Carbide Woes

A

Allan

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Greeting to the Brain Trust!
I have been turning down a hardened 5 x 3 MT sleeve to a 4 1/2 x 3 MT sleeve. I used a carbide cutter which worked surprisingly well at first. Once I had .1 to go I decided to try an HSS tool bit which was a poor performance. So then I switched back to my carbide but it doesn't cut well at all even at the same speed and feed as before. Instead of decent curly chips I get tiny "shards". I spent 45 minutes after my session picking them out of my hands with tweezers. What went wrong?

I had to reset the center height of the QCTP so I thought maybe I got it wrong. I messed around with that to no avail and called it a night before I got grumpy. The lathe is a 10" Tida. Not a bad lathe but not terribly robust. All the same it cut fine until I changed to the HSS and back again. Any ideas? :thinking:
 
Allan said:
Greeting to the Brain Trust!
I have been turning down a hardened 5 x 3 MT sleeve to a 4 1/2 x 3 MT sleeve. I used a carbide cutter which worked surprisingly well at first. Once I had .1 to go I decided to try an HSS tool bit which was a poor performance. So then I switched back to my carbide but it doesn't cut well at all even at the same speed and feed as before. Instead of decent curly chips I get tiny "shards". I spent 45 minutes after my session picking them out of my hands with tweezers. What went wrong?

I had to reset the center height of the QCTP so I thought maybe I got it wrong. I messed around with that to no avail and called it a night before I got grumpy. The lathe is a 10" Tida. Not a bad lathe but not terribly robust. All the same it cut fine until I changed to the HSS and back again. Any ideas? :thinking:
Click to expand...
Carbide is dull, in order to cut it needs a good depth. Initially you where cutting 1 mm and for the finish you went to .3 mm, you need a positive and small radious carbide with a sharp angle, that is, a finishing tool. Or well, increase your chunk of cut if posible.
regards.
 
Carbide chips very easily. I keep several 5-10 power loupes close at hand to visually inspect the edge. It is not unusual for things to be working well and then things change, the chips change and the finish is different. Take a look at the cutting edge of the carbide with a 10x glass.

Jim
 
I'm on the same page as Jim. check for chips.

I try very hard to keep my carbide tooling from touching metal that isn't turning. feeding them into touch the work is a very good way to chip them.

Steve
 
It's also possible that (by stroke of bad luck) your last cut went below the hardened area of the piece. I've encountered this several times. Turning a piece that is slightly hardened always produces a beautiful finish -and it's a pleasure to work with. As soon as you hit the soft, chewy inside... well, it might as well be a piece of gum.

EDIT: You mentioned "at the same speed and feed". Try increasing RPM's. Your last cut may have taken you below a decent SFM for that piece.


Ray
 
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Steve and Jimm are the winners on this.
I checked it out under magnification and sho 'nuff it was chipped. I was a bit stumped about how to remedy this problem since I have a dearth of diamond wheels, quorn grinders and such like. I spied a diamond tool for truing grinding wheels and tried it. I was a bit dubious as to the efficacy of hand honing a carbide tool but it worked a treat. I have since scored a cheapo sino made diamond cutting tool to fit into a hand drill. Should the need arise to touch one up again , I'll try it. If I can find it in my mess.
Thanks for all your suggestions.
Al
 
One more thing, you guys probably know this, but will add for the beginners. Be sure not to stop the spindle when the carbide is engaged in the metal as it will crack or chip the carbide. Make your pass and move it back or screw the tool back. Even dragging the tool over a stopped part will damage the cutting edge. I would also suggest you look at a Glendo Grinder for sharpening the tools, slow speed and no heat. I have one I have used for 20+ years and only replaced the 600 grit wheel 3 times, but I also use it in my scraping classes. For an occasional user a wheel would last for years.
 
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