Improving a lathe spindle head.

[tonyfoale]

For some time I have been planning to change the single phase, single speed motor on my lathe for a larger 3 phase motor driven by a VFD (Variable Frequency Drive). The infinitely variable motor speed would allow me to eliminate the multiple pulley speed change system, but there is a problem when running a motor at low speed with a VFD. For a given spindle speed the spindle torque from a motor running at slow speed is less than achieving the same slow speed by mechanical gearing, for a given motor. A way around this problem is to use a vastly oversize motor.

This post is not about fitting the motor but is about some changes to the lathe head that the motor and drive system enabled me to do. My motivation was to make the head stiffer and less prone to vibration and chatter. Not that I had particular problems in that area but I find it hard to resist when I see a better way of doing something.

Although I show how I did this on my lathe, a 1240 JET, the ideas are certainly applicable to those lathes which do not have a closed in spindle head.

Click images for full size.

I have prepared a PDF file with all the reasoning and details at Inflicting GBH on a lathe

I plan to put together a video about this but in the meantime here is a link to a 2 minute video showing some stiffness measuring.
https://www.dropbox.com/s/7t7ho5x1qq...Lathe.mp4?dl=0

Warning: The pdf shows images of severe GBH to a lathe and those with extreme sensitivity to mechanical abuse are advised to look away now.

[DIYSwede]

Thanks for yet another excellent and humorous writeup, Tony!

2 personal cents:
-Having the headstock trapezoid-shaped (rather than just box square) should (at least in my empirical theory) make it more stable,
and filling it with concrete will also add to that (apart of being over the top - but that goes without saying).

Q: -What about you workshops temp differences thru the year, combined with the "linear thermal expansion" of CI vs steel and concrete?
Any thoughts or hypotheses on that?

-Please keep up all your different projects and good spirits!

Cheers
Johan

[tonyfoale]

Thanks for yet another excellent and humorous writeup, Tony!

2 personal cents:
-Having the headstock trapezoid-shaped (rather than just box square) should (at least in my empirical theory) make it more stable,
and filling it with concrete will also add to that (apart of being over the top - but that goes without saying).

Q: -What about you workshops temp differences thru the year, combined with the "linear thermal expansion" of CI vs steel and concrete?
Any thoughts or hypotheses on that?

Cheers
Johan

thanks for the nice words.
I did think about thermal expansion differences. For the range involved I don't see much of a problem between the steel and CI. The concrete is an unknown quantity and my fingers are crossed as a preventative method. The internal shape will not allow significant movement even if the bond get broken but fretting could cause dust.

[olderdan]

GBH indeed Tony but I like it. Could be the first ever lathe to suffer from concrete cancer (joking). Good move to eliminate the back gearing and I am with you on the merits of poly V belts. Thanks for another suburb write up.

[DIYSwede]

Thanks for a quickie reply, Tony!
EngineeringToolbox re: Coefficients of Linear Thermal Expansion:

Cast Grey Iron: 10,8
Steel: 11- 12,5
Concrete (unreinforced): 13-14
Concrete Structure (assumed Rebar): 9.8

unit: micrometer/ meter x deg C diff

-How steel reinforced concrete can have a lower thermal expansion than any of its constituents has, was above my paygrade.
Then - checking into that* - aggregate/ filler material makes this difference - quartz/ gravel has the highest and granite the lowest LTE coeff.

Paper below measures differences in shrinkage in just-poured concrete, as well as LTE with different wetting methods applied,
this could perhaps be of interest?

*Shrinkage of Concrete and Development of Cracks | Concrete Technology

Cheers
Johan, "also a Reader - and I'm PROUD of it!"

[DIYer]

Thanks tonyfoale! We've added your Lathe Spindle Head Modification to our Lathes category,
as well as to your builder page: tonyfoale's Homemade Tools. Your receipt:

Lathe Spindle Head Modification by tonyfoale

tags: lathe, motor

[bruce.desertrat]

I think this is why many modern folks have gone to a granite-filled epoxy resin for this purpose...(and reading quickly I see that Tony has mentioned this in the PDF!)

[metric_taper]

Major modification to your lathe. I applaud your lathe improvement.
I have a 1976 Jet 10x24 lathe. Very similar head stock construction. I found when I had pulled the spindle many years ago (because of an oil leak), that the oil level sight glass, had a single hole going straight back to the area between the taper bearing cup, and the seal (annulus). If the level dropped below this hole, the sight glass indicated there was oil, but the bearing was dry. I modified my head stock by hand drilling a 1/8" hole from the bottom of the oil sight glass reservoir to the bottom of this bearing cup and seal annulus space (both front and rear bearings). I do see now when it runs that the bearing pulls oil in from the reservoir, and shows a low level, but returns when the spindle is stopped.
I too have installed a 3phase motor on my lathe, and a VFD, but still use the belts. I did change the standard v-belts with the segmented type, which greatly reduced vibration and noise.
This lathe was originally on a wood bench, and then a sheet metal stand. I could never do parting operations as it would go into a harmonic oscillation of the headstock and ways. I purchased a 2 inch slab of steel to bolt the lathe onto. I put that slab on my Wells-Index vertical mill, and faced the hot rolled slag off. Then drill and tapped the 6 mounting holes that mate with the mounting ears on the lathe way casting. This fixed the rigidity of the lathe to where I have no more issues with cutoff operations.

[wa0dw]

Your removal of the back gears puzzles me. They lock the spindle when trying to change the chuck or faceplate on a threaded spindle. They may not be needed for machining because of the VFD, however certain operations such as tapping would have much more torque with the back gears engaged with a higher motor speed.

[tonyfoale]

I found when I had pulled the spindle many years ago (because of an oil leak), that the oil level sight glass, had a single hole going straight back to the area between the taper bearing cup, and the seal (annulus). If the level dropped below this hole, the sight glass indicated there was oil, but the bearing was dry. I modified my head stock by hand drilling a 1/8" hole from the bottom of the oil sight glass reservoir to the bottom of this bearing cup and seal annulus space (both front and rear bearings).

I was preparing a post on this very topic. What a crazy design feature that was, I wonder how many other lathe models have the same problem. The degree of incompetence that exists in some design offices never ceases to amaze me.