View Full Version : Another threading question

11-16-2001, 06:45 PM
I've been practicing threading and have learned a lot [ what can one get $ for scrap steel these days ;-)] but have run into trouble at the end of a cut. I don't have a carriage stop, so how do I consistently stop at the end of a pass? I had a beautiful thread going and was doing final cleanup passes when I just bumped the uncut portion of the piece enough to torqued my tool holder just enough to eat the thread on the next pass :-( Sob...Sob...Sob!

11-16-2001, 07:25 PM
Well, the idea is to withdraw the tool at the end of the pass using the cross slide. Unless you're threading to a shoulder, you can shut down at your leisure. Getting the tool out can sometimes be tricky when threading to a shoulder. That's where a foot operated brake is the absolute berries. I sometimes tap the brake to cut power, coast the last couple of turns and then stab the brake just before it hits the shoulder. Watching the long feed dial helps too if your lathe has one. Set it to '0' at the shoulder before starting threading. With a lot of practice, it becomes easy. Undercutting a little at the shoulder is usually acceptable and gives you a gap to run into at the end of the pass.

When you're in doubt of the tool holder moving, just back the cross slide off a little, make a pass as usual, cut POWER somewhere in the middle of the pass and advance the cross slide to check positioning. If it hasn't moved, you'll slip right into the vee perfectly. If you're off, correct with the cross and compound slide until the tool goes to the bottom of the thread. You may need to resquare the tool also. Then reset the dials, back off the cross slide, disengage the half nuts and continue as before. Something like that anyway. :-)
Hope that helps a little.


PS: If your tool starts eating the thread, back that cross slide out fast! Keep spindle speed real slow until you get the procedure down pat.

[This message has been edited by snorman (edited 11-16-2001).]

11-16-2001, 08:04 PM
I've got a 1" travel dial indicator set up on the lathe to measure carriage position. I set the indicator so it reads, say, 0.500" when the carriage is at the end of the cut (i.e. end of the thread). I cut threads in slowest backgear (about 60 rpm), and I find it's fairly easy to watch the needle revolutions go 0, 0.100, 0.200, ...0.500, then:

1) Flip the halfnut lever to disengage the carriage and simultaneously
2) Wind out the tool with the cross slide feedscrew
I find I can stop within a thou or two of the same spot without much trouble.

This assumes you have a threading dial so you don't need to keep the halfnuts engaged to maintain the gearing relationship, of course.

[This message has been edited by SGW (edited 11-16-2001).]

11-16-2001, 11:13 PM
I do it similiar to SGW, with a small difference. With each successive cut, I start the back out/stop motion about an equal amount as the depth of cut. Lots of threading to a shoulder in rebarreling guns. Set up the poor man's indicator for one full revolution with tool bit as close as you dare to the shoulder, motor off. First pass zero, next pass .005" in, cut to 95 on dial, etc. As the tool goes in deeper, the 'v' of it will eventually crash the shoulder if you keep going to the same zero. By stopping a little shorter each time, it compensates for this a bit, and you get a tapered end to the thread up by the shoulder. Make any sense?
Takes some practice, but a nice result.
Oh, one more thing. If that cross slide handle is straight up, and your attention is on the dial, it's easy for that handle to start the wrong way. So set your cross slide zero off TDC, and save some future grief.

[This message has been edited by Gizmo (edited 11-17-2001).]

[This message has been edited by Gizmo (edited 11-17-2001).]

11-17-2001, 12:49 AM
As I heard it put one time, rythym, got to have rythym. Also have heard of making a mark on the chuck and watching it out of the corner of your eye.

If one was to mount a dial indicator on left of carriage, set it so needle starts to move at withdrawal point and then watch for needle to start to move out of corner of eye. An idea I just had.

I just pick out a point on stock at which to back off, mark a point at first with magic marker. When using a boring bar I have often used a piece of masking tape wrapped around to put marks on, or file a little shiny spot with a file. Gotta have a mark of some sort.

I like the dial handle straight down myself, personal preference.

11-17-2001, 11:31 AM
Gizmo...tnx ....always wondered how they got that pretty, smooth ,tapered end of cut on lathe spindle ...simpler in concept than execution....rebarreelling /shldr line reminded me of time i set up a win 52A for son & decided that tighter the hdspc the better.... turned up a .040 ga.& carefully faced shldr off.( only .002 under go)..shot like house afire ..3/4 10 shot group @ 100 yds TIL THE LEFT LUG SEAT CRACKED OFF!!!( was not uncommon w/ early 52's (ALWAYS check before purchase )..the safety went right thru lug seat weakening it... so much for x/tra tight hdspc..too soon oldt und too late schmart!

rmatel ....part of the learning curve ...probably everyone has done it; i have ,,,& probaply will get careless / confused again ....for the record ...for really critical stuff , can turn tool upside down,reverse hdstk direction & feed L to R AWAY from shoulder...real help w/ internal shldr....( screw chuck needs to be firmly in place), can also feed tool in upright position from rear of x/slide. w /practice u will be able to thrd in ist speed ,plain gear, but start w/ 30 or as near as u can get!!!

always wondered why bother w/ x/slide stop..found out45 yrs later when i returned x/slide one whole revolution off!!!finally realized reason my antique Reed had provisions for thrd stop screw was w/ lack of dials & compound, needed to know where last cut was after backing out, & screw was turned to provide x/slide movement for next cut..neat , as is the tilting cross slide allowing u to crank up tool bit to dead center!

while confessing,... few weeks ago,also cut most of an internal thrd w/compound set @30 deg . to spndl axis before noticed...really clever....works ,but i dont look at it when i put the dog plate on!

best wishes

11-17-2001, 05:51 PM
Thanx for the ideas. Unfortunately my slowest speed is 160 rpm and things happen fast. I learned the rhythm of disengaging the halfnut and backing out the crossfeed but the problem was "when". I like the idea of marking the stock with an "action" mark close to the cut 'cuz that's where I'm looking. Similar to docn8as's experience, the first attempts at threading I set my top slide 30* to the x-axis instead of the y-axis. Ended up with a cross between a buttress and a vee thread ;-)
Even better goof, while facing a piece, I set the x-axis power feed instead of the y-axis. Carbide tip "exploded". BTW, is the metal the carbide is brazed to good for anything??

Additional problem, what do you do when you can't use the threading dial and can't disengage the halfnut? My machine "coasts" quite a bit.

[This message has been edited by rmatel (edited 11-17-2001).]

George Hodge
11-17-2001, 06:59 PM
Rmatel,doesn't your lathe have a back gear? My Atlas will get down so slow,I forget where I am in the procedure. Start with the slowest speed you can,it gives you lots of time to react,then work up to a speed you feel comfortable with.

11-17-2001, 07:08 PM
First of all, understand that I'm not very smart but I like playing on this bbs. :-)

Just a few thoughts: 160 rpm is too fast for the "action mark" to be of any use. Boy, that is REALLY fast for a slowest speed. The cross feed should start out just a bit before disengaging the half nuts. Otherwise you'll stop the feed before the tool is off the work. Not good! In practice it's almost simultaneous.

Done that trick facing many times. Eventually you learn to back the tool off and start a trial cut on air. Well, most of the time anyways. :-) As for the tool bit, you could braze another piece of carbide on if you had some. Why bother tho since toolbits are cheap nowadays? Just another learning experience. And another reason to wear eye protection!

Re the dial/halfnut/coast problem: Huh, a real stumper for me. I suppose you mean you don't have a thread dial? Probably a work-around on that but it's beyond me. Probably no brake either. Hm, how about hand turning the spindle for the last few threads? Would that work? It might on light passes if you've got something you can grab ahold of like a chuck. That is if the tool doesn't break when the spindle stops. Possibly you could make a hand crank to fit the end of the spindle. I've seen that somewhere on the web. Have you considered a lathe upgrade? :-) :-)

11-18-2001, 06:49 PM
to George & Snorman,
My lathe is one of those 3 in 1's you guys are always warning people about ;-) so it doesnt have a backgear. Since I spent all my extra $ on accessories ;-) an upgrade is not an option. This is a hobby after all.

12-02-2001, 07:52 AM
Easiest way for me is to stop the lathe, and finish last bit by hand. I turn the chuck with an adjustable wrench. Crude, easy and works. Steve Acker and others have had articles on hand cranks for spindles as well. Acker turns whole thread. You don't have to release half nuts this way either.

Jim H.

12-02-2001, 08:47 PM
A few ideas. One - rhythm and practice. I love to thread, it is my favorite operation, and my students think I must be a thread fiend. All in the rythm, I also thread at 50 to 100 RPM to help cheat the rythm.

I also like the idea about the travel indicator. Works well.

The idea about turning the final few by hand with a wrench - i have done this tactic. My students call it my "caveman" method, but what a great thread.

I also like to put in a thread relief groove when possible, this is the best way when you can. A place to "Park" the tool. I use a calculation to figure this depth out, .613/number of threads, and add .0025 - this is straight on "One side reading" depth for those dasterdly .001 = .002 on the diameter lathes. Double this formula for .001 = .001 lathes. The groove also allows for a shoulder fit up tight. You can use a parting tool to make this groove, but I personally like a .030 to .060 radius tool for mine for appearance.

12-03-2001, 05:39 PM
I read about the groove in a discussion of left handed threads ( which is what my 1st non-practice thread will be once I get the courage to risk the piece. It worked for me. Thanks for the formula I'll give it a try. What I really need now is a "trigger" to initiate the final sequence rythym. Unfortunately my "asian" doesn't go slower than 160 rpm.

12-03-2001, 06:53 PM
The grove works well for both RH and left hand threads. I use the groove in teaching newbies, and in applications needing a fit to shoulder, or even on shafts.

I am also going to impart some formulas to you here, and some information for your consideration. The first being the one I told you above for groove depth - .613 / (divided by) the number of threads per inch =.0025 for the depth of thread relief groove. This is from the cross feed handle, and is good for a .001 on the crossfeed = .002 on the diameter lathe. Double the answer for a .001 = .001 diameter lathe.

The .613 / n formula is actually the thread theoretical height formula, the .0025 added is to allow for the tool to hit the groove without cutting the groove.

Here is more information for you. How deep do you feed the compound rest when threadding? I use the formula .708 / n (divided by the no. of thds per inch) for this figure. This is for a UN series thread, the depth being cut by moving the compound handle. You will find this works very well, and gets you to high limit of pitch diameter almost every time. I rarely have to re-cut when using this formula - watch your backlash on the compound rest though, or this formula is just trivia and will not work.

Final information. Start your thread right by using the right major diameter to start with. A rule of thumb is -.005 fom the nominal major diameter, but to be more precise, you can use charts from machinery's handbook to figure actual major diameter high and low limits. I use the charts whenever possible, and cut to "BASIC". This helps in final fit, and helps all items fit together right without having to monkey, sand, and re-cut. Also helps eliminate that nasty thread burr, and gives that nice "flat" on top of threads that look so professional.

want more? write me, un threads are my favorite thing to do in turning.

12-06-2001, 04:22 PM
threading toward the headstock / to a shoulder will surely test your hand/eye coordination. it will also be necessary to reduce spindle speed to the point where the finished thread looks like a blind blacksmith cut it with a dull cold chisel.
try mounting your threading tool upside down, (the point of the cutting face on center, of course), run the headstock spindle in reverse, and feed toward the tailstock. you will be able to thread at higher rpm because you can disconect the half nuts at your convenience. the only limitation is a speed slow enough to permit hitting the proper line on your threading dial. otherwise you can run the threading tool until you push the tailstock off the end of the bed (forget that. not a good idea). but you will get better finishes, and you won't have to stop breathing until the threading job is done.
i don't know what kind of a threading tool or tool holder you are using, but remember that if you are not using a zero raked tool, held at zero rake you will have to compensate for any rake angle with a corrected angle for the 60 degree form. hope you don't have too much trouble reading this, i'm a toolmaker , not a typist. it took me longer to type this than to cut the thread. By the way, you can use a similar method to cut an internal thread. you must use an internal left hand threading tool. if you dont want to grind one, micro 100's ILT's do well. use with cutting edge up ,point will be facing away from you, and thread to the tailstock. - Added bonus with the external thread the chips shoot right down into the chip pan.

[This message has been edited by pgc (edited 12-06-2001).]

12-06-2001, 04:38 PM
threading toward the headstock

[This message has been edited by pgc (edited 12-06-2001).]

12-07-2001, 12:32 AM

If your motor is 3 phase, consider a VFD to supply it. With one you can dial any speed you like starting at zero RPM. I use one with my SB 10" and the only thing I use the backgear handle for now is to lock the spindle when changing chucks.

12-07-2001, 02:15 AM

The groove is a standard machining practice and is normally cut to just below the threads maximum depth. This groove is essential for internal threads - especially in holes with shoulders or blind holes. Makes life easier and looks neat!

A radiused groove is not a bad idea - less likely to cause a stress riser on a smaller part.


12-11-2001, 06:10 AM
i too have this same problem..in one of the home shop machinist back issues it tells how to make a threading dial ...with out it you have a long hair pulling process . My old mentor told me once how to do it with out one, but i have never tried it ....ifelt the dial add on would help save my folicals....I have seen a threading dial as a add on for the sherline but i dont know what size lead screw they use it was around $110.00 You might be able to get/make the right gear and retro fit it....im like the others i like a very slow thread speed but i got your problem with this machine....


<font face="Verdana, Arial" size="2">Originally posted by rmatel:
to George & Snorman,
My lathe is one of those 3 in 1's you guys are always warning people about ;-) so it doesnt have a backgear. Since I spent all my extra $ on accessories ;-) an upgrade is not an option. This is a hobby after all.

[This message has been edited by wulfesinger67 (edited 12-11-2001).]

12-16-2001, 10:29 PM
I have never used any system other than the rythm method. So I say unto all you who would call yourselves machinists that is the only way to do it. Take a chunk of steel or whatever and practice stopping in the same place. Cut it off and start over and over until you get the feel of it. Once you got it you will never loose it. The other day I cut a 10/24 thread on a 3/8 brass rod. Thread was 1/4" long. I always make my undercut after finishing the thread, that way you always have a clean shoulder. Practice on an open piece of steel. Run it at about 60 RPM to start. If I had even attempted to use any of the systems you guys describe when I started out in the 50's my forman, a one eyed bastard would have had me on the bus fast! Things were tough in those days, no backtalk, just do it. Needless to say, after 3 years in that sweat shop I moved on to a more humane enviroment. But I certainly learned what makes a tradesman. I hope this will not offend anyone, it's just that I know the satisfaction you will get by mastering this technik.

12-17-2001, 08:35 PM
If your lathe won,t go slower than 160 RPM try mounting a jack shaft where the moter is, and hang the moter below, above, or behind. Use two pullies on moter and jack shaft 1-1 and 4 or 6-1, or 3 or 4 step pullies,one each of same size. For just a 2 speed set up pully seze table can be figured out from a formula in The Machinists Beadside Reader #1.

12-18-2001, 08:24 PM
I had the satan of all apprenticeship heads. He taught the rhythm method until I thought he was a crazy maniac. Problem was, I got it the first time. Had to grind our tools, and only once in the time I was apprenticing did I snap the tip of the thread tool, and that was when a girl I had eyes on walked upon me working on a thread and happened to want to watch. I was flustered, a bit "macho" and such. Problem, satans father was also watching. I nicked the tip, first time in three years, and boy did he let me have it both barrels with all the kings nglish and swedish words I had never heard before of since (asked my swede grandfather about one of two, he got out a bar of soap and tried to chase me down to wash out my mouth) in front of all who would listen. This was 24 years ago, he was 63 at the time, and by god he is still alive. Going to visit him this upcoming week, probably going to call me a few names when I see him - will feel welcome when he does. He still runs a shop, and is a miserable bastard to all, but boy do his apprentices get good job and pay. I know this first hand.

Bottom line, there are many ways. I like the grooves because it allows for a flush fit on the shoulder to the part, and a lock. I also like it because I can make a round groove (radiused bottom) because it actually adds strength over the cut to the end - proved it with many tests in class. BUT, for pure and solid satisfaction, and the thrill of craftsmanship, the rhythm method is the only way to go!!!!

Larry Miller
12-18-2001, 08:43 PM
When threading rifle barrels I mount a travel indacator left of the carriage. Zero it so when thread is full depth it won't hit the shoulder. Just kick the half nut out on zero & let her run a few turns. When thred is done you have a nice groove at the end.