Plywood Thickness Variation and Slot Cutting

Cutting slots for Shaker doors gets problematic when "quarter-inch" plywood is really more like 3/16" thick. In this thread, cabinetmakers discuss how they adjust to actual plywood thickness. November 12, 2005

I am building shaker doors with a 15 degree bevel using a bit set that cuts a 1/4" slot. The trouble I'm having is finding true 1/4" ply. Everything seems to be about 3/16". This must be a common problem, but no one seems to have any solutions. Does anyone have any suggestions?

Forum Responses
(Cabinetmaking Forum)
From contributor R:
Here is a common fix: take your slot cutter to your sharpener and have them grind it to the right width.

From contributor L:
There are a couple of ways to get around this. If it is paint grade then use MDF that is 1/2" thick and run it through your panel shaper and put the panels in backwards. If it is a stain grade door then use 3/8" to 1/2" ply and cut a 1/4" tongue on it so the rabbet is on the rear side. This way you don't need to have your cutter re-sized and the need for you to do a re-setup of your machine. I suppose you could run your plywood through the panel cutter if it is a baltic birch or other similar multi ply plywood.

From contributor W:
Most suppliers carry veneer faced MDF that is 1/4" thick.

From contributor L:
To contributor W: I find that 1/4" MDF barely fits into my 1/4" slots as it is. Put two faces of veneer on them and I can't see it fitting.

From contributor H:
1/4" veneer with MDF core is 1/4" thick.

From contributor B:
We've got straight bits that are plywood bits; they run a little closer than standard 1/4, 1/2, 3/4 and etc. Instead of plywood, I would suggest looking for bits. You might as well get used to it because plywood seems to get smaller and sloppier all the time.

From contributor L:
I didn't know that it would be 1/4" finished. I figured they took standard 1/4" MDF and applied the veneer to it making it slightly thicker than 0.250".

From the original questioner:
I have 1/4" maple veneered MDF from previous jobs and it also is roughly 3/16". I guess it can all vary.

From contributor D:
You could try one of Amana's new adjustable slot cutters so you could adjust the slot on each run to fit the panel that you have on hand. I saw them advertised for about $100 bucks in woodshop news

From contributor S:
I assume this is stain grade. I worked for a high end shop that puts the 3/16" thick panel in the 1/4 slot and pin tack the panel into the groove from the back. I think this is totally acceptable procedure. Fpr paint grade I take a 3/8" panel and "scoop" out the back to leave a 1/4" tongue.

From contributor M:
1/4 plywood will measure anywhere from .202 to .210. Most industrial cutters are set to cut your slot at these dimensions. I would look at getting a set of cutters with inserts and end any future problems

From contributor F:
I run my flat panel door parts on a shaper. Depending on what type of panel material I have needed or which of my suppliers have had the material, I too have had to accept the fact that the panel thicknesses are all over the map.

The way I have dealt with it is to buy two additional 2 piece sets of panel groover and tongue cutters for my Freeborn cope and pattern door knives. I can now run panel grooves and matching tongues at 1/4", 5 mm and 5.5 mm.

I personally like my panels to fit the groove snugly to eliminate panel rattle and I don’t want any brad holes in the back of the panel. So, if the panel stock doesn’t fit one of my three choices for groove width properly, I set the groover that is just a little smaller (thinner) than the panel thickness on the spindle and shim it to run eccentrically to machine the correct groove width.

This of course necessitates shimming the tongue cutters so that the tongue fits properly as well.
I haven’t tried shimming router bits that are made for doors, but I just looked at some in a catalog and I think it would be possible to shim the groover to run eccentrically and also shim the tongue cutter for the right fit.

From contributor T:
To the original questioner: Contributor W is right on about the 2 sided MDF veneer. The stuff that we use is .242 thick so you can just run your normal slot cutter if you want to run regular 1/4" (somewhere around 3/16") then do as others have suggested and use a 5.5 mm slot.

From contributor W:
1/4" seems too flimsy to me and I have an aversion to MDF in cabinets so I use 3/8" ply. I cut the groove on a shaper with a 1/4" slot cutter with the cutter height adjusted so that by running the piece through twice (each side up) I get a snug fit. Two problems I encountered that I've solved are: I was tormented by chip-out - solution: I make a 1/16" or so scoring cut by climb-cutting. Then cut the full depth the normal way.

For painted doors, any gap sticks out like a sore thumb - it's usually uneven and nasty looking no matter how well the panel to frame fit is. I solve this by first running a chisel brush around the gap with primer on it. I pack it in so it seals the gap, then spray the primer. Sometimes I'll have to repeat with one of the two top coats.

From contributor A:
We go about it from the panel thickness. We use the Freud adjustable dado set to dial in the thickness of the panel and then it’s off to the router table for the profiles wanted.