Plywood Thicknesses and Dado Widths
A detailed discussion of the fine points of making a well-fit dado joint. Calipers, shims, decimals, fractions, and more. May 14, 2006
I'm using dado construction and 3/4" plywood for my cabinet construction. My question is, seeing how 3/4" plywood is actually 23/32", should my dados be 23/32” or ¾”? Both work but 23/32” seems really tight and ¾” seems a little loose. Which is recommended?
From contributor A:
In a word (or two), dial calipers. Plywood, melamine, MDF, or whatever you use has always and will always be of varying thickness, and this tool will take the mystery out of the predicament. Get the exact thickness, and add .005" with shims to your dado blade.
From contributor B:
I agree 100% with contributor A. Once you put dial or digital calipers to use in the shop you will wonder how you ever got by with just a tape measure when doing millwork. It takes the guess work out of the equation. I machine a dado anywhere from .005" to .010" wider that the part thickness. A good rule of thumb for glue joint clearance on things like mortise and tenon is from .004" to .008". This is for Alaphatic resin glue (yellow). Most calipers also have a depth gauge that enables you to accurately measure the depth of dados, mortises and recesses. All of my face frame parts get edge planed to final width and I am able to hold a .005" tolerance on the final dimension using calipers.
Lastly, it would be a huge bonus if you were to learn and memorize all 15 decimal equivalents from 1/16" (.0625") to 15/16" (.9375") by the sixteenths. This enables you to read digital calipers easily and also to do fractional math using an ordinary calculator. If you learn this you can easily do the basic math with fractions without the need to find a common denominator or use a specialized calculator that takes fractional input. Decimal input can also be used in all kinds of cad programs and cutlisting programs etc. as well as CNC equipment.
From contributor C:
It doesn't matter how great calipers work and no matter many thousandths you shim for. The fact is any sheet, of whatever, is going to vary - not just one sheet to the next either. A single sheet will have thickness variations all over it. You just have to go with what works overall. It's wood, and the manufacturers will have standards that they meet, and those standards aren't really that high. They get lowered all the time. No matter how high your standards are they definitely aren't going to make the plywood any better. So you work with what you get, and try and make a dollar - maybe two - if you're good and fast.
From the original questioner:
I agree with all three postings. The calipers are nice but accurate and consistent readings are tough to get. What about test fitting the dado, how snug should it be? Should I have to force it in the dado with a hammer or should I only need a couple blows of my hand to position it?
From contributor E:
The rule of joinery is best summed up the way one of my old (and I mean a long time ago and he was in his 60's at the time) teachers put it: the joint should be just snug enough so that you can drive the boards together with your hat.
Too tight and you've left no room for the glue - too loose and now you're depending on the glue to fill the gap and to adhere the two pieces together. Here it'll do neither job well. Any joint that you have to jam, wedge, swear a little and then whack again with a deadblow hammer is too tight. There'll be no room left for the glue to do its job.
So be prepared to shim your dados and make repeated test cuts if you're doing this on the tablesaw, or to get a hold of so-called undersized plywood bits for your router if doing this with a router and a straightedge guide, and then maybe scoot the guide fence over and re-cut if needed. (CNCs are their own beast and you'd need to use the calipers to know what you need to program into the machine.)
From contributor F:
I totally agree with the above, but having said that, if you’re in a hurry about those pesky fractions and decimal equivalents, here are two solutions:
1. Get a vernier caliper – they’re plenty accurate enough for those types of jobs.
2. Get a Texas Instruments Ti- 30 or similar. It has a fractions bar, mixed fractions too. It's extremely handy to have in the shop. After all, can any of us find out what 32-7/16” minus 15-13/32" divided by 5 is quickly? I used to keep it in my apron for just that reason.
From contributor B:
The part of my post dealing with the knowledge of the decimal equivalent of fractions was only a side bar. That said, I personally find verneer calipers a pain in the rear to read. For less money a "General" brand plastic dial caliper that reads in fractions and thousandths can be purchased. But to the point I wanted to make is that yes, it is true that sheetgood thickness varies within a sheet and also from sheet to sheet. But, I find the difference to be a matter of a few thousandths most of the time within the same batch.
Yes, it’s true that you have to shim your stack dado to get the correct dado size for your material. With the use of digital dial calipers, I have learned how to make that process easy and fast. I have measured the width of the dados that my set makes with all the different combinations of chippers, unshimmed, and wrote the sizes down. I used the dial calipers to measure the dado widths in thousandths of an inch.
I have several shims, each in three different thicknesses and I have written down in thousandths of an inch the amount that each shim thickness changes the width of the dado after being installed and torqued down. Armed with that information, after the parts that will go in dados have been cut, I take several measurements from different places on the parts. I write down the thickest reading and add about .010" to it for clearance. Then, I select the chipper configuration from my list that that comes closest to that size but does not exceed it. I take its previously recorded dado width and do the math to determine which shims and how many are needed to shim the set to the needed dado width. It only takes putting the correctly shimmed dado on the saw once instead of several guesses like I had to do before I started using calipers.
The last thing is, yes, it does take some practice to learn the right touch to get an accurate reading with dial calipers. This is to be expected when learning to use any tool that you are not familiar with.
From contributor G:
Let’s go back to the best first answer: dial caliper plus .005 and add one more thing: a low angle block plane set to take .001 or so off - if its too snug, find the high part and bring the edge down a little bit.