Hypothesis of Obliteration

How To Machining
1

Well this project didn’t go as planned. Here are some facts:

  1. Stock is 1” thick
  2. Cross overall dimensions are 6” x 4”
  3. Cross pieces are 1/2” wide
  4. The thickness of finished cross is 1/2”
  5. Used 1/4” upcut end mill for rough pass
  6. Used 1/8” ball nose for first finish pass
  7. Used 1/16” ball nose for second finish pass
  8. Used 1/4” down cut for profile. (NB)
  9. I had dust collection turned off due to recent EMI issues

Everything went well until the profile cut. I started it, walked away and when I came back the cross was obliterated. I mean only a few small pieces were left on the table and the contour was full of saw dust. The machine stopped as expected.

My hypothesis is that I inadvertently used a DOWN CUT end mill for the profile and with this being a narrow contour cut 1” deep saw dust built up and the piece exploded.

I’m looking for validation or other ideas what could’ve happened in addition to anything else I should’ve done.

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2

There is a lot of short grain along one axis that make it a bit more fragile

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3

Divine intervention?

Or maybe the tabs broke? The extents of the horizontal bar are a decent length lever to those top taps, so any catching might have been amplified a bit?

If I were a betting man though, I’d guess you were correct. Probably packed chips/swarf that lead to the catastrophic failure. Though maybe we would expect some signs of heat buildup in that case? Like a smell or some burn marks on remaining pieces?

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4

Thanks. I didn’t see any burn marks in what was left. I think I may try it again with upcut bit AND sit there and watch it.

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5

I definitely use more tabs than that to hold the piece down. I suspect that the tabs broke, maybe one by one. Once the tabs start breaking late in the cut, the bit bites into the full depth of the wood all at once.

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Will definitely add more tabs. I have them set at .250 wide and .125 thick. Thanks

7

Are you using the new Cutout toolpath for the perimeter cut?

If not, try that.

That said, while making it of one piece has style points, the more typical construction is two separate pieces, each long-grain, and with a half-lap.

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8

@Jmcwoodworking

What species of wood was it? Some are more brittle than others and can exhibit this behavior, specifically with your grain orientation.

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9

It would be good to see the file with the toolpaths & model.

I question why you stepped down on the ball mills. Looks like the only tight corner that might need a smaller ball is the joint between the vertical & cross bar…

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10

There’s nothing inherently wrong with using the downcut bit to create the sharp edge. Then switch to the upcut bit to finish the profile.

Will is right, your vertical piece will be comparatively fragile.

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11

Note that it was @Stankus who first made that observation:

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12

I have made crosses as two parts but wanted to make this one on the CNC. I feel I should definitely be able to. I also wanted it with a little 3D aspect.

I have not tried the cutout toolpatyh yet. Is that ion the beta version or somewhere else?

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Can’t upload the file its too large. Appreciate the help. Ive made this exact cross before in an older version of CC on my Shapeoko 3 obviously just not 3D.

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See:

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Just saw this sorry. It was walnut.

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Here is a little more empirical evidence. As you can see below, it’s obvious where the toolpath went awry. I’m leaning even more toward the idea that not having dust collection on, with a 1” cut and using a down cut bit is what ruined this piece. It looks like sawdust just crammed in the toolpath and forced the bit off course.

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