threading problems.....

[savarin]

What about setting it to cut at 90' its not a problem on small threads.
Slow the speed right down, make the cut but stop the machine at the end.
back out the tool a bit so it doesnt rub, disengage the half nuts and move back to one or two threads from the start,
engage the half nuts and set the tool bit into the thread jiggle the feed to remove the backlash, then hand wind in reverse along a bit further out so the tool bit isnt touching the thread any more, increase the feed into the next cut and turn on.
Rinse and repeat.
This should tell you if its a machine problem.
Once you get the reverse working just keep the half nut engaged and reverse back to the start.
With a bit of care and taking regard of the backlash in the feed screw its very easy to pick up a thread.

 

[Bob Kelly III]

well I just came in from the shop I went out there to practice threading again.... wound up pulling the spindle completely out again because of that darn Bull pin....it came out too far again even with my indent !!!!!! so this time I fixed it ! I put a bevel on the end of the pin so I can get it back in even if I do pull it out too far ! that'll learn it !!!!! LOL.....( now I won't have to pull the spindle if the pin comes out too far I can just push it back in because of the bevel as it will raise the spring itself NOW !)
anyway got it back together and did some attempts at threading and had a good thread forming and that darn half nut skipped a tooth again and just wiped out the threads AGAIN ! .... so I am going to pull the apron , something I haven't done yet and have a look see and see if that half nut is buggered up or not.... I kind of doubt it because if I use the half nut for longitudinal feed it works great ! but who knows their may only be one tooth left on it ! LOL something ain't right ! maybe the mechanism is not sliding smoothly..... that would do it too I think !
when I tried it tonight I had it turning about 5 RPM really slow..... and it still did it !!!!!! something is borked ! it's gott'a be
going that slow I had time to stop it and only have a ring around the threads but at that point I said fooie on it 40 trys and 39 fails I don't think it is me anymore ! so the apron comes off tomorrow !
the one thing all the attempts have in common is the half nut, speeds were different, cutters were different , fingers were crossed , fingers uncrossed !
i used the cross slide only, I used the compound for thread depth only, yet they all failed and I used the half nut on all of them....
and they all failed for the same reason jumping out of sync !
....... I already know what I'll find.... "nothing wrong" the best possibility I can see is the mechanism that moves the half nut is locking up at times and I can't tell .... if so that should be repairable ! so wish me luck ! LOL.....
but at least that Bull pin is finally ,Finally Fixed ...I shouldn't have to pull the spindle again for that !....( I am getting good at it though ! )
HAHAHAHA
I was thinking of making a handle so I could wind the lathe backwards easy( a.k.a Steve Jorden)... but what's the point ? I'll get the 3 way switch soon I am sure !
but in back gears that beast does not want to turn easy ! I'ed have to raise that lid to take the tension off the belt each time.... that's no good !
.....i'll just get the stupid switch ! LOL....... I want to wire it properly anyway and take off the big clunky push to stop switch the last owner put on it
it just controls power to the VFD anyway.... i can't use it to stop the machine !
.....the stop button for the motor must be wired to the VFD and the VFD's power put on another toggle switch.
....
Also.....I used the tail stock when doing all my attempts by the way, and had it supporting the long rod about 8" of it sticking out of the chuck.....just to give me plenty of room sense the carriage was going toward the chuck !....call me chicken ! LOL.....
and the carriage stop was in place too !
.....
all in all it was a frustrating evening ! but clean living and perseverance prevailed ! HAHAHAHAHAH
we'll see what exploring that apron shows us tomorrow !
.....
Later !
Bob..........

 

[higgite]

59, 60, 61 I could be off a bit, like T Bredehoft said it's not set in iron, as long as you're close it should cut threads. But yeah it needs to be relative to the actual angle of the threads & I've noticed some of the newer lathe compounds just aren't marked correctly.

Just to avoid confusing newbies, I don’t think that’s what T Bredehoft meant. I believe he was referring to the protractor markings on the compound, which can be off by 90 degrees, depending on the particular lathe manufacturer’s way of marking their compound.

If you’re cutting threads with the compound, close is not always good enough if you want to cut good 60 degree threads. There have been a multitude of posts on here about folks cutting screwed up threads (pardon the pun) because they set their compound more than 30 degrees from perpendicular to the spindle axis, or less than 60 degrees from parallel to the spindle axis if you prefer. But, like others have pointed out, if you use the cross slide to advance the cutter, the compound angle is irrelevant. Hope this clears up more confusion than it causes.

Tom

 

[Choiliefan]

Lately I've been threading without the use of power.
Setting everything up as usual but am now turning the spindle by hand without disengaging the half-nuts between passes.
I started this technique using the Sherline threading attachment and have now transferred the same method to the Maximat 10p and my threads have improved (especially when threading up to a shoulder) and the process is far less stressful.
Every one should try this at least once.

 

[ttabbal]

If your chuck is not threaded on, Joe Pieczynski's reverse threading trick is nice. Thread away from the chuck, no stress about hitting the shoulder on the work or the chuck.

Of course, if you have the tailstock out, you can still hit that. I find I generally have more room even then though.

 

[Bob Kelly III]

I do have a threaded on chuck ! and it comes loose fairly easy ....I'll probably make a chuck lockdown arbor for it if I find it'a a problem
..... no confusion here about the compound protractors being off it's not a problem with this machine I don't think.....
it's gott'a be the machine somewhere ..... probably that half nut jamming on me ! I'ed just about bet on it !
.....when I get my act together I hope to produce good clean threads on demand from single point threading..... one of the main reasons I bought the lathe ! .... so I'll get there it's just a matter of time and money !( money being the hard part to come by ! )
..... there is a million opinions on threading do's and don't ....and I do appreciate the help !
I've tried cutting threads with the cross slide and with the compound, both ended up the same way wiped out because of the half nut skipping a tooth
..... it's gott'a be in that thing !
....
we'll see ( i wann'a pull it apart anyway and check it out !)
later Guys !
Bob.....

 

[brino]

I do have a threaded on chuck ! and it comes loose fairly easy ....I'll probably make a chuck lockdown arbor for it if I find it'a a problem

Bob,
Then you should beware that the chuck may try to un-thread itself from the spindle if you do wire up that reversing switch!
I have never seen it happen, but because I also have a thread-on chuck I do NOT run in reverse.
A chuck lock is a very good idea!
-brino

 

[RJSakowski]

It's time to approach the problem systematically.

As has been said, advancing the feed using the cross feed eliminates a compound angle error. You have played around with the form of the cutting tool, with rpm, and depth of cut without any improvement. That leaves loss of sync. between the carriage and the spindle. The half nut doesn't have teeth. It is a nut that is split in two. Disengaging the half nut pulls the two halves apart and away from the lead screw. Engaging the half nuts brings them together on the lead screw.

You can check if the half nuts are not engaging properly by closing them with the lathe stopped and trying to move the carriage. You should not be able to move the carriage except for a small amount of backlash. If you can move the carriage, you have to remove the half nuts and inspect them for wear and/or dirt. It is also possible that they are not adjusted properly. They should clamp down fairly tightly on the lead screw.

If the lead screw passes the test, then you need to look elsewhere for a loss of sync. One possibility would be a sheared drive pin. There may be enough resistance to drive the lead screw but under load it slips. Another would be one of the gears in the gear train is jumping teeth. This should be fairly obvious to spot but if you have an enclosed gear box, one of the internal gears may be slipping due to a sheared key. You might try another thread pitch to see if you can isolate the problem to a particular gear.

If the gear train passes muster, I would turn to the thread dial mechanism. A piece of crud in the thread dial gear could cause the gear to jump a tooth. Not having the gear fully engaged with the lead screw could also cause it to jump a tooth.

It appears that the 922 has an 8 tpi lead screw and that the thread dial gear has 16 teeth. This would mean there are four teeth between marks on the thread dial. jumping one tooth on the thread dial gear would advance or retard the half nut by 1/4 of the thread pitch causing you to cut a new track in your screw as you have observed.

 

[Bob Kelly III]

It's time to approach the problem systematically.

As has been said, advancing the feed using the cross feed eliminates a compound angle error. You have played around with the form of the cutting tool, with rpm, and depth of cut without any improvement. That leaves loss of sync. between the carriage and the spindle. The half nut doesn't have teeth. It is a nut that is split in two. Disengaging the half nut pulls the two halves apart and away from the lead screw. Engaging the half nuts brings them together on the lead screw.

You can check if the half nuts are not engaging properly by closing them with the lathe stopped and trying to move the carriage. You should not be able to move the carriage except for a small amount of backlash. If you can move the carriage, you have to remove the half nuts and inspect them for wear and/or dirt. It is also possible that they are not adjusted properly. They should clamp down fairly tightly on the lead screw.

If the lead screw passes the test, then you need to look elsewhere for a loss of sync. One possibility would be a sheared drive pin. There may be enough resistance to drive the lead screw but under load it slips. Another would be one of the gears in the gear train is jumping teeth. This should be fairly obvious to spot but if you have an enclosed gear box, one of the internal gears may be slipping due to a sheared key. You might try another thread pitch to see if you can isolate the problem to a particular gear.

If the gear train passes muster, I would turn to the thread dial mechanism. A piece of crud in the thread dial gear could cause the gear to jump a tooth. Not having the gear fully engaged with the lead screw could also cause it to jump a tooth.

It appears that the 922 has an 8 tpi lead screw and that the thread dial gear has 16 teeth. This would mean there are four teeth between marks on the thread dial. jumping one tooth on the thread dial gear would advance or retard the half nut by 1/4 of the thread pitch causing you to cut a new track in your screw as you have observed.

very astute observations ! .... I do have one questionable gear in the gear train and it IS in use threading toward the chuck ( it''s the upper idler gear on the logan 922 a 34 tooth'er.... and I have thought about the shear pin, on the lead screw ( if this thing actually has one) ....
i did turn out one rough but viable thread about 6" long when I first started this a left handed thread..... but the thing never skipped a tooth on me doing the left handed threads ! ...... I have a feeling you just found the culprit !
I have been gearing up to braze up that gear sense I got the machine !
it has been repaired by the screw /file method but it is a very crummy job way too much open spaces in that gear....
...... I looked at the half nut when I thad the thing all apart and the cross slide/apron on the floor it looked fine to me.
with the machine not turning I can move the half nut in and out of engagement easily...no binding at all but it may not be clamping tight on the leadscrew ! ( no, the carriage is locked to the lead screw when the half nut is closed it does not move hardly at all!)
....
I think I will swap idler gears.....and see if that cures the problem temporarily...
I have 2 more idler gears that came with the lathe but they are 36 tooth..... I could elongate the hole in the bracket and get one to fit to replace the bad gear..... but I thought that would screw up the thread count for the QCGB.... any thoughts on that ?
.....
I'll just fix the chewed up gear like I had planned..... and hopefully that will cure the jumping out of sync....
I think you hit the nail on the head ! ..... after looking at all the other possibilities that is the only weak link in the chain that I know of....
.......
before I forget.... what is the depth of cut when threading ?
I've seen a thousand threading videos and enjoyed all of them, but no one has ever said the depth of cut you should be taking....
This old Tony showed his dials and he took 10 thow but he's got a monster of a lathe and proper cutters
I don't .... steve Jorden mentioned 5 thow on his mini lathe....

thank you very much ! ..... I hadn't thought of that bad gear if any more consequence than Just looks bad.... but evidently it is much more serious than that !

time to FIX THAT !!!!!
LOL
Bob..........

 

[RJSakowski]

very astute observations ! .... I do have one questionable gear in the gear train and it IS in use threading toward the chuck ( it''s the upper idler gear on the logan 922 a 34 tooth'er.... and I have thought about the shear pin, on the lead screw ( if this thing actually has one) ....
i did turn out one rough but viable thread about 6" long when I first started this a left handed thread..... but the thing never skipped a tooth on me doing the left handed threads ! ...... I have a feeling you just found the culprit !
I have been gearing up to braze up that gear sense I got the machine !
it has been repaired by the screw /file method but it is a very crummy job way too much open spaces in that gear....
...... I looked at the half nut when I thad the thing all apart and the cross slide/apron on the floor it looked fine to me.
with the machine not turning I can move the half nut in and out of engagement easily...no binding at all but it may not be clamping tight on the leadscrew ! ( no, the carriage is locked to the lead screw when the half nut is closed it does not move hardly at all!)
....
I think I will swap idler gears.....and see if that cures the problem temporarily...
I have 2 more idler gears that came with the lathe but they are 36 tooth..... I could elongate the hole in the bracket and get one to fit to replace the bad gear..... but I thought that would screw up the thread count for the QCGB.... any thoughts on that ?
.....
I'll just fix the chewed up gear like I had planned..... and hopefully that will cure the jumping out of sync....
I think you hit the nail on the head ! ..... after looking at all the other possibilities that is the only weak link in the chain that I know of....
.......
before I forget.... what is the depth of cut when threading ?
I've seen a thousand threading videos and enjoyed all of them, but no one has ever said the depth of cut you should be taking....
This old Tony showed his dials and he took 10 thow but he's got a monster of a lathe and proper cutters
I don't .... steve Jorden mentioned 5 thow on his mini lathe....

thank you very much ! ..... I hadn't thought of that bad gear if any more consequence than Just looks bad.... but evidently it is much more serious than that !

time to FIX THAT !!!!!
LOL
Bob..........

If it is an idler gear, the tooth count shouldn't matter. It only counts when the gear is the driving another gear on the same shaft or it is the first or last gear in the chain.

Generally, you can start out with a heavier cut but once you are starting to remove a fair amount of material, you back off on the depth of cut. The actual depth of cut will depend on the lathe, the material, and the thread being cut. Advancing the cut with the cross slide rather than the compound set at 29.5º will require a shallower cut to prevent chatter.

I use the compound to advance the feed and after the first light pass to verify the thread pitch, I would start with a depth of cut between .006" and .010" depending on the thread pitch and reduce the depth of cut with each successive pass. As I approach the final cut, my depth of cut would be around .001".