I was doing some large cuts on the old Hercus 9, boring through 10mm plate with carbide cutters.
Started with a CCMT0602 on a 12mm bar, until the hole was large enough to move up to a 16mm bar:
IMG_4495.jpg
(Yes, I am using an internal threading insert for boring. It is my only 16mm bar, and I have plenty of 16IR inserts, and you can save some time by cutting in both directions - a deep cut toward the chuck, then maybe a gentler .2mm DOC away from the chuck)


I noticed that, when really pushing the larger cuts, the belt would try to twist off the side of the pulley!
IMG_4496.jpg


Experimenting with spindle speed, depth of cut, and feed rate showed that it was only the fastest pulley position (629RPM on my machine) and heaviest feed/DOC that caused it.



Got the job done eventually:
IMG_4497.jpg
(that is a 70mm hole, no coolant)

but am confused as to why the belt was skewing instead of just slipping!
Attached Images




Read the full thread at metalworkforums.com...