PDA

View Full Version : Thread Cutting Problem



bborr01
01-06-2010, 10:17 PM
Help.

I have a lathe giving me fits.

I recently bought a 18X40 Shen Jay lathe.

Today I decided to cut some 5/8 - 11 threads on a shaft to use as a plug to repair the 7/16 thread on a Bridgeport boring head arbor.

Could have just cut off a 5/8 bolt but that wouldn't be much fun.:rolleyes:

So.........I start cutting and made a few passed and all is going well and then I made a pass and it cut right down the middle of the thread. Looks like a 22 pitch thread.

The chart that I have says odd number threads can be cut on any main division, ie 1234.

So I cut the bar off and tried it again but this time I only engaged it on the 1 division.

I made a couple passes and then made a couple more without advancing the compound.

It kept cutting on every pass until the thread was almost gone.

I could go cut them on my SB heavy ten, but now this has got me sidetracked.

Any thoughts?

Brian

airsmith282
01-06-2010, 10:47 PM
your not supposed to advance the compound you advance the cross slide, but you have to have the compound set up right..

and you pick any number 123or 4 but what ever number you pick you have to use that number everytime,, you also need to have the right gears set up for the thread pitch you need

anyhow hope this helps..

i personaly perfer threading with taps and dies .

chriskat
01-06-2010, 10:54 PM
Is the material in a collet or a chuck? Is it good and tight? I've pushed material further into the chuck before; which, of course, changes the position of the thread relative to the movement of the carriage.

Just a thought but something has to be moving.

Jeff

doctor demo
01-06-2010, 10:56 PM
Check to see if there is a key missing from one of the gears between the head stock and qc box. It sounds like something is slipping.

Steve

bborr01
01-06-2010, 10:57 PM
Airsmith,

Thanks for the reply.

I think the subject of threading is not nearly as interesting as making a tapered shaft for a sex toy.

As to using the cross slide instead of the compound for threading, I have been cutting threads for over 35 years and have always used the compound to advance the tool bit. So did the dozen or so full time lathe operators that I trained under.

Also, as I said in my OP, I did try using the same number on the thread dial every time and it still wrecked the threads.

Brian

Ken_Shea
01-06-2010, 11:00 PM
If that lathe needs a gear change to turn metric threads perhaps they are still in there from a previous metric threading job, have you checked the gear train for gear tooth count as needed for 11tpi?

Arcane
01-06-2010, 11:00 PM
Since you had already done a few good passes before the bad one, that pretty well says everything was initially set up correctly. I'd guess what probably happened is you closed the half nuts in on the wrong place, but there are other things that could have happened too. The workpiece could have slipped in the jaws, (unlikely but possible)and if you had the compound set at a twenty nine and a half degree angle, maybe you had a brain fart and didn't get the infeed on the cross slide timed right with the advance on the compound. My number one guess would be wrong engagement point of the half nuts though.

J Tiers
01-06-2010, 11:01 PM
I am going out on a limb here and say that you probably have a clutch or shear pin somewhere in the drive to the leadscrew that has sheared or is slipping. Probably a pin, as a clutch would not be positive enough.

Slipping work is of course also possible.

That's assuming you are correct that you closed on the same number and still had the problem.

chrisfournier
01-06-2010, 11:06 PM
your not supposed to advance the compound you advance the cross slide, but you have to have the compound set up right..

and you pick any number 123or 4 but what ever number you pick you have to use that number everytime,, you also need to have the right gears set up for the thread pitch you need

anyhow hope this helps..

i personaly perfer threading with taps and dies .

Cross slide to remove and restore the origin of the cutter from the part while traversing to take another cutting pass and cross slide to advance cutter to achieve the proper thread depth. Only on really fine threads I might just use the cross slide if I'm feeling lazy.

The metric gearing sounds plausible as the culprit. I have poor engagement on my thread chasing dial on occasion and this can create a headache like yours but I doubt you're as sloppy as that given your experience.

oldtiffie
01-06-2010, 11:16 PM
Help.

I have a lathe giving me fits.

I recently bought a 18X40 Shen Jay lathe.

Today I decided to cut some 5/8 - 11 threads on a shaft to use as a plug to repair the 7/16 thread on a Bridgeport boring head arbor.

Could have just cut off a 5/8 bolt but that wouldn't be much fun.:rolleyes:

So.........I start cutting and made a few passed and all is going well and then I made a pass and it cut right down the middle of the thread. Looks like a 22 pitch thread.

The chart that I have says odd number threads can be cut on any main division, ie 1234.

So I cut the bar off and tried it again but this time I only engaged it on the 1 division.

I made a couple passes and then made a couple more without advancing the compound.

It kept cutting on every pass until the thread was almost gone.

I could go cut them on my SB heavy ten, but now this has got me sidetracked.

Any thoughts?

Brian
Brian.

Hooray for the "stuff the angled top-slide" Brigade - I am a fully paid-up foundation Life member!!

Anyway.

As you seem to have had only one "miss" and the later attempts were OK, I'd guess that the screwing/chaser dial is not correctly aligned to the lines or zero when the half-nuts were engaged. If it is the case its all too easy to "slip" a notch.

Engage your half-nuts, make sure the chaser gear is fully engaged with the lead-screw, rotate the head-stock spindle forward (by hand) to take up any "slack" and re-align the chaser dial to the index mark on the chaser body.

Some of the "chasing dials" are only a friction-fit - and can slip.

That's a big lathe - what is the lead-screw pitch?

kmccubbin
01-06-2010, 11:35 PM
Assuming this is the first time you've used this lathe for threading, it sounds like a metric/English transposition problem, and you got really lucky on your first few passes. Be sure the lead screw is fractional, re-check the gear train. Maybe cut a thread without dis-engaging the split nut. If it cuts a normal appearing thread, nothing is slipping. Then verify that the pitch is as intended. Or perhaps they put the wrong gear on the threading dial?

Let us know what you find, please.

Kerry

fasto
01-06-2010, 11:37 PM
It's also possible that the threading dial, or its gear is slipping.

bborr01
01-06-2010, 11:52 PM
Is the material in a collet or a chuck? Is it good and tight? I've pushed material further into the chuck before; which, of course, changes the position of the thread relative to the movement of the carriage.

Just a thought but something has to be moving.

Jeff

Hi Jeff,

I just double checked to see that the shaft was tight in the 3 jaw chuck.
It was.

Thanks for your post,

Brian

bborr01
01-06-2010, 11:57 PM
Check to see if there is a key missing from one of the gears between the head stock and qc box. It sounds like something is slipping.

Steve

Hi Steve,

I just put a channel lock pliers with a piece of leather on it and tried to turn the lead screw somewhat firmly.
It has the normal backlash in the gears. A few degrees or so.
It also has a degree or so that felt like it was slipping somewhere beyond normal backlash.

Thanks for your post.

Brian

darryl
01-07-2010, 12:15 AM
Just a possibility- could be that the half nuts have some sideways motion relative to the apron. Could be that the leadscrew has some endfloat. Could be that being new, some parts are tight and are just beginning to loosen enough for play to be taken up. Could be that being new, something is too tight altogether and a pin is shearing- soon to break. Well, it is a possibility. Could be that the first gear in the train is not properly keyed to the spindle, and it could be that the engagement of the half nuts is somehow backing off during the threading process. Could also be some effect of all the above mentioned parts wearing in. Could be that an intermediary gear on an independent shaft is floating sideways a bit.

bborr01
01-07-2010, 12:16 AM
Thanks guys for the quick response and all of the ideas.

I have checked that the part is tight in the 3 jaw and it is.

No slippage in the lead screw.

Threading dial is not slipping and has been engaged on the same number, one, pass after pass it now looks like it has about 3 or 4 threads in the area that it should have one.

I think it may not have been repeating the first few passes and I just thought that it was. I was feeding the threading tool in with the COMPOUND.

I am starting to wonder if it has something to do with the metric setup as has been mentioned a couple of times.

As far as i know to change from inch to mm I just change a few levers and start cutting.

Or am I missing something here?

Brian

bborr01
01-07-2010, 12:37 AM
That's a big lathe - what is the lead-screw pitch?

Hi OT,

Thanks for the reply.

It is a pretty good sized lathe, especially when I am used to running a SB heavy ten. It has a 2 1/4 hole through the spindle.

The lead screw is 1 3/8 dia. and 4 tpi.

I looked at the manual and the only thing it said is to change the levers to go from in to metric.

BTW. The lathe is not new. Just new to me. It is probably about 20 years old but is in new condition. It came out of a local community college.

Brian

whitis
01-07-2010, 12:40 AM
your not supposed to advance the compound you advance the cross slide, but you have to have the compound set up right..

and you pick any number 123or 4 but what ever number you pick you have to use that number everytime,, you also need to have the right gears set up for the thread pitch you need


The compound is normally used to advance the cutter when you are using flank infeed or modified flank infeed instead of radial infeed.

Threading dial usage may vary between lathes and the lead. You don't always have to use the same number on each pass. That is at least part of the point of having numbered marks rather than just a single mark.


On a logan, which has one unnumbered mark between each of the 4 numbered marks, you can restart the thread on any mark for an even number of threads, any numbered mark for odd numbered threads, and the SAME numbered mark for half threads (such as 7.5tpi), and you have to leave the half-nut engaged for quarter threads. If the TPI is divisible by 8, you can ignore the threading dial.
http://www.lathe.com/faq/index.html#_Toc95180303

On a south bend (according to 1950 edition of How to Run a Lathe), the instructions are similar for even and odd numbers but you use any odd numbered line for half threads and the same numbered mark for quarter threads. For metric, you leave the half-nut engaged or use a special metric thread indicator with multiple size gears that engage the lead screw.

If you have a metric pitched lead screw with a gearbox that is geared for imperial or imperial/metric, there may be issues with the threading dial. There are also lathes with rather odd numbers of TPI on the lead screw like 7.

For an 8TPI lead screw, any even number of threads will be in sync each 1/2" of lead screw travel (4 turns), odd threads 1" of travel (8 turns), half threads 2" of travel (16 turns), and quarter threads 4" of travel (32 turns) but a metric thread could take a very large number of turns (such as 254).

Slippage somewhere is a possibility. If it isn't the problem, but the threading dial is useless for imperial threads, then you can always keep the half nut engaged.

If you have the wrong gear combination or an inexact one for your thread pitch, you might actually be cutting some bizare thread pitch which will not sync up very often. Some lathes use an approximate combination (such as 47/37 instead of an exact one such as 127/100 for metric/imperial conversion. If that is the case, you may be able to cut threads that are within 0.02% but the threading dial may not function accurately.

Checking your leadscrew pitch would be helpful.

darryl
01-07-2010, 01:41 AM
Didn't think of it before- is it possible that the threading dial gear isn't tight to the shaft- if that moves it would sure throw things off. Or if the dial face is not tight to the shaft.

If there's any confusion between the crosslide and the compound, you could always set the compound at 90 then make some more trial cuts. You don't have to go deep, just see that the cutter comes back to the same groove each time. That's pretty basic, and I'm sure you're well past that by now-

I guess you could always try to cut the possible problem area in half- how about not disengaging the carriage as a test- back out the cutter and reverse the carriage under power, come in again. That should completely eliminate any effect of synchronization between gear selection and the threading dial.

bborr01
01-07-2010, 02:22 AM
Well, I was able to cut a thread that looked OK by not disengaging the half nut and reversing the spindle.

Getting late. I will try again tomorrow and report back when I know more.

Thanks again for all the ideas so far.

Brian

Peter N
01-07-2010, 02:50 AM
Sounds like you may have solved it, but I'll throw in another thing to check.

I had a similar problem last year when I was cutting a 3.5 tpi internal thread, and it puzzled me for a while until I finally traced it down to the meshing of the gears.
I hadn't set the mesh correctly - or hadn't tightened them up after setting them - so it was practically driving on the tips instead of the flanks, and as a result it was jumping a tooth under load.
Might be worth checking if you still have the problem.

Peter

NzOldun
01-07-2010, 03:26 AM
Help.

I have a lathe giving me fits.

I recently bought a 18X40 Shen Jay lathe.

Today I decided to cut some 5/8 - 11 threads on a shaft to use as a plug to repair the 7/16 thread on a Bridgeport boring head arbor.

Could have just cut off a 5/8 bolt but that wouldn't be much fun.:rolleyes:

So.........I start cutting and made a few passed and all is going well and then I made a pass and it cut right down the middle of the thread. Looks like a 22 pitch thread.

The chart that I have says odd number threads can be cut on any main division, ie 1234.

So I cut the bar off and tried it again but this time I only engaged it on the 1 division.

I made a couple passes and then made a couple more without advancing the compound.

It kept cutting on every pass until the thread was almost gone.

I could go cut them on my SB heavy ten, but now this has got me sidetracked.

Any thoughts?

Brian

Set the work up so that the tool is just about to make the first cut and ~" away from the end, with the half nuts fully engaged. Bring up the tail stock body and using a piece of wood (or anything convenient) as a packer in between it, just touch the back of the carriage. Lock the tail stock. This is to give you a stop to so that the carriage always starts from the exact same location!

Mark the chuck and headstock and and also the lead screw and gearbox with a line, using a with a felt tip you can rub off after-wards. With the lines on both the headstock and the lead screw in alignment (this is vital!), and the carriage back against the packer, you should be back at your original start point each time you take a cut. Try it half a dozen times to check, just scratching the surface of the work.

I've used this technique on an old (pre-WWII) Myford, which does not have a screwing dial and cut 11 TPI with no problems.

bborr01
01-07-2010, 03:49 AM
Well, its getting really late for me now. But i figured I'd lay in bed thinking about my problem so ........... I kept tinkering with it and found the problem.

The gear settings for an 11 pitch thread are KAF5. Four levers. I wondered what the lever was for that was marked UTSR. I figured it was used for metric threading. Then I saw an R marked above the inch threading chart.
Once I set the UTSR lever to R, it cut perfect threads.

Won't have to learn that one twice.:o

Thanks again for all the help.

Brian

wagnerite
01-07-2010, 03:59 AM
Brian, now that you got the threads cut... think you can help me with the tapered shaft? just kiddng :D

bborr01
01-07-2010, 04:16 AM
Brian, now that you got the threads cut... think you can help me with the tapered shaft? just kiddng :D

Actually, my southbend does have a taper attachment.:D

I can hear it now.

What you been doing in your studio lately?

Making parts for sex toys.

Oh boy.

darryl
01-07-2010, 04:18 AM
Is it possible that the thread dial you have doesn't match the leadscrew on that lathe? Or that maybe the gear on the thread dial doesn't match the dial itself. Just guessing at this-

bborr01
01-07-2010, 04:23 AM
Well, its getting really late for me now. But i figured I'd lay in bed thinking about my problem so ........... I kept tinkering with it and found the problem.

The gear settings for an 11 pitch thread are KAF5. Four levers. I wondered what the lever was for that was marked UTSR. I figured it was used for metric threading. Then I saw an R marked above the inch threading chart.
Once I set the UTSR lever to R, it cut perfect threads.

Won't have to learn that one twice.:o

Thanks again for all the help.

Brian

Hi Darryl,

The above tells it all.

Thanks for the help.

Brian