I’ve been using upcut endmills for a couple of years now with no issues. I seem to have gotten the hang of the S&F thing and generally get good results.
Yesterday I was cutting some plywood drawer dividers with an 1/8 flat upcut and was getting terrible chip-out, so I decided to try a downcut endmill I had bought several months back. I started with the same F&S and DOC I was using, and the dang thing went crazy. The only way I can describe it is like having a corkscrew in the collet. My material held fast to the waste board, but the entire gantry was shaking and the bit would not run straight. I slowed F&S and eventually DOC but to no avail. Finally switched back to the upcut mill and finished the pieces. I can see no obvious issues with the downcut bit. It is straight and dose not show any perceptible runout when inspected. Any suggestions?