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allowable taper when turning a shaft

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savarin

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#1
I'm making an extended cross slide leadscrew for my generic 9x20 lathe.
I've re-alligned the tail stock and have obtained the following dimensions over a 150mm long length of stainless after turning, testing adjusting etc to remove as much taper as possible.
Starting at the head stock and using a micrometer every 35mm I get :-
12.46, 12.47, 12.44, 12.40
Using the vernier I get :-
11.85, 11.85, 11.84, 11.84
Is this reasonable or should I aim for a closer match?
Thanks.
 

RJSakowski

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#2
A variation in diameter will have an effect on the class of fit along the length of the lead screw. If you are turning an Acme thread, the thread angle will be 14.5º per side. An increase of .1mm in diameter will decrease the thread clearance by about .026mm measured along the axis of the screw. If you adjust your thread nut for a tight fit at the maximum screw diameter, it will amount to about .26mm of play for every .1mm decrease in diameter. The effect would be similar to wear in the lead screw.

I would normally trust micrometer readings over those obtained with a caliper. From your micrometer readings, your total variation is .07 mm which would be about .18mm along the screw axis. This, added to any backlash associated with the thrust bearing and the lead screw nut, would be a theoretical worst case backlash.

I would be more concerned about the discrepancy between the micrometer and caliper readings. The average difference is .60mm. The micrometer and caliper should agree to within the resolution of the instruments. I would want to resolve that before cutting any thread.

My guess would be the caliper is the errant reading. A caliper can give a low reading if the adjustment of the beam is too loose. The far side of the movable jaw will lift with applied pressure which tends to pull the jaw away from the measured piece, giving a low reading.
 

Tozguy

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#3
Is it not the pitch diameter that is critical? Unless the screw threads will be bottoming in the nut threads, the o.d. of the shaft can vary slightly.
I suspect that getting zero deflection when cutting the ACME threads is the main objective.
 

Downunder Bob

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#4
Definitely as RJ said you need to resolve the discrepancy between micrometer and caliper, or, using a test piece confirm which one is correct, then only use it.

Interestingly the measurements with the vernier are quite consistent over the length, only .01 difference, this I would think is quite acceptable if it is in fact correct, so first you have to confirm which measuring tool is correct, one of them definitely has a problem, and like RJ I would tend to suspect the vernier, but you need to confirm it.
 

savarin

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#5
I know the calipers under report but are pretty consistent, the mic seems spot on but I'm not a good user. (never used one before and I might not be reading it correctly)
I revisited the tail stock adjustment and used a dial gauge to get the taper as best ever.
It now reads only 0.04mm difference over a 240mm length instead of 0.06 over 150mm, I'm calling that done. With the micrometer.
I will be cutting M10x1 standard metric fine for the lead screw
 

RJSakowski

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#7
Is it not the pitch diameter that is critical? Unless the screw threads will be bottoming in the nut threads, the o.d. of the shaft can vary slightly.
I suspect that getting zero deflection when cutting the ACME threads is the main objective.
The pitch diameter is the important parameter but my concern is that for whatever reason, the diameter is varying, it could also vary when you are cutting the thread. This would cause a varying pitch diameter.

Just looking at the micrometer readings and using the assumption that the tailstock is used for support, the following should be true. Deflection due to tool pressure should be minimal at the two ends of the shaft. This would result in a barrel shaped piece. If taper was also present, the piece could take on a bullet shape. Here is a drawing of the surface with the axial distance compressed by a factor of 100 to illustrate the point. Taper Curve.JPG
While the curve is greatly exaggerated, the curvature would be sufficient to cause difficulties with backlash in the lead screw.

If I were trying to adjust taper of the cut, I would make my measurements as close to the headstock and tailstock as I could. In this case, it looks like the tailstock is too close to the front by .06/2 or .03mm.

The other concern is the bowing of the workpiece. Going from a 12.4mm diameter to a 10mm diameter will make the bowing worse. Definitely, a follower is is recommended.
 

savarin

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#8
Thanks Bob, I plead a seniors moment that I didnt think of searching for that. Doh! Looks like I did get it correct after all but need way more practice holding it square.
Thanks RJ, I've only used a follower once before and it was not a happy experience.
I will be turning the shaft down to 10mm using the tail stock and a travelling steady to get as accurate as possible but cutting a thread with the steady will be my main challenge.
The first cuts to get to size yesterday without the steady took 4 spring passes to get to size.
I think I will set up the steady with the fingers on the left hand side of the work and the tool bit exactly opposite the fingers and have more shaft spare on the right hand side that can be removed once the length of thread required is cut to size.
As its a left hand thread I think this will work.
My biggest worry is the roughness of the cuts wearing the brass fingers. I've read somewhere that running a file after every pass mitigates this.
But, as its a metric thread being cut with an imperial lead screw on the lathe so hopefully there will be sufficient space and time to do this on the return without disconnecting the half nuts.
so much to think about.
 

Downunder Bob

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#9
I've always found it interesting how some people prefer micrometers over verniers and vice versa. When I trained back in the early 60's I was taught micrometers,

Only the senior toolmakers used the verniers. A few years later, late 60's, when I went to sea, and initially on Swedish ships, I was expected to learn to use vernier calipers, it was a bit of a learning curve but eventually got there, and over time preferred the calipers for speed, but when it comes to real accuracy I still prefer the micrometer.

By the early 70's I came back to Australia, and started working on Australian ships, and again I found the majority preferred the micrometer. Eventually with the electronic digital calipers becoming popular and affordable a lot are changing over, as they are quicker, and nearly as accurate as a micrometer. I would still prefer a micrometer for any thing better than 0.001" My old Moore & Wright from 1961 still reads to 0.0001
 

Downunder Bob

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#10
Since you want an M10x1 thread a pretty popular size why don't you just buy a length, not the rubbish from the hardware shop but get a good quality from a machinery suppliers?
 

RJSakowski

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#11
Thanks Bob, I plead a seniors moment that I didnt think of searching for that. Doh! Looks like I did get it correct after all but need way more practice holding it square.
Thanks RJ, I've only used a follower once before and it was not a happy experience.
I will be turning the shaft down to 10mm using the tail stock and a travelling steady to get as accurate as possible but cutting a thread with the steady will be my main challenge.
The first cuts to get to size yesterday without the steady took 4 spring passes to get to size.
I think I will set up the steady with the fingers on the left hand side of the work and the tool bit exactly opposite the fingers and have more shaft spare on the right hand side that can be removed once the length of thread required is cut to size.
As its a left hand thread I think this will work.
My biggest worry is the roughness of the cuts wearing the brass fingers. I've read somewhere that running a file after every pass mitigates this.
But, as its a metric thread being cut with an imperial lead screw on the lathe so hopefully there will be sufficient space and time to do this on the return without disconnecting the half nuts.
so much to think about.
Your lead screw is a left hand thread so you will be threading towards the tailstock. I would thread a longer length than necessary to give you additional time to clean up the thread burrs. If necessary, you can run back and forth with the half nuts engaged but the threading tool backed out to clean up those burrs.
I would encourage you to watch the video by Tom Lipton (OxTools) on metric threading with an Imperial lead screw.
By follower, I meant the rest that mounts on the cross feed ad travels with the cut. The steady rest mounts on the bed and is stationary. It seems that you have the two interchanged.
 

RJSakowski

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#12
An interesting tale about measurement instruments.
In a nearby farm town a machinist friend decided to open a job shop but was struggling to make ends meet due to lack of business. To augment his walk-in business, he decided to design and build a mortising machine for sale to woodworking shops. The machine was awesome, using hydraulic motors for cutting the mortise and tenons on aqll joints in a single setup and would cut the mortises and tenons to .001" accuracy.
He took the machine to a woodworking convention and was demonstrating it to his potential customers. As he was touting the accuracy of the joints, one of the owners of a furniture manufacturing company picked up one of the samples and proceeded to measure the joints with a wooden yardstick. My machinist friend decided right then and there that the industry would never appreciate the accuracy that he had built into the machine. True story
 

hman

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#13
Savarin -
I just thought of something. Have you added a "reverse tumbler" to the gear train in the headstock of your lathe? Most 9x20s (mine included) don't have any way to cut left hand threads! I added a reverse tumbler to a previous 9x20 that I owned, but later sold. My current one doesn't yet have one.

////OOPS//// Never mind! I just went searching on the forum, and the first hit was your 2012 thread. Good for you!
 

Downunder Bob

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#14
RJ, I do not doubt it.
 

savarin

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#15
I must admit John I find it difficult to believe I've only been doing this machining stuff for 6 years (I think I joined when I purchased the machine)
 

Downunder Bob

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#16
I find it difficult to believe also Charles. You have come up with so many new, different ways of doing things, all because you had no preconceived ideas about how it should be done, so you just did what you thought might work. And look what you have achieved. A Master machinist by any book.
 
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savarin

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#17
Carefull Bob I wont get through the door in a minute. :beer:
 

Downunder Bob

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#18
No need to get a swelled head over it Charles,, it's just fair comment where it's due, and I'm sure every one here who follows you will agree.
 

P. Waller

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#19
You are making this part for personal use?
If so the allowable taper is what You Require, there is no standard. If you are producing this from a Customer drawing that requires a lead accuracy that has a value, say .001 +- .001" per 12" of length then you know where you need to be with it.

If you try to produce a leadscrew on a lathe to very high accuracy. sub .001" taper and lead you will quickly go mad.
This is what cylindrical grinders and thread grinders are built for.
 

savarin

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#20
Ha Ha, too late, I'm probably mad already looking at what I'm working on.
 

Chipper5783

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#21
Turning a straight shaft and threading a relatively "long" shaft are two quite different exercises. The straight shaft is much easier because it is easy to measure and you can greatly reduce the tool push by using a pointy tool. With threading, the concern is the pitch diameter, which others have done a good job of pointing out. When threading it is nearly impossible to eliminate the tool push. Making numerous passes at the same setting will not address the spring of the tool and work piece. The issue is that the tool shape is generating the form of the thread, so there is a lot of the cutting edge in contact at once. For a long thread, you have to work out a follower rest. Also, make sure the tool is very keen, use lots of back rake, consider cutting only half the flank at a time. Another point is that you only need the follower rest near the end - most of the thread can be cut without the follower rest (the pitch diameter will be barrel shaped, but so what, you are still roughing - that makes it much easier to manage the brass tips of the follower rest).
 

savarin

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#22
Sorry for the delay I forgot to mention I've done it.
I was very impressed with the quality of the thread and how straight it came out, only 2 thou difference between the ends and the centre.
I used the tail stock centre and the travelling steady and took light cuts and spring passes.
After the first pass I used a very fine file to remove the burr (if there was one) and again halfway through.
I would nominate this thread as the best I've ever cut and how smooth it is.
 

ch2co

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#23
Come on Charles. You keep talking about what you’re working on but what’s with the lack of photographic proof? I’d love to see it.

The Grumpy old Chuck
 
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