# Power Feed to Make Threads (English or metric threads) Curiosity



## B2 (Feb 5, 2022)

Hi Folks,

I am just curious.  Does anyone use the Power Feed feature on their lathe to do threading?  You might ask why bother, but I am curious for a "Universal" Threading chart/spread sheet I have been investigating.    What are the issues and problems?  Is it ever useful?  Clearly you give up the thread dial as you are not using the lead screw. There may also be some looseness or backlash, but is this really an issue for fine threads where you have gotten a running start?  Clearly an "Electronic Lead Screw" would make this a moot point, but if you do not have one....

By "Universal" I mean any of a large number of lathes. The thread chart would include all of the change gears etc in all combinations.  Clearly on many lathes there are far more possible "custom" threads that could be made.  Does anyone make custom threads or ever run into odd ball threads that they need to make?  If and when I finish it I will post it for folks and I am trying to decide if I should include the additional threads that you could possibly make using the Power Feed rather than the lead screw  1/2 nut.  

Dave L.


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## matthewsx (Feb 5, 2022)

I’m probably the last person who should answer this but I think you wouldn’t end up with anything repeatable. That’s the point of a lead screw, to be physically linked to the spindle.

For custom threads a lot of folks are happy with electronic lead screw conversions, look at the Clough42 project. It’s a fair bit of work but for machines where the change gears are missing or just a PITA probably worth it.

I’m looking forward to other answers here but just don’t see how simply power feeding would work for threading. 

John


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## RJSakowski (Feb 5, 2022)

The Clough42 ELS is great and I really like it.  One shortcoming is that the feed table for power feed and for treads has preset values.  You can cut a feed or thread that isn't in the list but it involves going back into the source code, modifying it to include your desired feed or thread and reflashing the Launchpad.  A feature that I wold really like to see added is to be able to jog the feed rate or thread pitch via the user interface.  There is an unused button on the user interface that could be used to toggle a jog mode and use the + and - buttons to jog.  It is a fair amount of modification of the source code and I am not familiar enough with it to do so (or I would have already  )

As to the need for this, winding springs or electronic coils would be the obvious ones.  I could also see where a custom thread might be desired.


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## B2 (Feb 5, 2022)

Thanks.  I am a ware of Clough42 and may some day build one, but not yet.  Anyway, the Power feed rod is "physically connected" to the spindle via the power feed rod in a predictable manner, so I am a little confused by your  phrase. 


matthewsx said:


> physically linked to the spindle.



At least on my PM lathe and others that I am familiar with the power feed rod is the one below the lead screw which has a slot in it running its length.  The Apron attaches to this rod via the power feed lever but uses a gear box at the apron to move the apron along the lathe bed.


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## B2 (Feb 5, 2022)

RJSakowski said:


> jog mode and use the + and - buttons to jog


Since I do not have and ELS yet, I am not for sure what you are getting at in your quote.  My lathe, with my VFD conversion has a nice spindle jog and so would move the threading tool along at a slow pace.   Maybe you mean at a fast pace?  Or maybe this + and- is a feature on the ELS for changing the thread number?    Do you ever cut threads using the Power Feed on the apron?


B2 said:


> VFD conversion using solid state electronic components


However, I am not for sure what I would use it for, but I have thought that being able to cut a spiral where the pitch is changing with distance along the bed.... might be interesting.   I am thinking toys.   I suppose a ELS could be reprogrammed to do so.  Or maybe one should just have an entire CNC lathe!?


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## matthewsx (Feb 5, 2022)

Sorry, probably should have “geared to“.

The gear in the apron which drives the feed screw does move the carriage at a consistent speed but with backlash it’ll be hard to get back to the starting point for the thread and it wouldn’t be as consistent as direct gearing through the lead screw.

I might be wrong about this because I’m a total novice with threading but if someone has done it successfully done it I’m sure they’ll chime in here.

John


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## B2 (Feb 5, 2022)

By the way, I do use my lathes to wind coils.  I find that I usually do this at extremely slow speeds so as to keep the wire nice and neat.  But I usually guide the wire by hand.


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## B2 (Feb 5, 2022)

matthewsx said:


> Sorry, probably should have “geared to“.


So on the PM1440GT and the PM1340GT the feed rod is geared from the spindle.  It is connected to the knobs that control the 2x factors on the threading gear box.  So its number of possible speeds does not include all of the levers that select the fundamental TPI  values.  but is is also varies by the possibilities of the change gears.  So there are a lot of them.  In the PM1440GT there are 4 sets of factors of two plus the WXYZ threading settings, but is not connected to the final set of threading gears, PQR(I)T.  In the 1340 I think there are 5 factors of two but because it has a Norton style gear box and I am guessing that these are not connected to the feed rod  it so it would be more limited in its possible feed rates.  By the way, the best I can tell the feed rates provided on the 1340 face plate are inaccurate and inconsistent with the factors of 2.


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## RJSakowski (Feb 5, 2022)

B2 said:


> Since I do not have and ELS yet, I am not for sure what you are getting at in your quote.  My lathe, with my VFD conversion has a nice spindle jog and so would move the threading tool along at a slow pace.   Maybe you mean at a fast pace?  Or maybe this + and- is a feature on the ELS for changing the thread number?    Do you ever cut threads using the Power Feed on the apron?
> 
> However, I am not for sure what I would use it for, but I have thought that being able to cut a spiral where the pitch is changing with distance along the bed.... might be interesting.   I am thinking toys.   I suppose a ELS could be reprogrammed to do so.  Or maybe one should just have an entire CNC lathe!?


The jog refers to setting the lead screw speed and is separate from the spindle rotation.  The ELS has no control over the spindle. It just monitors the spindle rotation via the encoder and syncs. the lead screw to the spindle, eliminating the gear train.


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## B2 (Feb 5, 2022)

On my old South Bend 10 there is no separate power feed rod.  Just the lead screw.  So you do feeding and thread cutting are the same function and use the 1/2 nut for both.  However, the Norton gear box is fantastic.   It has 10 fundamental thread gear positions, but 7 factors of two.  So there are both lots of threads up into the several hundred TPI values and corresponding sloooow feeds.


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## RJSakowski (Feb 5, 2022)

B2 said:


> By the way, I do use my lathes to wind coils.  I find that I usually do this at extremely slow speeds so as to keep the wire nice and neat.  But I usually guide the wire by hand.


I wind coils by hand as well.  Sometimes though, you want to lay down a coil that doesn't have close spaced windings.  It also would be useful if you wanted to wind non overlapping, close spaced windings in multiple layers.


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## JimDawson (Feb 5, 2022)

Sounds like its time for a CNC lathe conversion   

Extra credit:  Convert a manual lathe without disturbing any of the manual functions and be able to switch between CNC/manual operation in under a minute.


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## 7milesup (Feb 5, 2022)

With using the power feed, wouldn't you have to leave it engaged, stop the lathe, reverse and repeat?   Not sure what you would be gaining, but then again I am a complete novice when it comes to threading.


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## Christianstark (Feb 5, 2022)

I’m not sure, but I do not believe the straight gear teeth on my 1340 lathe bed are involute. I would think that may be enough to ruin some finer threads, but I’m a novice.


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## mikey (Feb 5, 2022)

Wait ... what? You want to use power feed instead of the leadscrew to cut threads ... why? The gearing for thread cutting is tied to the gearing on the *leadscrew* to give you the correct pitch. While the power feed is also tied to the gearing, *the ratio is not the same as the leadscrew* so you will not have accurate pitch results. Moreover, the thread dial indicator does not function if the leadscrew is not used so how are you going to time your cuts?

I must be missing something here so color me dumb but I don't get where you're going with this.


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## cwilliamrose (Feb 5, 2022)

I have done odd ball threads on my Emco V13 lathe. The plate on the machine only addresses the most common pitches. I made a spreadsheet to search out other possibilities. There are a couple of customizible tools available to do the search using computer power rather than patience and a bunch of manual iterations. This started for me needing to cut 3.3333 TPI threads which was not on the plate but was possible with the right combination of change gears.


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## mikey (Feb 5, 2022)

cwilliamrose said:


> I have done odd ball threads on my Emco V13 lathe. The plate on the machine only addresses the most common pitches. I made a spreadsheet to search out other possibilities. There are a couple of customizible tools available to do the search using computer power rather than patience and a bunch of manual iterations. This started for me needing to cut 3.3333 TPI threads which was not on the plate but was possible with the right combination of change gears.


Yes, many threds can be calculated but did you use power feed or the lead screw to cut it?


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## cwilliamrose (Feb 5, 2022)

I used the lead screw. I agree with those other comments regarding it being impractical to thread with the power feed -- unless you can do it in one pass. Not sure about the backlash part of the question but it could be a problem too.


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## RJSakowski (Feb 5, 2022)

JimDawson said:


> Sounds like its time for a CNC lathe conversion
> 
> Extra credit:  Convert a manual lathe without disturbing any of the manual functions and be able to switch between CNC/manual operation in under a minute.


I thought about a CNC lathe but I like the hands on machining.  In fact, most of my lathe work doesn't even involve using power feed.  Old habits die hard.  

I should point out that if/when I need the capability of a CNC lathe, I can use my CNC mill as a vertical lathe.  True, I don't have through spindle capability and turning is limited to about 9" and I don't have spindle sync but for most needs, it would suffice.


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## B2 (Feb 5, 2022)

Thanks folks,  Good comments and appreciated. I'll respond to them a bit below.  



JimDawson said:


> Extra credit: Convert a manual lathe without disturbing any of the manual functions and be able to switch between CNC/manual operation in under a minute.


Hi Jim,  Yes, I once saw a U-Tube video where some one had done this.  It looked cool.  But then I lost the link so have to search for it again. Sometime.  



7milesup said:


> With using the power feed, wouldn't you have to leave it engaged, stop the lathe, reverse and repeat?


Yes, 7mileup.  You would want to leave it engaged.  The reason for doing it might be that one could get TPI values which are not available other wise.  On my PM1440GT, and including the change gears that come with it that amounts to about 1500 additional TPI values (It is possible there are some redundancies.).  Without the change gears there are 16 possible feeds and so TPI additional values. 



Christianstark said:


> I’m not sure, but I do not believe the straight gear teeth on my 1340 lathe bed are involute. I would think that may be enough to ruin some finer threads, but I’m a novice.


Hi Christian, 
I am not for sure what the effect of the different kinds of teeth on the lathe bed would have.  Anyway, if you keep the feed lever engaged, back out the tool a bit, and backed up the apron, put the tool back to a cutting position, and go again...  I would think the results would be repeatable.  This is what some folks advocate when  using the proximity stop approach.    However, maybe the Feed threads are not always uniform?   I would think that the fact that the 1/2 nut grips multiple lead screw threads that it tends to reduce backlash and cutting resistance, as well as other possible clamping issues.   I am not an expert on threading, but sometime I will try this out and just see if it is repeatable.  On my old South Bend where I need to replace the 1/2 nut along with a bunch of other rebuilding,  I can hardly cut a thread anyway.  (The lead screw on it seems to be in very good condition.)



cwilliamrose said:


> This started for me needing to cut 3.3333 TPI


Hi CWilliam, 

Cool, so there is sometimes a need for rather unique TPI values. 

Using the spread sheet tool that I have been working on I find that there are several external gear setting where I can get 3.3333 or 3.33?? threads.  ??=> other digits.  But you cannot get either of those with the 60/30 external gears that are normally used on the PM1440GT.   The tool is nice as it generates all possible TPI values for all possible gear settings as well as all possible external gears that I possess into a table.  Then I have a macro command that will search for almost anything you request for in the way of TPI.  It appears that there are 5 possible unique gear and lever combinations out of the 6144 possible combinations of gears and settings to get the 3.33?? values.  The PM1440GT has 16 fundamental TPIs that can be reached with the main gear box, then it can generate times 4 possible factors of 2 of these.  So that yields 64 possible threads without changing an external gear.   Suppose you have inserted external gears, like 65T/30T,  69T/28T, 60T/28T, 60T/35T, or 60T/42T which are not the more common 60T/30T gears of the 1440 where you would cut a 24TPI .... and you wanted to cut it without taking the cover off and removing gears.  Would it not be nice to just change the gear box levers and get the standard thread?   What gear settings do you use?  Just search for 3.33?? in the tool and you find either 24.000 or 24.00?? for each of the external gears I just mentioned.  The Power Feed values just make more entries to that search.  The PM1330GT manual says that you cannot get to the 13.0000 TPI with the standard 60T/30T external gears.  However, I have found that there are 60T/28T, 60T/30T, 60T/35T, and 60T/42T combinations will get the exact 13.0000 TPI value.  If you are willing to drop the preciseness to 13.0??? then I find 9 unique settings.  Even the standard 60T/30T yields 13.0909 TPI which is with in 1%.  Maybe the manual should have said that, but maybe the author did not know?




RJSakowski said:


> if/when I need the capability of a CNC lathe, I can use my CNC mill as a vertical lathe.



Yes, I prefer to use my lathe by hand too, but I do find that if I want a really smooth surface the power feed does a better job of making the cuts uniform.  This seems to be especially true on soft materials like brass.  My CNC mill has enough backlash that the circle cuts are not really round!  Also the final surface always has cutter marks.  The nice think about that CNC lathe video was that you could effectively turn the CNC off and it worked like a manual lathe.  

Dave L.


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## xr650rRider (Feb 5, 2022)

There is often times a clutch on the feed mechanism, not going to be a very good thread when your clutch slips.


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## B2 (Feb 5, 2022)

Thanks.  True on my PM1440GT also.  Interesting thought!  I had not though of this.   I guess one has to be aware of it for sure. The way the manual describes it ("The clutch comprises a pair of spring loaded steel balls bearing on a detent disc driven by the saddle feed gearbox.").  It does not sound like a high performance device and that a variable load might make the phase of the feed rod motion modulate without actually slipping out of the detent.  If it is like the detent mechanism in my speed control lever it is not all that great.   So I will have to engage mine to see what it sounds like when activated.  I suspect it makes a clicking noise.  I had thought that there was one also on the lead screw, but I guess not.  It would just crash a tool into the spindle if it were not for my proximity sensor system.  I suppose with the proximity sensor in place one could get rid of the Feed shaft clutch or redesign it.  One might ask, "Does one need a safety clutch on the slow feed mechanism why is there not one on the lead-screw as well?"  Maybe just for this reason!

Dave L.


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## RJSakowski (Feb 5, 2022)

B2 said:


> Thanks folks,  Good comments and appreciated. I'll respond to them a bit below.
> 
> 
> Hi Jim,  Yes, I once saw a U-Tube video where some one had done this.  It looked cool.  But then I lost the link so have to search for it again. Sometime.
> ...


By vertical CNC lathe, I mean put the work on the spindle and mount the cutting tools on the lathe to turn a part.  I had designed a tool post for my mill (yet to be built as I haven't needed it) that will mount cutting tools using the old OEM 4 way tool post from my lathe.


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## xr650rRider (Feb 5, 2022)

B2 said:


> It would just crash a tool into the spindle if it were not for my proximity sensor system.  I suppose with the proximity sensor in place one could get rid of the Feed shaft clutch or redesign it.  One might ask, "Does one need a safety clutch on the slow feed mechanism why is there not one on the lead-screw as well?"  Maybe just for this reason!
> 
> Dave L.



  What happens when a piece of swarf triggers your proximity sensor and your in the middle of say a $795 carbon fiber barrel blank thread?  Not enough material for a do-over.  You just created a very expensive tomato stake.  Might not ever happen until Mr. Murphy pay a visit.
  I thread away from the chuck and don't have to worry about crashing.  If you have a chuck that is not screwed on, I don't see why anyone would ever feel they need the stress.

Joe says he's been doing it this way for 30+ years, so if good enough for him, I know it works for me.





Try cutting an internal Acme 4 thread into a blind hole at 60 rpm.

Joe Piecynski single point threading playlist


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## Christianstark (Feb 5, 2022)

Gears that are not involute move at different rates of speed depending on tooth engagement. The saddle would speed up and slow down very slightly throughout the cut, but the spindle would be uniform. It would cause some thread deformity. Would it be inaccurate enough to ruin a thread? Probably not. Could be for fine pitched threads though. I don’t think the clutch slipping would be an issue though. I would think single point threading would be less taxing than rough cuts with power feed.


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## B2 (Feb 5, 2022)

Thanks guys,

@xr650rRider 
Yes, I like the inverted threading concept and know that you are a believer!  However, because the chuck spins on and off to mount/dismount on my old lathe spindle I was afraid of it causing the chuck to come loose so have not moved over.  However, more modern lathes do not have spin on chucks so maybe I can give it a try one of these days on my PM1440GT.  I may have to get some extra tooling.  
.  
Yes, I have been concerned about the magnetic metal cutting debris getting into the inductive (magnetic) proximity sensors that are used by essentially everyone.  For this reason I have been thinking about building a new sensor which is more like a limit switch but more precise.  Most of the VFD conversions discussed on HM are using mechanical relays rather than solid state electronics.  For this reason, the relay contact bounce at the VFD latching relay input can result in slow relay response or even chatter of the relay, so a mechanical switch's performance might not be great for repeatability, reliability of the relay, nor over all performance.  However, my solid state VFD conversion and its transistor latch system does not have this slowness limitation.  The solid state latch is so fast that it would unlatch during the first part of any contact bounce.  So while any debris might still be a concern about repeatability, at least it would not prevent it from working all together.  


B2 said:


> VFD conversion using solid state electronic components.



@Christianstark 
I just went an looked at my PM1440GT.  I cannot say that the lathe bed track uses involute thread shape, but they are some what like them.  I would say they look more like triangles with the tips cut off. (There is probably a name for this style thread.)  Meanwhile the Lead screw thread is square.  In is my understanding that an involute tooth shape is used to prevent wear of gears.  A round gear against a round gear will have no/little friction, during the gear turning, at the contact point when an involute gear is used as the surfaces are essentially rolling over each other.  No friction means no wear.  However, in order not to have back lash they have to be very tightly fitted, which we never see so wear does develop.  So the straight feed lathe bed gear teeth are on an essentially infinite radius circle.  For this am not for sure that an involute gear tooth shape would work well and raises the question which gear tooth size should  even be used?    Since my lathe is pretty new, I feel no backlash on my apron when the Power Feed lever is engaged but I have not measured it.  So maybe the motion uniformity is pretty good??!  On the other hand perhaps any backlash would not even be due to this gearing interface.  Their are more gears inside the apron!  I suppose gears in the lathe gear box also have lots of backlash so maybe the motion of the lead screw is not perfect either when under load?!  So maybe it goes like this.... when threading course threads where the load is usually high the errors in the gears do not mater much as a percentage of the thread length.  When threading fine threads the load is always small so maybe the back lash does not really kick in much.    Win-Win?

Dave L.


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## mksj (Feb 5, 2022)

There is a reason why you never see a feed rod used for threading, it is not accurate, has too much play and the loading would probably change the position. My lathe in feed mode has about a 1/4 turn wind-up in the chuck before it engages the carriage going from reverse/forward, and cannot be used with a threading dial if one was to use it. Clutch only engages with a hard stop, it doesn't do little slips unless one is doing really deep cuts, I have yet to get to that kind of load. Fundamental difference is that feed rates are an much slower then thread rates, the maximum feed rate on my lathe is 0.033 IPR,  which would be a 30.333 pitch. If you want to go faster, you may be exceeding the design limits of the feed gear train. If one wants a zillion threads, then go to an ELS, or buy a lathe with more capabilities.

Use of a proximity stop has no effect on screwing up threads, it is only the user that screws up threads. Upside down reverse threading you can't see the chips/what the cutter is doing, and metric threads would be even more difficult. Make a chip shield if you are worried about chips triggering the sensor. I am not looking for a Rube Goldberg machine to do things I will never need.


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## Christianstark (Feb 5, 2022)

@B2 the involute shape also ensures movement is consistent regardless of what position a tooth is in during engagement. There are videos that demonstrate this if you Google it. Lower friction is also a benefit as the teeth don’t rub.


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## Bi11Hudson (Feb 6, 2022)

There are several methods for threading vs carriage travel. The two I have are vastly different. Either one works well enough for small threads. The smallest(?) thread I have cut on both machines is 80 TPI. They will both cut smaller, the problem is my skill grinding tools.

The Atlas 12 inch (Craftsman version 101.27440) cuts both feed and threading from one point, the half nuts.  There is a follower gear/dial that is geared to the lead screw but I have taken it off and use motor reversing for the subsequent passes. Metric gears on a U S lead screw must do so. I do both for convenience to me and just ignore the dial. Plus, the machine is old and sorta "loose".

The earlier model Grizzly (G-1550) is a 9X19 Asian model. Essentially a G-4000 but made in Taiwan rather than China. I have personal perceptions of the Grizzly vs a Horrible Fright equivilent, having had both. But they are not the point here. The Griz has a "feather" that runs in a slot in the lead screw and is fed from a different lever than the longitudinal feed. I changed one of the gears in the outboard chain to get finer threads. 80 TPI is the smallest I dare cut. It will go to 160, *I think*.

There are larger machines that have two lead screws. One for feed, one for threading. I would love to have such a machine, but am limited by weight in a wood frame shop (old house) in addition to cost. The Atlas is heavy enough to rate reincorcement under the floor, I don't dare put a heavier machine in place. From experience, I consider the Atlas to be a "light" but quite servicable machine.

The Atlas is the key to the original question. It cuts threads using the same process as when doing longitutinal feed. On that machine, the only difference between the two is that the threads cut on the lower end have a sharper, well pointed tool and lighter cuts. The idea of cutting threads with the "feed" function would be limited only by the pitch of the thread. TPI is a function of how far the carriage moves per revolution of the spindle. Feed is no different from threading beyond the depth at which it is cut. At 40 TPI, the carriage is moving 0.025" per revolution. The depth of cut is on the order of a couple or three thou per pass for threading. And, in my case, the speed at which the spindle is running. I'm an old man and don't think/move that fast, so need to run the machine slower. Beyond that, as long as the feed gearing will align with the proper pitch, there is no reason not to cut threads with longitudinal feed.

.


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## B2 (Feb 6, 2022)

Hi Bi11


Bi11Hudson said:


> There are several methods for threading vs carriage travel.


Thank you for your response!  It is very through, useful and very informative.  Base upon the input from the forum, an especially your experience, I have decided I will include the possible threads one can get from the FEED position in my over all charts.  It seems clear that some folks have lathes which can be used for this and their operators are sufficiently skilled to make use of the information. Others may not need it or be sufficiently skilled to use it.     My spread sheet tool will also be constructed so that if one does not want the FEED thread info they can always leave the data out of the search or the overall threading table.  

I too have had need for and cut some high density treads in my optics work while at the Eastman Kodak Research Labs, as well as since then.  It turns out in some optics applications you wish to turn or move a lens to adjust a very tight focus, especially in applications like microscopy.   I first ran into this when I had responsibility to find materials to use for optical disk recording back in the early 1070's (think Blue Ray DVD type devices).  Here one employs the focusing of a laser spot down to the "diffraction limit" to make a dot or stripe exposure on the medium/disk surface.  The depth of field over which the beam can be focused to that limit is approximately the wavelength of the light/laser being used.  For green or blue light that is approximately 1/2 of a micron.  So you wish to be able to move a lens on the 1/2 micron scale or even better.  In my case, I had to do this in the total dark when doing the recording as some of Kodak's potential materials were very light sensitive.  There are optical tricks, primarily looking at the reflected portion of the beam, to determine if you have reached this focal point or not.  Anyway, I had access to a really competent technician who was also an excellent machinist, and he built me something that at that early stage of my life I would not have been able to do. (I am sure that even now I could not make as fine a device even with more than one try!)   I also had the help of a very good mechanical engineer, who the technician worked regularly for.   He designed a spring loaded air pressure floating foot that would stay at, and repeatedly go back to a constant distance from the surface it was setting on.  (Think of a game table where you float a hockey puck like object around on the compressed air coming out of the holes in the table.)   This worked well enough that I could find a focus position while in room light, then in the dark I could pull back on air-spring device and insert fresh materials to be exposed. The air foot on the device was less than 2" by 2" with air ports at the corners and the then lens mount in the hollow center.   The material was on a cheap, motorized, X transnational stage table which conveyed the material to be expose... so I could make a string of exposed spots by moving the stage while modulating the laser on and off at very short, micro-second variable, exposure times.   The lens barrel was to be mounted into the air spring foot and was turned in a threaded mount to adjust the focus.   The Tech. took the microscope lens that I was to use and machined off most of the barrel of it and re-threaded it with a very fine thread and, of course a new outer barrel to receive these threads, which was held in the air-spring mount.   He also machined a hardened steel wheel handle for the lens which was about 1.5-2" in diameter and he put 360 cut lines on the outside of the wheel.  Into these he fashioned a spring loaded detente point.   So as you turned it one turn it made 360 clicks, which you could feel and count.   As I recall, each click represented a motion of the lens of less than or about 1/2 micron.  

It turned out later that a couple of the best materials I found for this application was very thin films of Bismuth and amorphous Tellurium and Selenium which were not really light sensitive at all.   They formed images by melting the material and making tiny holes in the thin films.   Hence, to this day, Bismuth is one of my favorite materials.  Everyone should have a favorite material!  =-) !

So after all of these years,  if I still have that right,  a rotation of that wheel by one full turn would have moved the lens in less than 180 microns.  At 40 microns per 0.001" that meant that it moved 0.0045" per turn  ... which is 222TPI!  Maybe my memory of the numbers is off by a factor of 2, but even if so, this is some pretty fined threads in that brass barrel!  More a kin to what Bi11 was talking about.

 I got to work on a lot of neat projects while at EK!  So I was able to learn a lot of stuff. 

By the way, the Norton gear box of my Old South Bend 10K heavy that is in need of a rework has 10 fundamental threads and 7 factors of 2x.  Hence,  the highest thread density listed on the plate is 480TPI or 0.0021" per turn with a corresponding feed rate of 0.0007"!   That is without any exchange gears! I am sure that position has seen very little use! None by me.  Love that Norton Gear box and wish I had one on my PM1440GT!

Thanks to everyone for their responses.   I think I got what I needed to know.   If others have had experience with Feed threading I would still be interested in hearing about it.

Dave L.


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## Christianstark (Feb 6, 2022)

“So after all of these years, if I still have that right, a rotation of that wheel by one full turn would have moved the lens in less than 180 microns. At 40 microns per 0.001" that meant that it moved 0.0045" per turn ... which is 222TPI! Maybe my memory of the numbers is off by a factor of 2, but even if so, this is some pretty fined threads in that brass barrel! More a kin to what Bi11 was talking about.”

This was likely accomplished with a differential screw, rather than threading so fine.


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## B2 (Feb 6, 2022)

Maybe, It has been a long time ago. However, there was only one shaft (the brass that mounted the lens) and one outside barrel that was fixed to the handle.   I could turn it fairly long distances, like a centimeter, and I do not recall any intermediate barrel or nut. I never took it apart as I was afraid I would never get it back together.   There was a substantial mount of grease which I assume was to take up the spacing of errors and to make it stiff to turn.  A way of mounting one piece relative to the air-bearing so that it would move longitudinally along the center axis. I do not recall.  Don't for get that the whole thing had to be hollow to allow the light beam to pass to through the handle, the barrels and finally the lens.  The OD of these high power lenses are less than 1cm and you cannot block any of it without giving up some numerical aperture and hence the minimum size spot that can be made.   The lens barrel was the original from the microscope objective but had been turned down to just a shell to allow for the threading etc.   The OD of the outside barrel was not very large either.  But all of this is just a distant memory now.  So....   Later I had several other interactions with the Tech.  He was really good!


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