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Threading Question

Henryrifle

I have an inexpensive Grizzly lathe, a G4003G. I've switched to an inboard spider to keep the workpiece as close the headstock as possible. I use a laydown full profile threading insert and make threading passes as prescribed in the Standard of Depth of Cut document for American UN external threading.

There does not appear to be a problem with the threading work except for the way it looks. I used to use an on-edge HSS threading tool with similar results. I thread at the slowest speed my lathe will run - 70 RPM. I use Viper's Venom on the last few passes but not the first ones lately.

I am guessing this is some kind of chatter. You can see what looks like ridges in the valleys of the threads. What is causing this?

IMG_8264.jpeg

Thank you,
Hank
 
Just before you achieve the desired pitch dia try running several spring passes Sometimes it will clean up that chatter. A little more speed might help. Check the gibs on the compound and crosswise make sure nothing is loose. Just a couple thoughts
 
I know everybody is ate up on vipers venom, but I have found the best cutting oil to use in threading, especially if you use HSS tools, is Rigid Black Sulphurized Pipe Thread Oil.

But I do not think that is your problem. Get a quality insert holder and crank up the surface feet per minute.

Also, there is no law against single Point claiming a center and using your live center While threading.
 
+1 to Rigid over V.V.

I recommend Carmex BCX grade inserts. They're designed for slow speeds and cut beautifully. The partial profile inserts are sharp and have high rake angles on them so they cut well.

The full profile / TPI specific inserts don't seem to have the same angles and I have had that same "wavy" finish. They need a little more speed, and feeding them in a 29.5/30 degrees fixes it (for me)
 
It is a single phase motor.
I am using a TPI specific insert
Good thought on the gibs--I do adjust them often as I assume that is a source of chatter.

Jackie - I will use the live center next time. I had that thought but thanks for the reinforcement.

So, maybe I am trying to justify spending money on a better lathe, but is that what you might expect with a less precise lathe or is that a speed, technique or other user induced issue?

Thank you,
Hank
 
I've seen some pretty old well worn lathes cut some good looking threads. Try advice above and you'll likely come out with a set up that will produce smooth threads.
 
One more suggestion, make sure your cutting tool's top cutting surface is slightly below center line.
Few thou will do, Use a live center to support if you can, and find inserts that are sharpened, ground an all cutting surfaces. the last few passes should be .001 or so , less pressure on tool.
Carbide inserts made for high speed threading....
 
I use the compound the whole way and it’s set to 29.5.

Looking forward to supporting with live center in tailstock next time.

Thank you all,
Hank
 
If you choose to use a live center as Jackie suggested don’t overlook the part about single point claiming the center after you have it properly indicated
 
To slow of an rpm for a carbide insert. In my personal experience, HS is the way to go for our less that stellar engine lathes. Or lathes are not even remotely close to a Hardinge or Monarch lathe. If you insist on a carbide insert go as fast as you can, rpm wise. Or, if feasible, run the tool upside down, crank up the rpm and feed away from the shoulder. Good luck. And as Jackie said, high sulphur content is your friend.
Paul
 
boltfluter said:
To slow of an rpm for a carbide insert. In my personal experience, HS is the way to go for our less that stellar engine lathes. Or lathes are not even remotely close to a Hardinge or Monarch lathe. If you insist on a carbide insert go as fast as you can, rpm wise. Or, if feasible, run the tool upside down, crank up the rpm and feed away from the shoulder. Good luck. And as Jackie said, high sulphur content is your friend.
Paul
Click to expand...
I agree. I think more speed is the very likely fix. Yes, HSS is a lot more forgiving to low speeds. Carbide needs the heat that comes with higher speed to give a good finish. There are some grades of carbide that are more forgiving to low rpm as well.
 
jackieschmidt said:
I know everybody is ate up on vipers venom, but I have found the best cutting oil to use in threading, especially if you use HSS tools, is Rigid Black Sulphurized Pipe Thread Oil.

But I do not think that is your problem. Get a quality insert holder and crank up the surface feet per minute.

Also, there is no law against single Point claiming a center and using your live center While threading.
Click to expand...
My findings with laydown inserts is as above, rigidity and proper speed, more speed than HSS.
It’s really easy to thread fast on my lathe so I run 800-1000+ Rpm.
 
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