I've noticed that the material I use varies from sheet to sheet in thickness. It happens in both MDF and plywood. Of course the import seems to be worse than the domestic but I can't always say that. I've tried to use digital calipers to measure the edge of the material before I program dados. When the stuff comes off the machine the edges do fit just fine. However, the parts cut from the center of the panel don't seem to fit. They are usually too fat for the dado.
MDF seems to vary from sheet to sheet as well - same bundle, sheet to sheet. One will leave a slight skin on the bottom, and the next will through cut slightly into the spoil board. I could touch off the spoilboard for the through cuts and retoolpath everything. I really don't want to do that. It would be hours of toolpathing for the number of programs I use. It seems there has to be a better way.
From contributor A:
It depends on how many programs you reuse on a regular basis and how long you have been referencing from the top of the panel face. I always prefer to touch off the spoilboard. It just seems that there is less messing around with programs and any operator input that wastes time - unless of course, you have a control that lets you use parametric variables to control tool depth. That would mean re-toolpathing as well though.
Is the problem that the parts don't fit in the dados, or that you are not through cutting consistently? I don't understand why you would get a different depth of cut from one sheet to the next. Are you calipering and changing cut depth for each sheet?
ďIs the problem that the parts don't fit in the dados, or that you are not through cutting consistently?Ē
It's both problems. Letís say I'm cutting parts out of .5 MDF. I toolpath the part with a cutting depth of .505 inches. I cut one sheet and the parts come out fine with just a slight imprint of the toolpath on the spoilboard. I load the next sheet and cut the same program, the same way. Now this time the parts have a paper thin skin left. Now I have to cut the parts out of the sheet.
As far as the dados, letís say I am cutting out a bookcase and need the center shelf to be fixed. I want to dado the shelf for support. I pull a sheet of ply off the shelf. I want the material to fit the dado correctly but when I measure the material. Letís say it measures at .7253 in. on the edge and I cut the dado at .726 inch - the parts cut from the center of the sheet are thicker than the .726 inch. Now the dados are just slightly too thin to fit the material.
To fix this, I currently have to adjust on the table saw to get it to fit. It seems there should be a way to account for this. I just don't know what it is. But I do like your lock dado idea. I'll re-toolpath everything if I have to and touch off the spoilboard.
I also meant to say that you can also purchase material that is sold as exact thickness. (I forget the term for it.) It's a bit more expensive, but that would solve a lot of your problem.
This sounds like you are having a problem with your Z encoder and it is not hitting its depth consistently each time. If it were working properly it should not matter how much variance in thickness you find in one sheet or a whole bunk and the type of material won't matter either. The top of your spoil board does not change (unless you do a fresh flycut). If your spoilboard is moving you have other problems to work out, but I would first investigate your Z-control.
I always use air to blow off the touchpad and the material surface before touching off. Lately, I've been touching off in the material surface and cutting. If the sheet gets through cut like I think it should I do not rezero and just keep loading and unleading the machine. This seems to work better. However, I feel this is a band-aid solution. Could it be that maybe the touchpad or has a layer of dust that elevates the pad? This would give an artificial high thickness for the material so maybe that's it? Maybe?
What about the material thickness differences from edge to center? I have verified the change using a digital caliper. Seems if I had a caliper with a 24 inch reach the problem would be solved. Also, I'm in Florida in a non-air-conditioned shop. Could the humidity moving in and out of the edge of the material change the thickness enough to cause this? Supplier has an ambient air warehouse. Thanks for all the suggestions.
Are all of your parts dadoed or are some cross rail just doweled? This was always a problem. You could take a sample of say 25 or 50 sheets and come up with a range and average thickness of a sheet. At one point we were machining 250 sheets of Baltic Birch a day. You talk about a thickness problem!!! Many of the cabinets were dado construction and we found a dado with that worked. 15mm BBB Baltic Birch measured anywhere from 14.1mm to 15.7mm we used 14.7mm because the range was on the higher side although the average was lower. QC at our plant just had to live with it - after all we make cabinets and not rockets. At that volume you canít adjust for each sheet or part. This is the fun part of what we do.