Thread Dial

[petertha]

This post made me go out & check my own lathe. I engaged thread mode lever, clamped the half nuts so they were fully engaged, moved the carriage laterally left till it stopped, zeroed the DRO. I get about 0.017" left/right float between end positions. Not sure if this is normal but is why I thought 0.125" sounded high.

 

[Kpar]

Thanks guys I'm just a novice at threading and your comments has helped me more than Mr.Gogle & youtube.
I'm getting on a bit & mind is slow but these posts have helped me a lot.
Kpar

 

[guzista]

Thanks djc, Leadscrew is 24mm Dia, Pitch 3.0mm Dial Indicator gear 24 teeth.
I guess this is how you measure pitch "center to center of thread"

That doesn't look to be matching the 3.0mm pitch guage to me. The points of the guage need to be slotted into the thread grooves rather than the top of the thread.

 

[Kpar]

My understanding is if you use a normal metric thread gauge and align the tips to the center of leadscrew thread will give a correct
pitch. I used a 3 pitch tab and it matched perfectly to the center of each leadscrew thread.
Is this incorrect ?.

 

[Kpar]

This is where I got my info from.

 

[Chiptosser]

You may be able to judge the thread pitch this way, on low count coarse threads.
Also, you are measuring a Acme thread,15 degree angle with what looks like a Vee thread gauge

The gauge should fit into the grooves and match the angle perfectly. If you try to gauge a 1.25, 1.50 pitch it is going to be harder to measure over top of the threads.
Try measuring a .09 pitch thread . Worse yet, if you are trying to determine if you have a metric or an imperial thread. You need the right pitch gauge for the type of thread that you are trying to identify. Good Luck.

 

[guzista]

My understanding is if you use a normal metric thread gauge and align the tips to the center of leadscrew thread will give a correct
pitch. I used a 3 pitch tab and it matched perfectly to the center of each leadscrew thread.
Is this incorrect ?.

It doesn't really matter where you align the guage to on the thread but the points should align at exactly the same position on each thread. In the photo it clearly shows an increasing misalignment with each consecutive thread. Obviously it's far better/easier if you have the proper thread form guage, as already pointed out by Chiptosser.

 

[awake]

I agree that the end float will be taken up when the carriage begins to move in response to engaging the half-nuts. That said, 1/8" is excessive and unnecessary. Were it me, I'd try to reduce that to no more than .010. Really, .001" should be enough if everything is machined and aligned properly - you just need enough that the leadscrew doesn't bind as it turns.

What is part # 48 in your diagram? It looks like this could be a set screw that adjusts the endplay - ?

 

[petertha]

From what I have read on multiple forums, it wouldn't be the first time & likely wouldn't be the last time an IMP leadscrew was packaged with a MET lathe or vise-versa. Seems to be a thing on some of the Asian hobby lathes where they serve 2 markets. I've also read of IMP leadscrew and somewhat poorly made or installed MET half nuts caused grief. When run that way (or attempting to), luckily damage was confined to the brass followers, not the leadscrew.

The measurement photo might be a parallax issue but totally valid point to take nothing for granted. As mentioned the common leadscrew pitches are 3mm (0.1181") and 8 TPI (0.1250"), the difference being 0.0068" over a single thread. Nothing wrong with using a thread gauge but a more accurate method is use the calipers crest to crest encompassing more threads to improve accuracy. For example 25 threads of 8 TPI = 3.125" vs 25 threads of 3mm = 2.953". Now the difference is 0.172".

I'm hoping the floating leadscrew is the main issue but this is good stuff to check in any event.

 

[Richard Hed]

My understanding is if you use a normal metric thread gauge and align the tips to the center of leadscrew thread will give a correct
pitch. I used a 3 pitch tab and it matched perfectly to the center of each leadscrew thread.
Is this incorrect ?.

In the photo, the left thread does not seem to match the screw gauge like the right one does.

 

[Kpar]

I think you will find it's the angle of the camera shot
Kpar

 

[roncohudd]

That doesn't look to be matching the 3.0mm pitch guage to me. The points of the guage need to be slotted into the thread grooves rather than the top of the thread.

 

[Kpar]

An update to my issues. I ended up disconnecting the lead screw and found that there was a fair amount of float in the main drive shaft. Removed the forward reverse gear lever (BIG MISTAKE) and the yoke that slides the gear into place fell off. Had to dismantle a bit more to get it back. Anyway set it all back to normal and presto only things are a lot better ??.
Adjusted the gibs on the half nut and with about .040" float.
Ran a test 1.5 pitch thread as per DJC's suggestions with the thread dial disconnected. Done 5 passes engaging the halfnut at random and thread came out ok. I may try and stick with those metric threads that do not require a thread dial.
I do remember crashing the toolpost into the chuck when doing my first thread, maybe that caused misalignment ?.
Thankyou all, I 've had this lathe for a few years now and have learn't something new.

 

[petertha]

Sounds encouraging. What do you mean by 'the main drive shaft'?
Were you able to examine the condition of the half nut threads? If they have been seeing wear, particularly after a crash, that also affects how good or bad thread engagement occurs.
Yes a crash could have disturbed something for sure. Usually the shear pin goes (as its supposed to) but I guess crashes can vary. I've also heard of cases where they discover steel shear pins or roll pins & the instructions say 'brass pin' so sometimes its a bit of detective work sometimes.
Did you end up validating pitch over larger thread count for certainty?

 

[Kpar]

Sounds encouraging. What do you mean by 'the main drive shaft'?
Were you able to examine the condition of the half nut threads? If they have been seeing wear, particularly after a crash, that also affects how good or bad thread engagement occurs.
Yes a crash could have disturbed something for sure. Usually the shear pin goes (as its supposed to) but I guess crashes can vary. I've also heard of cases where they discover steel shear pins or roll pins & the instructions say 'brass pin' so sometimes its a bit of detective work sometimes.
Did you end up validating pitch over larger thread count for certainty?

Done a couple of more threads this morning, all ok. The drive shaft I mentioned is where the coupling for the leadscrew which has a sleeve & two roll pins (maybe I should change one to brass). Unable to get to the halfnut looks like a major job. Yes I managed to confirm the lead screw with a mates leadscrew gauges, it is a 3mm pitch.
I'm now feeling a bit more confident in single point threading. The next lot of test threading is comparing Crossfeed to compound feed. It seems there are different opinions ?.
Thanks all for your support.
Kpar

 

[Harglo]

I had excessive wear in a lead screw. Removed it to turn it around as chances are a lead screw being worn near the tail stock are slim. A bit of adaption but it sure helped as the 1/2 nuts see threads that may have never been used.
Harvey

 

[Kpar]

I had excessive wear in a lead screw. Removed it to turn it around as chances are a lead screw being worn near the tail stock are slim. A bit of adaption but it sure helped as the 1/2 nuts see threads that may have never been used.
Harvey

Good one Harvey

 

[HCNZ]

I have the same lathe, I made a table to show what threads I could use with the thread chasing dial and what segment I could engage the half nut in. My thread chasing dial has a 20 tooth gear and 4 segments on the dial, not every metric thread pitch works with this setup, so I made another dial with a 21 tooth gear and 3 segments on the dial. It’s a shame they don’t include any information about this with the lathe, when I first got the lathe and started learning to cut threads I started with a M12 thread and trying to use the thread chasing dial which was never going to work. I’m not sure if the table will make much sense or if it will be of any use but here is a picture. It’s interesting that your dial has 6 numbers on it, how many tooth gear does it have?

 

[Kpar]

I have the same lathe, I made a table to show what threads I could use with the thread chasing dial and what segment I could engage the half nut in. My thread chasing dial has a 20 tooth gear and 4 segments on the dial, not every metric thread pitch works with this setup, so I made another dial with a 21 tooth gear and 3 segments on the dial. It’s a shame they don’t include any information about this with the lathe, when I first got the lathe and started learning to cut threads I started with a M12 thread and trying to use the thread chasing dial which was never going to work. I’m not sure if the table will make much sense or if it will be of any use but here is a picture. It’s interesting that your dial has 6 numbers on it, how many tooth gear does it have?

Thanks HCNZ, I have copied your table & will check it out.
The thread dial has 24 teeth and as per previous post from djc I followed his suggestions. It's so much easer to drop the
halfnut randomly & cut several met. threads.(thanks djc) Other threads I will leave the halfnut engaged & reverse back until I become more confident in using the dial indicator.
It seems like when these lathes were assembled they just picked any marked dial gauge & leadscrew.
Kpar

 

[HCNZ]

Thanks HCNZ, I have copied your table & will check it out.
The thread dial has 24 teeth and as per previous post from djc I followed his suggestions. It's so much easer to drop the
halfnut randomly & cut several met. threads.(thanks djc) Other threads I will leave the halfnut engaged & reverse back until I become more confident in using the dial indicator.
It seems like when these lathes were assembled they just picked any marked dial gauge & leadscrew.
Kpar

When I purchased my lathe I was told the lathe is made in different factory’s at different times, sometimes they come with a 16mm tool post and sometimes a 12mm tool post, could be a similar thing with the dial. It’s a bit odd though the 20 tooth gear works out a bit better. I modified my table to (I think) suit your lathe, it’ Give you an idea of what threads will work anywhere and which ones won’t. When I get my computer back on the internet I’ll post the spreadsheet if that’s useful.