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Internal threading

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32chevy

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#1
One of the last task of a project I'm building requires threading on the lathe. So far I have found it challenging. I need to turn a 1.5-20 male thread on a 4140 bar stock and the female thread in separate part that threads on to the first. This is my first project requiring lathe threading

The first part I made had the male thread. I used a pitch micrometer and go/no go thread ring gage. I next proceeded to make the female part. I bored to the minor dia , created a clearance grove and planed on cutting thread until the mating part could screwed on.

Without a male thread gage I didn't know how else to cut the thread to size

I used boring bar with thread insert at 90rpm. The tool was on center. I ran machine in reverse with lead screw turning the same direction as chuck. Cutting tool was upside down. I was able to produce correct pitch but I could not make the parts fit each other.

After threading I noticed the minor diam would reduce in diameter by .030" so I had to switch tool and bore back to original minor diam. i think it was a burr created from threading. I then had to change back to thread form tool and cut thread deeper. I am happy that I didn't manage to cross thread.

I need help/tips? Do they make internal pitch micrometers? I would hate to have to sub the threading out , this is a personal project.
 

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4ssss

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#3
Use your male threaded part as your ID gage, as it's the right size with the ring gage. Don't worry too much about the fit of the nut as the bigger the bolt, the more clearance they have. A nice threaded fit will be fine for this size bolt unless it calls for a specific type thread. Nice job on the carbide thread boring bar, but in the future you may want to use a cobalt or HSS tool as if the carbide chipped you'd be hard pressed to pick up the thread again. That's probably why it doesn't fit now as intended, the burr you found was the thread not being picked up. It's not an easy thing to do if you have experience, let alone a first timer.
 
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Doubleeboy

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#4
If you have your compound set parallel to ways its a lot easier to pick up a thread than with compound set at 29 degrees. If you have not seen Joe Pie's videos on threading on youtube, do so, it may be a big help.
 

32chevy

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#5
Yes , I followed Joes Method and it worked for the male thread. Does he show it for internal threading.
 

Tony Pisano

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#6
We always used 29-1/2 degree offset on the compound. Use a center tool to set the toolbit and zero the crossslide and compound. That way when you crank the tool out to back off, you always bring the crossslide back to zero, then You feed the compound a little each time so you aren't cutting with boh sides of the toolbit, and there is a little play in the thread angles. Are you sure the threading bit is big enough to cut the full depth of thread? It looks kind of small but that may just be the photo. Also curious why you are running in reverse with the tool mounted upside down?
 

Wreck™Wreck

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#7
Also curious why you are running in reverse with the tool mounted upside down?
Internal threading. to a shoulder makes it difficult to see when to stop the tool.

90 Rpm's is slow for that tool at that diameter which often leaves a burr, since he is threading from left to right he can go much faster. If a small lathe where one may spin the chuck by hand and drive the lead screw I run the tool into the undercut then rotate the chuck by hand until the thread dial reaches the correct position, close the 1/2 nuts and let it rip thus leaving the entire lead screw length to stop it. Some lathes do not allow this approach
 
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