Need some help or suggestions, when carving lithophanes this weekend the machine stopped and a message to clean Y rails came up, I did, let the machine keep going and broke the tip of my first bit. Did a wood project no problem then attempted another litho different picture and the same message and same result broken bit number 2. I know not to bright! Different spots on the litho run 1st one about 1/4 of the way thru 2nd one about 3/4 thru. I am pretty sure its slight high spots on the litho boards but can't be positive. However I stopped the litho runs and ran two more wood projects no problem. I am using a sled that's .25 inch's deep to set my material in. This would be my 6 and 7 litho's using the sled no problems on the first ones. Any suggestions?