Crazy Threads...

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Hukshawn

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hello all. Im new here. i come here in search of knowledge and solutions.
i am a beginner/intermediate. i have a reasonably solid handle on the lathe, but still have years to go before i call myself anything close to an expert.

My issue...

i am trying to thread a piece. it is not going well. i cannot seem to get a consistent cut resulting in cutting in between threads.

i believe i have my machine set up properly. i am trying to cut an 18 thread, the quick change gear box and levers are all set according to the label plate on the lathe. it also has a diagram of the gear setup on the side, i counted the teeth to ensure it hasnt been tampered with previously. all seems well, mild slack in the gears, but nothing serious.
im also following the appropriate instructions with the thread dial indicator and engaging the half nut when required. i have my compound set to 61, which would make the tool 29 degrees to my work piece and have my 60 deg cutting tool set in my qctp perfectly at 90 deg set with the thread ruler tool thingy (ya, i cant remember the name of it). So, all seems to be well as far as the settings.. but the threads are still not consistent. in fact, theyre a mess...
ive retried a few times, flattening the area, and trying again. the first few times, the threads look like they were stepped down away from the tool. as if the angle was too sharp. other times, the tool would just cut out the previous thread. ive included a picture of my current mess and some set up photos

some history.... when i purchased the lathe, i disassembled the table, cross slide, compound, apron, tail stock... everything except the gear box and the head stock. cleaned all the chips out, tightened ways, and keys and made it all nice, solid, clean, and oiled. i also took apart the thread dial indicator... im wondering if this is where my problem is.

its hard sometimes to get the half nut to engage exactly on the thread dial indicator line. it would snug in just past or just before the line. obviously that's not going to work... i loosened the allen screw and aligned the tick with where the half nut seats fully. tried again. the half nut engages nicely right on the dial line. but still, threading is way off. so im wondering if i messed up the thread dial indicator positioning...

sorry my first post is so incredibly long. but this is driving me nuts... please folks, help me out.
thanks
shawn

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On a new bit of stock with some run out room so your not threading to a shoulder try again only this time leave the half nuts closed when you reverse back to the beginning only run the tool bit out so it doesnt contact the threads in reverse.
If this gives a good thread then try again using the thread dial as usual.
That should narrow it down a bit, say, if the first test also fails then maybe the lead screw is slipping a bit or the half nuts are too worn or they dont fully engage (the problem with my lathe when I first received it new)
I'm not a pro this is just a guess.
 
Here is another try.
10 threads per inch. slowest speed. all engaged on number 4

IMAG0664.jpg
 
No wear to speak of on this lathe. if there is it's miniscule at best
 
savarin said:
On a new bit of stock with some run out room so your not threading to a shoulder try again only this time leave the half nuts closed when you reverse back to the beginning only run the tool bit out so it doesnt contact the threads in reverse.
If this gives a good thread then try again using the thread dial as usual.
That should narrow it down a bit, say, if the first test also fails then maybe the lead screw is slipping a bit or the half nuts are too worn or they dont fully engage (the problem with my lathe when I first received it new)
I'm not a pro this is just a guess.
Click to expand...
How do you move the carriage with the half nuts closed...?

Edit... you said reverse. I'll try that


Edit edit... that works perfectly... on the test piece, was a but tricky to get set up and advance the compound before running out of time, but the tool did follow the same cut line.. a bandaid fix to finish this project I suppose. would still really like to know why it's happening. I bought this lathe specifically because it was leaps and bounds better than my old one.
 
Last edited by a moderator:
Hukshawn said:
How do you move the carriage with the half nuts closed...?
Click to expand...

You revers the motor. If you have a reverse. Also it looks like you aren't backing the tool out quick enough. What depth of cut and what rpm?
 
Are you setting the cross feed to zero on the dial each pass and feeding in with the compound?
 
kd4gij said:
You revers the motor. If you have a reverse. Also it looks like you aren't backing the tool out quick enough. What depth of cut and what rpm?
Click to expand...

Ya I realized the reverse fiasco after I read it a second time.
There's no shoulder left on my work piece after an hour of ruining cuts.
I had only been cutting 3-4 thou each pass. rpms are about 45. the slowest my lathe will go.
 
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