Dados with a router - straight or spiral bit???

[Bob Elliott]

That absolute best results for this kind of work will come from using a jig that permits you to use a much smaller cutter than the material with multiple passes to get the total groove/dado/rabbit width that exactly matches the actual material you are using. Unfortunately, sheet goods are variable in thickness, even in the same stack and sometimes even on the same sheet...including with the highest quality "domestic" product. The undersized bits are certainly closer to reality, but they often still leave gaps or do not provide the best fit.

I agree that downcut spiral or a compression bit that's designed for the limited penetration one uses for dados/grooves/rabbits will provide the best quality edges. It may be easiest and simplest to acquire the downcut spiral than to find a compression bit that is "short" enough for the job...many compression cutters are designed for full thickness cutting out of sheet goods on CNC rather than non-through cutting jobs. Straight bits or those with slightly skewed straight edges can cut pretty cleanly if they are sharp...like with a dado stack, running masking tape along the line can also help with keeping things clean.

I should have been clear with what i wanted to do.

I am doing exactly what Jim suggested. I am using 3/4" plywood for the cabinet boxes. I made a dado jig that I clamp the plywood in to match the size. I clamped the jig to the plywood that I want to cut the dado in and used a straight bit with a bearing that rides along the jig to cut the dado. I used a 1/2" bit when I made the cabinets for the bathroom.

I am getting some new tools for my kitchen cabinets. If I can get a downcut spiral bit with a bearing I would like to because I want to have the cleanest edges possible on the plywood dados. Is a bit like this available?

[Jim Becker]

Spirals tend to not have bearings, especially top bearings. What you might need to do is "re-jig" your jig to work with a guide bushing instead of a bearing toward the same end.