# threading problems



## Fixnair (Feb 17, 2017)

Good afternoon folks! I haven't been around for awhile but I've been moving my household and shop from California to Oklahoma. It;s taken me a couple of years but I'm finally making chips again. I have a South Bend 13 X 40 lathe from WW II. I know there is a lot of wear in the machine but the machine was free to me. 
I've done a lot of turning, facing and boring since I've owned the machine and I feel I'm ready to learn how to thread. This is where the machine will not cooperate with me. It will not track properly. I'm trying to cut 1/2" - 20 threads and the first pass looks OK and measures OK with a pitch gauge but when I go for a second pass I'll engage the 1/2 nuts on the #3 & it will just rip the existing threads to bits. 
I know the gears in the single lever quick change gearbox are worn out. The 1/2 nut is new though. the gear train between the head stock and the quick change are a little sloppy also. the lead screw has some wear too, how much I don't know.
I guess it's time to rebuild the QC gear box. Can you guys turn me on to where I can purchase new gears? Bushings and shafting I can make with the exception of the splined shafts. Also is there any way to determine wether the QC gears are at fault or the power train gear set between the head stock and the QC box?

Thanks in advance for any help you experts may pass on


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## T Bredehoft (Feb 17, 2017)

Don't be too fast to blame the worn out machine.   
I'll engage the 1/2 nuts on the #3

I always use the same number, but with an even numbered thread you should be able to use any number. 

Are you advancing the tool with the cross feed or the compound? do you retract the tool at the end with the cross feed?  do you have a stop set on the compound? or do you return to the same number every time?


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## Fixnair (Feb 17, 2017)

I'll answer in the order you asked; Yes, I'm advancing the compound feed, Yes I'm backing the cross feed out to return, I do not have a stop on the compound, Being the first time I've tried to cut threads I engaged the 1/2 nut on the #3 each time.


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## JR49 (Feb 17, 2017)

Fixnair said:


> Thanks in advance for any help you experts may pass on



I Am certainly no expert (very far from it), but I figure I can get the ball rolling for you.  You need to give the experts a lot more info so that they can help.  T Bredehoft asked some very good questions, but there are more.  Basically, you need to describe EVERYTHING you do from the time you put the work in the chuck, until the time when the tool tears through the thread you cut on the first pass.  I guarantee they will find the problem, once you do that.   Good luck,   JR49


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## cathead (Feb 17, 2017)

Have you tried cutting some threads leaving the half nuts engaged?


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## Glenn Brooks (Feb 17, 2017)

Also not an expert on threading, but, here is a possible test of the machine.  Try threading on a setting that isn't worn out much in the QCGB.  Say, 36 TPI or higher on. Any diameter.  1" would be fine.  If you find a gear setting that isn't badly  worn, and the machine still wipes the thread, then either the lead screw/half nuts are really buggerrd or perhaps something in your process isn't as consistent as it should be.


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## markba633csi (Feb 17, 2017)

Could one gear be missing a keyseat and be slipping? I can't think of anything else.  Can you back out the tool, reverse the lathe keeping the halfnuts engaged, and try another pass? If it still wipes the threads something must be slipping.  You are using the backgears, aren't you? 
Mark S.


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## T Bredehoft (Feb 17, 2017)

One more ask. 
Is your compound set at 29 1/2º or at 60 1/2º?   Not all compounds are numbered the same. The compound should be not as much as 45 º to the work.


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## Charles Spencer (Feb 18, 2017)

Fixnair said:


> Thanks in advance for any help you experts may pass on



Well, that certainly isn't me.  I did have a similar problem for a bit on my SB 9 though.  It turned out that my threading dial wasn't tight enough against the lead screw, causing it to slip.


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## Reeltor (Feb 18, 2017)

I don't see how a worn gear in the QCBG could cause the wiping of the previous thread as you describe.  Can anyone enlighten me on how this would occur?  I am thinking that the first pass using the "worn" gear in the QCGB would cut the thread and all subsequent passes would follow the same path.

From the info you provided it sounds like you are doing everything correctly; do you have a copy of SouthBend's How to Run a Lathe?  While I haven't gone looking for it on this site, I'd bet a nickel that you can find a copy.  Years ago I watched a guy cut threads on his SB, on his machine the thread dial was on a pivot.  He pulled the thread-dial away from the lead screw when he wasn't threading and pushed it in to make full contact when he wanted to cut threads.  If the thread dial wasn't fully seated onto the lead screw, the dial would turn but it wasn't accurate or repeatable.  Is your SB setup this way?

Mike


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## Charles Spencer (Feb 19, 2017)

Reeltor said:


> He pulled the thread-dial away from the lead screw when he wasn't threading and pushed it in to make full contact when he wanted to cut threads. If the thread dial wasn't fully seated onto the lead screw, the dial would turn but it wasn't accurate or repeatable.



Yeah, what he said.


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## Fixnair (Feb 19, 2017)

JR49 said:


> I Am certainly no expert (very far from it), but I figure I can get the ball rolling for you.  You need to give the experts a lot more info so that they can help.  T Bredehoft asked some very good questions, but there are more.  Basically, you need to describe EVERYTHING you do from the time you put the work in the chuck, until the time when the tool tears through the thread you cut on the first pass.  I guarantee they will find the problem, once you do that.   Good luck,   JR49


Thanks for responding JR. I mounted my part in a 4 Jaw and indicated it in to within .001". Then I set the quick change gearset to produce 20 TPI. I then engaged my back gear ta the slowest speed.  Then set my compound @30 degrees & backed it out alacthe way. My  headstock has about .003 clearance. I then turned down my part to 1/2" & made a 1/8" cwide undercut 3/4" from end of part about .100" deep. Touched off & set my crossfeed to make a .005" cut.  Engaged the half nut on #3 and at the end of the cut I disengaged the half nut. backed out the crossfeed & returned the carriage back past the end of the stock. Measured with a thread pitch gauge and it looked OK. Returned the crossfeed to zero & advanced the compound .010"and engaged the half nut at #3 again. disengaged the half nut, backed out the crossfeed, returned the carriage to beyond the end of the part & shut the lathe off. Looked at the thread and it looked as if it was a 40 pitch instead of a 20 pitch. I wised I could forward a pic to you but that's not possible.


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## Fixnair (Feb 19, 2017)

cathead said:


> Have you tried cutting some threads leaving the half nuts engaged?


Sorry but I cannot reverse my lathe. My chuck is threaded on & I don't want it unscrewing in me. thanks for the reply though.


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## Fixnair (Feb 19, 2017)

Glenn Brooks said:


> Also not an expert on threading, but, here is a possible test of the machine.  Try threading on a setting that isn't worn out much in the QCGB.  Say, 36 TPI or higher on. Any diameter.  1" would be fine.  If you find a gear setting that isn't badly  worn, and the machine still wipes the thread, then either the lead screw/half nuts are really buggerrd or perhaps something in your process isn't as consistent as it should be.



Good idea Glenn. There is some wear in the lead screw but I don't know how to quantify it. The half nuts are new and I just replaced the driving key in the lead screw. I do have a question regarding the half nut indicator, is it supposed to engage exactly in a mark? I engage mine just before the dial reaches 3 but it doesn't engage 'till something after 3. I'll work on it some more during the weeek. Thanks.


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## Fixnair (Feb 19, 2017)

markba633csi said:


> Could one gear be missing a keyseat and be slipping? I can't think of anything else.  Can you back out the tool, reverse the lathe keeping the halfnuts engaged, and try another pass? If it still wipes the threads something must be slipping.  You are using the backgears, aren't you?
> Mark S.


Yes Mark, I'm in back gear. Maybe I'll make sure my chuck is very tight and try that backing up trick.


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## Fixnair (Feb 19, 2017)

T Bredehoft said:


> One more ask.
> Is your compound set at 29 1/2º or at 60 1/2º?   Not all compounds are numbered the same. The compound should be not as much as 45 º to the work.


Well, it's set at 30. Thanks for asking


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## Fixnair (Feb 19, 2017)

Reeltor said:


> I don't see how a worn gear in the QCBG could cause the wiping of the previous thread as you describe.  Can anyone enlighten me on how this would occur?  I am thinking that the first pass using the "worn" gear in the QCGB would cut the thread and all subsequent passes would follow the same path.
> 
> From the info you provided it sounds like you are doing everything correctly; do you have a copy of SouthBend's How to Run a Lathe?  While I haven't gone looking for it on this site, I'd bet a nickel that you can find a copy.  Years ago I watched a guy cut threads on his SB, on his machine the thread dial was on a pivot.  He pulled the thread-dial away from the lead screw when he wasn't threading and pushed it in to make full contact when he wanted to cut threads.  If the thread dial wasn't fully seated onto the lead screw, the dial would turn but it wasn't accurate or repeatable.  Is your SB setup this way?
> 
> Mike


Yes Mike. Mine is set up the same way and I do disengage it when not threading. I consider your suggestion later this week when I get back on this. I do have the South Bent book on how to run a lathe.


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## markba633csi (Feb 19, 2017)

Starting to sound as if there is something wrong with the threading indicator- ?  If you can run the spindle back by hand (kind of a hassle but possible) and the recut thread is ok then that's the only thing left...
Mark S.


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## coffmajt (Feb 19, 2017)

I have read about the lower gear on the threading dial not being pinned or worse, shearing the pin .  That could cause some problems, It sounds like the rest of your process is good except I try not to take more than .003 when threading, use lots of cutting oil, and if possible support the end of the shaft on a live center == Keep us posted -- Jack


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## brino (Feb 19, 2017)

Fixnair said:


> I do have a question regarding the half nut indicator, is it supposed to engage exactly in a mark? I engage mine just before the dial reaches 3 but it doesn't engage 'till something after 3.



Mine engages slightly after the numbers too. It always has and it works just fine.

Any chance that a previous owner changed the lead screw to a metric one?
That might mean the thread dial gear is not meshing properly.

-brino


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## jocat54 (Feb 19, 2017)

You should be able to reverse without any problem with chuck coming loose after you back out the cross feed. I use to do it all the time with a SB9A.
I would check for any problems with thread dial----have had the problem of not engaging the feed screw when pushed in and being too loose and disengage.


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## bfd (Feb 20, 2017)

after the first pass return to the next pass but leave the tool bit back and start the next pass. when the tool would start cutting shut the lathe off  and coast to a stop. then turn the cross slide in to see if the cut is in the right place. if not try to move the carriage back and forth to see if there is anything that skips, half nut or slop in the lead screw. too many things could be wrong tool bit loose or moved. do this a few times to see if it ever comes back into alignment if it does there is something wrong with your geartrain. bill


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## cathead (Feb 20, 2017)

The "Process of Elimination" is a very useful tool in solving many kinds of problems.  Once you determine all the
things it isn't, you end up with what it "is".  I'm sure you are well on your way to figuring this out and I for one will
be watching this post to find what it is.


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## Tozguy (Feb 20, 2017)

Fixnair said:


> Yes Mike. Mine is set up the same way and I do disengage it when not threading. I consider your suggestion later this week when I get back on this. I do have the South Bent book on how to run a lathe.



When the second pass rips into the threads made by the first pass, is it ripping into the exact middle of the first pass?
How many teeth are on the thread dial gear? The problem almost certainly lies with the thread chaser dial gearing.


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## stupoty (Feb 20, 2017)

might be worth checking another TPI or visually inspecting the gears ,  their might be a gear with a missing tooth some ware between the spindle and the lead screw.

Stuart


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## Fixnair (Feb 20, 2017)

Wow! I'm overwelmed  with the number of replies to my problem. I'm impressed with the interest you guys have shown. I will work on some of the suggestions this week & let you guys know what I find.


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## neilking (Feb 20, 2017)

I'm having good luck on my 9b, no thread gage. Cross slide in - cross slide out, leaving the compound alone and leaving half nuts engaged.

Sent from my SM-G920P using Tapatalk


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## Chuck K (Feb 20, 2017)

Sounds like the same problem as trying to cut metric threads with an imperial lead screw.....


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## neilking (Feb 20, 2017)

A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.




I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.




Here's what a 6" vise looks like on another PM45 (gt40's)
View attachment 253544


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## Fixnair (Feb 22, 2017)

Thanks for all you r input. I have finally made something that looks like a thread! 3/4 - 10, it is a little undersize (.015") and the nut fits snugly so I'm sure it is not the correct form. My single point threading tool I made myself and all I have to make such is a bench grinder. The root and the crest of the threads come to a point so my tool needs some work. I think I'll purchase a carbide threading tool. I'll post a picture below.

I think I'll work on the grind for my tool and try the  3/8"-20 thread again, the one I tried first and wouldn't work. Stay tuned & I'll let you how that comes out.


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## JR49 (Feb 22, 2017)

Fixnair said:


> Thanks for all you r input. I have finally made something that looks like a thread! 3/4 - 10, it is a little undersize (.015") and the nut fits snugly so I'm sure it is not the correct form. My single point threading tool I made myself and all I have to make such is a bench grinder. The root and the crest of the threads come to a point so my tool needs some work. I think I'll purchase a carbide threading tool. I'll post a picture below.
> 
> I think I'll work on the grind for my tool and try the  3/8"-20 thread again, the one I tried first and wouldn't work. Stay tuned & I'll let you how that comes out.
> 
> ...



Hold on there, Fixnair, You're not getting off so easily. We all want to know what the actual problem was, and how you fixed it.  Thanks, JR49


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## Reeltor (Feb 22, 2017)

Yea, what he said; what did you do differently to get the thread to cut correctly?

Hit the tops of the thread with a file to flatten out the tops of the "V", if your tool fits the fishtail then it's fine.


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## Fixnair (Feb 23, 2017)

OK, I not trying to bail on you guys. I know we all want a resolution to this thread. I tried several thread forms and had success with most of the selections of the QC. When I used the far left row on the QC (4-8-16-32-64 and 128) the handle would jump up & down and make a knocking sound. It seems that the QC is at fault. I'm going to have to work with this some more before I actually condemn the QC. Now that I have a decent threading tool I'm going to try the 3/8-20 form again.


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## cathead (Feb 23, 2017)

This is just a guess but it sounds like that knocking sound might be the QC gear slipping a cog or two.  I would take a close look at
that area.  I could be a bearing or bushing or worn gear.  The handle jumping indicates to me a slipping cog...


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## Fixnair (Feb 24, 2017)

Thanks Cathead! I'm getting kind of discouraged with this. I've been working on this all week now and have not made a lot of progress. I have been able to make a few fair threads, but not what I would call good threads. My 3/4-10 looks fair and a nut threads on albiet kind of loosly & the OD is .013 undersized when compared to a stock bolt.

I tried to make a 3/8 16 which again was only fair. I tried 3/8-20 which was unrecognizable as a thread. All my previous practice was done on aluminum which I was running low on stock so I switched to using steel. I tried to make another 3/4-10 and that was a mess.

When I look at the steel, under a magnifying glass, part it looked more like a buttress thread that anything else. I also tried something else that was suggested. Someone advised me to shut the lathe off during a cut and allow it to coast to a stop W/O moving any if the feed dials. Then reverse the lathe & see what happens. It should track back I'm assuming. Well my lathe is not wired to reverse so I stopped the lathe during a cut, disengaged the back gear and manually rotated the lathe backwards and observed the tool point/work piece interface. Under the glass again I could see that the piece rotated backwards an amount W/O the tool moving 'till the tool point contacted the thread in the left side of the point then dug into the work piece. This tells me there is excessive clearance in the whole drive train. Where I don't know. I've had the whole gear-train apart and I know they're worn as well as the shafting. I guess its time I learned how to make gears. Making shafts and bushings are no problem.


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## cathead (Feb 24, 2017)

Fixnair said:


> Thanks Cathead! I'm getting kind of discouraged with this. I've been working on this all week now and have not made a lot of progress. I have been able to make a few fair threads, but not what I would call good threads. My 3/4-10 looks fair and a nut threads on albiet kind of loosly & the OD is .013 undersized when compared to a stock bolt.
> 
> I tried to make a 3/8 16 which again was only fair. I tried 3/8-20 which was unrecognizable as a thread. All my previous practice was done on aluminum which I was running low on stock so I switched to using steel. I tried to make another 3/4-10 and that was a mess.
> 
> When I look at the steel, under a magnifying glass, part it looked more like a buttress thread that anything else. I also tried something else that was suggested. Someone advised me to shut the lathe off during a cut and allow it to coast to a stop W/O moving any if the feed dials. Then reverse the lathe & see what happens. It should track back I'm assuming. Well my lathe is not wired to reverse so I stopped the lathe during a cut, disengaged the back gear and manually rotated the lathe backwards and observed the tool point/work piece interface. Under the glass again I could see that the piece rotated backwards an amount W/O the tool moving 'till the tool point contacted the thread in the left side of the point then dug into the work piece. This tells me there is excessive clearance in the whole drive train. Where I don't know. I've had the whole gear-train apart and I know they're worn as well as the shafting. I guess its time I learned how to make gears. Making shafts and bushings are no problem.



Hmmm.  A couple things come to mind.  Assuming you are cutting from right to left, the 60 degree cutter is cutting only on the left
side of the thread since you are advancing the cutter with the compound at 30 degrees.  The cutter needs a little relief angle on the left to cut properly.  
Also, your cutter needs the tip ground off about 5 thousandths at a slight relief angle for what you are doing.  You need to advance the
compound no more than about 5 thousandths on each pass.  It will take at least a half dozen passes to get to near size.  If your stock is
under size, the threads either will not form completely or end up under size.  If you only take off a few thousandths on each pass, it is hard
to understand why the threads could end up that much under size.  It's handy to have a nut that fits the intended threads and do a test
fit or two as you get close to finished.  The last pass usually will be just .001 or so for a clearance fit.  Also, use some lubrication 
suitable for the metal you are cutting and relube on each pass.  

Also, when backing up the lathe, the cutter probably won't follow the threads due to lost motion.  If you do that, I would recommend
backing out the cutter and  then back up.  It's pretty normal to have some slop.  Once you have overcome the lost motion, the 
cutter will back up at the thread rate but may not look right in line with the thread grooves.  When advancing forward again, the 
cutter will line up with the grooves.  Even with a lot of wear on the drive train, the lost motion will catch up and cut decent threads as
long as you don't have something slipping.  I hope this makes sense.


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## Fixnair (Feb 25, 2017)

Thanks Cathead. I think next week I'll purchase a carbide threading tool so I can take the tool out of the equation.


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