Lesson Learned

[Bobby O'Neal]

Made a couple sets of cabinets for my mom's garage storage this weekend. Nothing special. Just some paint grade carcasses and poplar face frames. Only took two sheets and ply for the carcasses. Joinery was nothing more than some dadoes and a rabbet or two. After cutting everything up to the point of a dry fit, we noticed some severe fit issues in the joints. Turns out we had two different thickness of plywood. Some were super sloppy and some had to be tapered to even fit. All I can figure is that one got wet.

So, the other thought is this. Making a test cut to check our dado size (in a piece of MDF) we had a very nice fit. I suppose it doesn't matter which sheet that particular piece we tested came from because neither fit well during out dry run. So, does my little piece of labeled MDF to remember different sizes of dado setups have a flaw? Does the test cut always have to be in the same material you are using for that particular project? I wouldn't have thought the different materials would accept a dado blade in different ways but it seems they did.

[Bill White]

I'd fire your supplier. There's some real crap out there so, as the noted Roman scholar (pick one) said,"caveat emptor".
Ya gotta check everything ya buy today.
Bill

[Larry Fox]

I agree with Bill. This is also why I use pocket-hole joinery for almost all my cabinetry work.

[glenn bradley]

A lot of ply is irregular and as you say; checking each joint is the way to a better fit. Even then you can have issues as the ply will not be consistent along the length of the joint. Those special undersized bits help but, only if YOUR ply is the amount under size that the bit manufacturer calls "under size". I tend to use a 3/8" spiral for most dados and make two passes for a better fit. I do not have trouble with ply bought in from the same bundle from a decent lumber yard. The consistency from my local guys is pretty good.

[Jay Jolliffe]

I use a lot of 3/4" birch plywood cabinet grade. That's suppose to be the best. I've had them come right on 3/4, 1/32 less & 1/16 less from the same supplier just different batches.....How about very small pieces of metal in the layers. Looks lie maybe a 23 gauge pin or something that small...I've had the in Birch ply, cherry & vg fir.....What really gets me is I buy A1 ply which is suppose to be good both sides. One side is nice & the other is 6-8" strips

[Jim Rimmer]

Lots of good comments about plywood, but if I understand Bobby's question, he is wondering how he could make a test cut in MDF that seemed to fit and then cut a dado in the project wood and it didn't fit. Does wood cut differently with a dado blade than MDF does? I would be interested in the comments on that particular phenomenon myself. I could understand how one piece of plywood will fit and the next one wouldn't but shouldn't some of his plywood have fit since it fit the test cut? :confused:

[Jim Underwood]

The only way to really tell what's going on is to get a caliper or micrometer and measure both the cut and the sheet goods.

I swear if I had a dime for each I've seen this issue come up I'd be able to retire....

You'd think us woodworkers/cabinet makers would get together and put the hurt on manufacturers to get their act together and make sheet goods to standards.

[Steven Green]

Ply has been getting worse year by year. If you intend to use your dado blade I'd say the measure first cut once rule applies. I've been burned by the same thing once and I'm not about to let it happen again. A big plus for pocket holes, they really take the headache out of joining ply.

[Paul Steiner]

Another possiblity is that your vendor sold you MDF that at somepoint got moist or damp. Although it was not enought moisture to cause a lot of expansion it was enough to throw everything off.