Threading with lathe

Surprise, once I return the carriage to the beginning of the screw, I can't
register the cutting tool on the next pass with my first cut.
This is on my new old lathe (Reed Prentice, 100 yrs. old), the Monarch at my
last job had a nifty indicator just to find the proper moment to engage the
screw, but what does one do without that.
And there must be a decent way cause they had screws 100 yrs.
ago.
Help Uwe
Reply to
Jaggy Taggy
Loading thread data ...
Worst case you can leave the half nuts engaged and put the lathe in reverse to wind the apron back for the next pass. If you don't have a threading dial it is possible to make one. Google tells all. - GWE
Reply to
Grant Erwin
They had thread dial indicators like that 100 years ago!
Seriously, without a thread dial indicator, you need to leave the half nuts engaged and reverse the spindle to bring the carriage back to the start point.
There is a very neat article in HSM magazine describing how to fabricate a thread dial indicator for nearly any machine, it ran about two years ago or so. Maybe another poster has the reference?
Jim
Reply to
jim rozen
Don't disengage the half nuts, back the lathe up and take the next cut.
Reply to
Nick Hull
There is a way to do it without a thread dial indicator *and* be able to disengage the half nuts. Once all is set up to make the first pass position the carriage where you want to start (LATHE OFF). Pull the lead screw to the LEFT by hand to cancel out any end play. Engage the half nuts (hand rotate the chuck until they drop easy). Position a carriage stop against the right of the carriage. Make a register mark on the chuck/workpiece/or whatever (top, side or whatever is convenient). Make your first pass (just a scratch pass to test the setup). Disengage and return the carriage to the stop. Lathe off now. Again pull the lead screw to the left.. Hand turn the chuck (normal direction) until the register mark lines up in the original postion. At this point the half nuts should engage. If not rock the chuck back and forth slightly until the half nuts drop easily into engagement. All should be well now to resume threading. Do another (or several more) scratch cuts to verify. For left hand threads reverse left/right hand instructions.
Hope this helps. Ken.
Reply to
Ken Davey
All the other posters are correct: without a threading dial, you will have to leave your half-nuts engaged throughout the entire threading operation; reversing the spindle between cuts to back the carriage up to the starting point for the next cut. (Btw, you must also use this procedure if cutting metric threads on a lathe with an inch-calibrated threading dial.)
However, they failed to point out one critical procedure. (I'm sure they assumed you knew this, and you probably do.) Make sure you completely disengage your cutting tool from the workpiece thread you're cutting (by backing off the cross slide or compound) before reversing the spindle. It might seem that this would be unnecessary since reversing the spindle (with half-nuts engaged) should cause the tool to follow the path of the previously cut thread backwards to the starting point. The problem is that tolerances in the lathe drive train will result in the reverse path not coinciding exactly with the forward (cutting) path. If you fail to withdraw the cutting tool from the work, then at best you will bugger the thread you are trying to cut; at worst, you will bugger the thread AND you will damage/break your cutting tool.
Regards, Michael
Reply to
DeepDiver
Good point, except for one minor detail: Do not back off the cross slide. Do all the backing and feeding with the compound. Feeding with the cross slide will usually result in the point being broken off the tool...
Jerry
Reply to
Jerry Foster
No, one uses the cross slide for that, and feeds successive passes with the compound. By setting the cross slide dial at 0 you have a ready reference to which you return each time. It's not only faster, but easier. That's the way threads are properly chased. By the way, it's a very good idea to set your cross slide such that the handle registers at roughly 10:00 and then set your 0 reference. That way if you're chasing a thread that requires a rapid withdrawal of your tool (such as chasing to a shoulder) it's very easy to withdraw the tool. After setting your cross slide and dial, pick up the item to be threaded by feeding the compound until the tool makes contact.
Harold
Reply to
Harold and Susan Vordos
Yep. It's a sort of bicycle pedal thing. You can find a spot to put the handle such that an applied force can get it going in either direction, with about as much effort.
Not that I've ever *done* that of course. But it sure does make a mess when it happens!!
IIRC, there is one case where one does not need to use a threading dial - that is when one is cutting the same tpi as the lead screw. I think you can close the half nuts at any time then, and the tool will pick up properly.
Jim
Reply to
jim rozen
The only way I know is to back the tool out, and run it in reverse to get it back to your starting point - without disengaging the carriage from the leadscrew, of course. If there's another way, I'm looking forward to learning it.
Dave Hinz
Reply to
Dave Hinz
This won't always work. Imagine cutting an 11 TPI thread on a machine having an 8 TPI leadscrew. Let's say you cut exactly 5 threads, having progressed axially 5/11 inch. The leadscrew will have turned 5/11 * 8 turns or 3-7/11 turns. The register mark on the workpiece is in the right place but the leadscrew won't line up with the halfnut when you return the carriage to the start position.
A method that would work is to use a 1" dial indicator on the carriage and have it always travel exactly 1 inch (or an integral number of inches) even if it isn't cutting the whole way. For threads divisible by 2 it could be an integral number of half inches, for threads divisible by 4 it could be an integral number of 1/4 inches, and so on.
Reply to
Don Foreman
I figure that when copious spare time becomes available (yeah, well...) making a threading dial for add-on to the old FE Reed is a project well worth trying. I suppose one might be able to bastardize one from another lathe with the same pitch leadscrew, but that would take a pretty good junkyard.
Reply to
Ecnerwal
That HSM article I mentioned had the information needed to hob your own gear for a thread dial.
Jim
Reply to
jim rozen
BTW, the fractional inch trick only works if the thread pitch is divisible by a factor of the leadscrew pitch. For an 8 TPI leadscrew the factors are 2, 4 and 8. If you had something wierd like a 9-TPI leadscrew, then you could cut thread pitches divisible by 3 by moving in increments of 1/3 inch.
When cutting metric threads with a TPI leadscrew or inch threads with a metric leadscrew, the only course is to leave the halfnuts engaged from start to finish.
Reply to
Don Foreman
There is one other way, using a single point engagement dog clutch like hardinge uses on the HLVHs.
I'm pretty sure that inch/metric HLVHs have a single leadscrew, and it's english. And the threading is carried out in the same way in either inch, or metric mode.
Basically the leadscrew phase is always maintained when its feed clutch is released. When it closes back up, the spindle and leadscrew phase is exactly the same each time.
Jim
Reply to
jim rozen
On a HLV, the half nuts are never opened during threading.
You power forwards and backwards. Rather ingenious, very quick, bit hard to get used to if you have used the normal style.
Gunner
Rule #35 "That which does not kill you, has made a huge tactical error"
Reply to
Gunner
My old F.E. Reed had no threading dial, either. But it's a simple affair to make, and add to the machine's capabilities, with no mods other than two tapped holes in the right side of the apron.
LLoyd
Reply to
Lloyd E. Sponenburgh
The half-nuts are always closed, right. But the top lever actually unclutches the leadscrew from the spindle! Think about it, the spindle is turning, and the leadscrew is stationary when the lever is straight up.
The only way I could do that with my southbend (halfnuts closed of course) is to put the reverse tumbler in the center postion. Once I do that, I've lost the thread lead, if I re-engage it again the thread dial won't be right. The gears can engage at any random point besides the point where they were the first time.
But because the hardinge has a *single*point* engagement clutch on the shaft that feeds the gearbox it can only drop in in one position - which preserves the relative phase between leadscrew and spindle. Ingenious. The only other lathe that I've ever seen do that, was a round bed Drummond, I think.
I always wondered how hard it would be to retrofit a system like that to my south bend.
It was clear that hardinge bros figured out that threading was a 'big deal' and designed their machines to do it well. Even their very old toolroom lathes (like that one that sold on ebay recently) had the quick-retract for the compound slide, so that threads could be done up near a shoulder easily.
Jim
Reply to
jim rozen
I'm intrigued by this concept (especially since I cut mostly metric threads)! Two questions:
1. Does it matter where in the leadscrew power train that the single-point engagement clutch is located? For example, does it need to be at the input (where the spindle drives the gear train), or could it be at the output (where the QC gearbox drives the leadscrew), or perhaps even somewhere in between?
2. After you disengage the clutch (spindle still turning, but leadscrew stationary) how do you reset the carriage position since the half nuts are kept engaged? I'm guessing there is some additional reverse drive mechanism for the leadscrew. Do you just throw the tumbler into reverse and re-engage the single-point clutch? The disadvantage there is that the reverse motion of the carriage would be no faster than the forward. Or is there a higher-speed reverse driven by the spindle or, perhaps, by an electric motor?
Regards, Michael
Reply to
DeepDiver

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