Dados are too tight in MDF

[Michael Spurgeon]

I am working on a project that involve the need to create long 3/4 inch wide dados in MDF and need them wide enough to insert 3/4 inch MDF panels into the dados for glue-up. Since MDF dulls blades fast, I prefer to cut them with a straight cutting carbide router bit instead of dado blades. The new Bosch 3/4 inch straight cutting carbide router bit leaves a dado that the 3/4 inch MDF cannot fit into -- VERY close. I used a couple of scraps and found that if I beat the MDF into the dado, it slightly bowed the backside of the piece with the dado cut in it = tooooo tight and not enough room for glue.

Before somebody asks, yes, it is a true 3/4 cutter and not the undersized plywood bit.

While I cannot locate a slightly over-sized 3/4 cutter, I assume that my only choices are to either sand one side of the routed dado OR sand the MDF that is going into the dado.

There has got to be a better way -- suggestions/opinions?

Thanks,

Mike

[David Hawxhurst]

are you using a router table or edge guide? either way adjust the fence or edge guide a couple of thousands and make a second pass.

[Bob Vallaster]

If you're using a straight-edge as a guide, make first pass as before, then adhere a layer of masking tape to the face on which the router bears against the guide to gain a few thousands in a second pass.
Not enough? Use another layer of masking tape and go again.

BobV

[glenn bradley]

One or two swipes with 200grit on a ROS should cure this. You are right to want the joint to have adequate room for glue. MDF ends will soak it up and can make for a starved joint. For a perfect fit, you can make one of these (http://www.sawmillcreek.org/showthread.php?t=46406&highlight=auto+dado) in under an hour.

[John M Wilson]

Not sure how wide the piece is you are putting the dado into, but I gotta give a plug to a fellow creeker with one of the most handy jig designs I have made. It's called the "Auto Width Dado Jig", and here is a link:
http://www.newwoodworker.com/autodadojigpln.html

This jig will allow you to custom cut a dado to the precise width of the board you want to fit into it. It's a great concept, and even if you don't buy or use the plans, it might give you an idea for a way to solve your problem (make two passes).

Hope this helps!

[Michael Spurgeon]

are you using a router table or edge guide? either way adjust the fence or edge guide a couple of thousands and make a second pass.

For the test, I was using my router table, but there is not nearly enough table for 7 foot dados in a piece of 3x7 ft. MDF:D

Mike

[Michael Spurgeon]

Not sure how wide the piece is you are putting the dado into, but I gotta give a plug to a fellow creeker with one of the most handy jig designs I have made. It's called the "Auto Width Dado Jig", and here is a link:
http://www.newwoodworker.com/autodadojigpln.html

This jig will allow you to custom cut a dado to the precise width of the board you want to fit into it. It's a great concept, and even if you don't buy or use the plans, it might give you an idea for a way to solve your problem (make two passes).

Hope this helps!

Now that was an awesome jig -- just a couple of additional steps to the 8 ft. circular saw guide that I made

Mike

[Michael Spurgeon]

If you're using a straight-edge as a guide, make first pass as before, then adhere a layer of masking tape to the face on which the router bears against the guide to gain a few thousands in a second pass.
Not enough? Use another layer of masking tape and go again.

BobV

Bob--since I will be using an 8 ft. edge guide that I created for my circular saw, this method should work great and now I wonder why I did not think of that (easy answer = age:confused:)

Will also see what happens if I use a ROS lightly as also was suggested.

Thanks to all who answered.

Mike

[Chip Lindley]

Just a thought Michael. Could be that your sheet of MDF might have picked up some moisture and swelled a bit. This MDF width may be a bit wider than the next sheet you buy. Cut a dado into a scrap of MDF with your Bosch 3/4" bit and take it to the BORG next time you go. See if your scrap dado fits any of the sheets better than the one at home.

[Chris Parks]

I bet it is imported METRIC sized MDF and you are using an imperial bit. .75 inches = 19.05 mm.

[Sam Babbage]

I bet it is imported METRIC sized MDF and you are using an imperial bit. .75 inches = 19.05 mm.

Were that the case, then it would fit. :p

[Chris Parks]

I thought about that after I wrote it, brain fade!!

[Jerome Stanek]

That jig is nice but you have to only use the bit that you made the jig with because bits vary in size. Once you have the bit sharpened the pieces will not fit.

[Rod Sheridan]

I bet it is imported METRIC sized MDF and you are using an imperial bit. .75 inches = 19.05 mm.

Chris, isn't the North American standard metric thicknesses for sheet goods?

Regards, Rod.

[glenn bradley]

I like Bob V's solution. Easy to perform on your long cut. Good one Bob.

[Will Overton]

I don't use a lot of MDF, but the last 2 times I bought it, both this year, the sheet were slightly thicker than marked. This held true for 1/2" and 3/4". I wound up cutting the dadoes in 2 passes.

[Chris Padilla]

I use my dado blade on my TS for things like this since these can be dialed in a tad wide so the MDF fits more easily. As you probably know, MDF tends to be dead nuts on at its stated dimension.

Otherwise, lots of great ideas here!

[Gerry Grzadzinski]

I think that metric MDF comes in 18mm thickness.

[Thomas love]

Adjustable rabbit bits go along way in my shop , the ones with interchangeable bearings or a 3/4" straight bit with an edge guide for the router.
Just clamp the shelf to the table or bench and route the slightest rabbit on the edge, do it on a test piece first... once you get the fit go to town.

When I make dadoes on the table saw I purposely leave one of the chippers out of the stack in order to make sure I under size the dado then adjust the male edge to fit with the above mentioned router bits.
Once you become familiar with setting up (quite simple) you won"t ever think about all these jigs and adjustable dado blades >

Of note I would like to mention, I have the Freud adjustable set as well as the Forrest dado king set with magnetic shims, no adjusting for me i just grab the ones i need to under size and go. Depending on the size of the components a router table works great but no necessary if you do not have one. Just my 8 cents.
tom.

[Michael Spurgeon]

Adjustable rabbit bits go along way in my shop , the ones with interchangeable bearings or a 3/4" straight bit with an edge guide for the router.
Just clamp the shelf to the table or bench and route the slightest rabbit on the edge, do it on a test piece first... once you get the fit go to town.

When I make dadoes on the table saw I purposely leave one of the chippers out of the stack in order to make sure I under size the dado then adjust the male edge to fit with the above mentioned router bits.
Once you become familiar with setting up (quite simple) you won"t ever think about all these jigs and adjustable dado blades >

Of note I would like to mention, I have the Freud adjustable set as well as the Forrest dado king set with magnetic shims, no adjusting for me i just grab the ones i need to under size and go. Depending on the size of the components a router table works great but no necessary if you do not have one. Just my 8 cents.
tom.

I could not use such quality (and expensive) dado sets on MDF -- cheaper to toss out a $20 router bit after a project like this one.

At least I found it was not my imagination that the MDF was thicker than rated and certainly could have picked up a little humidity and swelled 1/32 although I suspect quality control was the actual culprit.

I'll try out the tape and the sanding and see which one works best for my situation -- the tape method worked great on my router table.

Thanks,

Mike

[Thomas love]

Mike, you can do the whole thing using the method i explained with a router bit. Nix the 3/4" bit and use a 5/8 for the dado then put an edge guide on the router and make the rabbit to fit the the 5/8 of the original dado, Approx an 1/8 rabbit by the depth of your dado. By the way I don"t throw away router bits either, a diamond card in the vise and a few passes will tune it back up.

I am glad the tape method worked for you. I suspect humidity was the actual culprit not quality control. Put mdf in water and see what it dose, Summer time, high humidity, same thing just takes a bit longer than actually putting it in water
I started using the method i described due to the fact that i have cut up and dado-ed cabinet parts and had the pieces that where designated to fit in the dadoes swell enough from the time i dado-ed in the morning up to assembly time, enough to make things difficult. The fact that you have a router table makes it even easier.

I do not view making a dado-ed cab joint any different than making mortise and tenons,( just a tenon with one shoulder) you must make one fit into the other, to think that the bits are going to match the thickness of your materials on any given day is wishful thinking.

By mentioning my dado sets I was trying to show they are not necessary. good luck my fellow creek-er.
tom