- Joined
- Nov 21, 2015
- Messages
- 56
So I had yet another "moment" while trying to part off on the lathe... again. I had a piece of 2" EN8, bored out ready to take a 1.25"x24TPI internal thread. Rather than thread into or out of a blind hole I thought I'd part it off first rather than afterwards, then remount, clean up the face and thread. My parting blade was a 3mm wide x 5/82 deep M2 HSS. I'd sharpened it before starting the cut. I had chatter at 85rpm so I dropped the speed to 58rpm and had a good cut going, liberally lubricated with cutting oil. I widened the slot by 10 thou to reduce the temptation for it to bind. The chips were coming out nicely, then it grabbed the workpiece, which spun in the 8" 4-jaw, creating some deep grooves matching the chuck jaws. By the time I'd worked out what was happening and turned the lathe off there was a loud bang and the tool shattered.
This isn't the first time I've had issues with an HSS parting tool, which I assume is losing its sharp edge during the cut. Finally it was time to try something different, so I dug out an old circular saw ripping blade that had gone dull. I could cut it with a hacksaw, although the steel was hard. I cut a generous width so that I could mill it to size, trying to keep the top of the carbide tooth parallel to the cut. I milled it top and bottom to bring it close to 5/8", in the process blunting a 10mm HSS end mill. This is despite running it at about 300prm (which I calculate is about 30 sfpm) and drizzling cutting oil on it continuously.
I then mounted the new tool on my TC grinder to grind the top and bottom smooth and to exact height, and then ground the carbide tip top parallel with the base and put 12 deg clearance on the end. The holder raises it by about 5 deg. so it has that much back rake plus about 7 deg front clearance.
I then fitted the new blade into the parting tool holder, remounted the remnants of the 2" stock in the chuck and tried parting it off again.
The long curly chip strategically draped over the compound is the result of the cut. Same speed, same manual feed, but this time the part came away cleanly with a good finish. The tool at the end felt as sharp as it was beforehand.
I thought I'd share this as I'd read about the suggestion of doing this but not found any actual threads or decent examples of anyone actually having documented it.
Rob
This isn't the first time I've had issues with an HSS parting tool, which I assume is losing its sharp edge during the cut. Finally it was time to try something different, so I dug out an old circular saw ripping blade that had gone dull. I could cut it with a hacksaw, although the steel was hard. I cut a generous width so that I could mill it to size, trying to keep the top of the carbide tooth parallel to the cut. I milled it top and bottom to bring it close to 5/8", in the process blunting a 10mm HSS end mill. This is despite running it at about 300prm (which I calculate is about 30 sfpm) and drizzling cutting oil on it continuously.
I then mounted the new tool on my TC grinder to grind the top and bottom smooth and to exact height, and then ground the carbide tip top parallel with the base and put 12 deg clearance on the end. The holder raises it by about 5 deg. so it has that much back rake plus about 7 deg front clearance.
I then fitted the new blade into the parting tool holder, remounted the remnants of the 2" stock in the chuck and tried parting it off again.
The long curly chip strategically draped over the compound is the result of the cut. Same speed, same manual feed, but this time the part came away cleanly with a good finish. The tool at the end felt as sharp as it was beforehand.
I thought I'd share this as I'd read about the suggestion of doing this but not found any actual threads or decent examples of anyone actually having documented it.
Rob
