cutting threads on lathe

I finally attached the thread cutting kit to a sherline lathe. The kit
came with a 60 degree carbide cutter.
Even with 1/4" brass rods, that broke in probably minutes, followed by the
spare I had.
the cuts looked like shit too. They weren't really shiny and the "feel"
from the handcrank on the headstock/gear train didn't seem right. I was
only cutting 2 to 3 mils per pass.
I decided to gring my own with 1/4" HSS steel blanks on a Tormek knife
sharpener.
That worked much much better. The cuts look nice and shiny.
So what's the deal with the ugly finish I was getting with carbide? I
think I know why the tips snapped off, but the finish part I don't
understand. A mild steel rod looked awful with the carbide cutter too. It
looked like there was more tearing or smearing than cutting going on.
Reply to
Cydrome Leader
Loading thread data ...
Carbide is a matrix of grains glued together, like concrete, and it doesn't take as sharp an edge as HSS. I doubt you'll ever need carbide on a lathe with so little power anyway. It's likely that if the material was that difficult to cut the lathe would twist away when you tried to force a carbide bit into the work.
High Speed Steel will cut stainless steel and Grade 8 hardened bolts, it just dulls quickly.
jsw
Reply to
Jim Wilkins
The 60 degree cheapo brazed carbide lathe bits I got were similarly awful stock and the point wasn't fine enough for smaller threads. I honed one of the bits with a diamond hone for a few minutes and the results were much better.
Reply to
Pete C.
I though they were giant crystal or something like that. The "like concrete" part makes sense.
Does HSS leave nicer finish when sharp than carbide on these materials? Considering tiny pieces of the tips broke off the carbide cutters I have, I can't really test this anymore.
Reply to
Cydrome Leader
Carbide is fine under the right conditions. My old 10" South Bend lathe is on the border where sometimes it works and sometimes it doesn't. It's been better since I ground the uneven wear off the compound slide and tightened the gibs. I get nearly a mirror finish with carbide on stainless steel.
jsw
Reply to
Jim Wilkins
Wrong carbide, wrong rake, wrong lubricant, too high or too low, or crappy chinese carbide without a good edge. Lots of possible reasons for a crappy cut.
Reply to
clare
Make sure any cutting edge is sharp sharp. Not just "marketing sharp". You want it scary sharp. Even touching it up with a bench grinder is probably going to get it sharper than it was when you got it. If I really care about a cut I rough the tool out on a grinder, but then I hone it by hand on a stone.
If you were trying for something like a #5-40 thread, then be aware that the thread is really deep compared to the material diameter, and that's going to steal lot of strength from the piece. Consequently, your piece will start bending away from the tool, and Really Bad things will happen. I wouldn't try that deep a thread without using a center in the tailstock. (actually for that sort of thing I use a die!) The finer the thread is in proportion to the OD of the piece the less trouble you're going to have doing the cut.
Make sure the tool height is correct -- you want to be bang on the center of the work; get it off and your tool won't work right.
Reply to
Tim Wescott
The carbide was not ground for cutting the brass. You need a honed edge and high rake angle brass. You are better off using HSS because you can grind the proper angle on the tool. You could order inserts designed for cutting brass too.
John
Reply to
John
I much prefer HSS to carbide for a lot of jobs and especially threading. You have to have rigidity squared to use carbide well and horsepower too. Great for removing a lot of material quickly or production jobs. I've got a 7.5 HP Reed Prentice with the rigidity and HP and only use carbide 25% of the time.
Reply to
Tom Gardner
Positive rake for brass is trouble, due to hogging. That's especially true on a small, light duty machine.
Beyond that, threading tools should not have rake unless the rake angle is compensated in the included angle, and then you are limited to the questionable performance you'll achieve. The best policy is to keep rake at 0 degrees when chasing threads, and to feed by the compound, which would be set at 29 -1/2 degrees. I prefer 29 degrees, to insure the back side of the thread cleans up.
Using a carbide tool for brass after it has seen steel is a huge mistake. The keen edge that is necessary for the tool to cut well will have long since been dulled, yielding poor performance. Brass tends to act like a bearing, and will readily float a tool that is slightly dull, yielding inconsistent results.
A person with little to no experience in chasing threads is never wise to experiment with carbide. It takes almost nothing to chip the fine tip of a carbide threading tool, which, once chipped, will perform poorly, if at all. Stick with HSS unless you have more than a good reason to go to carbide, then insure that you use the proper grade. That often spells the difference between success and total failure.
Harold
Reply to
Harold & Susan Vordos
NOT, repeat NOT for the original poster. I recently bought carbide threading inserts for my CHNC lathe. WHAT AN IMPROVEMENT! On a machine built for it, these are great. Of course, I never thread at less than 500 RPM on larger diameters and into the 1000s if going under one inch. Don't know if the reflexes could ever be fast enough on a manual machine.
Karl
Reply to
Karl Townsend
They couldn't.
For the OP, what Harold said. And don't forget to radius the tip of your HSS threading tool. The correct amount for each thread pitch is published in the manuals, and you can make a close approximation with a good loupe and a caliper set to the right width.
First, though, just get the darned thing cutting right with a HSS tool, even if you leave the tip sharp to start with. It's tricky on a very small lathe. Follow Harold's recommendations.
Reply to
Ed Huntress
FWIW, in the shop where I sit, they have a diamond wheel where they can grind new points on carbide inserts; carbide is the only thing anybody's allowed to use it on.
My first thought was rake, chip relief, and whatever you call the height of the cutter, but then again, brass is kinda mushy in those circumstances. I was once trying to turn down a piece of copper, and the bit bit into the copper and the part bent like toothpaste.
Good Luck! Rich
Reply to
Rich Grise
Good reason -- high speed steel, or any ferrous metal, will eat your diamond wheel alive. Steel really likes carbon, when you get it hot, and it doesn't care how much the carbon costs. d8-)
Copper or any soft-to-medium copper alloy, especially yellow brass, will grab a tool if the work or the tool aren't rigidly supported (MUCH shorter overhangs are allowed than with steel), and if there is enough positive rake to make the tool want to grab. Small lathes, including my SB 10L, are among the machine tools that are vulnerable.
Standard lathe-bit side- (usually) or front (sometimes, as in plunge cutting) rake for turning brass, as a safe starting point, has been negative 5 degrees, for over 100 years. However, most applications are done closer to zero rake. Negative rake is tricky, too, sometimes causing the tool to skate.
Harold's recommendations were spot-on.
Reply to
Ed Huntress
I noticed this too- copper seems really sticky, and the bronze bolts used by utility companies are hard to even drill.
Reply to
Cydrome Leader

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.