When a 1/4" isn't a 1/4" ?

[dennis thompson]

So I bought some 1/4" mdf and cut some 1/4" dadoes, to fit shelves made from the mdf but the mdf is slightly thicker than the dado I cut. I have a lot of dadoes to cut, I guess I'll just return the mdf? Any suggestions before I return the mdf?
Thanks

[Davis Young]

Usually mdf should be dead on. It could be the mdf swelled around the edges where it absorbed moisture, the middle may be okay but you won’t know unless you cut into it or measure with deep reach calipers. Exchanging for another sheet isn’t a guarantee that you won’t have the same problem. If you do so I would suggest bringing a dadoed sample piece to the lumber yard to test on their stock.

[Jim Becker]

With any kind of sheet goods, it's necessary to measure the thickness (in multiple places) to be sure that any grooves, dados and rabbits you cut are of matching dimension. Most sheet goods are thinner than the nominal dimension in the store (they are actually metric, but even then, there is variability). MDF is "often" true, but as you have noted, the material you got is "more generous" than you expected in thickness. Any time you're cutting grooves, dados and rabbits for sheet goods, you may need to cut them with two passes of a smaller tool so you can get the width exact for your material.

[Dan Hahr]

Just recut the dados a hair wider.
Dan

[glenn bradley]

MDF also comes in many flavors/makers. I have only occasionally gotten MDF that is out of dimension enough to be an issue. Be sure you haven't used an 'undersize' bit intended for plywood. In practice I always slap a caliper on anything I am fitting into a captured joint; dado, mortise, etc.

[johnny means]

A quick buzz with an ROS with 320 grit is my answer to tight fitting slots. I'll also add a tiny chamfer to ease insertion. Expecting machinist tolerances here is not going to work. .25 keys won't easily slide into a .25 slot, a clearance fit is necessary.

[Richard Coers]

If you need to take a fare about off the MDF, you can cut a rabbet on the bottom surface of the MDF

[Jared Sankovich]

Most of the 1/4 mdf I've come across is .255-.260".

[Rob Sack]

Assuming the MDF is consistently slightly over-sized, what about adding shim(s) to your dado setup?

[Brice Rogers]

Could you perhaps put a caliper on the slot and the mdf and tell us what each of those measurements are?

[roger wiegand]

I think one of the few things in life that you can depend on is that the nominal dimension of sheet goods will not match the actual dimension. Nor will it be the same from lot to lot.

[Steve Jenkins]

I was a partner in a company that manufactured veneered panels. We primarily used mdf for the core and the first step was to calibrate the mdf by running it through a wide belt sander to insure they were all the same thickness.