Erratic cuts on pocket floor

As you can see, I am getting erratic cuts on the floor of the pocket on this plaque. This sign was cut 1/4" deep into 3/4" pine with .100" passes with a new 1/4" end mill. A similar plaque cut from HDU was much better, but not as good as it should be. The Z belt appears to have normal tension. Any thoughts / suggestions would be appreciated.

The Z needs to be pretty tight…moreso than X & Y.


Almost looks like it could use a finishing pass. Was this done on plywood?

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I will test it again. Other than breaking the belt, is there an indicator that says it is too tight?

This piece was cut using beautiful New Zealand Pine. I did not do a finishing pass here…I didn’t think it necessary until the prime coat was applied.

It doesn’t look like a machine issue… if it was a loose belt, you would have shallow sections at the start of your toolpath and deeper towards the end. I would guess it’s a feeds/speeds issue. See that diagonal line above the first “3”? That looks like the marks left by chatter, a result of going to deep and too fast. While 0.1" depth is fine for clearing, you really need a finishing path.

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Easiest way to do a finishing pass on the bottom of a pocket is math — I believe it would be:

For a given depth, D, and a given depth per pass, P, and a desired roughing clearance, R, determine how many passes there will be, D/P, and let that be N and then reduce each pass by R/N.

(unfortunately, Disqus doesn’t seem to have an equation option)

This is quite likely what happened… I normally work with HDU and it is much more forgiving. I will back off on the feed and try Will’s finishing path approach as well.
Thank you!

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Will, I don’t quite understand how P is entering into this equation? Would you kindly plug in these factors and show me the equation: Assuming a total desired .25" depth with max roughing passes of .10", I am guessing a finishing pass of .04" would be good?
Thanks again!

Okay, one wants to cut down to 0.25" — currently this is done in 3 passes:

0.0–0.1" – 0.1" pass
0.1"–0.2" – 0.1" pass
0.2"–0.25" – 0.05" pass

P == 0.25"/0.1" (round up)

To reduce that finishing pass to 0.4" we’d want to increase the number of passes to 4:


so you’d make the depth per pass 0.07"

Got it… thanks!

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