Author Topic: My tailstock is off  (Read 3619 times)

Offline steamer

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Re: My tailstock is off
« Reply #15 on: July 02, 2018, 01:57:12 PM »
Head stock should point up and out 0.0005/12"   This allows for cutting pressure and assures it will cut concave and not convex.
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Offline steamer

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Re: My tailstock is off
« Reply #16 on: July 02, 2018, 02:20:55 PM »
See reply #354

http://www.modelenginemaker.com/index.php/topic,369.350.html

I have pictures of the procedure.

Dave
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Offline zeeprogrammer

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Re: My tailstock is off
« Reply #17 on: July 02, 2018, 03:57:41 PM »
Head stock should point up and out 0.0005/12"   This allows for cutting pressure and assures it will cut concave and not convex.

That confuses me. I hadn't seen a spec like that.

1) Is Rollie's Dad's Method affected by that?
2) At the distances I usually cut (1" to 4") does this really have that much affect?
3) I don't understand concave versus straight. And is this when turning, facing, or both?

Thanks.
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Offline steamer

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Re: My tailstock is off
« Reply #18 on: July 02, 2018, 05:21:30 PM »
Head stock should point up and out 0.0005/12"   This allows for cutting pressure and assures it will cut concave and not convex.

That confuses me. I hadn't seen a spec like that.

1) Is Rollie's Dad's Method affected by that?
2) At the distances I usually cut (1" to 4") does this really have that much affect?
3) I don't understand concave versus straight. And is this when turning, facing, or both?

Thanks.


Is rollies Dad's method affected.     NO.  It's unlikely you will even see it while performing Rollies Dad's method......Rollie is a great guy!   You should meet him sometime.

At the 1-4" distances it shouldn't unless it's really out.     The spindle nose should point up and towards you  .0005" at 12 inches from the spindle nose.   It's not a lot

If it cuts convex, during a facing cut, the center of the part will be longer than the OD, and the part will not sit flat on a flat surface.  If it cuts perfectly flat or concave, it will set on the face or the perimeter of the part.    Again...  This dimension is specified to allow for deflection of the lathe subjected to cutting forces during facing primarily.


I can sketch it and post if you want me to.


Dave
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Offline petertha

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Re: My tailstock is off
« Reply #19 on: July 02, 2018, 06:38:03 PM »
I'd like to see your sketches Steamer, I'm interested in this stuff.

I've read through RDM, both in what I think is the official procedure link below and on other forums.
http://manuals.chudov.com/Rollies-Dads-Method-of-Lathe-Alignment.pdf

I guess I don't quite understand the nuances of RDM. When a headstock axis deviation is measured using RDM (notwithstanding all the shaft averaging business which I still can't quite get my head around), at the end of it all the recommended 'fix' is to start 'leveling' the lathe. Which is another way of saying introducing or removing lathe bed twist to compensate. But I can envision a 100% planar lathe bed resting on a 1-degree slope without introducing taper cutting issues. Providing the lathe bed is designed to be sufficiently rigid under its own weight which it should be. How is the lathe being adversely affected?

But as I mentioned in an earlier post, if your headstock is adjustable in the yaw plane viewed from top, which many machines are, I would think all levelling bets are off & this deviation trumps shimming the lathe feet. Maybe RDM was documented in a time or specifically for lathes that have the spindle axis cast in place (meaning there is no HS adjustability). This I can see as a correction of last resort. But for 2-part headstock/bed lathes, I would think this would be the focal point especially if they were ever moved or never adjusted over time. Think about it exaggerated: if my lathe was 100% level & 100% untwisted, then I loosened the HS bolts & cocked the spindle axis 10-deg in our out relative to lathe bed, how much shimming & jacking of the feet would have to be done to twist the lathe bed to compensate? Answer: a lot. Unless I'm missing something fundamental, its attempting to solve the problem with the wrong lever.

Back to convex/concave I think this might be related to facing cuts on the end of stock? The way I envision it is:
- if extended HS axis is pointing to front of lathe then a facing cross cut will yield a shallow inward concave cone. In non-TS supported longitudinal cutting, the carriage travel will cut a taper where the TS side diameter is smaller than the HS side, no different that TS supported offset taper cutting method.
- if extended HS axis is pointing to rear of lathe then facing cross cut will yield a shallow outward protruding convex cone. Longitude cutting will yield taper in opposite of above, TS diameter larger than HS diameter.

Of course cutting forces & material & cantilevered vs. supported stock can mask what are probably tiny deviations.
« Last Edit: July 02, 2018, 06:43:45 PM by petertha »

Offline steamer

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Re: My tailstock is off
« Reply #20 on: July 02, 2018, 07:35:32 PM »
I'd like to see your sketches Steamer, I'm interested in this stuff.

I've read through RDM, both in what I think is the official procedure link below and on other forums.
http://manuals.chudov.com/Rollies-Dads-Method-of-Lathe-Alignment.pdf

I guess I don't quite understand the nuances of RDM. When a headstock axis deviation is measured using RDM (notwithstanding all the shaft averaging business which I still can't quite get my head around), at the end of it all the recommended 'fix' is to start 'leveling' the lathe. Which is another way of saying introducing or removing lathe bed twist to compensate. But I can envision a 100% planar lathe bed resting on a 1-degree slope without introducing taper cutting issues. Providing the lathe bed is designed to be sufficiently rigid under its own weight which it should be. How is the lathe being adversely affected?

But as I mentioned in an earlier post, if your headstock is adjustable in the yaw plane viewed from top, which many machines are, I would think all levelling bets are off & this deviation trumps shimming the lathe feet. Maybe RDM was documented in a time or specifically for lathes that have the spindle axis cast in place (meaning there is no HS adjustability). This I can see as a correction of last resort. But for 2-part headstock/bed lathes, I would think this would be the focal point especially if they were ever moved or never adjusted over time. Think about it exaggerated: if my lathe was 100% level & 100% untwisted, then I loosened the HS bolts & cocked the spindle axis 10-deg in our out relative to lathe bed, how much shimming & jacking of the feet would have to be done to twist the lathe bed to compensate? Answer: a lot. Unless I'm missing something fundamental, its attempting to solve the problem with the wrong lever.

Back to convex/concave I think this might be related to facing cuts on the end of stock? The way I envision it is:
- if extended HS axis is pointing to front of lathe then a facing cross cut will yield a shallow inward concave cone. In non-TS supported longitudinal cutting, the carriage travel will cut a taper where the TS side diameter is smaller than the HS side, no different that TS supported offset taper cutting method.
- if extended HS axis is pointing to rear of lathe then facing cross cut will yield a shallow outward protruding convex cone. Longitude cutting will yield taper in opposite of above, TS diameter larger than HS diameter.

Of course cutting forces & material & cantilevered vs. supported stock can mask what are probably tiny deviations.

Working on loading a sketch

RDM will minimize your errors....all the errors.    Most beds are not perfect, and RDM will just show the errors but won't tell you where they come from.  If you have a perfect bed, dead level, then that shouldn't show up at all...but few beds are perfect.


As to the result of having the TS engaged or not with different HS alignment conditions....In general I agree...but the work piece will influence the out come in non-intuitive ways depending on how stiff it is, how it's mounted to the headstock  ( chuck, what kind of chuck, condition of the chuck, just a dead center in the spindle....ect)

My numbers come from the SB test sheets, and "Machine Tool Reconditioning"   
They are based on a test bar mounted in the spindle nose.




« Last Edit: July 02, 2018, 07:39:16 PM by steamer »
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Offline petertha

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Re: My tailstock is off
« Reply #21 on: July 02, 2018, 09:45:53 PM »
Here is my attempt at putting picture to (my) words. Click PT headstock alignment PDF, its kind of hiding right on top of pics. At this level it has nothing to do with chucks, test bars, cutting pressures or tail stock support interaction... Just focusing on the headstock alignment axis relative to the bed way axis in planar 2D. And the usual caveats; bed ways are straight, carriage travels along them straight & cross feed travels perpendicular to straight.

My 14x40 lathe pics. Kind of hard to see, but these are the 4 HS hold-down bolts. The shimming grub screws are deeper down the rabbit hole.

Offline steamer

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Re: My tailstock is off
« Reply #22 on: July 03, 2018, 01:18:57 AM »
The Southbend doesn't have that adjustment.   It has a flat and V way, so once the HS is down....it's not moving.    The only way to move it is to shim it,,, :vomit:


Or

Scrape it    Which I did.

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

 

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