Consistent Dado Joints with Inconsistent Material

Thickness variations in sheet goods make for loose dado joints. Here's how to adjust your CNC settings to deal with the problem. March 27, 2008

Question
I am using Microvellum and an Onsrud CNC. I use blind dado construction for my cabs. We use melamine board, and it varies in thickness from .73 to .78. So of course this affects my dado being loose or too tight. Does anybody have a suggestion on the best way to deal with this? I have tried just going with the thicker stuff, but it makes some of the sheets' dados to loose. With using Mv, I can't adjust tool diameter on the fly to compensate for different thicknesses. Maybe I am being too picky on how tight I like my dado...

Forum Responses
(CNC Forum)
From contributor F:
If you index off the top of the spoilboard instead of the top of the part, the tenon will machine to the thickness that you have set in the program, no matter what the thickness of the material.

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From contributor M:
There are a couple of ways to go. The best is of course to write your programs after you know the thickness of the material you are going to use. It is a bit of a pain to monitor thickness, but if you can establish a routine, you get used to it. I use Alphacam with constraints. I am pretty sure Microvellum has some sort of parametric you can use to control dado width.

Usually if you stick with one manufacturer of melamine, you have no huge problem, as the variance is rarely more than a hundredth or so anyway. Another way is to machine the tenon face as well as the tenon itself to, say, .7 or so. You need to work from the spoilboard up for this. In other words, expose the core on one side of the tenon to a constant thickness and let the bottom of the dado take the inaccuracy. Say you use .75 as a nominal thickness. If the tenon is .25 long, make the dado .28 deep. .72 material will actually make the dado from the surface .25 again. Thicker material will make the dados deeper. If you stop dado both sides of the tennoned part, you also don't have to worry about the tenon bottoming out on one side only. Then again, there is always confirmat.

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From contributor B:
I agree with contributor F. You have to touch off of your spoilboard, then come up .735 and set that as your part height or material height. Don't touch off the material and set it there. The problem is with your tenon and not the dado. You have to work from the spoilboard up. Not from the material down.

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From contributor T:
Just to add fuel to the question... When he indexes off the spoil board without relocating the dado, won't the top and bottom of the upper be too high or low when assembled? I have/had the same concern and went back to screw construction. Maybe I am missing some of the advantages. I also don't like the double process for the t&b of the upper and base deck (tenon first, then flip or change to the saw for the groove). Looking forward to more input!