allowing you to? I understand you have 3/4” solid stock, and undersized plywood is causing an issue for you, where is it the issue? IOW what assembly, or part of an assembly are you stuck on.

I know where the ply goes, as opposed to the solid stock on cabinets I make, and I am not seeing where these 2 meet that is causing an issue? Perhaps a glimpse of your plan, or a sketch of what you see the joint where you are seeing a problem would help resolve this.

I am probably wrong, but I am wondering if you are thinking of making a panel, either a door, or a side where you abut solid stock on the edge of plywood? Is that even close to what you are thinking? The only other place where they come together on my cabinets would be as an edge banding, on the edges of plywood, and for those either a tape, or solid wood you want them to be oversize compared to the ply as you can then trim them to be exact. Actually there is another place I would see them, on an inset door/panel, but if you have 3/4” solid stock, you would want 1/4”, 3/8” or at the biggest 1/2” thick plywood for them.

-- Think safe, be safe

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