Thread cutting question

I have done a fair amount of wood turning but have only a little experience with metal lathe work. One aspect of cutting threads leaves me a bit puzzled. Small lathes like the ShopFox or MicroLux 6"x10" do provide the gearing for a variety of TPI. However they do not have the thread dial for aiding in knowing when to engage the carriage movement. So what is the preferred method to align the multiple cutting passes on a lathe without the thread dial? John

On Wed, 11 Apr 2012 21:53:12 -0400, John wrote:
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1: Set up your cut. 2: cuss 3: rotate the lathe, _by hand_ the requisite number of turns. DO NOT disengage the half nut (as if you had one) 4: cuss 5: rotate the lathe backwards until you're at your starting point Don't crash the cross-slide into the tail post, unless you like cussing and buying new gears. 6: advance the compound a bit. 7: repeat steps 2 through 6 as necessary to finish your cut.

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OR... find a gear that mates with your lathe's lead screw and has an even multiple in teeth to the pitch of the lead screw. (say, if your lead screw were 10tpi, then a handy tooth count would be 20 teeth.)
Then take a couple of hours off, and MAKE a threading dial.
LLoyd

"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message
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They certainly are handy, but I think thread-cutting has enough pitfalls that a beginner should keep the lead screw engaged and clean and oil the thread while waiting for the bit to return.
jsw

On Thu, 12 Apr 2012 08:51:59 -0400, "Jim Wilkins"
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I must still be a beginner. I find it easier and F-up proof to just engage once. Stop, back off, and reverse lathe at end of thread. I do have a rocker on/off/reverse lathe switch to make this fast. Set carriage to a "0" so you can just give it a whip to back off, then back to "0" before next cut. Move compound for more cut depth.
This took longer to write than it does to do one cycle.
Karl

On Thu, 12 Apr 2012 10:23:55 -0500, Karl Townsend wrote:
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Hey! I have a rocker switch on my lathe, too. Sitting on the bench right by it is a back gear with three teeth broken out, to remind me not to use it when I'm threading in a tight space...
One of my back-burner projects is to build a crank for the spindle, so that when I have threading to do I can just attach it and do the whole hand-threading thing easily. Of course, it'll also be a mechanism to break arms or hands if I forget and leave it on when I then turn the motor on. Life is full of compromises.

All I can say to all the above is: WUSS!
C'mon! I learned to thread with a dial in the first two days I owned a lathe! (That was back in 1971)
I cannot (_CAN_NOT_) understand why anyone would waste time, risk breaking tools, risk buggering up a threaded piece, and expend all that _effort_ on doing a simple single-point thread by "backing".
The _only_ place that has, even in amateur machining, is doing some odd- ball irrational-fraction thread that cannot be accommodated by a dial.
F'gosh sakes! Make (or buy) a threading dial, do it right, and do it fast. Threading isn't a hobby, it's just one thing to do to enjoy your hobby. Why not enjoy it?
LLoyd

Lloyd E. Sponenburgh wrote:
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My Harrison M300 requires that you keep the half nuts engaged when doing metric threading and it is an inch lead screw machine or vice versa. It has a universal thread cutting gearbox so can cut metric and inch threads without any additional gearing as it is built in but you have to be aware of the caveat with the pitches. Personally I've always used a threading dial but my neighbour has not and now I find backing is often easier/quicker than the threading dial as the Harrison is 3 phase and can be reversed easily. Having done both I think it more likely that one would bugger a thread by using a threading dial than "backing" but of course both procedures require some understanding of the machine operation and getting that wrong will screw the result, no pun intended.

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Yeah... well, I was being over-enthusiastic with the term "irrational fraction". Almost any metric thread on an English screw will be irrational, and require backing.
But again... why would _anyone_ use that method unless it was absolutely required?
Necessity isn't the "mother of invention"; laziness is!
Lloyd

On Thu, 12 Apr 2012 19:14:01 -0500, Lloyd E. Sponenburgh wrote:
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I have no half nuts, I'd have to make that, too.
Besides, most of the threading I do is short, fine, and comes to within a thread pitch of some wall or another, so hand-cranking makes oodles of sense to me.

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    :-)
    So -- make a storage place for the hand crank which is connected to an interlock switch, so if the hand crank is not in that storage place, you *can't* start the motor.
    Enjoy,         DoN.

In rec.crafts.metalworking, you wrote:
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Depends on how the chuck attaches to the spindle nose, and the motor.
If it is bolted on, or you have one of the various fancy noses (D-series camlock, L-00, or similar) you can do that.
If the chuck screws onto the spindle nose, you risk unscrewing it under power when you reverse.
And -- if the motor is a capacitor start induction motor, and you switch quickly from forward to reverse (or vice versa), it is far more likely to keep running the same direction it was already going. Such motors require the motor to slow down far enough for the centrifugal switch to click closed so the starting winding can force it to run in the other direction. Otherwise, you change the polarity of the starting winding, but the centrifugal switch keeps it disengaged, so it does nothing to reverse the spindle.
Also -- if you succeed at this frequently, you are overheating the start capacitor, and likely to need a new one when the "magic smoke" escapes.
If the motor is a DC one, or a three phase one, or three phase under the control of a VFD, yes you can switch to reverse at will.
So -- it is a full case of "it depends" whether your approach will work for someone else.
I'm planning to substitute a three phase motor for the current centrifugal one in my 12x24" Clausing lathe for this very reason. I've got a L-00 spindle nose, so the chuck won't unscrew, but the current motor won't reverse like that. :-)
Good Luck, DoN.

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Good points, DoN. I'm lucky to have a D1-3 nose with 3 phase and VFD.
I'm curious, I can't engage my half nut with the lathe running. Have to line it up and rock the carriage slightly to drop it in. No big deal for me, but it is a big reason I just do it once each time I thread. Will other lathes let you engage on the fly? But then I'd have to remember which thread I was doing. Thats starting to get hard for an old guy.
Karl

On Fri, 13 Apr 2012 05:00:55 -0500, Karl Townsend
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My old Delta allows engaging on the fly. I thought that was common, but it's the only lathe I've ever used. Once I did an internal thread, 5tpi. For that, I threaded away from the headstock, cutting on the backside, spindle in reverse, compound angled toward tailstock. Engaging on the fly with the threading dial certainly made that easier.
The Delta has a threading stop on the cross slide which enables you to make a quick withdrawal and return when engaged. Very handy.
Pete Keillor

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I didn't find a threading dial for my South Bend until a few years ago.
The half nuts engage easily while running, though for the short, odd pitch threads (1/2-13, 5/8-11) to a shoulder that I cut in low-speed back gear there's little time difference between backing the carriage with the leadscrew and waiting for the one proper line to come around to the index. If I move the carriage to align the dial the tool may be further away than the length of the thread. I can clean, examine and oil the cut and advance the compound while it's backing, but not while watching the dial.
OTOH the dial was very helpful for threading a 1/2"-12 horizontal mill drawbar with a long thread that ended at a wide groove.
My lathe was missing the threading chart, so I made one up in Excel and added the 29 degree infeed distance and the threading dial rule for each pitch.
jsw

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    The best setup, then.
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    Hmm ... half nuts will only engage when the threads of the leadscrew line up with those in the half nuts. This normally happens when the threading dial is at one of the marks (any one, though for many threads, you need to restrict the selection of numbers. IIRC, yours is a metric lathe, not imperial, and the rules for those often have multiple threading dial gears -- often on a spline which slides up and down to select what you want. Which gear you use depends on which threads.
    So -- first check is to see whether the index plate on the dial is properly set. With the pickup gear engaging the leadscrew, and the spindle stopped, crank the carriage until an index mark lines up with the reference one on the housing of the dial. Then check whether the half nuts will close. (Note that they should be interlocked with the feed system in the apron so you can't engage both the feed and the half nuts at the same time.
    Assuming that the half nuts engage on a line -- you are fine. If they only engage somewhere between the lines, you need to loosen the screw holding the dial to the top of the gear shaft, and rotate it until a line is in position and re-tighten it.
    At this point -- it should be possible to close the half-nuts at *any* line (though not the proper choice for all threads, of course).
    Normally, I start to close the half nuts just before the desired index line comes up, and it will resist until the line is in position, and then just drop in.
    And if I'm too lazy to look it up, I will simply close always at '1' on the dial. (If the Imperial thread being cut is a multiple of the pitch of the lead screw, say 16, 32 and 48 for example on an 8-pitch leadscrew, then you can close it on any line.) For Metric, you have to make sure that the proper pickup gear for the thread you are cutting is engaged on the threading dial. This information should be on a data plate on the lathe -- either on (or near) the threading select controls or on the threading dial housing itself.
    Good Luck,         DoN.

If you're comparing/shopping for a light duty metal lathe, consider the overall characteristics of the 7x10/12/14" mini lathe models to see if a 7x would fufill your needs.. and they generally have a threading dial. The 7x minis are widely used by a large following of DIY-HSM types with many modifications and upgrade improvements easily found online.
The Shop Fox and Microlux 6x10 models are micro models which will have some limitations and workpiece size restrictions which may become disappointing in time. The basic versions are lacking a compound slide and don't appear to have a method to disengage the carriage from the leadscrew (handwheel on right end).
The basic threading method of make a cutting pass, then back out the cross feed, reverse/back up to a starting point.. has already been mentioned, and is widely used by many small lathe owners.
There are other aspects, such as always resetting everything so gear and feedscrew backlash doesn't cause mistakes.. although backlash is a main consideration when using most metalworking machines. These setting adjustments aren't complicated, just a matter of always keeping in mind that the settings are made after the backlash has been "taken up" or passed beyond.
So after the workpiece diameter has been turned to the correct size, the cross slide dial is typically set to zero with the tip of the thread cutting tool just skimming the area to be threaded. The compound slide, preset for an appropriate angle (for SAE, Acme or British threads etc) will also be adjusted past the backlash point, and it's dial is typically also adjusted to zero.
The apron leadscrew feed is engaged at a point beyond where the actual thread cutting is to begin, so that as the spindle power is applied (by motor or hand crank) the backlash in the gear train will be taken up before the thread cutting tip contacts the thread area on the workpiece.
The zero setting on the cross slide dial makes it easy to return to the same in-feed position after the cross slide is backed out far enough to clear the thread's cut depth, when returning/reversing the leadscrew to carry the carriage to the next starting point.
The zero setting on the compound slide makes it easy to see how far the cutting tool tip has been advanced into the workpiece.
The threading dial indication relative to the point of engagement for the carriage feed will differ depending upon the pitch of the desired thread, and the specs of the leadscrew and the thread indicator's gear.
FWIW, lathes utilizing change gears are generally capable of cutting more thread pitches than the user manual or supplied thread charts will show.

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    I'm not familiar with those particular lathes. If the lathe has the normal half-nuts, your best bet is to keep the half nuts engaged the whole time, and crank the spindle by hand to avoid the risk of running too far in the thread on one pass or another.
    However, *some* lathes instead have a dog clutch between the gears and the leadscrew -- one which only engages in one position, so with that, you can close it when you want -- it won't start driving until the gears line up with the leadscrew.
    And others may have something like that, but engage at one of four positions. On a ShopTask, I've seen a four LED counter to tell you which position to engage the dog clutch in.
    The best bet, if you can find a group which uses you particular model of lathe, to find out what they do.
    Good Luck,         DoN.

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Rotate the chuck by hand till one of the jaws is pointing straight up, mark that jaw with felt tipped marker and then engage the half nut....next, make another mark on the bed and then scribe a line on it so that you can always return your carriage to the exact same spot.....after each pass, disengage the half nut, rotate the chuck by hand so that your marked jaw is again pointing up, move the carriage back to your scribed line, and finally, re-engage the half nut....

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A lot of food for thought. Since I just received the unit it will be a while until I have it set up and try any of the suggestions. This lathe does bolt the chuck on so danger of coming loose in reverse. It uses a clutch. It may be necessary to turn the speed down to 0 before reversing. It is a variable speed DC drive. John