Fixturing and an Oops

I love the DeStaCo quick clamps. They have many varieties. I recently bought some of the push-pull style to make a fixture to mill my .50 BMG inserts.

I used some MDF to make the plate, and glued two blocks on to form a 90 degree corner. Then I added some #8-32 threaded inserts to bolt down three DeStaCo push-pull clamps.I make two different size inserts, so I installed two different sets of threaded inserts so I can use the same fixture for both.

fixture1

I just bolt it to the table, indicate it square to the machine and pick up the zero using the corner blocks.

Then I clamp in an insert and start milling.

fixture2

And now for the Oops…

You should never run your Shapeoko 1, 2 or 3 (or any other variant, or any other CNC machine for that matter) unattended.

This is one example of why.

too-fast

I was milling 0.830″ diameter 1.100″ deep holes in MDF with a 1/4″ two flute carbide router bit. The cutter was a little dull at the start, and not really the one I should have been using.

I was helical milling the holes in two passes.

The first pass was 0.450″ with a 0.100″ ramp down per revolution. I ran it at 39 IPM and it seemed to go okay.

The second pass was much more aggressive (because GWizard and the CNC Cookbook blog said it should work – with an end mill, not a router bit…) I helical milled to 0.830″ diameter with a 0.300″ inch ramp down per revolution at 32 IPM.

You can tell by the first hole that I was in trouble. It’s oval shaped.

I was in the shop but not really watching the machine too closely. I heard it when it got into real trouble and ran over and hit the reset button (no E-Stop wired up yet – though it was almost on fire.)

Let’s break it down a little:

  • Wrong cutter
  • Dull cutter
  • Straight flutes on the cutter did not expel the dust well enough
  • Overly-aggressive cut
  • First run of a new tool path
  • MDF

The oval shape suggests to me that the dual Y motors were able to push the dull bit hard enough but the single X motor was not. Then it started losing steps and got totally into the weeds.

If I had not been in the shop it probably would have caught on fire after a bit as it tried to drive the end mill sideways at 1.100″ deep through solid MDF. You can see where it started burning.

As a side note: MDF is terrible stuff to mill. It dulls the cutters almost right away. I’ve been doing some reading and 2 flute end mills with TiAN coating are suggested. I have one coming to try out.

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