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Why do lathe spindles use coarse threads?

M

Mike McDonald

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Feb 8, 2011
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I was trying to adapt a lathe face plate to another purpose yesterday and ran into a fine/coarse thread mismatch.

Eventually this lead me to wonder why lathe spindles are coarse threaded. I have no argument for why fine threads would be better, (honestly, I don't have a good understanding of the mechanical trade-offs between the two) but I know that coarse/fine threads are (almost) always chosen for a reason..


Does anyone know why, or have a plausible theory why, lathes use coarse threads?


MZR
 
I think it's just to make it quicker to get the parts on and off. A course thread is probably a little tougher and we are putting some pretty large and heavy pieces on and off but as you said I'm not an engineer and fine threads could be , well fine.
I did have a mini lathe with 3/4 x 16 tpi spindle. Those threads were awfully thin but were a standard nut size so it was easy to make my own faceplates.
Now why everyone picked 1 1/4 x 8 instead of the standard nut size of 1 1/4 x7 I don't know. I do know that a 1 1 /4x8 nut is made from a special extra tough steel for special purposes. That way they won't accidentally use a 1 1/4x7. Maybe that's why they picked that size.
1"x8 on the other hand is a pretty standard size.
 
Mike, my guess would be that the courser thread will stand up to more
abuse. If anybody can cross thread something I can and if my spindle threads were finer, I would have.
I've gotten a 73lb chunk of wood and face plate on the lath with a bit of finagling (I did weigh it, Just had to know...)
The courser threads helped by not having to have the face plate presicely in line when I was trying to thread that thing on.
I know people turn heavier stuff, but for me that was big enough for me...
c
 
I suspect that part of the reason may be cost.

Consider that an Acme thread (shaped like a trapezoid rather than a "v"), used on metal lathes' tool feed and the spindles on table saws, would be much more precise and durable, and almost impossible to cross-thread. More expensive to cut, however.
 
I dont know the why either. Years ago Jack Straka decided to change his spindle to 12 threads. I forget why. He said that lasted about a week. He said the extra on off effort was not worth it. The machinest got paid for a new shaft and paid to put it in and take it out and put back the other one.
 
Acme threads are usually used where the screw and nut will be used repetatively, as in feed and lead screws. The more vertical sides of the form have a better wear characteristic than a V form, especially when the load is carried for substantial distances as in a feed screw. The more vertical sides of the thread do not help to center the nut on the pitch diamter of the thread as well as a vee form. In terms of cutting the threads on a lathe spindle, there would be more expense involved with an Acme than a V form. With the typical use of a lathe spindle, centering of the chuck is a more important function than wear, especially when the load applied is only at the moment of contact with the spindle shoulder. Also, the pitch of an Acme is typically more coarse than a V form, and the root diameter is correspondingly smaller. In a lathe spindle, that likely would result in less rigidity in the spindle nose.

Here is a link to commonly available Acme screws. Note the diameter and tpi compared to the common spindle sizes and tpi's.

http://www1.mscdirect.com/eCommerce/NavigationServlet?ta=Y&N=12101927
 
Mark Mandell said:
I suspect that part of the reason may be cost.
Click to expand...

Cost really! They don't make thousands of these lathe a day. And if there was ANY benefit to an Acme thread for a lathe spindle I am positive that many lathe makers would use Acme threads and make sure that "advantage" was front and center in advertising.

Coarse threads are deeper than fine threads. That yields a large bearing surface.

There are three three threading standards. UNC = Coarse, UNF = fine and 8UN = 8tpi.

8UN is only for 1" and above. UNC at 1" and 8UN at 1" have the same specifications.

There are loads of different spindle threads found on lathes.
 
Since I have plenty of 4", 5", 6", 8", C-clamps that have a true Acme thread, I have a difficult time accepting the hypothesis that overall cost is the reason why coarse threads, instead of Acme threads are used in wood lathe spindles. The threading of the spindle is a very small portion of the overall cost in making a lathe, but the Acme threads used in the C-clamps can't cost that much to produce, considering we're talking about an item that sells for around $20 +/-.

I never really considered the "why" in the choice of threads, but it may be simply that an industry has been built around attachments and spindles that were originally pioneered in certain thread styles and sizes......no more "whys and wherefores" than that!

My spindle is 1 1/4 x 8tpi, and the threads are definitely trapezoidal in shape. I had thought these threads would be appropriately called "Acme", but I guess it's actually a coarse thread 8UN, as Gynia describes.........funny how I never gave this a thought until this thread!

.....and, it's a very good question!:D

ooc
 
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Acme threads would be the worst possible choice for a self-centering connection, such as a lathe spindle. Unified screws have a 30-degree "ramp," whereas Acme has 14.5 degrees. (60/2 vs. 29/2). Think about mating two cones (sort of). Steeper has a better concentric fit.

Odie, your "trapezoids" are mostly the sharp points reduced to small flats.

Coarse for strength and quick assembly, fine for precision without as much strength. "Standard" for some camera lenses are as fine as 1"-32tpi; 28 tpi in larger sizes.

Acme, as well as other shapes, can be rolled instead of cut (stronger and less waste too); so cost of manufacture is less important than demand for product.
 
Thanks, all, for sharing your thoughts.

I think Joe had the most likely explanation:
Joe Greiner said:
Coarse for strength and quick assembly, fine for precision without as much strength.
Click to expand...

..but, if coarse is stronger, why would every trailer hitch ball I looked at recently be fine-threaded - wouldn't this be an area where you'd want the most strength, and precision isn't as important?

I'm not sure we're going to figure this one out, but I'm enjoying reading everyone's theories!

MZR
 
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Mike McDonald said:
..but, if coarse is stronger, why would every trailer hitch ball I looked at recently be fine-threaded
Click to expand...

While fine thread isn't as strong as coarse threads, fine threads are less likely to loosen catastrophically (more turns on the fine threads). The strength of the ball comes from the diameter of the threaded portion not from the threads,
 
john lucas said:
I think trailer hitch balls have fine thread so you can apply more torque so the nut won't come loose.
Click to expand...

And then there's the lock washer....

Coarse thread is less vulnerable to crud and cross loading/threading than fine. Flat-topped rather than sharp threads make it much easier to pass the thread through a circular opening or a circular opening over the thread. Think of those clamps and saw arbors.

Less maintenance and chance for screwing (! up favors the coarse.
 
I"m no engineer and I could be totally off, but if you think of the angles created by coarse and fine threads, the fine thread (with less angle) would be more likely to bind up and not release as easily. (better for a hitch ball than faceplate)

The coarse threads with a slightly larger angle would have less rotation from initial contact to tightened than a fine thread, and would also be easier to break loose.
 
For clarification, the angles Darryl refers to are the helix angles of the thread around the shaft. This is independent of the angle or shape of the thread form itself.


Machinery's Handbook offers some distinctions for coarse vs. fine threads:

Coarse is preferred for threading into lower strength materials such as cast iron and mild steel; and into softer materials, such as brass, aluminum and plastics; to obtain optimum resistance to stripping. Also for rapid assembly and disassembly, and for corrosion allowance.

Fine is suitable if the strength of the thread equals or exceeds the applied axial load. Also appropriate for short length of engagement (to put more threads to work), thin-walled material (less reduction of material), and for finer adjustment.


The down side of fine threads is that they're vulnerable to cross threading. To insure against that, rotate opposite the direction of tightening until the threads click into engagement, then tighten.
 
Joe It seems like I read somewhere many years ago that a shaft only needs 3 threads to provide maximum strength. Did I read that wrong or is my memory fowled.
 
Cast Face Plates

I have seen lots of cast face plates which I suspect is the reason coarse threads prevail on lathe spindles. Coarse threads also tolerate the less than perfect conditions that I have seen on wood lathes without binding. I have restored some vintage lathes and chased the threads on the spindles. This removed years of rust, wax and crud. I like the look of clean threads on the spindles but most of the spindles were OK before I cleaned them up.
 
I don't want to start an argument but Gynia I hope you don't mind if I
correct you but a fine thread will stand more torque than a coarse thread.
I believe John already metioned that fact.
 
Not to add fire to this, or cause an argument, but when somebody says one thread is stronger than another, How? In what direction? Twisting torque applied to tightening the fastener, at right angles to the spindle/bolt? or pulling force? Curiosity.
 
Bruce Smith said:
I don't want to start an argument but Gynia I hope you don't mind if I
correct you but a fine thread will stand more torque than a coarse thread.
I believe John already metioned that fact.
Click to expand...

Which becomes unimportant as soon as the nut meets the shoulder of the spindle. Especially as it attempts to tighten with power to the lathe.
 
Strength is measured as the pulling or clamping force. Torque vs. pulling force has a boatload of factors, including geometry, cleanliness, grip length, with or without washers, etc. Best established by testing the actual conditions (and required for structural connections using torque wrenches). It's generally preferred to break the fastener instead of stripping the thread.

Regular nuts usually span 5 or more threads total. Discounting the first and last (as incomplete) would provide 3 as a minimum engagement.

Carried to extremes, a fine thread would have no strength at all, so I'd hesitate to make a blanket declaration.
 
Bruce Smith said:
I don't want to start an argument but Gynia I hope you don't mind if I
correct you but a fine thread will stand more torque than a coarse thread.
I believe John already metioned that fact.
Click to expand...

I have zero problem with you correcting me. My experience is that I can strip out fine threads with less torque than coarse threads. Often times, experience leads me to correctable conclusions.
 
The explanations were why I asked. It seemed like either one carried to the extreme would not be strong. Twisting strength would appear to be different from tension strength. Interesting questions that I don't have engineering training for.
 
john lucas said:
Joe It seems like I read somewhere many years ago that a shaft only needs 3 threads to provide maximum strength. Did I read that wrong or is my memory fowled.
Click to expand...

Minimum thread engagement is a complicated subject. At a minimum, if the bolt and nut are the same material, you should have a length of engagement equal to 65% of the nominal diameter of the bolt.

Basic design standards take the premise that you have enough thread engagement to ensure that the bolt fails at the first exposed thread rather than shear the threads.

A higher engagement % reduces the risk of vibration back off. In our situation we are applying torque to tighten the nut under turning, so this is not really a factor unless we are running in reverse.

The load applied to threads is not uniformly distributed across all engaged threads. Typical thread loading by thread is

1st 33%
2nd 27%
3rd 20%
4th 12%
5th 8%

The strength of the nut / bolt system is the lesser of the strength of the thread and the strength of the bolt.

The strength of a thread is proportional to the Thread Shear Area which is a function of the length of engagement, pitch, major and minor diameter.

For 1" of engagement the Thread Shear Area

1 1/4-7 => 3.75
1 1/4-8 => 3.55
1 1/4-12 => 3.07

So while the coarser threads are stronger, they are not dramatically so, with the 1 1/4-8 only about 16% stronger than 1 1/4-12

Given the big hole (MT2) in the middle of the bolt (spindle nose), I suspect that the threads are actually much stronger then the spindle nose, thus you are more likely to shear the spindle under excess load then shear the threads.
 
While a lot of analytical rationale could be presented to explain why lathe threads and other things are the way that they are, I think that historical evolution and various practical considerations played much bigger roles than a deliberate engineering design process -- sort of like the way that we arrived at the spacing of railroad rails.

I don't believe there was any point in lathe development where engineering designers started from a completely clean sheet of paper, ignoring similarity and compatibility with any and all previous designs.

I can envision from a practical point of view that production turners would want spindle threads that were durable and not something that could be easily cross threaded or simply wear out too quickly from heavy use. Also, these same production turners would want a spindle that only required a few turns to thread a faceplate onto it. Eight turns would be preferable to twelve, sixteen, or thirty-two turns.
 
I agree with Bill about the way these things likely developed.

It could be as simple as a machinist asking the lathe manufacturer if the new spindle would be 1.25".

The manufacturer says "of course", the machinist hears it as "coarse", and the rest is history.

Had he said "that's fine", we might have 16 tpi... < s >
 
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Bill is probably right folks didn't use mathematical modeling and cad cam programs to design the perfect thread.
They evolved over time.

The Great Baltimore 1904 fire was the birth of national standards for fire hose couplings

Fire companies came from New York and Philadelphia by train. But their hoses could not connect to Baltimore hydrants.


al
 
So from what I can gather from the postings, it sounds like the strength of the threads really doesn't come into play. The hollow spindle would be the weak point. So the threads are probably coarse as they are probably less likely to cross thread, and would be much faster to turn things off and on the spindle. Or do I have it all wrong?
 
Michael DeWald said:
So from what I can gather from the postings, it sounds like the strength of the threads really doesn't come into play. The hollow spindle would be the weak point. So the threads are probably coarse as they are probably less likely to cross thread, and would be much faster to turn things off and on the spindle. Or do I have it all wrong?
Click to expand...

It sounds to me like you might be reading too much into some of the comments. If I may continue my evolution analogy, woodturning lathes have progressively improved over time -- bad ideas fell by the wayside while good ideas found favor. Modern lathes have also borrowed much of their design (including the spindle thread size and Morse taper socket) from metalworking machines. The end result is that the defacto standard interface for connecting a load to the spindle on modern lathes is probably about as close to ideal as we could expect.

I don't see anything that I would consider a design weak point and certainly not the fact that the spindle is hollow. Things eventually wear out and a poorly made lathe can hasten that along. A woodturner can also abuse his lathe to speed up its demise.

The last two points are just my conjecture. They seem plausible, but I didn't present any evidence to support that notion. It is mainly just the absence of a contradictory argument that led me to that idea. But, I could be all wet.
 
Michael DeWald said:
So from what I can gather from the postings, it sounds like the strength of the threads really doesn't come into play. The hollow spindle would be the weak point. So the threads are probably coarse as they are probably less likely to cross thread, and would be much faster to turn things off and on the spindle. Or do I have it all wrong?
Click to expand...

Well everything has a weak point. The question is "Is that weak point relevant?" I've never heard of anyone shearing a 1 1/4-8 spindle, I suspect that the force needed to shear the spindle would require such an imbalance that the lathe would fall over or the piece would fly off the lathe first.

I think the metalworking history is more relevant, there is little need for a large spindle bore on a woodworking lathe, so an MT2 spindle bore is enough. Given an MT2 spindle, you really don't need much bigger than 1 1/4 spindle. Most metalworking lathes have larger spindle bores, even tiny 7x10 metal lathes have an MT3 spindle bore to allow feeding of 3/4" stock through the headstock.

Given the existence of an 8UN thread standard for 1" and larger bolts, it was probably just force of habit that made early lathes have 8 TPI threads.
 
Plus 1 (and then some) for evolution. Until about 1800, threads were hit-and-miss, occasionally deliberate mis-matches to foil competition. Modern "standards" needed about 150 years to develop. As of 1992 (24th edition of Machinery's Handbook), there were 11 "standards" for 1" alone, including 1-27UNS and 1-28UN. It's hard to believe those two arrived by a rational process.
 
Joe Greiner said:
Plus 1 (and then some) for evolution. Until about 1800, threads were hit-and-miss, occasionally deliberate mis-matches to foil competition. Modern "standards" needed about 150 years to develop. As of 1992 (24th edition of Machinery's Handbook), there were 11 "standards" for 1" alone, including 1-27UNS and 1-28UN. It's hard to believe those two arrived by a rational process.
Click to expand...

And, machine tapers are not any better -- there are more "standards" than you can shake an arbor at. In addition to the familiar Morse taper used on woodturning lathes, some others are Brown & Sharp, Jacobs, Jarno, NMTB, and R8.

I think that originally the Morse taper was envisioned to have a fixed taper ratio of ⅝" per foot, but it did not work out quite that way, probably because convenient rounding was done for diameters and lengths of the seven different sizes of Morse tapers.

The Jarno taper uses a fixed ratio of 1:20 for all of its sizes and is the most logically organized of all the tapers, as if being logical counts for anything.

The most illogical of all has to be the Jacobs taper where the assigned number has nothing at all to do with its size. The Jacobs taper continues to have broad acceptance especially for drill chucks. Score one for the opponents of a logical approach.

The R8 taper is the invention of Bridgeport. The logic in this case was financial rather than technical. If you owned a Bridgeport milling machine then obviously you should buy Bridgeport cutting tools. I used to work where we had huge Cincinnati Milicron machines, but I don't know if they had their own special tapers.

The NMTB tapers are used in CNC milling machines. They have a very fast taper which is 3.5" per foot which means that they look more like cones. They are not self holding, but were designed for accurate alignment and fast release.
 
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More info that you wanted to know about tapers.

The NMTB tool holders I'm familiar with use a draw rod to hold them in the machine.

I think the Cat and BT tapers are usually found in CNC machines that have tool changers.

R8 is not only a taper, it also funtions as a collet. Some of the older Bridgeports had a #2 Morse taper in the spindle and use a morse taper collet with a drawrod. They held very well, but were limited in size to 1/2". The R8 system will go to 3/4" without using the extended nose collets.

Never have understood the Jacobs taper thing. It is amazing how well they hold if the parts are perfectly degreased before assembly.
 
Yes, I think my post is being a bit misinterpreted. My "weak point" comment was not that anything was likely to fail. It was only that the failure point for any commonly available thread were likely to exceed the failure point for the hollow spindle. Because of that, strength of the threads becomes irrelevant, and convenience becomes the primary concern.
 
Michael DeWald said:
Yes, I think my post is being a bit misinterpreted. My "weak point" comment was not that anything was likely to fail. It was only that the failure point for any commonly available thread were likely to exceed the failure point for the hollow spindle. Because of that, strength of the threads becomes irrelevant, and convenience becomes the primary concern.
Click to expand...

It is quite likely that I misinterpreted what you meant. I'll have to admit that I am still not quite clear on what you mean by "failure point", but the essence of the matter is that a lot of other things on the lathe will probably break before the spindle does.

This doesn't mean that you may not need to ever replace the spindle. I had to replace a spindle on my almost new Delta 1440 lathe years ago -- not because it failed, but because it never was right from the beginning. It was slightly crooked from being improperly manufactured. It also came with a badly scored bearing land so that the existing bearing could not be removed. Delta sent a replacement spindle and set of bearings which ran like it should and I was a happy camper.
 
The only problems I've seen with lathe spindles is being bent, or the threads not matching a particular chuck or faceplate. The later is probably a tolerance thing since every screw and nut has a tolerance and if you happen to get a spindle on the plus side and chuck on the negative side then problems arise.
The bent spindle seems to be more common. I have not seen it yet on 1 1/4, m33 or 1 1/2 but have seen it several times on 1" spindles. Usually from putting too large a piece of wood on a lathe.
 
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