i must be wrong...threading

[janvanruth]

I have been thinking about the way the leadscrew / halfnuts work.
If i could engage the halfnuts at exactly the same position, reference apron to the bed, there would be only one exact moment the halfnuts
would engage.
That would be the moment the position of the leadscrew was when i engaged the halfnuts the first time.

My thinking is to have a stop to the right of the apron, hold the apron fix to the stop, engage halfnuts.
Thread going towards headstock, disengage halfnuts.
Take the apron to the fixed point and engage the halfnuts when possible.
Only at he exact moment the leadscrew would be in the position it was when it was engaged the first time it would
be possible to engage the halfnuts.

I just cant imagine this procedure to be right, as i know it isnt done, so it must be wrong.
But i dont see the flaw in my way of thinking...

Where do i go wrong???

[Old Hat]

Picture two objects that need to be syncronised, any-time they are brought together.
The point in time, isn't important, but the frequency is crucial.

So you use the thread-dial to reference the frequency, and engage when it is a match-up.
Increment is unimportant.
Any line for even threads, BUT, has to be a multiple for odd threads, so you have only
half the opportunities to re-engage for odd threads.

===========================

Manual transmition!!
You need the gears at the same surface feet per minute to shift gears and re~engage.
It doesn't need to be the same tooth-to-tooth as last time.

That Help?

[Noitoen]

Never tried but, I was told that if you can divide the lead screw pitch by the thread your cutting's pitch with a whole number result, you can engage the half nut anywhere and it will always be correct. This is easier with metric threads.

[Old Hat]

Never tried but, I was told that if you can divide the lead screw pitch by the thread your cutting's pitch with a whole number result,
you can engage the half nut anywhere and it will always be correct. This is easier with metric threads.

Now I have a head-ache!

[RussZHC]

Don't know there is a flaw in the logic, per se BUT you would be, essentially, determining a stop for a single length of screw you were cutting for a single thread pitch/count.
So lets say you wanted a given thread a given length, what happens if you want another screw, same pitch/count but way shorter or considerably longer?
What happens if you want a screw, same length as the first but a different pitch/count?

The implication of the first question is you would need to have this stop as far from the headstock as possible (to accommodate the longest screw said lathe could cut). And all the hassle that would go with that esp if you were winding it back by hand.
The implication of the second question is the position of the stop would have to be moved ever so slightly and then you run into the issue drmico60 points out.

My logic could easily be flawed however...to me what you suggest is more of a reason the set-up with a threading dial works well, you've taken its features and removed them for something that sounds and looks simpler but isn't...

[Richard P Wilson]

The position for re-engaging the half nuts has to synchronise the original position of the leadscrew AND the original position of the spindle/work. Your method only gets you back on the leadscrew, it doesn't even attempt to synchronise with the work. For imperial threads on an imperial lathe or metric threads on a metric lathe, this is the job of the thread dial indicator. For metric threads on an imperial lathe and imperial threads on a metric lathe, even this doesn't work, and the normal method is not to disengage the half nuts, but just to withdraw the tool, and run the carriage back with the lathe in reverse, feed tool in again, and another pass forwards. This is the absolutely fool proof method.
My grandfather told me that when he started as an apprentice, they didn't have thread indicators, so they would make chalk marks on the gear train at the beginning, and only engage the half nuts when all the chalk marks lined up.

[George Seal]

My take on the OP is that I am ass-u-me-ing that he does not have a threading dial. If I stop carriage at point A. My half nuts will engage 8 different positions.

Clear as mud to me

[janvanruth]

The position for re-engaging the half nuts has to synchronise the original position of the leadscrew AND the original position of the spindle/work. Your method only gets you back on the leadscrew, it doesn't even attempt to synchronise with the work. For imperial threads on an imperial lathe or metric threads on a metric lathe, this is the job of the thread dial indicator. For metric threads on an imperial lathe and imperial threads on a metric lathe, even this doesn't work, and the normal method is not to disengage the half nuts, but just to withdraw the tool, and run the carriage back with the lathe in reverse, feed tool in again, and another pass forwards. This is the absolutely fool proof method.
My grandfather told me that when he started as an apprentice, they didn't have thread indicators, so they would make chalk marks on the gear train at the beginning, and only engage the half nuts when all the chalk marks lined up.

right on the money!!!
I knew i was missing something.
Not synchronising the spindle/leadscrew was the flaw.
Your granddad got it right

thanks

[cameron]

Never tried but, I was told that if you can divide the lead screw pitch by the thread your cutting's pitch with a whole number result, you can engage the half nut anywhere and it will always be correct. This is easier with metric threads.

It's just as easy with inch threads. Divide the thread tpi by the leadscrew tpi.

[mixdenny]

The method works just fine as long as you index the spindle as others have pointed out. This is the standard method used on on a Shoptask, as it does not use a threading dial due to the different way it uncouples the leadscrew. The Shoptask makes it easier to do with adjustable stops and a spindle brake/clutch. I don't use the stops since I have a DRO.

Dennis

[Toolguy]

The easy way to avoid problems is to always engage the half nuts with thread dial on the same mark for everything. Once you have done some threading that way, it will be easy to use different marks for different threads. This is with inch leadscrew and inch threads. For metric you still have to leave the half nuts engaged all the time.

[Old Hat]

The easy way to avoid problems is to always engage the half nuts with thread dial on the same mark for everything. Once you have done some threading that way, it will be easy to use different marks for different threads. This is with inch leadscrew and inch threads. For metric you still have to leave the half nuts engaged all the time.

There's some patience required waiting for the God Mark to come around,
BUT he is right, It's the best Way.

[Paul Alciatore]

Actually, you can synchronize metric threads with an English lead screw, it just takes a lot more time waiting for the right point to come around - and if you miss it, you have to wait all over again. And, of course, you would need a really complicated threading indicator: the normal ones just wouldn't work. Certainly not good for a commercial environment.

Leaving the half nuts engaged is simply the cheapest, simplest, and often the fastest way.

The easy way to avoid problems is to always engage the half nuts with thread dial on the same mark for everything. Once you have done some threading that way, it will be easy to use different marks for different threads. This is with inch leadscrew and inch threads. For metric you still have to leave the half nuts engaged all the time.

[PStechPaul]

There was quite a bit of discussion about this in another recent "thread":

http://bbs.homeshopmachinist.net/threads/63943-Single-point-threading-Not-understanding-the-59-5-degrees/page5

There is a very good explanation in an article that was linked there:

http://metalworkingathome.com/?p=11

I still don't totally understand it, however, and at some point I might dig a little more deeply into the mechanics involved. In the case of my lathe, cutting 8 TPI with a 16 TPI leadscrew, the leadscrew rotates at exactly twice the speed of the work, so the correct alignment occurs at every other full rotation of the screw. But my thread dial has 64 teeth and 8 tick marks, and it was too difficult to engage the half-nut at the exact location because the angular rotation was only 5.6 degrees between engagement points and on the small (about 1" diameter) dial these were only about 0.05" apart.

With the same change gears, I can select 12 TPI (0.083" pitch), in which case the carriage and cutting tool will move at 1.33 times the speed of the work. Each rotation of the leadscrew is 1/16 or 0.0625". It seems that what is needed is for the turns counter to move a distance that is the least common denominator of the two pitches, so 4 turns at 0.0625 would be 0.250 while the tool moves three turns which will be 0.0833 * 3 = 0.250 or three thread pitches. Since there are 8 tick marks on the dial, any one of them would be OK, and even half marks would be OK, but there are three other engagement points that are wrong.

At 10 TPI, the ratio is 16/10 or 8/5, so the every 8 turns is at every 5 threads on the work, and thus only the 8 marked ticks will match, and there are 7 incorrect possibilities in between each tick.

At 13 TPI, the ratio is 16/13, so it takes 16 turns of the screw to get the 13th thread, and this will occur at every other tick mark. The manual states that only tick marks 1, 3, 5, and 7 can be used, but I think 2, 4, 6, and 8 are also OK.

It makes sense, then, that at half-pitches like 9-1/2 or 11-1/2, it takes twice as many turns to achieve synchronization, and the chart shows 2 and 6 as the marks to hit, but I think it could also be 1 and 5, 3 and 7, or 4 and 8.

Now this brings me the question of how the threading should be started. Do you need to wait for the indicator dial to hit one of the allowed tick marks to start? I think this is the case, but it seems to be not fully explained in the manual. The manual also suggests that the same tick mark should be used for each pass for best consistency, although I don't think this is really necessary unless the indicator dial or the gears are messed up.

I haven't figured out if the same principles apply to the settings for finer threads. For 16 TPI, same as the leadscrew, obviously any position would be OK. For 32 TPI, each turn is two thread pitches, so again it does not matter. But for 18 TPI, synchronization occurs at 9/8, so it takes 8 turns of the dial and any of the 8 ticks are OK.

For the finest thread range, 32, 40, 48, and 56 are multiples of 2, 2.5, 3, and 3.5, so each 32 and 48 are OK on any alignment, and 40 and 56 are OK on every other. For 36, 44, and 52, I think it is every 4 turns (since they are all evenly divisible by 4), while 38 and 46, evenly divisible by 2, it would be every 8 turns, or every tick.

I think I understand it now, but I think I broke my brain (or sprained it a bit).

[Old Hat]

Another threading thread! Cool.
I took some dramamine, this time.

Let's Do~It!