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Spherical grinding HELP!

Luke93

Plastic
Joined
Jul 19, 2017
Hey guys new here, cannot find any info on manual machining any more!
My work has just picked up a new spherical ball turning/grinding lathe I had the fun task of setting it up I'm having some major issues with the grinding side of it.
We're using a 500mm diameter wheel, 2inchs thick at 1300 RPM on a 4kw motor.
We're trying to grind a 50+inch stainless 316 ball valve ball. No matter what wheel, what the feed/speed/depth of cut is it just chatters immensely, I'm dressing/balancing the wheel even dressed the wheel so it was thinner. No matter what I do it just chatters for the life of my I can't figure it out and no one seems to have a clue either.
I've aligned the pulleys on the grinder and tensioned the belts and tried all sorts of grades of wheels.
Is there any diagnostics that can help me isolate the source of the chatter?
 
I'm no grinder hand, but a 2" wide wheel sounds a bit much, even though grinding the spherical face is
point contact, I would dress away some of that face width.
 
I'm no grinder hand, but a 2" wide wheel sounds a bit much, even though grinding the spherical face is
point contact, I would dress away some of that face width.

Neither am I but yes that thought is in my mind, I did dress it down to about 10mm wide it improved but was still chattering :( it also seems to leave a vibration finish after the wheel passes, but whilst cutting it leaves chatter marks

A picture of your actul setup with part in machine would help to diagnose the problem.

Shared album - Luke Van Den Bos - Google Photos

Sorry that's the only one ive got, behind the grinding attachment is a solid steel Arbor bolted onto the chick face with another 4jaw chuck on the end (gripping the ball)
I didn't design this set up but I too think it's not rigid enough especially with all the weight in the middle, the problem is it needs to be there for the table to be able to rotate the full circumference of the ball otherwise I'd only be able to grind half at a time (with such tight tolerances it has to be finished in one shot)
I've since shortened the tail stock to absolute minimal but still same result.

Part holding.

it also seems to leave a vibration finish after the wheel passes, but whilst cutting it leaves chatter marks, could that indicate the rigidity of the job being the cause?

I literally only just barely touch on with the grinding wheel and it instantly chatters, I thought that even if it was the rigidity of the job it wouldn't do it whilst barely touching on the job

It also turns fine with a cutting tool.in there without vibration but I do understand that's only a 0.8mm tip compared to a grinding wheel

Sorry guys just trying to give as much info as I can, been trying everything with very little resources or other professional opinions

Thanks!
 
A picture of your actul setup with part in machine would help to diagnose the problem.

Shared album - Luke Van Den Bos - Google Photos

I didn't set this up, Ive shortened the tail stock since, only picture I have too, but you can't see there's a solid steel mandrel bolted to the faceplate of the machine with a chuck on the end gripping the job which I think is a huge part of it but it has to be there to be able to finish grind the ball in one pass otherwise the table won't have enough room to be able to rotate all the way around it

Part holding.

I was confused by even touching the wheel.on the job (barely touching) it chatters instantly
At the right speed and feed it gives a vibration finish but chatter marks where the wheels contacting the job, is that a indication?


I'm no grinder hand, but a 2" wide wheel sounds a bit much, even though grinding the spherical face is
point contact, I would dress away some of that face width.

That was exactly my thoughts too! I've dressed the wheel to be about 10mm wide and still didn't have any luck, instant chatter still I'm not sure if that's too thin or not, I was contemplating dressing the middle away so it was cutting on the leading corner (will be hard to get the correct sizing) but would have absolute minimal contact point


Thanks alot guys it's so hard to find useful opinions and thoughts
 
The whole set up screams vibration especially being near on hollow, but I didn't think it would cause such chatter in the wheel even at minimal contact or am I just flat out wrong?
 
Also since that photo Ive shortened the tailstock to minimum and I've moved the bed forward removing the overhang of the grinding attachment
 
My experience tells me that you are going to have nothing but problems. I would find someone with the correct machine to grind the part. Tool post grinding even in a machine as big a robust as yours is always a problem. I'm sure you'll find someway to get through the job.

Good Luck

Athack
 
Wheel force down going, and has the weight hanging down like a spring with no support under so certainly it will like to chatter and wobble. Better would be to have the tail not be a tail but a standing vertical post and the wheel head going round a horizontal plane ..but done is done...Thinking... Wheel down pressure perhaps 40 to 80 pounds. Set an indicator under ball and set a 50lb weight on top to see indicator movement. Make a fixture coming off bed way to become a steady to support tail quill straight up and the outboard to near the part.

Set an indicator to the top of rotating table and set an 80 lb weight on it to see reading amount of down movement...Set an iron pad on the swing table perhaps 6" to 12"(?) in diameter having three set pads doweled and screwed to the table.. rough surface grind it to .002 or so then scrape it to be flat. now you need to invent or buy a screw jack.. like a mill jack with having a motor to rise the screw with a clutch like drive that would rise at perhaps 30 or 40 pounds with the screw dead solid to what ever up movement is made.. Jack top perhaps try Nylon for starts and perhaps going to bronze..that may solve the down going spring..

Likely need two such devices with one mounted at the opposite of the grinding wheel..

Yes you might just try the wheel going the other direction of rotation first..yes the wheel nut may be going wrong and the sparks would go up and I don't think that would help..Might try the wheel .005 or .015 above center also..You said any wheel.. perhaps call Radiac 800-851-1095 and ask about wheel choices.

Agree SS is a bugger to grind on any machine..

Buck
 
My experience tells me that you are going to have nothing but problems. I would find someone with the correct machine to grind the part. Tool post grinding even in a machine as big a robust as yours is always a problem. I'm sure you'll find someway to get through the job.

Good Luck

Athack

Thanks athack, 2 weeks in it's been nothing but unusual complex crappy problems this machines new and was purpose built for these jobs (chinese) haha think she's a lemon

Wheel force down going has the weigh hanging down like a spring with no support under so certainly it will like to chatter and wobble. Better would be to have the tail not be a tail but a standing vertical post and the wheel head going round a horizontal plane ..but done is done...Thinking.

I can always just stand the machine up on its end right? Hahaha kidding
Ive been trying to think of a way to support its weight but I'm still in disbelief that just the set up can create the level of chatter it's producing !
 
I didn't set this up, Ive shortened the tail stock since, only picture I have too, but you can't see there's a solid steel mandrel bolted to the faceplate of the machine with a chuck on the end gripping the job which I think is a huge part of it but it has to be there to be able to finish grind the ball in one pass otherwise the table won't have enough room to be able to rotate all the way around it
I can't see google so pulling idears out my ass here but you know the open-front supports for grinding long shafts ? Any chance you can make something similar ? Not likely the contact surface is too big, grinding a sphere it's going to be real narrow no matter what. 90% chance your part is quivering the instant you touch it with the wheel.

Even a support from below with a spherical socket shape might help ? Top and bottom would be even better, with a little bit of drag between them ?

Not to stand up for China but I don't think the machine can keep the part from vibrating. That's a workholding problem :(
 
I can't see google so pulling idears out my ass here but you know the open-front supports for grinding long shafts ? Any chance you can make something similar ? Not likely the contact surface is too big, grinding a sphere it's going to be real narrow no matter what. 90% chance your part is quivering the instant you touch it with the wheel.

Even a support from below with a spherical socket shape might help ? Top and bottom would be even better, with a little bit of drag between them ?

Not to stand up for China but I don't think the machine can keep the part from vibrating. That's a workholding problem :(

If this is a custom machine for doing ball valves I would expect the machine to have suitable workholding as part of the machine.

Sounds like it is indeed a lemon. Can we have some more detailed photos ,my bet is that some of the experienced guys will spot where the machine is lacking.
 
Not to stand up for China.. Cash would have designed a better machine..Looking at the concept I would have said No....A special machine like that I would had gone to a German outfit...Yes we had outfits in the USA that would have done much better..I can think of many names..Gone is gone.

If that is a ball way table yet another wrong design, A roller way tail? another wrong...Richard would have done better...

at 1300 RPM.. the part? Might slow that down. lock set the table off the balls if so and solid..to rough grind each a series of lines around then finish tickle with rotating table..Might look at Blanchard table as example of a solid oil movement rotary tabel.. but done is done and try to make that machine work is the OP.

Perhaps show the chuck end.

And I am going off with the Boy Scouts so I can't come.
 
With that periodicity of chatter, and that it seems consistent all around the ball (which has varying natural resonance due to the through hole), I'd think the issue is in the grinding wheel side.

Are the wheel spindle bearings tight? If you 2/side tape a flat metal plate to the wheel end and side, and mount a sensitive indicator against the plates and lever against the wheel with a lenght of wood, do you see easy movement? Try this at a couple of rotation points in case the bearing inner race is out of round.

What linear/rotary slides does the wheel move on? Are they tight? Analyze each axis of motion for free play and lack of preload.

Wheel balance might be an issue (but that would be a lower frequency effect, I'd think), and also the dressing method - is dressing done with a slide, with a small rotatory nib (for a concave profile), or a crush roller at the end? It might not have good control of the geometry, and is dressing in a periodic wave into the wheel.

More and better pictures of the setup, including of the dressing mechanism and wheel mounts and free axis of motion would help.

Lastly, you could try to contact one of the guru's in the field, Jeffrey Badger, the Grinding Doc. He posts here on occasion. Welcome! - Metal Grinding Expert Classes and Clinics - TheGrindingDoc
 
Why don´t you make simple non-contact supports, of high accuracy, zero contact, pwr-4 rigidity and low cost ?

Make some surface-inlets, shapelets, of near -net shape, of appropriate size, ie 50" inside radius as you said, as near as cheap-easy-practical.

Suggest about 50x50 mm squares, or == 60-80 mm D round, turned to near-net shape of the ball surface.
Drill 30-40 holes, about 4x D of the hole, in depth (reservoirs).
So maybe 1-2-3 mm D holes == 4-10 mm deep, in the pads, on the inside, and a bit smaller through hole, say 0.6 D or so.
Feed in air, at around 0.5 bars.
It´s called an air-bearing, easy to make, and *very, very* rigid, if you add suitable pressure, volume, or reduce clearance.

Someone smart might make it in 2 parts, with the reservoir inners being per-job, and the rest being at-need.
The second part has a cavity to distribute air to the ports, and a single drilled hole to an air nipple, in the center.
Bolt or glue the shoes together, yes (industrial) glue will hold fine.

Would probably take about 1 day to make a fancy-pansy great looking job with 4-6 supports, with any cnc equipment.
Or anyone with a lathe about the same 1 day to make perhaps fewer, equally-functional parts, for one who can turn some inside-radius stuff somewhat competently.
On such a big surface, the clearance can even be quite large, if you have a bigger compressor, perhaps even around maybe 0.5 - 1 mm clearance would likely work, at higher pressure/flow/noise.
(6 supports with 60 mm D surface each, makes for a lot of surface-area, and very very high support load capacity, in an air bearing).


? wont work ??
From my tlar, 6 supports of 60 mm D, at 1 bar / 0.2-0.5 mm (depending on airflow in l/min), would probably support 20-40 metric tons before contact.
Much more if the clearance is around 0.02 mm, somewhere in the ballpark near it.
(More clearance needs more airflow and a bigger compressor.)

The inner-liner holes act as dynamic reservoirs, and stiffness goes up as clearance-distance to power-of 4, while flow decreases with distance.

One could use ball-joints of any type, to mount the supports-- just use big thick ones to get the most benefit.
Ie the post below the joint must/should be thick !

If using say 60 mm D round bar to make the inner liners, I would aim for support posts of 40-80 mm D, in steel, and 40-100 mm D ball-joints.

Or just make a 2-axis clevis joint / universal joint of choice, for each support, sized for carrying 20-30 tons each.
Say 20 mm thick steel plate, 100x100 mm square platefor the joints, drill and ream holes for joint axels, TGP bar/drill rod/linear bearing rod, of about 30 mm D as the pivot pins.
Strength is not the issue, avoiding bend/jitter is the issue.
To support say 5000 kg theoretical for 50 kg practical, or 1% load, per joint.

Might cost 10-15 kg mass / joint and 40$ in materials, each, for 3-6 of, and maybe 1 day of work manually on proper equipment.
The air distributes the errors and geometrical-averages-it in a quite wonderful way.

All above is related to my spindle-building exercises, examples I have seen, and the math / examples in university industrial engineering books.
And lots of hours of learning experiences.

The advice is worth what You paid for it so far ...
and practical demos can be made available.

Multiple benefits, esp. if arrayed semi-hemispherically, for 3-4-5-6 pieces:
Self-centering,
(strongly) self-dampening,
cheap,
arbitrarily strong/stiff,
low cost to make,
low materials cost,
low skill needed to make,
relatively low work-hours needed,
arbitrarily accurate to even below sub-micron range, if needed,
( ..with some secondary reverse processing (grind the inside pads)(lap the pads).)

Ie gets stronger and better the harder You press down, a 60$ air manometer will regulate it, no electronics needed.
+Improves tir, despite spindle errors.

+Improves apparent or functional machine rigidity, perhaps 10x.
Because a lot of the stress is now spread outside the contact point, in a geometrically-increasing way, on perhaps 50x50 cm area vs a tiny contact point at the grinding wheel.

+All the bits can and will be used in the future, since You already bought a machine to do this stuff, so obviously You will encounter these issues all the time.
And the clevis/pads/joints means 95% of all work can be used with many-or-most-all new jobs.

Just make the support arms for each pad flat bars, clamped in place, from outside.
Say 40-50 mm x 80-100 mm arms, held in place at the periphery of the machine, thus nothing gets in the way.
Just my 0.03€.

If it is unclear, I can make a 3D model privately.
Take 1 day, 2 at most.
 
Use a cup wheel, and straddle the ball ? (feed spindle radially into centerline)

Springfield made vertical grinders that looked like Bullards.
 








 
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