Help with unexpected overcut (broken bit)

Howdy! I’m new to CNC, and I decided to “go big or go home” with my Shapeoko Pro rig… I guess since my shop is setup in my garage I was able to “go big AND go home” :man_shrugging:

I ran a few test projects with some different bits and I seemed to be doing ok, so I decided to try my hand at an 18" sign. This project took 2 bits: a 60deg VCarve endmill (#302) and a 1/8" square endmill (#102). The stock was white board plywood 19-1/2" by 24" and 3/4" thick.

There were no problems with the #302, but after the circle was cut with the #102, the machine started cutting along the bottom of the board outside the circle

I’m using the free version of carbide create, and there don’t appear to be any toolpaths outside or on the edge of the stock other than the circle cut out (see follow up post in this topic for picture - new users can only post one image per post)

I’m rerunning the machine now to see if I can identify what might have happened, but any thoughts, recommendations, or things to check out would be incredibly helpful!

Thank you,

Here is a screenshot of the toolpath (simulation view) project in Carbide Create

What seems to have happened is the machine was making a full depth pass, didn’t quite make the turn at the bottom of the circle, and because the stock was thicker than the flutes are long, was guided by the stock until the endmill broke.

Avoid slotting (cutting a toolpath as narrow as the endmill w/ a single set of stacked/successive paths) and add geometry so as to cut as a pocket down to at least tab depth or the penultimate pass:


and consider leaving a roughing clearance and taking a finishing pass.


Project is too close to the edge of the material used. On that pass, it looks as the direction of rotation caused the bit to push the uncut edge into the workpiece. Since the bit was already too low and rubbing the shank, the bit could not cut into the material and the machines y-axis lost steps.
the help part…
1] Try and use oversized stock on projects so that you can keep support at the sides. ( I try to use stock at least 1 inch larger than the project)
2] Do not leave the machine unattended while it is working. It can burn your house down.
3] Avoid slotting. When you are doing the profile cut, add an outside contour cut at least 25% wider than the bit you will use and cut down far enough to leave enough material to hold the project, then do the profile with tabs for the last 0.1 " or so.

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Thanks for the feedback - it looks like I was a little aggressive in saving material. Moving forward, I will make sure to add a little space between the cut and the edge of the stock.

@WillAdams - Thanks for the links… I’ll check these out after work and see what I can learn.
@MindlessCorpse - (great username!) - Thank you for the good info. I’m not yet comfortable leaving the machine, although I was in another room when it started making horrendous noises.

Is there anything I should do to make sure I didn’t mess up the machine? Since it appears to have been pushing against the stock, would that cause belts to slip or something else to get bent / broken?

Usually the belts slip w/o harm.

I’ve found that most infelicities from a crash are resolved w/

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Yep, stay with this forum. Any problem or advice that you might need can be found here.


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