Subject: Re: Warm Bearings

From: [email-masked] (David Erickson)
Date: 13 Jul 1995 06:40:45 -0700

In article <[email-masked]>, Mike Rehmus  <[email-masked]> wrote:
>
>Course the biggest problem most of us face is the poor surface finish 
>caused by the synchronous 'beat' of the feed-system gear train.  That is 
>why the better toolroom lathes (Clausing for example) have a separate motor 
>drive for the carriage and cross-slide feed rod and only use a gear train 
>off the spindle for threading.

I had been wondering about that problem.  Also the problem of resonant
frequencies.  I find that I get a much worse finish at certain rpms, and
I currently have a problem which I believe is vibration-caused, which is
shortening cutter life drastically (when facing a very hard cast iron
chuck backing plate, using a c5 carbide cutter which is quite sharp and
which has a normal amount of relief and a slight back rake, the cutter
will begin to dull after cutting about 1", and looking at it shows no
chips, just the beginning of a flat spot parallel with the facing cut).  
This is at 900 rpm, cuting an 8" backing plate from inside to outside
using the power cross feed, taking a very light cut.

This is on my Sheldon 11" lathe, which has a peculiar bed-mounting 
configuration (or it may be common and just seems peculiar to me).  
The lathe bed is mounted rigidly to the base at the headstock end, but the 
tailstock end has a single spring- loaded mount that is intended to let 
the bed avoid twisting forces, I guess.  

How I am supposed to level this bed is a mystery to me - I
leveled the headstock end, and the tailstock end didn't exactly follow.
I finally took a 2x4 and "tweeked" the end of the tailstock and got
it so that it was pretty close.  Adjusting the tailstock end of the base
>from  side to side had an imperceptible effect on the bed level, but may
have contributed to the vibration problem.  I think I will start over,
this time leveling the headstock end of the bed, then the tailstock end
of the base, and then tweeking the tailstock end of the bed if it is
needed.  I may try changing the belt tension too, to see if that has
any effect on vibration, and I will try removing the very small amount
of endplay that I left in the headstock thrust bearing (currently .002").

Which brings up my question: how do you locate the cause of a vibration
problem?  Is it a process of trial and error, like I am trying, or are
there better approaches, which the HSM would have access to?

-Dave Erickson

---

From: Mike Rehmus <[email-masked]>
Date: 13 Jul 1995 16:02:36 GMT

>frequencies.  I find that I get a much worse finish at certain rpms

The contibutors are: Gear train noise, belt noise (Use the best belt you can find 
and inspect it for 'lumps' where it was vulcanized (ends joined), bearing noise, and 
the big killer when it appears, chatter from underdamped bits and pieces.

, and
>chuck backing plate, using a c5 carbide cutter 

I think you need C2 carbide for cast iron.  I just cut down a 10" backing plate for 
my 8" chuck with C2.  I took 0.125 to 0.150 inch deep cuts at slow speeds with high 
feed rates with no problem.  Course the cast iron chips flew half way across the 
garage.

>configuration 

I essentially mounted my 6" Atlas that way at the tailstock end by just allowing the 
mount pad to slip between the surface of the aluminum plank on which it is mounted 
and a steel washer under the head of the mounting bolt.  This way I avoid turning it 
into a rather large bi-metalic strip (cast iron lathe bed bolted to a 2" thick, 
Blanchard-ground aluminum plank)
>
>Adjusting the tailstock end of the base
>have contributed to the vibration problem.

Doubt that had much to do with it assuming that the spring setup is a factory 
designed system.

  I think I will start over,
>needed.  

I may try changing the belt tension too, to see if that has
>any effect on vibration,

Probably won't change anything.  Just make certain the belt isn't so loose that it 
more or less describes an oval when under power.  Also make certain it is of 
constant cross section.

 and I will try removing the very small amount
>of endplay that I left in the headstock thrust bearing (currently .002").

Be careful with removing all the end play.  Thrust bearings are very sensitive to 
loads and can heat up very quickly.  They don't seem to mind the more or less 
intermittent cutting load but they don't seem to like a constant load.  If you want 
to try an experiment, put a bar between centers and then overtighten the tailstock 
ram.  The thrust bearing will get warm and you can even stop the spindle on smaller 
lathes with enough overpressure.

I'd leave a bit of play in that bearing.  0.002 does not sound excessive.

>there better approaches, which the HSM would have access to?

Professionals use a variety of tools including CAD programs and other, more specific 
analysis tools to predict vibration and minimize it or its effects.

Sometimes they can place accelerometers on a machine and, looking at the frequency 
of the vibration (and vibration waveform shape) determing the source of the 
vibration.  Doesn't mean they can remove the source (an car engine is a source that 
cannot be made vibration free, they just add things on to smooth it out)

It can be a real tricky problem.  On several models of Mercedes, so I'm told, there 
is a weight on a rod mounted behind one of the headlights.  That weight, in that 
place, solves a nasty harmonic vibration in the car's chassis.  You also see weights 
sometimes hung off of transmissions or rear-ends on automobiles to quell driveline 
vibrations.

If everything on the lathe is in good shape and operating at the correct tolerances, 
you should not have any vibration problems except chatter which can normally be 
cured by changes to the setup, speed and or feed.  When in doubt, go up in size.  
Stiffness goes up by the *fourth power* of the diameter of a bar.  That's why a 3/4 
inch boring bar is so much stiffer than a 5/8" bar.

Best,

Mike Rehmus

---

From: Colin Morton <[email-masked]>
Date: Thu, 13 Jul 1995 23:29:30 +0800

In article of 15:32 8/7/95, Mark McDade  x36841 <v040601@den writes:

>I'm bringing up a "new" ancient South Bend "heavy 10" lathe.  After
>about three minutes of running, the bearing caps are warm to the touch -
>after a long time (20 minutes or so), they don't seem to be much
>hotter.  Are they ok that way?  My guess is that they don't exceed
>110 degrees F.  The bearings run free and have no perceptible slop
>(though I haven't actually measured it).  I have been told that taking
>the assembly apart is a nightmare and to avoid it - also told to
>"flush" the assembly with kerosene - any ideas?
>
>Mark McDade
>
>[email-masked]
>

 Mark  You dont mention where the bearings on the lathe are I presume you are
talking about the Main spindle Bearings ?  do the bearings run inan oil bath ie
are they lubricated from the main gearbox? If they are they sound to be running
on the hot side
Colin Morton Uno engineering Western Australia