Bit "popping" back up instead of continuing depth of cut

I apologize if this has been discussed before - I couldn’t find anyone with a similar issue.

I am cutting corbels out of 4/4 white oak using a 1/4" end mill. To make a completely through cut, the machine will make 10 passes. However, after approximately 3 or 4 passes, upon establishing the new depth of cut at the beginning point, as soon as the bit contacts the workpiece, it “pops” back up ever-so-slightly so as to continue the path without contacting the piece. It continues to do this every single pass until the job is complete, leaving the piece only approximately 30% cut.

I have tried increasing the spindle speed to no avail. I have no clue what may be causing this issue and cannot afford to continue sacrificing one inch pieces of white oak.

Any suggestions? Thanks in advance.

I had a similar issue. My Z belt was not tensioned properly. So it would work down to about .35 to .4 inches, but would not cut deeper. Set the tension properly and the problem was solved.

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I had a similar issue. Check out this post…

I too think I have been a vicim of this issue. Although it took me a little while to figure it out.
Then when I homed my machine I think it over traveled and mashed my limit switch

I was considering trying to figure out a way to make a couter balance system instead of the springs.

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The problem w/ a counter balance is that it will almost double the amount of mass which the machine has to move for the Z-axis carriage.

This sounds like a belt skip, but I would like to mention - sometimes the Router itself needs to be positioned up or down in the clamp to allow for the maximum depth of the job to be reached.

A cheap air cylinder with a plastic reservoir to keep the force constant would work. I don’t know if it would be worth the pain of having to adjust the pressure every few days though.

I have my belt tight and I have a lot of problems with my SO3 losing steps on the z axis when I cut with a 1/4" endmill any material harder than plywood. Its the only persistent problem I haven’t messed with to see if I can fix it. If I’m cutting plastic or something harder than plywood, I typically just use a 1/8" endmill if its not time prohibitive and it won’t lose steps. Otherwise I set my plunge rate to 5 IPM. Sometimes I’ll just use a ball mill if I can.

If MeshCAM had an option to ramp or spiral into cuts it probably wouldn’t be an issue but it doesn’t have that option.

I might try just removing one of the springs to see if that helps. I’m guessing that will introduce some cocking since the z carriage isn’t that stiff. Or, maybe just sliding the router further down so that the springs aren’t under as much tension.

The springs are commodity parts from a hardware store, and available in various tension ratings.

One can also adjust them by clipping out a few coils (same number on each side).

Thanks for the advice. I used an 1/8" end mill and it’s working well. Once this project is finished I will try to address the belt skip. Thanks again to all who responded,