Single Point Threading Issue on Lathe

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r2784c

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Hi,

Was hoping to get some feedback on why the single point threading I was doing on mild steel stock caused the workpiece to be pried from the chuck on my lathe. This has not happened to me before, but I don't have a ton of experience. Here are the particulars:

  • Material: 1018 mild steel, 3" round bar
  • Chuck: 6" 3 jaw. Given the diameter of the stock, I had to flip the jaws such that the workpiece was only held by 3/8" depth of the jaws.
  • Cutting tool: Kennametal carbide single point threading insert. Using a 12TPI insert to make a 1/2-13 thread as they don't make a full profile 13TPI insert and the 12 is all I had.
  • Threading passes: Following Kennametals suggested pass count and cut depths. With that said, I did a couple additional spring passes in between increasing depth of cut.
  • RPM: 200
  • Lathe: Hardinge HLV-H
  • Overall length of workpiece: 5"
I had no issues on the first three passes, but on the forth pass, the tool seemed to catch and the workpiece spun out of the chuck. You can see in the picture where the threading tool dug into the piece.

While there was only 3/8" depth of the chuck jaws holding the piece, I was surprised that it so easily yanked the piece out of the chuck (which I had checked the tightness on prior to threading). Looking for some advice on what to try to do differently next time.

Thanks.
 

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You're not using a tailstock center to support the workpiece?
-Mark
 
As I remember, that lathe has no tailstock, thus no support for a long workpiece and the use of the chuck jars as stated provide little support, the result is not surprising. The only advice might be tapping the part for a draw bolt on the back end and using it to retain the work in the chuck.
 
Long overhang + short grip - tailstock support = successful launch.

I would expect to need 2 in. gripped in the chuck to get away without a tailstock center.
 
Last edited:
29 1/2 degrees
Is your compound offset by 29 1/2 degrees?
If you don't understand the question, just ask :)
 
Thanks for all the responses. The compound is set at 29.5 degrees. The lathe does have a tailstock, but with the centers I have I can't get enough access to thread the entire length that I needed threaded. I figured there was just not enough gripping power, but was surprised that I didn't have any issue taking the 3" diameter down to 0.5" with 0.080 Depth of Cut passess. I was not expecting the single point threading tool to exert more force.
 
The next question is, are you advancing into the work with the cross or the compound
 
I am advancing with the compound and using the cross slide to back the tool out at the end of the pass.
 
So far so good, The HLV on the original cabinet is a fairly robust base, however make sure it's all locked down.
Even the Kennametal carbide single point threading insert is still just that, carbide.
I'll go against the grain a bit here and suggest grinding and honing 5% (or better) cobalt HSS.
I'm just not a fan of carbide on smaller lathes unless you have the ability to grind and hone with some precision.
Just can't seem to get the carbide as sharp as the HSS.
The top can be fairly flat compared to a turning tool. The relief is dictated by the thread pitch. The undercut can be nearly straight down as the radius takes the stock away/off the cutting point quickly.
By my terminology you may guess I was taught by the navy and not a traditional trade school or apprenticeship.
You'd be correct.

In short HSS is your friend.
 
Just curious, how large of cut were you making with your threading tool?
 
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