magnetic fixture

[[Lo...]]

Been looking around on info for recommended pull force for magnetic fixtures. We had a company 3-D print some custom V-Blocks with a small Neodymium magnet imbedded in the base of the V. Well, they printed blocks didn't hold up. The part moves up to .0005 (inches) when the stylus engages and we have to hold .0008 axial runout, to the centerline, on several features. We are remaking the fixture out of steel and I need some advice on the pull rating of the magnet that should be used, the part weighs .7lbs. I now wish I had spent more time with the old tool maker and engineers that used to design these things because now it's my job. Thanks

[[Ch...]]

If you have very tight tolerances then a magnet will impact you measured values.

[[Cl...]]

I design a lot of fixtures that use magnets use magnets. Generally they are far enough away from the stylus that
it doesn't affect accuracy. When not practical, I use a plastic screw to secure the part? I just replaced a magnet
with a plastic screw and it works much better.

[[Lo...]]

I am no where near the actual magnets when measuring, that's burned me in the past, and when measured the part shows minimal on a gauss meter. Just having difficulty figuring out how strong the magnet should be to minimize movement. The company that made these seem to be ignoring my request for the specs on the magnets, my boss told them where they can place these expensive, paperweights, fixtures. I would love to strap them down with a wire clamp but I need to turn this over to production and I cant have an operator crush the clamps or forget to engage them.

[[Ja...]]

did u try using less probing force (in setting this defined by probing dynamics), so that part does not move while probing?

this is just a suggestion, I am not sure if this would be a good way to go about it since using less force may affect your dimensional measurement.
Any opinions on this suggestion?

[[Lo...]]

I did, it moved less but not good enough when compared to an off the shelf V-Block. I used some unidentified magnets we had lying around for this test. Off the shelf Block had zero movement. So I ordered magnets with a 6 pound pull force. I will adjust the height of the magnets in the block to apply the correct holding force. I was hoping one of you wonderful people had a calculator or some similar formula. Like I said earlier, I had someone smarter than myself figure all that out. When I asked the current Engineering group for help, all I received were blank stares. But, please, keep any ideas coming. Thanks

[[No...]]

I'm far from being an expert in magnets, but how is the magnet placed in the V-block? Can you post a photo? Because that matters a lot in order to make the magnetic field exert its full strength. For example, if you hide a strong neodymium magnet under a 10 mm solid steel plate, you won't get much force on the top side of the plate, because the plate literally "swallows" most of the field. The magnet needs some kind of air gap (or a gap filled with non-magnetic material) where the field can come into effect.
That's why most off-the-shelf blocks look like this: http://de.bms-industrie.com/images/pres ... ent-77.jpg

[[Je...]]

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Hi Chad.

I have been advised otherwise by Zeiss technicians, so I'd be interested to find out more. We measure magnetized parts quite often, we use magnetized fixturing, and we have observed high repeatability for measurement results. I wonder if Zeiss has any documentation on this?


Jeff Frodermann
Windings, Inc.
New Ulm, Minnesota

[[Ch...]]

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The only time I have ever witnessed the magnet having an impact is running gage rings and under +-.002mm tolerance. AND only when the gage ring was in the upright position. There was no impact when the gage ring was laid flat.

My theory is that when the gage ring is flat, then the magnet is pulling on the probe equally at every point. If the ring is in the upright position, then impact the magnet has on the probe will change as the probe get further from the magnet.

Full disclosure, I use magnetic fixture quite often. Just no longer for gage rings in the upright position. My guess is that magnets are probably ok for 99.99% situations.

[[Da...]]

We use a clamp fixture for this type of part.
if you're interested I can elaborate...

[[Je...]]

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Thanks, Chad. That's actually very interesting and makes sense. I suppose if mag. force is equally pulling down at each point, interference would be less likely.

I should probably mention that we use a VAST XXT passive sensor. I didn't ask the Zeiss technicians about active sensors. I wonder if active sensors, having more components, would be susceptible to magnetic force, or if the sensor would somehow compensate (or if the components are mostly non-ferrous and not significantly affected).


Jeff Frodermann
Windings, Inc.
New Ulm, Minnesota

[[Ow...]]

A Vast XXT passive sensor can only carry up to 15g of probe weight and most, if not all of the probes have carbon stems that are not magnetic.

A Vast Gold XT active scanning sensor can carry up to 600 grams of probe weight and a lot of the longer bigger ruby probes have carbide stems. Pure tungsten carbide itself isn't that magnetic but I believe carbide probe stems contain 3 to 20% Cobalt in the mixture and is ferromagnetic.

A lot of it depends on the magnets force. In a pinch for time, I once used a 1,000lbs two-pole lifting magnet to hold a part and when one of the carbide stems got close enough to it, accuracy went to hell in a handbasket.

[[Da...]]

I use fine pole magnetic chucks for almost all on our parts. These chicks we're designed for use in edm machines. They are quite strong but also clamp very small parts. We regularly get very close to the mag with no degradation on accuracy or repeatability. They're very versatile. You can see the small aluminum parallels that we use to space a part away from the side rails. They can be easily removed after the magnet is turned on, and allow full access to the parts right down to the tables surface. Earlier in this post I showed some clamp style fixtures that I use when I want to get access to both end faces. They hold the parts horizontally and the clamp fixture is simply put on to the mag chuck and away you go... I also hold very close tolerances and these fixtures provide incredibly stable, repeatable measurements even on non ground surfaces. I actually got the idea her on the forum and have fine tuned them to meet our needs ! 😃[img]
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[[Je...]]

That's a cool setup, David! Thanks for sharing.

It's actually been working well for me to palletize parts using permanent magnets that create downward pull on steel standoffs. We have plenty of strong magnets, since my company provides electric motor solutions. So, instead of purchasing custom-made standoffs, I wrap our own magnets in Kapton tape and locate them on or near the standoffs. I've gotten fairly good at adjusting the mag. force depending on the size and arrangements of magnets.

What's nice about your setup, Dave, is that you can demagnetize the fixturing surface with a lever, whereas I have to physically remove a magnet to break the field.




Jeff Frodermann
Windings, Inc.
New Ulm, Minnesota