[Emc-users] medium frequency vibration, spindle bearings?

[Gene Heskett]

Greetings all;

I am having a vibration when cutting a harder steel, (old worn mine
shafting stuff, rusty surface too) seems to be fairly fixed in frequency
to somewhere in the octave below middle C.

I have no "shake" in the rear spindle bushing and it runs about 10F
higher temp than the front bearing in terms of temperature rise from 10
minutes at 1600 revs, but reading on the rings I made to lock the
spindle and chuck together for prevention of a chuck unscrew on a fast
spindle reversal, I can see under a thou of movement when a 2 foot piece
of pipe is chucked up and used for a lever with 50 or so lbs of lift on
the end of the pipe. I do not feel this vibration at the toolpost but on
the front bearing cap with a much weaker feel on the rear bearing cap.

Is this just barely measurable movement the likely suspect?

Thanks everybody.

Cheers, Gene Heskett

[andy pugh]

Post by Gene Heskett
Greetings all;
I am having a vibration when cutting a harder steel, (old worn mine
shafting stuff, rusty surface too) seems to be fairly fixed in frequency
to somewhere in the octave below middle C.

Can you kill it by oscillating the spindle speed over-ride?

[Gene Heskett]

Post by andy pugh

Post by Gene Heskett
Greetings all;
I am having a vibration when cutting a harder steel, (old worn mine
shafting stuff, rusty surface too) seems to be fairly fixed in
frequency to somewhere in the octave below middle C.

Can you kill it by oscillating the spindle speed over-ride?

yes, momentarily Andy, but it right back as soon as the speed steadies,
at rpms from 50 to 600. And the oscillation frequency is quite constant
over that range. Feeling the spinning workpiece, its much stronger
there, than anyplace else.

But, while I was out and 40 miles down the interstate, to a D.M.V. office
taking care of fresh handicapped hanger tags for the rear view mirrors,
and a 2 year license sticker for the Toy Rav-4, I stumbled over a
locally owned Ace HDWE dealer, and found 1 only 1.25" mounted stone I
can put in the die grinders chuck, and maybe I can cobble up some code
to make this 30" barrel lose a bit of weight. A pound would be a lot,
hauling it around in the deer woods. I'd settle for half a pound even.
Even that is still a pound more than it weighs now.

If this grinding wheel works and leaves a usable finish on the SS, that
negates the need to make an arbor. For a small time at least.

Thanks Andy.

Cheers, Gene Heskett

[andy pugh]

Post by Gene Heskett
yes, momentarily Andy, but it right back as soon as the speed steadies,

Don't let the speed steady?

Are you running a tailstock centre? if tightening that (which, depending on
the bearing type, might preload the head bearings) then I would suspect the
head bearings.
What type of bearings do you have?
http://www.lathes.co.uk/sheldon/index.html doesn't seem to say.

[Gene Heskett]

Post by andy pugh

Post by Gene Heskett
yes, momentarily Andy, but it right back as soon as the speed steadies,

Don't let the speed steady?
http://youtu.be/dXR2wDDFFm4
Are you running a tailstock centre? if tightening that (which,
depending on the bearing type, might preload the head bearings) then I
would suspect the head bearings.
What type of bearings do you have?

Bronze with adjustable by shims caps. No shims at all in the rear, left
end, and 4 mills or so in the front,right caps after some adjustments &_
monitoring with an IR thermometer. Any tighter and the heat rise at
speeds above 500 rpms seems too much. It came into my possession with
at least 20 thou of slop in the left bearing, and perhaps -3 thou too
tight in the right bearing which ran burn your fingers hot. Ed Nisely
was nice enough to donate some shim stock he no longer needed, nearly a
year back. Between that and some Reynolds Wrap I thought I had it pretty
close.

Post by andy pugh
http://www.lathes.co.uk/sheldon/index.html doesn't seem to say.

Various production runs had different bearings according to John Knox on
that mailing list. And many times the manuals that might come with it
have bogus bearing info. Such was the case with the in the desk spindle
drives, the countershaft bearings may be shown as torrington needle
cartridges, but mine came with a single oilite bushing long enough to
fill the space that a pair of torringtons plus a space in the center for
grease distribution. That didn't work so well when it was setup with
grease zerk fittings, the viscosity of the grease created a lot of heat.

On disassembly to drive out the bronze and pull the torringtons back in,
I found the real problem was the lower pulley carrying the two belts
that spin the spindle, had been reversed on the shaft, so the key was
not even in the slot, and the setscrew that was supposed to be locking
the key in place, was the only thing spinning the pulley with the shaft,
with at times obvious slippage which also generated heat that
telegraphed to the outer pulley and into the bearing pockets of the belt
tensioning yoke. Made the belts stink like hot rubber etc etc.

So I made a browning style tapered collet in the left face of the pulley
because thats the only chance I had of making it run semi-true on the
shaft, due to both the shaft wear, and wear in the plain hub of the
pulley. That shaft would be simple to make but is RC60+, so its quite
close to being made from un-obtainium as there is no more N.O.S. of it
left on the planet according to John Knox.

Cheers, Gene Heskett

[andy pugh]

Post by Gene Heskett
Any tighter and the heat rise at
speeds above 500 rpms seems too much.

What oil are you using? Some lathes need very light oil. I run ISO 32
hydraulic oil in the Rivett. (10 wt on the other scale)

[Gene Heskett]

Post by andy pugh

Post by Gene Heskett
Any tighter and the heat rise at
speeds above 500 rpms seems too much.

What oil are you using? Some lathes need very light oil. I run ISO 32
hydraulic oil in the Rivett. (10 wt on the other scale)

Exactly whats in the can, 10w non-detergent for air compressors. Seems
thick and I may switch to 0w-20 synthetic when this is used up.

Cheers, Gene Heskett

[andy pugh]

Post by Gene Heskett

Post by andy pugh
What oil are you using? Some lathes need very light oil. I run ISO 32
hydraulic oil in the Rivett. (10 wt on the other scale)

Exactly whats in the can, 10w non-detergent for air compressors. Seems
thick and I may switch to 0w-20 synthetic when this is used up.

Looking further. On the Holbrook list it appears that the manufacturer
specifies ISO 5 oil for the plain bearings. Folk with those lathes use ISO
5 hydraulic oil.
I can easily find it in 1L quantities in the UK:
https://www.alexoil.co.uk/hydraulic-oil/548-hydraulic-oil-iso-5-1l.html
But can't work out how to localise the search to your area.

(I was initially thinking that 5-wt motorcycle fork oil would be an easy
source, but that appears to be SAE 5wt, so not the same thing at all.)

[Gene Heskett]

Post by andy pugh

Post by Gene Heskett

Post by andy pugh
What oil are you using? Some lathes need very light oil. I run ISO
32 hydraulic oil in the Rivett. (10 wt on the other scale)

Exactly whats in the can, 10w non-detergent for air compressors.
Seems thick and I may switch to 0w-20 synthetic when this is used
up.

Looking further. On the Holbrook list it appears that the manufacturer
specifies ISO 5 oil for the plain bearings. Folk with those lathes use
ISO 5 hydraulic oil.
https://www.alexoil.co.uk/hydraulic-oil/548-hydraulic-oil-iso-5-1l.htm
l But can't work out how to localise the search to your area.
(I was initially thinking that 5-wt motorcycle fork oil would be an
easy source, but that appears to be SAE 5wt, so not the same thing at
all.)

Knowing a wee bit, probably enough to be dangerous, about the
hydrodynamics involved, those are quite huge bearings to be set at sub
thou clearances where the oils viscosity can represent a considerable
portion of the drag involved, 10w in those clearances might as well be
90w gear oil. At 2k revs, and no cooling circulation of the oil from a
larger oil pan and pump like an IC engine has, theres no way in hell its
not going to get hot. Recommendations not withstanding, 10w is too thick
IMO. So when that pumper can is empty, it will get refilled with the
lightest syn-oil I can locally source.


Cheers, Gene Heskett

[Cecil Thomas]

Gene,
I think you are dealing with ringing as opposed to chatter. Chatter
is usually a low frequency vibration caused by the tool biting in and
releasing repeatedly. The frequency and intensity will change as you
change spindle speed, cutting depth and feed rate.

Ringing on the other hand is just like the ringing in an LC
circuit. It is a function of the stiffness of the material, its
diameter, its length and how it is restrained at each end. Given an
appropriate text book and a calculator you can actually calculate
what the frequency should be. The only way to stop the ringing is to
add damping, a lossy element (resistor)to dissipate the energy.

Think about a bell or a guitar string. As long as the work piece is
significantly longer than its diameter it will ring like a bell.
The only way to stop the ringing is to absorb the vibrations, think
putting your hand on the guitar string. I have solve "ringing"
problems by working only on one half or less of the stock and
sticking gobs of modeling clay on the other half and wrapping all
that up with duct tape. That kills the vibration. when I finish the
first half I move the "damper" to the other end.

The simplest way is to use a follower rest. That stops the vibration
immediately. Didn't you get a follow rest with the lathe?
This might call for a few manual passes with the follow rest before
you get fancy with the CNC. That's why I insist that my CNC
conversions retain handwheel operation.

Cecil

Post by Gene Heskett
yes, momentarily Andy, but it right back as soon as the speed steadies,
at rpms from 50 to 600. And the oscillation frequency is quite constant
over that range. Feeling the spinning workpiece, its much stronger
there, than anyplace else.
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[Gene Heskett]

Post by Cecil Thomas
Gene,
I think you are dealing with ringing as opposed to chatter. Chatter
is usually a low frequency vibration caused by the tool biting in and
releasing repeatedly. The frequency and intensity will change as you
change spindle speed, cutting depth and feed rate.
Ringing on the other hand is just like the ringing in an LC
circuit. It is a function of the stiffness of the material, its
diameter, its length and how it is restrained at each end. Given an
appropriate text book and a calculator you can actually calculate
what the frequency should be. The only way to stop the ringing is to
add damping, a lossy element (resistor)to dissipate the energy.
Think about a bell or a guitar string. As long as the work piece is
significantly longer than its diameter it will ring like a bell.
The only way to stop the ringing is to absorb the vibrations, think
putting your hand on the guitar string. I have solve "ringing"
problems by working only on one half or less of the stock and
sticking gobs of modeling clay on the other half and wrapping all
that up with duct tape. That kills the vibration. when I finish the
first half I move the "damper" to the other end.
The simplest way is to use a follower rest. That stops the vibration
immediately. Didn't you get a follow rest with the lathe?
This might call for a few manual passes with the follow rest before
you get fancy with the CNC. That's why I insist that my CNC
conversions retain handwheel operation.
Cecil

I think you're right, and yes I have both steady and follower, but nut
enough workpiece length to make use of them.

And I still have cranks, but they are mpja encoder wheels that I can set
to is little as .0005" per click.

I have the part turned to a much better major diameter, which when turned
around in the chuck, ought to be much more rigid than it now is as its
being gripped on an about 3/8" extension that will be turned off
eventually. If I even use it. I found a mounted stone this afternoon,
which if I can source more of, might be just what Dr. Gene wrote on the
prescription pad. I have already mounted a big HF die grinder to an AXA
toolholder.

The toolpost, a plunger style Phase-II, is being a problem child, not
wanting to pull the plunger in to release the toolholder on one face,
and the pad holding screw feels gauled or something in the plunger as it
comes loose but will not come clear out. Fells like its full of grit
and eventually starts spinning the plunger, or feels like it.

Do you have any ideas on how to disassemble and clean/lube one of those?

Post by Cecil Thomas

Post by Gene Heskett
yes, momentarily Andy, but it right back as soon as the speed
steadies, at rpms from 50 to 600. And the oscillation frequency is
quite constant over that range. Feeling the spinning workpiece, its
much stronger there, than anyplace else.
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Cheers, Gene Heskett