Plywood back on cabinets

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Forum topic by bill66 posted 07-20-2019 10:42 AM 367 views 0 times favorited 9 replies Add to Favorites Watch
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10 posts in 894 days

07-20-2019 10:42 AM

I am building several cabinets from 3/4 birch plywood. Knowing that plywood is undersized I purchased a set of Freud plywood bits which are about 1/32 undersized. Now the carcass are complete with dado for back board recessed for the stretchers in the back and shelf pin holes drilled. The plywood from box stores is thinner the the dado made from the 1/4 undersized bit resulting in a gap. The dado will not accept a true 1/4 inch material. What should I do now?
Thank you. Bill A

9 replies so far

View therealSteveN's profile


3365 posts in 1023 days

#1 posted 07-20-2019 01:17 PM

Using plywood, especially for panels is a great thing. Dealing with the crazy sizes, that always seem to be moving, sux.

If it inserts, but is slightly lose. To check this, let go of the piece you insert, does it flop over, or lean more than a 10* angle from 90, then go back to drawing board, and adjust Dado, leans a little, it will go together fine, and adding space balls or something similar will take the rattle out. Optimally when inserted you can lift both pieces by just picking up the one you inserted, but careful not to have a jam fit, because glue will swell that, and you can get into a working open time jackpot with the glue on final assembly.

Off size is truly off size, I am not aware of a cheat to cover all the possibilities, Every plywood type will be a different shade of 23/32. Sure they are basically 23/32, but there is enough small difference the only positive way to sneak up to the exact size is to use a dado stack, and shim it out. Router bits are all going to be their own variety of 23/32.

FWIW MDF, and BB ply are the 2 most consistent, so if you get something dialed in for them, you will be good. The XYZ BORG plywood, especially from HD are what I call Forrest Gump sized. It’s more fun if you say it with a Gomer Pyle accent though….

-- Think safe, be safe

View Fred Hargis's profile

Fred Hargis

5637 posts in 2942 days

#2 posted 07-20-2019 01:28 PM

If you could find true 1/4 material, you could rabbet it around the edge to fit the dado you have, but I’m not hopeful you could find true 1/4” material. That, BTW, if a workaround that a lot of folk use for any thickness of sheet stock. Like SteveN said, precise thickness is a moving target, so making the dado a known width (say, 3/8” dado for 1/2” ply panel), and then rabbeting the panel to fit it is about as good as anything.

-- Our village hasn't lost it's idiot, he was elected to congress.

View jdh122's profile


1087 posts in 3266 days

#3 posted 07-20-2019 01:42 PM

If the cabinets are disassembled (I wasn’t sure from your post whether this was the case or not), you can glue in a piece of solid wood the size of the dado and then redo the dado (I’d use the tablesaw for this rather than the router table, since I’ve had bad experiences with 1/8 inch router bits breaking off – use regular blade, multiple passes after moving the fence slightly).

-- Jeremy, in the Acadian forests

View bilyo's profile


774 posts in 1551 days

#4 posted 07-20-2019 01:47 PM

therealSteveN has it right. There are two ways I can think of to “fix” it. First, you can cut a strip to fit the groove and glue it in, sand it smooth, and then re-cut a groove the right size. Second, since your back panel will only be seen from the inside, install the panel and then tap in a series of thin wedges at about 3” intervals all around on the back side. This will make the panel fit tightly against the inside edge of the groove. Or, once you install the panel and have it captured on 4 sides by the cabinet case sides, top, and bottom, If there is not a disturbing gap visible on the inside, leave it alone. It won’t go anywhere. Hope this helps.

View LeeRoyMan's profile (online now)


207 posts in 175 days

#5 posted 07-20-2019 02:13 PM

Slide the panel in and attach stops on the back side pushing the panel tight against the front.
Buy thicker material and rabbit the edges that will be sliding into your groove to fit your groove.
Just put them in and caulk around the edges with matching caulk after you finish them.

View bill66's profile


10 posts in 894 days

#6 posted 07-20-2019 02:36 PM

Wow. You guys are great with the ideas. I kept digging around and found enough in the back of storage bin to get through the completed cabinets thankfully. Going forward only consistent sized plywood is coming into the shop.
The size that matches my router bits for dado of plywood.
Thanks again.

View Rich's profile


4681 posts in 1038 days

#7 posted 07-20-2019 02:45 PM

I’ve found 1/4” plywood to be the most inconsistent of the sizes. The 1/2” and 3/4” stay pretty true to being 1/32” under, but 1/4” can range from 1/32” under to exactly 1/4”. I have two 4×8 sheets for an upright pantry I’m building for a client and they are 1/4” exactly. I passed on the so-called plywood sized bits since there is too much variance in the sheet material, as you’ve found. Instead I use a home-built dado jig for my router that adjusts to the thickness of the piece that will fit in the dado and routs an exact dado.

Given that your gap is likely 1/32 or less, why are you so concerned? It’s the back of a cabinet after all. Folks don’t generally pay that much attention to the back of the cabinet. If it really bothers you, go buy some 1/4” plywood that’s either the same thickness as your router bit (take calipers with you), or get some material that’ll be exactly 1/4” and use a 1/4” bit to widen your dadoes.

Finally, to clear up some confusion, what angle the board leans at when it’s in the dado is meaningless. A set of calipers to compare the width of the dado to the thickness of the board will tell you how far off you are. Also, space balls won’t do anything for you. I’ve never seen a cabinet back that rattled…lol. And do not glue it.

Good lesson to be learned here though. Do not cut your good material until you know what size. You could have made a pass through a piece of scrap and used that to check the dado size compared to the ply.

And finally, technically what you’re cutting is not a dado, it’s a groove. Dadoes go cross grain and grooves go along the grain.

-- My grandfather always said that when one door closes, another one opens. He was a wonderful man, but a lousy cabinet maker

View Rich's profile


4681 posts in 1038 days

#8 posted 07-20-2019 03:37 PM

Slide the panel in and attach stops on the back side pushing the panel tight against the front.

- LeeRoyMan

This is a great idea. Glazer’s points would work for that since it’s recessed.

-- My grandfather always said that when one door closes, another one opens. He was a wonderful man, but a lousy cabinet maker

View MrRon's profile


5621 posts in 3692 days

#9 posted 07-20-2019 11:15 PM

I would use a feeler gauge to find how much of a gap there is and then rip thin strips of wood and press them into the gap.

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