litho breaking while carving

I'm not exactly a pro, but I have carved approximately 10-15 lithos in the past few months. However, this weekend I had two break in the final few minutes of a three hour carve, both of the same picture. I don't have a clue what is happening. Each time the bit finishes the carve, but the top 3 or 4 inches breaks. Any ideas? I am using a 3/4" piece of mdf with an 8x10 recess in it as a sled. I wondered if somehow the end of the corian is rocking off the bed of the sled, but I haven't had that problem before, so I'm stumped. Pictures of the cracked lithos are attached. As you can see, the first carve has an unintended carve through near the upper center. Both breaks appear in the same approximate area, though the first fractured across the entire width, while the second took out the upper right corner, then the upper border. Both carvings are displayed 90 degrees from their position during the carve. The broken pieces are actually on the left end of the carves. Thanks for any advice. BTW, I did not use "bit optimization." Don't know if that might have helped. Also, I think the actual break occurred when the bit reached nearer the end than what appears on the carves. I checked the progress of the carve last night at about 97% and there was no problem.

Attachment 74072Attachment 74073

That is strange, I have not heard of this problem before. On your sled, does is have a solid bottom or does it have a cut out? Any chance that the corian is thicker than your cut out in the sled?

My first thought was bit optimization. That vase in top right has some deep holes in it. Possible that it is rocking or not tight enough in the sled.

How new is the bit? Dull bits create heat and larger forces.

bottom of sled is solid. Don't think corian is thicker than cut out, but I'll double check it. Thanks.

Bit could be dull. I have several, and I think the one here is probably about nine months old. I could retry with newer bit and optimization.

Success! I used a sharper bit and used bit optimization on "best", and it worked! Thanks for the help.

This is the second time I have been directed to use bit optimization. I have not been using this feature. Should I use this feature for everything, or are there specialized needs/uses?

Thanks, again!

Quote:

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Originally Posted by t-townwilson

Success! I used a sharper bit and used bit optimization on "best", and it worked! Thanks for the help.

This is the second time I have been directed to use bit optimization. I have not been using this feature. Should I use this feature for everything, or are there specialized needs/uses?

Thanks, again!

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I would say use it for everything unless you run into a specific and unusual situation where the project looks better when you turn it off.

dave

Quote:

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Originally Posted by t-townwilson

Success! I used a sharper bit and used bit optimization on "best", and it worked! Thanks for the help.

This is the second time I have been directed to use bit optimization. I have not been using this feature. Should I use this feature for everything, or are there specialized needs/uses?

Thanks, again!

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Like Dave said! Very seldom will I NOT use bit optimization.