Help please? Threading on a 618

[machinistnoob]

It is possible to slip in the chuck if the work piece jams between two jaws rather than three. That's when I learned to twist the work piece a little while tightening the chuck as it gives me a better feel that the jaws are closing symmetrically about the piece. I've had that happen to me with drill bits as well. I've made many beginner mistakes starting out, so I don't take all of this for granted, every lesson was hard earned.

Will check and update

 

[machinistnoob]

Ok. I went back with all the suggestions in mind. Didn't find anything obvious. Tried again and got the same results. I will note that my trial piece is long and unsupported in the middle which will allow deflection. The results are the same at the supported ends. These are my second attempts. I fill the rod with attempts, turn smooth and try again.
Pics:
#1- scratch pass
#1A- dial on first scratch pass
#2- second pass. No adjustment or advance of compound between 1st and 2nd pass. Just ran again in the same manner.
#2A- dial on 2nd pass

 

[WobblyHand]

Please explain, exactly what you do after the first pass. I'm trying to understand everything you did, don't leave a single boring detail out, right down to scratching your nose. Do you disengage anything besides the half nuts?

On the last picture, on the bottom gears, the middle one, there's another gear on the middle shaft? Can you take a picture of that? Also, your lead screw, can you show a detail of the section, do the threads look sharp, or flat topped? The picture may be playing tricks on my eyes, so it would be good to get another, closer view.

One thing that is interesting is the second thread is not exactly in the middle of the original thread. It's closer to a third. If you do a third pass, does it shift further and fill in the blank area? Or are the thread starts more random, rather than simply linear offset?

 

[machinistnoob]

Please explain, exactly what you do after the first pass. I'm trying to understand everything you did, don't leave a single boring detail out, right down to scratching your nose. Do you disengage anything besides the half nuts?

On the last picture, on the bottom gears, the middle one, there's another gear on the middle shaft? Can you take a picture of that? Also, your lead screw, can you show a detail of the section, do the threads look sharp, or flat topped? The picture may be playing tricks on my eyes, so it would be good to get another, closer view.

One thing that is interesting is the second thread is not exactly in the middle of the original thread. It's closer to a third. If you do a third pass, does it shift further and fill in the blank area? Or are the thread starts more random, rather than simply linear offset?

First pic is middle of lead screw, 2nd is Tailstock area of lead screw

 

[machinistnoob]

First pic is middle of lead screw, 2nd is Tailstock area of lead screw

Could that be my problem? Also, when I leave split nut engaged, turn off motor and reverse saddle back and repeat going forward, there is no issue. Tool follows same path

 

[WobblyHand]

Could that be my problem? Also, when I leave split nut engaged, turn off motor and reverse saddle back and repeat going forward, there is no issue. Tool follows same path

Your lead screw is worn, but have seen worse. I don't think it's responsible for the weird threads. Think it is something else.

This is a good clue. Sounds like the the basic lead screw and half nuts are ok. But, what do you mean there is no issue? Can you successfully cut a thread using the technique you stated? If you haven't tried it, please do so when you get the chance. And measure the thread pitch, say over 10 threads. 10 threads = x inches.

I am wondering if you have set up the gears correctly, or inadvertently used a combination that may result in metric threads (or something like that). What thread do you think you are cutting? Have you measured it? Does it match your expectations?

 

[machinistnoob]

Your lead screw is worn, but have seen worse. I don't think it's responsible for the weird threads. Think it is something else.

This is a good clue. Sounds like the the basic lead screw and half nuts are ok. But, what do you mean there is no issue? Can you successfully cut a thread using the technique you stated? If you haven't tried it, please do so when you get the chance. And measure the thread pitch, say over 10 threads. 10 threads = x inches.

I am wondering if you have set up the gears correctly, or inadvertently used a combination that may result in metric threads (or something like that). What thread do you think you are cutting? Have you measured it? Does it match your expectations?

16TPI is what going for. Thread pitch gauge confirms that is what I'm getting on my scratch pass. My problem is tracking the original pass. I just figured out today, that when I cut forward and reverse back to starting position, and then repeat a second pass (without disengaging the half nut) it tracks properly and there is no error. So I can (begrudgingly) successfully complete the threads in this fashion. However, I should be able to be successful while using the thread dial. I was successful the first few times I attempted to cut threads. And now, zilch. So, staying engaged works. If the gears are incorrect, it would just be a TPI error, correct? The error should still be repeatable, regardless? Furthermore, if the gears were not meshed correctly, say, sloppy, the backlash would be taken up and still repeatable. Correct? Although im a total noob, i am handy and I think my procedure for cutting is on par.

-proper gears in their proper position
- touch off, zero cross and compound
- engage half nut at desired mark on dial
- scratch and measure
- withdraw cross, return carriage to starting pos.
- return cross to zero
- advance compound appropriate distance
- engage half nut at the same mark
- repeat
- scratch head

...and thank for your time with helping me

 

[WobblyHand]

Think you are understanding correctly. Thanks for confirming your thread settings.

Wondering if you need a wee bit more distance to cancel out your backlash. How about trying an offset of a quarter to a half inch to the right before starting? You won't be able to use such a long rod. How about having the rod stick out four diameters and unsupported for a test. Start your threads a half inch to the right of the rod. Thread like you think you should. What happens?

If you can properly thread using the half nuts "on" all the time, then basically everything is fine. It's some subtle thing that is not quite right.

I had to wake up this morning at 4am this morning and I'm dragging right now. Hope someone can help you some more tonight. Otherwise I'll help out in the morning.

 

[machinistnoob]

Think you are understanding correctly. Thanks for confirming your thread settings.

Wondering if you need a wee bit more distance to cancel out your backlash. How about trying an offset of a quarter to a half inch to the right before starting? You won't be able to use such a long rod. How about having the rod stick out four diameters and unsupported for a test. Start your threads a half inch to the right of the rod. Thread like you think you should. What happens?

If you can properly thread using the half nuts "on" all the time, then basically everything is fine. It's some subtle thing that is not quite right.

I had to wake up this morning at 4am this morning and I'm dragging right now. Hope someone can help you some more tonight. Otherwise I'll help out in the morning.

More distance to run out backlash, makes sense. Thanks again. It's back to work in the morning. I'll keep y'all updated with how it goes. Thanks everyone.
M. Noob

 

[Nutfarmer]

Look at the half nuts and check for wear. Also see if they fully engage. I don’t know on the 618 , but the 10 inch atlas uses the half nut for regular turning and will show wear sooner than other lathes