unbalance in magnetic lathe chuck

michiel · Jun 5, 2021

Hi everyone

,

In my small machine shop (Weiler Matador, Aciera F3, Deckel SO clone, large drill press with XY-table, MIG and TIG welding equipment), i build, modify and repair machinery and tooling for my company.

Recently i bought a used magnetic lathe chuck from a dutch brand (forgotten the name). This was fitted with a D1-d Camlock adapter plate, i think for a AI Hembrug lathe. Because this will not fit my lathe, i turned a new adapter plate to measure and fitted the chuck. The face of the adapter plate turned out perfectly flat, the shoulder was accurate to measure.
The problem was that the chuck had a lot of runout although i think it was mounted correctly to the plate. Also, unbalance occured which causes the lathe to vibrate above 500 RPM. The runout was on the outside perimeter of the chuck, not on the face. After taking the assembly apart, indicating the plate on all sides (nothing wrong), I concluded that the best way forward was probably to machine off the runout.
Now the chuck has no runout, not on the face nor the side. But the unbalance remains: on 800 RPM the whole lathe vibrates. Maybe, components inside the chuck cause the unbalance. Also a good chance is that I did something horribly wrong. But what?

L Vanice · Jun 5, 2021

If this is a permanent magnet chuck, there will be a device to shift internal parts to turn the face magnetic field on and off. I would expect the chuck to be in fairly good balance with the magnetic field on and possibly bad with the field off.

In any case, I would not trust a magnetic chuck on a lathe to hold work at high RPM. Flying parts can be dangerous.

I have two round magnetic chucks. One is Eclipse brand and was specifically designed to be used on a 5C spin/index grinding fixture, so zero to hand crank speed and low cutting force. The other is, I think, an 8 inch diameter flat back for lathe mounting. It is still in the box after thirty years or so because I never really needed it.

Larry

thermite · Jun 5, 2021

L Vanice said:
If this is a permanent magnet chuck, there will be a device to shift internal parts to turn the face magnetic field on and off. I would expect the chuck to be in fairly good balance with the magnetic field on and possibly bad with the field off.

Near as I can tell, my 6" Walker-Magnetic ... built for and branded by, Hardinge, UK, and square-key operated is in balance in either mode. Pull is supposed to be 750 lbs, Avoir.

OP:

Something was probably adrift inside the chuck.

When you lathe-turned the outer housing, the walls were no longer true to the original thickness nor balance.

Wrong solution to the problem.

You will now have to detect and correct the ORIGINAL issue, and then, re-balance the now-assymetrical body with added weights and/or drilled lightening divots.

That is easy work for a proper balancing shop. It is "doable": but far from easy for an amateur with no "real" equipment for it beyond need and dedication to see it through. Read that as "tedious, very-damned."

Your call if it is worth it. It might not be.

Larry:

I surely do not have another 30 years off a 1945 vintage, but you have done YOUR loyal duty protecting that 8" magnetic for 30 years, so if you'd care to price it not too assertively, I could take a turn at tool-whore/packrat "guard-duty"?

The 6" is a good match for the 10EE's. An 8" not, but...I've finally gotten OEM Cazeneuve backplates out of France and on the way, will be ordering more.

Ray Behner · Jun 5, 2021

I had the same problem with my 10" Japanese chuck. Beautiful piece. Thanks Acme Thread! I thought it was only when out of field, but no. I drilled several ¾" holes around ½" and more deep in the back plate. Kept drilling and kept getting better. I drilled in an arc of maybe 40/50º each side of center of imbalance. Although it will spin dead smooth at 2500 rpm, I don't go that speed. It has a 20mm hole in the center where I put a close fitting pin, so everything is anchored to it. Either by a bushing or smaller pin.

plastikdreams · Jun 5, 2021

Who in their right mind would use a mag chuck on a lathe?!?!!?

thermite · Jun 5, 2021

plastikdreams said:
Who in their right mind would use a mag chuck on a lathe?!?!!?

Leftists, maybe?

Polarity thing.

michiel · Jun 6, 2021

L Vanice said:
If this is a permanent magnet chuck, there will be a device to shift internal parts to turn the face magnetic field on and off. I would expect the chuck to be in fairly good balance with the magnetic field on and possibly bad with the field off.

Larry

I tested it both in 'on' and 'off' position, same unbalance.

thermite said:
OP:

Something was probably adrift inside the chuck.

When you lathe-turned the outer housing, the walls were no longer true to the original thickness nor balance.

Do you mean to say that the manufacturer of the chuck deliberately created a runout of several mm to counteract the unbalance in the mechanism?

michiel · Jun 6, 2021

Ray Behner said:
I had the same problem with my 10" Japanese chuck. Beautiful piece. Thanks Acme Thread! I thought it was only when out of field, but no. I drilled several ¾" holes around ½" and more deep in the back plate. Kept drilling and kept getting better. I drilled in an arc of maybe 40/50º each side of center of imbalance. Although it will spin dead smooth at 2500 rpm, I don't go that speed. It has a 20mm hole in the center where I put a close fitting pin, so everything is anchored to it. Either by a bushing or smaller pin.

Thank you.
How did you find the side of imbalance?

Ray Behner · Jun 6, 2021

michiel said:
Thank you.
How did you find the side of imbalance?

michiel
Very unprofessionally. I jury rigged a set of bench centers and let it find the heavy side. It always ended up precisely opposing the chuck keyhole. Made sense to me. The movable piece wasn't accounted for when built. Or more likely the balancing step was forgotten. Good luck!
Ray

thermite · Jun 6, 2021

Ray Behner said:
michiel
Very unprofessionally. I jury rigged a set of bench centers and let it find the heavy side. It always ended up precisely opposing the chuck keyhole. Made sense to me. The movable piece wasn't accounted for when built. Or more likely the balancing step was forgotten. Good luck!
Ray

Nah. You know young "Design Engineers?" Those who had NOT come up off the shop floor - never gotten chips embedded in their tender slide-rule-munging hands?

Some naif had likely seen a picture of a chuck with the KEY hung in the hole... and expected it to be run that way. But for only a fraction of the duty-cycle. Before it was flung out.

So they "did the math" and tried to average-out the offset by count of revolutions, with and without the key!

Predictable results ensue.

Ever try to shift a pushbutton Edsel motorcar out of "PARK" whilst on a hill?

Didja know that Olympic, Titanic, and Britannic were designed with rudders too small for adequate maneuvering of hulls of their waterline length? As-in to miss grazing an Iceberg or a sea-mine?

Be glad it was only a magnetic chuck, this go!

michiel · Jun 6, 2021

This morning I managed to open the chuck. Nothing is loose or anything, but there is nothing counterbalancing the moving weight of the magnets. Stupid design.
Inside, there is a bit of room for those led balancing weights they use on car rims. I will get some of them tomorrow.

@Ray Behner: thank you.

Ray Behner · Jun 7, 2021

michiel said:
This morning I managed to open the chuck. Nothing is loose or anything, but there is nothing counterbalancing the moving weight of the magnets. Stupid design.
Inside, there is a bit of room for those led balancing weights they use on car rims. I will get some of them tomorrow.

@Ray Behner: thank you.

Keep us posted. With pics if possible?

thermite · Jun 7, 2021

michiel said:
This morning I managed to open the chuck. Nothing is loose or anything, but there is nothing counterbalancing the moving weight of the magnets. Stupid design.
Inside, there is a bit of room for those led balancing weights they use on car rims. I will get some of them tomorrow.

@Ray Behner: thank you.

"Stupid" and also surprising.

"Ordinarily" the effective way to release a Permanent-Magnet chuck is to rotate half of the magnets right in-place so the poles are opposed and the magnetic fields simply cancel each other out in the de-activated position .... or gang-up to pull as a team in the active position.

No "magic" involved.

They NEED to remain exactly in-place with respect to their fixed mates for that to work well.

So you'd have to "work at it" to make the mass-balance of the active magnets change on even a rectangular grinder mag chuck that is NOT spun-up when in-use.

Why a lathe mag chuck was not made to be inherently balanced .. or "at least".. statically if not also dynamically balanced as Ray did and you are about to do?

Well there is really only one word for it:

"unforgivable"

.. as proper Engineering goes.

It just ain't that HARD ... if you but make it a routine part of OEM production.

Conrad Hoffman · Jun 7, 2021

Are you sure this thing was intended for lathe use? Seems like an accident waiting to happen. Better than double-stick tape though.

thermite · Jun 7, 2021

Conrad Hoffman said:
Are you sure this thing was intended for lathe use? Seems like an accident waiting to happen. Better than double-stick tape though.

Does seem like a rotab, DH, or grinder animal?

Not as good as tape on plastics, Bronze, or shiney-wood though!

"Joe pie" demos an old technique I'd personally have kept my 'mout shut about knowing, lo these many scores of years.

Because it can be very-damned risky.

Pressure Turning - YouTube

Conrad Hoffman · Jun 7, 2021

Huh, so it has a name. I've done that a lot over the years, but never with anything more than a few inches in diameter. Works fine even on my lightweight Logan, but you should choose your materials for good friction.

thermite · Jun 7, 2021

Conrad Hoffman said:
Huh, so it has a name. I've done that a lot over the years, but never with anything more than a few inches in diameter. Works fine even on my lightweight Logan, but you should choose your materials for good friction.

"Pressure" turning? We called it "friction chucking".

Conveyor belting? Paper scares me what with zero resilience, and I'm chicken.

Well. We didn't usually have a "live"center, either...so..

Besides.. tough-as with the cord-in, but smoother-than..... old tires ... and there is always somebody selling the cut-offs.

Except in Hong Kong.

The demand to glue a layer of it on to deaden raindrop noise on 15 or 20 million s**t-metal through-wall and/or "split" unit air conditioners (our 3 br flat has four..) means they have to buy NEW conveyor belting! But every one-person no room for a hamster to run the abacus hole in the wall stocks it. AC units don't last long and the box-knife opening to match louvers on the old sheet won't match the new unit anyway even if one COULD get that insanely durable adhesive to part!

Acts like 1950's Goodyear "Pliobond" and could was it IS .. a local copy?

I mean these sheets of conveyor belting stand several typhoons and years of rain and baking in the tropical sun?

Conrad Hoffman · Jun 7, 2021

Ha! Pliobond; I keep some around for re-bonding leather to cameras and such. You can still buy it.

thermite · Jun 7, 2021

Conrad Hoffman said:
Ha! Pliobond; I keep some around for re-bonding leather to cameras and such. You can still buy it.

Yes, and it isn't half bad, but .. what we can "still buy" isn't the same chemistry as it was back when it was becoming a legend.

Carcinogens and such have forced more than one change.

China still uses the original nasty, copied or not, 'coz it works better.
And the population isn't stupid enough to eat it nor desperate enough for peace as to have to glue their kids feet to the overhead, either.

Screwmachine · Jun 8, 2021

Just for info- I have a 100mm mag chuck from Bohli with a w20 backplate to fit my Schaublin 102, Schaublin supplied these but Bohli offers them with and without the backplate as well. No vibration anywhere, checked up to 1500rpm, seems pretty darn balanced and that's with the magnet both on and off.

They do charge 230 bucks to fit the backplate to the chuck- maybe they balance them?

When I do use it I am usually well under 1000 rpm as typically the work itself isn't centered.

bohlimagnete.ch

unbalance in magnetic lathe chuck

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