View Full Version : Help with threading needed WARNING large pics

Your Old Dog
10-01-2006, 06:38 PM
I'm home, still nurseing a broken ankle. I thought it might be a good time to spend 2 or 3 months trying to figure why I can't thread on the SouthBend 9.

Can someone with a SB9 tell me if this looks like their drive setup?

This is my chart on the machine and when I follow its instructions and then put in a threaded rod it don't come close tracking properly (is that even possible?) The threaded rod I used was a chunk of 1/2" by 13 tpi.

I was using the rocker style SouthBend threading tool but am going to the type that I can use in the tool block as I know it's more steady. My rocker is homemade and not nearly useable. What do I want to waste, the red or the yellow? Note the red has some backangle introduced. I have a pink wheel I can dress this on. The blank is cobalt.


I am setup on 29 1/2 near as I can tell. The compound angles off to the right. I am assuming this is right as I face the lathe.

All of my previous attempts were with the lantern tool holder and now I'm thinking some of my ratty looking threads had to do with unstable tool rest.

As mentioned earlier, putting a section of threaded rod into the 3jaw and setting the bit properly in the thread I noticed that it would not track properly. Was that even a valid test?

Any help is appreciated. I have several books on the subject and still haven't worked it out. Im convinced the secret to lifes riches, at least for me, lays in learning to thread. Am I destined to die a pauper? Thanks in advance.

10-01-2006, 06:44 PM
YOD, Either HSM or MW had a good article on threading a few months ago and it focused on the SB. Worth looking up if you can.

By not tracking, do you mean not giving the pitch you expect? Check the gears driving the leadscrew, back to the tumbler reverse gears. OOPS, that's probably the info you want !

Found it ... here you go: http://www.lathes.co.uk/southbend9-inch/page6.html

It looks like you're set up like figure 2 in the gear chart.

I think your compound is angled the wrong way. As you advance the compound, you want the left edge of the tool to advance into the left wall of the thread being cut. The right edge should just skim the right hand wall of the thread being cut, due to the 29.5 degree compound angle.

For a final cleanup pass, the cross slide can be advance a slight amount so that the tool cuts BOTH walls at the same time, giving the final profile.


Your Old Dog
10-01-2006, 06:52 PM
My head hurts :D I read thru that site but I'm hoping maybe someone with a SB9, take Evan for instance, would maybe mount up on that big white steed with shineing armour and come romping down the forum lane to confirm that my gear train looks right or I got something switched !! I think theres several guys on this site with SB's. Thanks for the effort, I scratch you down in the "favors owed" booklet I keep !!


10-01-2006, 06:57 PM
All looks ok to me .Just grind the top of the tool down to center of the center I my self would go to the yellow line. dont see why it wont work .Unless you got wore out half nuts . and i would not thank so looking at condition of lathe. hope this helps. Set your compound the other way it should read 60 not 30 as in picture. that is the problem . OLD LANE

10-01-2006, 06:58 PM
Ok, hold your head with your left hand and give us a tooth count on the large gear to the left. Also, what's the tooth count on the driven gear to the right, mounted on the leadscrew? The numbers should be stamped on the gears somewhere but the photos don't show it. Looking for 80, 72 or something like that.

10-01-2006, 07:05 PM
NO your gears are RIGHT.

10-01-2006, 07:09 PM
Counted the left one at 80. Right one at 56. The stud gear looks wrong. I'm counting 20 teeth. Its the outer gear on the shaft that's driving the largest gear. From the chart, for threading it should be 16, 24 or 32.

At this point, I'll step aside for other SB comments ;)

10-01-2006, 07:09 PM
YOD do you have a thread dial on your machine? Placing a hunk of all thread in the chuck and hoping it lines up is a long shot. The best way is to place a piece of 1/2 HR in the chuck and cut a relief about a 2" in or so move your carriage back a about 1" or so from the end using the thread dial and engage the carriage and thread to the relief... Withdraw tool reverse the carriage back before the stock. Advance your cut in and engage the carriage when the mark comes around again. The lead in will take-up and play in the nut and if you miss the mark you can reset. Also put the lathe in back gear to slow things down.

Your Old Dog
10-01-2006, 07:18 PM
Thanks for the interest in this problem.

I will count up all the gears tomorrow and label them. I do have a thread dial on this machine and I don't think it's my problem. I'll also post a shot of one of my past attempts to thread to show you how ratty it is. It may offer a clue as to whats wrong.

I was concerned that mounting a section of 1/2 x 13tpi threaded rod in the lathe, closing the half nut and then positioning the bit in the thread using the cross and compound slides and the machine would not trach properly when turned by hand. It makes me think there is a problem with the gearing.

I'll get more infor for you all tomorrow.

Thanks again,

10-01-2006, 07:26 PM
Your gearing looks correct to me. The stud gear (the one that engages with the tumbler reverse gears) should be 20 teeth. The screw gear (the one that goes into the QC box) should be 56 teeth. The number of teeth on any intervening idler gears don't matter.

To cut the top row on the threading chart (4, 4 1/2, 5, etc.) you change the 20T stud gear for the 40T gear that's stored beside the 56T gear.

10-01-2006, 07:35 PM
I'll have to look it up ( minds like a sieve at times ) but depending on odd or even pitch you engage the thread dial on different marks. Doing otherwise will make for something that looks like a rat chewed on it

10-01-2006, 07:36 PM
YOD & SGW, Sorry, my mistake on the stud gear. The 1939 "How to Run a Lathe" book did not show the stud gear for QC machines (too old) and I grabbed the non-QC chart. Been on the road all afternoon and should just READ only :) Den

10-01-2006, 07:57 PM
I have a model C, so no quick change gear box for me. I have threaded on my lathe. You NEED a threading gauge, you know so you engage the half nuts at the correct time. Your tool needs to be 60 degrees, can use one of those fish tail gauges to grind to. the 29.5 is half of 60 plus a little allowance. You cut only one side of the thread, plus a little bit more each pass by feeding in the compound until you have a full thread. Really amazing the first time you do it as you see an actual thread starting to develope.

10-01-2006, 07:59 PM
YOD, the easiest way to find out if it is threading the same pitch as you have the tumblers set on is with a dial indicator. Mount an indicator parallel to the movement of the saddle and engage the half nut. Then make a mark on your chuck and headstock with a pen so you can see when you have turned the chuck one complete turn. Make sure to turn it a couple of times to get out any slack out of the gear train. Now zero the indicator and measure exactly how much the saddle moves per one revolution of the chuck. This will tell you exactly the how much the saddle is moving per rev. Multiply this movement by the number of threads you are trying to cut and it should equal one inch. Now compare it to what you have the gear box set to. If it is not right then you definitely have a problem in the gear train. Example: if you set the gear box to thread 10 threads per inch, the saddle should advance exactly .100 per turn of the spindle. If you multiply 10 times .100 then you will get 1 which represents one inch. Make sure and turn the chuck in the same direction all of the time to prevent getting any slack in your measurements.

10-01-2006, 08:47 PM

I can cut threads on my lathe,(not a SB) and some are rough looking. I just buff them on the buffer wheel and they work fine.

I normally only cut threads that are above 1/2", on the lathe. I use a die for 1/2" and under. The threads look better.

Just my 2 cents.

Good luck on your threading.

charlie coghill
10-01-2006, 09:30 PM
YOD if you lived any where close to Medford Or. I would come over and give you a hand.

10-01-2006, 09:56 PM
The picture of the gear train appears correct. My Model A 9" has a 24 tooth gear in mesh with the tumbler gears and it is on a common axis with a 20 tooth gear which meshes with an 80 tooth gear which meshes with a 56 tooth gear that is on the gearbox input shaft. I have cut many threads with this configuration and they always come out exactly as the threading chart says they should.

J Tiers
10-01-2006, 10:05 PM

Your gearbox needs a certain gear ratio versus the spindle on its input shaft. I don't know what that is, but it is find-out-able

That is the ratio of the "stud gear", the one on top of the 4th gear of the cluster at top, to the one on the QC box input. The others are just filler, and do not count.

You need the info as to the proper gear on teh QC box input shaft, and then to use the gear given as stud gear at top. The middle gear is just whatever fits. That should do it for you.

Someone with an SB should be able to enlighten you as to the QC gear.

If it stilldoes not track, you must be following directions wrong somewhere, as tothe QC levers.

Since 8 tpi is the top left hole on teh right hand lever, with left lever in A as it is now, move the right hand lever to set upfor 8tpi. Then manually turn the spindle. The leadscrew should turn same speed as the spindle if your leadscrew is 8 tpi as I think it is.

If not, the QC input gear may be wrong.

C. Tate
10-01-2006, 10:15 PM
How are you engaging half nuts? Do you know the proper way to "pick up" a thread? Could be method and not set up. I can nto speak for the gears on a SB.


Your Old Dog
10-02-2006, 07:25 AM
Thanks a million guys. It's early in the morning here so I got all day to try some of the great suggestions. By this afternoon I hope to be making threads or turning my small machine shop into a pool hall !!

Charlie, thanks for the offer but I'm in Buffalo, NY area.

Evan, I haven't seen you show up. Hope you don't think I was being flip with my comment. I wouldn't want you to take my feeble attempt at humour as a slap in the chops. I greatly respect your abilities and I knew you had a SB.

Thanks all, I'll keep you posted. I just got to blast my way thru this threading thing.

10-02-2006, 10:54 AM
Not to worry. I have a model C and it's been too many years since I ran a model A to remember what the setup is for the quick change. Also, your pictures aren't showing up, perhaps you have exceeded your bandwidth or something?

You needn't worry about the lantern tool holder, it is plenty strong and rigid enough to do excellent threading. It's what I have used for decades and works fine.

All SB lathes are even geared. If the ratio of the gears from the spindle to the lead screw is 1 to 1 then the lead screw makes one rotation for every spindle rotation (not counting the gearbox setting).

With a 1 to 1 ratio the lead screw will cut whatever thread it is cut with. The SB9 is 8 tpi so it will cut 8 tpi with a straight ratio.

Your Old Dog
10-02-2006, 11:43 AM
Thanks Evan.

Im in for lunch now from the shop. I found out that it is tracking fine but the problem may be in my in-ability to hit the thread dial correctly. According to the manual I'm supposed to hit any numbered line while trying to cut odd numberd pitch. I practiced engaging the half nut so it went in smooth each time instead of clashing the gears together. I can only hit it about half the time.

I made about 6 passes on one attempt and it was going smoothly and then I started a entire new thread on top of the one I was working on. It's starting to look alot like pilot error. That reminds me, I want to die peacefully in my sleep like my grandfather did and not like the 150 screaming passengers on his plane!!

10-02-2006, 12:01 PM
Threading involves several different distinct parameters. When learning to thread, it is best to address one at a time.

You are obviously having a problem with gearing the lathe to produce the desired thread pitch, and this should be the first thing to address.

Get rid of the toolpost and place a magnetic base or something similar to hold a pointer that will show the cutter's path.

The 1/2"-13 all thread is a good way to see what is happening, but merely placing the cutter in the thread may be what is holding you back.

Don't worry about the counter or anything else, but with the lathe turned off, move the carriage away from the chuck a couple of inches and engage the half nuts. Turn the lathe on and let it run a couple of revolutions until the carriage moves. Stop the lathe. Don't touch anything, but move the pointer into a thread groove. If the QC is set to that pitch, the pointer should follow when the lathe is restarted, if not there is a problem in the gear combination.

Give this a try and get back to us.

10-02-2006, 02:49 PM
YOD (Ray) -
I just wanted to say 'thanks' for the excellent photos of the gear train setup. Between that and the other replies here, I will have an easy time putting a QC on my 9C. (No, I still haven't done it, despite having everything but the 20T stud gear.)

OT - I hope the ankle is coming along well and heals completely. It's great that you're putting the time to good use! :D


10-02-2006, 03:37 PM
YOD (Ray) -
I just wanted to say 'thanks' for the excellent photos of the gear train setup. Between that and the other replies here, I will have an easy time putting a QC on my 9C. (No, I still haven't done it, despite having everything but the 20T stud gear.)

OT - I hope the ankle is coming along well and heals completely. It's great that you're putting the time to good use! :D


The feed rate in turns per inch is found by calculating the total gear ratio of the spindle and the lead screw. You divide the lead screw by this figure. My cheapy Grizzly lathe at the slowest feed is calculated thusly:

L / ((a/b) * (c/d) * (e/f))

bc and de are keyed on their own shafts. An idler gear is ignored as it has no effect on ratios.

L = leadscrew in tpi. In my case it is 12 tpi.
a = 40t, b=80t, c=35t, d=90t, e=30t, f=100t

a is the spindle gear, f is the lead screw gear. This produces a feed rate of appx 205 turns per inch, or very close to .005" (1/205)

10-02-2006, 03:51 PM
For the pitch, if it's like a Heavy 10, it MUST be in backgear to get the proper pitch.

10-02-2006, 04:28 PM
Backgear makes no difference to the spindle/leadscrew ratio.

J Tiers
10-02-2006, 04:56 PM
As far as the threading dial.....

First, thread in back gear, just because it is easier to see what is going on in time to do something. Ratios will be Ok normal or back gear.

Then, be sure your dial is correct. For an 8 tpi screw, there should be a 32 tooth gear on teh dial, and 8 marks, 4 numbered, which I think is standard for S-B (can also do with 16 teeth and 4 marks, 2 numbers as with Logan).

If the dial gear is NOT an even multiple of the screw tpi, don't use it. You'll have to just reverse the lathe and not disengage the halfnuts for multiple passes.

Taking up the slack is important too. If you don't allow the leadscrew to take up all the slack in the nuts before starting the cut, things will get messy, with wobbly threads.

And, if there is slack in the screw etc, adjusting straight in for each cut will also get you drunk threads. Advance for each cut with the compound and set it to 29.5 deg... just steeper than 30.

Don't get caught by the angle trap.... you want 29.5 deg off a line perpendicular to the work.... NOT 29.5 deg from axis. You want 60.5 deg from axis. Some machines are calibrated one way, some the other as far as the degree marks.

10-02-2006, 05:45 PM
This sounds like a case of, thread dial needing to be moved slightly sideways on its mounting pin to get mark aligned to the proper point the half nuts actually engage. The marks are a guide, it might engage a 1/16 before or after the stationary mark on the thread dial. Try (without cutting) engaging half nuts a little before or after the stationary mark to see where the half nuts engage smoothly and fully. Don't ride half nut handle, snap it in.

Lew Hartswick
10-02-2006, 05:56 PM
You did put the qc handles in the right place I trust. They aren't in the pix.

10-02-2006, 06:22 PM
YOD, I'm not entirely certain you have the angle set right on the top slide. I am not at home so I can't go look at my lathe but this is how it should be set.


10-02-2006, 06:46 PM
YOD's gears are set correctly (as already stated). With the 20 tooth stud gear he has in place, his QC box will do ALL of the threads on this chart (for the 20 tooth stud column):


The 40 tooth stud gear, used only for the coarsest threads in Row 1 on the chard, is stored on the outside of the leadscrew gear, not in use.

I've got three copies of the SB book from 1939 (too early), 1944 and 1966. None of them show the model A QC chart for some reason. Tony's UK site is a blessing to have.

Your Old Dog
10-02-2006, 06:48 PM
Thanks Evan, you're right. Someone else caught it and I corrected it today. I did have the wrong angel set.

With the 1/2 x13tpi threaded rod in the chuck I hand spun the carriage down to the thread by turning the chuck with the halfnuts AND thread dial engaged. When it got to an even number on the thread dial I stopped. Then, using the cross slide and the compound I rest I adjusted them so that the bit was in the threaded rod where it belonged. Then I started turning the chuck by hand and the bit tracked perfectly.

I don't know for sure but I think what happened before was that on the first pass I did not use the thread dial and then tried to use it for the subsequent passes.

I do have the spuds for the QC in the correct places. It's been checked a couple of times.

I think the feel of the thread dial engaging may be much more fussy than I thought. I'm trying to practice to engage it on a numbered line without it bumping into a gear. Some times I can snap the halfnut in and it goes smoothly and other times jerky.

Someone suggested using the back gear (is that the bull gear?) That might give me the advantage to snap the halfnuts shut at precisely the same spot each time.

Thanks for everyones help. You've been great and I owe you all a big thankyou. I promise to post the first decent threads I get. I hope it's tomorrow or the next day!! I have a million projects in my head that need threaded :D