Mortice and Tennon Tolerances

Hey gang.

Working with MDF and Melamine and locking them together with mortice and tennons (I think that’s what they’re called). Where you have a square hole in one piece and a tab in the other. The tab slides into the square hole. I’m using 5/8"x1" holes and tabs but the fit is super tight (like you need a hammer…). Designed in Fusion 360.

Is there a good way to incorporate tolerances?
I’m using a Kyocera 1/8" bit. Never measured it. Could this be the issue? I assumed it’s actually 1/8".

Id measure the bit.

Do you use parameters in fusion?

I have at times, yes. Why?

Do you have some digital calipers? Can you see if they are cut to the size you expected them to be?

And I second what fish tacos said, you can add a variable specifically for your tenon tolerance, and then add it to each dimension related to your tenons and mortises, and then you can adjust the tolerance by changing that variable. But if the error is that your bit is smaller than the CAM software thinks it is, then a tolerance is just going to cause you headaches in the future.

Also check that you are cutting square, you should be able to see where it is wearing/rubbing and that might point you to some issues with the bit not being perpendicular to the surface, or the XY isn’t square. Both of those could cause tigher joints than you expect.

And just to check, you are overcutting inside corners, or undercutting outside ones, right? So that you aren’t fitting a square peg in a 1/8" rounded rectangle hole?

It make it easy to change tolerance if its not your bit you could just change a simple figure you your model till you got the fit your looking for.

Ya, I’m using the dogbone script on all corners (set to 1/8"). I will measure the bit tonight and see if that’s it. Also measure the parts to see if they are too large or small. If not, then add a parameter. Have either of you done these and not had to add parameters for fit ?

Like I mentioned, I’m using the Kyocera. But in fusion I just grabbed an 1/8" bit. However, 1/8"=3.175 mm in actual fact. So if it’s actually a 3mm bit, then my holes would be too small, and my tennon would be too big, correct ?

And your mortise would be too small. Because it’s not removing as much material as it thinks it is.

I haven’t made any mortise and tenons with my machine, but I made some finger joints (or box joints) on the edge of a ply to make a 3 sided box. I can’t find the file right now though, so I couldn’t tell you if I added any tolerance, but if I was to make it today, I would add some, so I probably did… Not very confident though. Box joints aren’t affected by material thickness either.

The bit measures 1/8 or 3.12ish on my caliper. Since it really can’t give accurate readings past 2 decimal points, looks like I’m going to have to look at including a tolerance parameter in my dimensions.

I wish there was a universal tolerance setting somewhere…

If you set your bit size smaller, you are effectively changing every cut. Every part will be a little smaller and holes will be a bit bigger.

Ah yes of course ! Brilliant !