What Am I Doing Wrong

Fortis64

Fortis64

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Dec 8, 2016
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I've tried threading a few times with some success but more failure . I set the lathe up as is shown all over the internet and youtube . . My threads always seem to lean to the right as I'm looking at the lathe ,the thread is formed just doesn't look right. I spent a good couple of hours making a backplate for my 4 jaw ,everthing was perfect and then i started threading the 7tpi 1 3/4 " bore ,then the problems started . The back plate is junk now . I should've taken photos . My thread dial doesn't line up bang on the mark ,it's more like a 1 1/16th of an inch either side ,i can never get my half nuts to engage right on the line . I'm staring to think it's not the correct tread dial . . Any advice welcome .....

Sean
 
Without photos it is hard to tell what is wrong. But as far as the dial not lining up on the mark. The dial could be the right one but the housing for it may be off a bit. Mine was just a bit and I made a shim for the housing so now it reads right at the mark when engaged. It is still usable the way it is as long as you keep engaging at the same place. It shouldn't matter if it is before or after the mark as long as it is the same. So if your first engagement is just before the mark you should always be just before the mark when engaged. Other than that I am not sure why your threads may be off without photos.
 
The thread dial can be adjusted to engage on the marks. Usually by moving the index mark or by rotating the gear on the shaft.

Threads leaning right; using method shown all over the internet(?); need more details on this.
Are you are feeding with the compound set at 29,5 deg to the spindle axis or 29.5 deg from the cross slide axis?

As Bill said, pics would help a lot.
 
Sean,
Could you supply photos of your set up please?
 
Hi Sean (@Fortis64)

Do your threads look like this:
http://www.hobby-machinist.com/threads/threading-help.49488/

That problem was caused by setting the angle of the compound slide wrong.
It is easy to do, because some machines measure it relative to the lathe axis (the imaginary line from headstock centre to tailstock centre), while some measure it with respect to the cross-slide motion (ie. 90 degrees perpendicular to lathe axis!).

You need to know which kind of machine you have.

Hope this helps,
-brino
 
Yeh that's how they look ,a bit worse than that . Maybe I need to try the 90deg method then I'll find out I suppose.

Thanks guys .

Sean
 
I set the compound to 29.5 deg. as mentioned above for external threads. If I'm reading your post correctly, you are threading internally. In that case the compound is swung so the crank will be under the work at the 29.5 deg. angle or whatever the protractor reads on your lathe (if that makes sense). You'll want it to be slightly under the 30 deg. angle from square so one edge of the cutting tool is doing all of the heavy work, other side is just scraping the opposite side so you don't have steps in the thread from multiple passes.

Sounds like your tool is not square to the work or you are getting deflection while threading. I verify the tool is set at the proper angle to the work with a center gauge like the one pictured below. Set the flat edge on the work and set the 60 deg. V of your tool into the V of the gauge. Adjust the tool post (not the compound) so the tip of the cutting tool fits into the gauge V. For internal threading, you'll set the gauge on the back side of the work, front side for external threads (assuming you have a cylindrical part in the lathe).

If you don't have a center gauge, you can use a hardware store bolt. Lay it on the work same as the center gauge and set your tool bit into the threads of the bolt.

If you are square but are still getting an off-angle thread, you might try less depth of cut. I usually go around 0.002" - 0.003" on the radius per pass. Others may go 0.005" - 0.010", but I'm conservative when it comes to depth of cut. The deflection could also be from the boring bar sticking out too far from the tool post.

Hope that helps, good luck with the backing plate!

Bruce

upload_2017-1-8_8-26-28.png
 
All might not be lost with your backplate. By installing a threaded bushing in the back plate might be salvaged. It would be a threading exercise for you.
 
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