The last hard layer line on the bottom is probably from me cutting the last 2mm as a separate toolpath. I am not using tabs, just tape, and try to cut that last bit of thickness out very slowly as to not cause the pieces to move.
My trays are just under an inch thick (21mm) so the #201 can not do the cut out in a single pass. Without buying a bit with a longer cutting length, is there a way to not get these layer lines?
In addition to Will’s comments it’s possible you may also benefit from “tramming”. That means insuring the Z-axis is perpendicular to the X-Y plane and that your work surface is parallel to that plane.
This could explain some of the step like pattern you see along the vertical. It’s hard to tell from your photo since you focused on the vertical issue but when you’re out of tram you will also see a tiny step-like pattern on the flats with a square mill.
Thanks Will, that last thread you posted with the info on how to do a pocket cutout made a big difference in my results.
Bryne, good thinking about the tramming. I don’t think that was my particular issue as I get really smooth results on the flats with square mills. I’ve also surfaced a board with nice smooth results.
Quite a few different aspects of CNC setup and use can make elimination of these marks seem tough. It has already been mentioned that tramming your spindle is a necessary place to start. If the spindle is not truly vertical in both the X and Y planes, then marks like your workpiece displays are probable.
If the cutter is not sharp, then there will be slippage as the cutting edge bites into your workpiece and that can cause the marks, which look like they might be from cutter chatter. If the cutter is not sitting tightly in the spindle, that is another possibility for the marks you can see. Feeds and speeds may also cause the cutter to grip and release the workpiece if your movement speed is too fast or the stepdown and stepover settings are too much for the cutter diameter.
Looking at the stepped lines, I would think your stepdown setting (which may be easily achieved on your workpiece material) demonstrates that the cutter is either not at optimal sharpness or needs tramming because it is leaving witness lines showing each step.
Just in case you thought that was it… if the workpiece is not held securely enough, then chatter can be the result. Another point that is sometimes forgotten is cutter engagement. If you are slotting, the cutter is 100% engaged all of the time… that is to say the cutter face is in contact across the exposed cutter blade and the workpiece face without any break. This heats up and dulls the cutting edge.
Cutting additional geometry around the slot can really help this issue, If you are only viewing your piece from a single aspect, a downcut bit will give you a clean entry to the surface of the part. A finishing pass will help to clean up any machining marks although my preference is not to create machining marks in the first place, that will later have to be removed.
A final word is always run a test piece of material if you can. It will help you to narrow down the correct feeds and speeds.
