Another threading question

[rmatel]

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!

[snorman]

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.

Steve

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).]

[SGW]

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).]

[Gizmo]

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).]

[halfnut]

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.

[docn8as]

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
docn8as

[rmatel]

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]

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.

[snorman]

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? :-) :-)

[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.
bob