Material left in full dado

[Joe Soto]

In the photo the dado on the left is a toe kick, the one on the right is a stretcher that we use for a back toe kick. Same material, same settings, cut with the same tool. Both are within .0005" of the width they should be so my tooling is set correctly. .75 material + .015 clearance=.765 Both cuts are .7645
I am using a 3/8" bit and realize the cut is more than twice the cutter width but can not figure out why the toe kick gets a cleanup pass and not the stretcher. What am I missing?

[Donald Thomson]

I have the exact same problem. I've looked at machine flex and bit deflection. The only solution I could come up with is to slow down my feed rate and increase my spindle RPM's a little to reduce machine/bit deflection. Sometimes it works, sometimes it doesn't. And like your situation, it is random. Some mortises are clean, others have that ridge in them. Don't know what to do about it, other than break out my chisels and clean up the mortise.