Crazy Threads...

[Tozguy]

so, here are my two main issues that i just cant wrap my head around...

first - with the half nut engaged the whole time and reversing the lathe back to the start position. i had the machine set to 16 tpi but it cut a 15... how can that be?

second - when i use the thread dial and disengage the half nut each pass, i have to use the same number each pass, without changing any depths or positions, each cut is 1-2 thou in front of the previous cut. if i carried on long enough, i would eventually over lap my first cut. and if i try to use a different number or line as you can with an even number tpi, the cuts are in a totally different place. ill end up cutting between threads.

I can reproduce this problem exactly on my lathe.
My lathe also requires a different gear alignment when changing to metric and back. The same gears but meshed differently. Sometimes I overlook having to realine the drive train back from metric to inch.
The problem of getting an offset with each pass when using the half nuts is typical of this situation.
The 'good' threads shown in your post 68 when not using the half nuts but producing the wrong TPI is also typical of having a metric gear mesh when cutting inch threads.

Your posts 21 and 105 show the thread charts but I am suspicious that the gear diagrams might be mislabelled. The top diagram is labelled mm but might be for inch threads. Compare your diagram with Pierre's in post 102. What if there is nothing wrong with the lathe and simply using the 'other' diagram solves the problem?

 

[higgite]

Hukshawn,

Set up the lathe to cut 1.5 mm pitch threads with the half nuts engaged. Do not use the thread dial. I’ll bet you get exactly 16 TPI. If so, that means the lathe is geared for a metric lead screw with 3mm pitch, not for the 8 TPI leadscrew that is on it now. If so, divide 24 by the TPI that you want and set the lathe up to cut that mm pitch. For instance, if you want to cut 12 TPI, set up for 2 mm pitch. You may not be able to cut TPI's that aren't evenly divisible into 24 nor metric threads with that gearing/leadscrew combo. I haven't worked that out yet and don't have the time for it right now.

Tom

 

[Bob Korves]

It is really easy to read gear train drawings incorrectly. I have done it. There are multiple (at least two) levels of gears paths. Two gears meshed on one level create simple ratios. Using both levels of any gear results in compound gearing. Having the bigger or smaller tooth count facing inwards or outwards matters. You must make sure that not only are the tooth counts correct, but also which level of each gear is being used.

Searching for problems in general also requires a certain mindset. When I learned to fly 40 years ago, my first instructor, Dave Morss, taught me to inspect the aircraft before flight with the attitude that there is something or multiple things wrong with it that will try to kill me. It is my job to find them without fail. That is a very different mindset than seeing that everything is correct. If you are looking for correct, you will usually find it, whether it is true or not. Thanks, Dave...

 

[Tozguy]

And when you get to a dead end starting to doubt that the problem can be fixed, go back to review some of your earliest assumptions to find out which one sent you down the wrong path.

 

[Hukshawn]

I can reproduce this problem exactly on my lathe.
My lathe also requires a different gear alignment when changing to metric and back. The same gears but meshed differently. Sometimes I overlook having to realine the drive train back from metric to inch.
The problem of getting an offset with each pass when using the half nuts is typical of this situation.
The 'good' threads shown in your post 68 when not using the half nuts but producing the wrong TPI is also typical of having a metric gear mesh when cutting inch threads.

Your posts 21 and 105 show the thread charts but I am suspicious that the gear diagrams might be mislabelled. The top diagram is labelled mm but might be for inch threads. Compare your diagram with Pierre's in post 102. What if there is nothing wrong with the lathe and simply using the 'other' diagram solves the problem?

I'm going to try that. I did switch the gearing to try to cut a metric thread, I had the same problem with the thread dial, but I did not try not using the thread dial. I will exparament more in the "metric" setting.
Id find it rather appalling that the chart could be wrong. but anything is possible.

The gear train only has two options. That big gear sides back and forth on a bracket. It either engages one gear or the other. There does not seem to be an option to flip gears around. And the gears appear to be set up according to the label. I'm only a novice, I don't know her math to actually math out the ratios to see where things have gone wrong. I will have to research.

 

[pdentrem]

I have a question. If the leadscrew and gears were original set up for metric first, just changing the leadscrew to Imperial will not make an Imperial lathe. Correct?

For your reading entertainment. I have these books in my library from long ago. This one is on threads Imperial and Metric.

https://www.amazon.ca/Screw-cutting-Lathe-Martin-Cleeve/dp/0852428383

Pierre

 

[Tozguy]

One last question, I'm working two jobs now starting this week for a few weeks, so it'll be some time before I can devote any time again to solving this again..

even if it has an imperial lead screw, it appears the lathe is designed to cut both imperial and metric threads, based on the label plate... so why would it matter? that's the hard time I'm having with this. according to the label plate, I have the gears set to cut imperial, with an imperial lead screw, disregard the thread dial, using positive drive the whole way, it SHOULD work.... no?

Everyone's saying recheck the setup, check the gears, I did that. I even supplies pictures, they look right... I don't know where to look for problem.

Could the gear box be metric? should I look for a metric lead screw and change this back? I know nothing about metric threads. I'm at a total loss at how to fix this and am fearing this lathe is useless for threading.

The quick change gear box is neither metric or inch. It is just a bunch of different gear ratios in the secondary drive to the lead screw. The primary (outboard) gear ratio is designed to get a useful but limited range of thread pitches out of the gear box. To get a broader range of thread pitches (including metric ones), gears can be changed in the primary (outboard) gear train.

It is standard procedure to check thread pitch (with a thread gauge) after a light first pass to make sure that it is what you want. Why? Because it is too easy to get the gear choice wrong, either in the primary drive or in the secondary one.
Because metric pitches don't coincide with inch ones (although 16tpi and 1,5mm are close) the primary gear ratio has to be different for metric threads. This metric gear ratio puts the lead screw drive out of sync with inch threading and means the half nuts and inch thread dial, still in inch sync, cannot be used.

So if you are getting 15 tip from 16 tpi settings in the secondary gear box then the ratio through the primary gear train is wrong.

 

[davidh]

i crashed my lead screw drive a couple times too many and the lead screw itself move and broke the bracket on the rh end of it, the leadscrew itself actually pulled away instead the quick change housing and would shift back and forth depending the the amount of torque that the tool applied to the work surface. that was my first guess for you after reading all the posts. . . . .

 

[John Hasler]

why would it cut a thread the machine wasnt set to cut? i was set to 16 tpi and it cut 15 tpi... regardless of the thread dial. also note, this thread was cut by not disengaging the half nuts. pulling the tool back with the cross slide and reversing the lathe back to the start. so the thread dial was not used at all.

The threading dial and the incorrect TPI are two separate issues. The dial does not affect what thread gets cut. What I mean is that if there are transposing gears installed to make it cut metric then it will always cut metric no matter how you set the gearbox.

I suggest that you set it up for 8TPI and then verify that the carriage moves exactly one inch for every eight spindle revolutions.

 

[John Hasler]

even if it has an imperial lead screw, it appears the lathe is designed to cut both imperial and metric threads, based on the label plate... so why would it matter? that's the hard time I'm having with this. according to the label plate, I have the gears set to cut imperial, with an imperial lead screw, disregard the thread dial, using positive drive the whole way, it SHOULD work.... no?

Unless it was originally designed to have a metric leadscrew and someone replaced it with an imperial one.

What do you do to switch between metric and imperial threads? Whatever it is set it for imperial, cut some 8TPI threads, set it for metric, and cut more 8TPI threads (don't use the dial). Measure both.

[Edit] With a 14 tooth gear and an 8TPI screw your threading dial should be correct every four revolutions, if I've done the arithmetic right.