Threading problem

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kmanuele

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A newbie, sorting out my new PM-1130 lathe on a 1" piece of 360 brass. It is going well, but having some problems threading. It's a 5/8-18 external thread (size is arbitrary -- just for test), threaded right to left. It fits the test nut well, but thread quality is poor and the first few threads seem to have doubled peaks. Any ideas? Thanks in advance

IMG_3946.jpg
 
It almost looks like there is a groove at the crest of the thread. This could be due to engagement on the wrong number early in the process or tool pushing up a burr. Flat file the crest of the threads and see what it looks like.
 
How are you measuring the threads? It looks to me like your threads are not deep enough. A test nut may fit a not deep enough thread. Run a file over the top of the finished threads to clean them up a bit. Need better pics of threads to really diagnose the issue.
 
What makes you think the thread is of low quality?
Both faces are smooth.
only the top is a bit ragged--easily fixed with a file.
Could us a bit of chamber at the front leading edge.

As to the double edge:: start the lathe a bit farther back from the thread so all the parts of the lathe 'train' get stretched before tool engages.
 
Maybe you hit a half number on the thread dial early on.
 
That looks better than my first attempt!

I see what you mean about the "double peak". Is it possible that the feed lever was engaged on the wrong number on the threading dial early on during a shallow cut?

A few questions.
1) what tool are you using? HSS? carbide insert ? Pictures of the tool would help.
(no top rake and no cutting fluid is typical for brass)

2) what size and type of lathe? Is there much wear in the cross and top slide screws/nuts? I want to get a feel for size and rigidity of the machine.
(EDIT: never mind, I re-read and see a new PM machine!)

3) What technique? are you plunging straight in with the cross slide, or setting the top-slide/compound to 30 degrees from perpendicular and feeding with it?

It is fairly standard practice to finish threads with a wire brush and a flat file to help clean the threads while still on the lathe.
Just watch for those chuck jaws, and never use a file on the lathe without a handle.

Brian
 
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Wow, five responses in a five minute span.
We were all typing over each-other.

Brian
 
Wow is right ! Thanks for all the responses ! I filed off those burrs, and this is what I have --- zoomed. Besides the double peaks, the root seems too wide?

re: tooling, HSS tool (3/8"), no rake, no fluid, compound at 30 degrees, feeding with compound, tool post perpendicular

IMG_3951.jpgIMG_3953.jpg
 
The look of the root, it would seem that the tool must have a flat or radius ground on the point, or that it might have been dulled by another threading job, brass would be an unlikely candidate to dull a tool. the problem with the top of the thread does indeed look like a first pass engaged on the wrong place on the thread dial.
 
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