Ballbars

[small.planes]

Evening all,
I have to make (or more accurately get made ) some spherically concave magnets.
Ignoring the magnet part for a second these things have to be *super* accurate. They are to hold Balls for a calibration artifact for the metrology system I work on.
The exact specs are at work, but basically they are for a ball bar (thats a bar with a ball on each end, common metrology artifact) 1 meter between ball centers, which needs to have an accuracy of 1 part in 100,000 (IIRC, might be more accurate than that) so the bar has to end up at 1m +-0.005mm, but with removable balls...
Ideally we'd like to be able to 'wring' the balls into place so they are repeatably positioned (this is a stepping stone to a more complex artifact - hence the removability) and then the magnet and wrung force will hold the balls in place. IIRC the balls are a few Kg heavy, so probably will end up using neodynium or samarian magnets.
The bar part will be a Carbon fibre tube, and these spherical seats will be fixed to the ends, and set to the correct length by measuring with an interferometer against an Invar standard. Of course that means glue near some very expensive kit...

So, as you can probably guess this has started come from an Academic, and although I think its probably possible Im guessing doing it this way would be expensive.

Any ideas on whether getting spherical concave magnets is even possible, where to look for them, and how to make sure they end up on the ends of the tube positioned to super tiny tolerances very gratefully received

Dave

[TGTool]

I don't know if this is feasible as you envision the part, but the first thing that occurred to me was making the spherical part a laminate stack like a magnetic parallel and installing the magnet on the opposite side, that is, inside the tube. That way you can deal with some of the issues separately from complications of magnetism.

[John Garner]

Dave --

I don't think you want the ball sockets to be spherical, nor do I think you want them made of the magnetic material itself. Instead, take a look at a magnetic "nest" for a Laser Tracker's "Spherically Mounted Retroreflector" (SMR) and then at the Bal-Tec website.

Hubbs Machine and Manufacturing may be the US's premier maker of SMR nests. http://www.hubbsmachine.com/

Bal-Tec specializes in precision balls, but do know a few things about mounting them. http://www.precisionballs.com/

John

[Just Bob Again]

Rare earth magnets are sintered and relatively fragile. They'll shatter easily and have to be plated to avoid corrosion. They'd have to be ground before magnetizing, then plated. Far simpler to machine the actual contact surface from something more reasonable and have the magnet behind it in a counterbore from the back side. You'll need a substantial magnet. A least an inch diameter, maybe 2. You could do the laminated alternating pole stack (harder) or just use a single recessed magnet and have less holding power. Without the laminations, the external field will be much higher and you may have real problems with the standard being attracted to other objects. With the laminations, the field won't be a problem more than an inch away or so.

[winchman]

It seems like you could make a nest for the ball and turn the magnetic attraction on/off with something like a magnetic indicator base.

It's been a while since I handled gauge blocks, but I'm doubtful you'll be able to wring the matching spherical surfaces together without some flat starting points on the balls. If you ever did get a ball wrung into a spherical socket, I'm almost positive you wouldn't be able to get them apart without some sort of release area.

Roger

[Evan]

I very much doubt you can have spherical concave magnets made to the required accuracy. Magnetic materials all exhibit an effect called magnetostriction. This is an effect caused by the rotation of the magnetic domains when subject to a magnetic field. In the magnet material itself it causes an anisotropic change in dimensions when the magnet is magnetized. Since no work can be done on a magnet to refine it's shape after magnetization it isn't likely possible to attain the required figure.

Also, the balls will exhibit the same effect and it also will be anisotropic meaning the amount of magnetostriction produced will depend on orientation re the magnet. This isn't a minor technical effect. It's widely used in various types of transducers and is the effect at work that makes transformers hum. It applies to any and all magnetic materials and happens in direct response to the strength of the field, alternating or not.

[Benta]

Did anyone notice the "1 m +/- 0.005 mm" specification? This metal bar would need to be in a 100% temperature controlled environment and nobody should be allowed to touch it.
This requirement is completely unrealistic! Even an invisible dust speck between ball and bar would violate that tolerance.
Academic indeed.

Benta.

[Just Bob Again]

Difficult but not unrealistic. A 5 ppm tolerance on length. The tempco of carbon fiber is around 1.5 ppm/degree C. Some special weaves and fillers can achieve less than that, or even negative tempco. It would work in a toolroom with normal care. That's why they chose carbon fiber. It can be more stable than metal by a wide margin. Expensive and hard to work, but a good choice here.

[small.planes]

Thanks all, some good ideas for me to investigate further

Yes, the layup of the CF is special, it has 0 coefficient of thermal expansion, the only expandy bits will be the endplates and the balls. The Metrology system is also CF, and it also has a very special layup. The accuracy required is high, but this is the artifact used to calibrate the system, and as such the accuracy obtainable from the system depends on how accurate it is.

Ultimately there will be a 'mesh' of these artifacts, as we need to calibrate a system to measure the wing jig of a certain composite airliner, and thats very long. This is (one) of the reasons behind the removable balls, its (theoretically) possible to make a single artifact big enought, but theres no way you'd be able to move it around...

I hadnt considered distortions on the balls due the magnet. Ill look into it and see if that will cause a problem.

The nest idea is similar to the first thing I though of, which was trapping 3 balls between the Big ball and the endplate, which can then be flat, and hence is 'relativley' simple to make and size.

Im really enjoying this job, I write embedded software for a living, but my 'unique' skill set (the recruiters words...) means Im also getting some of the more mechanical items to do as well.

Dave

[Evan]

The bar isn't metal. It is specified as carbon fibre. If the correct carbon fibre layup is used it will have a nearly exactly zero coefficient of expansion with temperature. This is what I used on my telescope for the truss frame. Pulltruded carbon-epoxy rods have a very slightly negative CLE meaning they expand very slightly as they grow colder. Combined with the positive CLE of the aluminum parts at each end my telescope doesn't change focus over the entire possible range of temperature when taking astrophotos.

Dang, fell asleep for a few minutes before posting this.

[small.planes]

Evening All,
Ive been pondering on this, and after a little digging about looking into methods of making hemispheres I found a thread here showing how to do it on a mill.
So I knocked up a seat to fit one of my daughters rubber balls:



Ignore the turned finish, it was a scrap offcut of mystery metal, and was like that when I started.
Looks like the seat part *should* be possible.
Of course they'll need to be cut (or at least finished) in situ on the end of the bar, as the glue will have set then...

Dave

[Evan]

Need to borrow my 1 micron per division dial indicator?
Nah, foget it, nobody can measure to those tolerances in a home shop.

[small.planes]

Thanks for the offer, but I hope the Optical Research group at Loughborough Uni has access to the required measuring tools
This isnt a home shop job, but there are a lot of practical knowledgeable people round here to pick the brains of.

Dave

[Peter N]

Just had a thought Dave.
You could rough it to near shape, a few thou' or less, using conventional machining, harden it, then send it off for Electron Beam Machining for the finished surface. Capable of nanometer level machining so I've been told.
I've known of the EBM process for some time, but can't point you in the direction of anyone actually doing it.
A friend of mine who works for Dage designing ultra high resolution digital x-ray equipment switched me on to it some time ago.

Peter

[small.planes]

Cheers Peter, Ill look into that.
I did wonder about spark erosion process as well, but Im not sure of the tolerances obtainable, and essentially its a form tool process, so would need new electrodes for each part I imagine?

Dave