Surface grinding/lapping
Surface grinding/lapping
(OP)
I'm designing a laminated superconducting magnet which is 2m long, made up of around 250 individual plates. The required flatness of the pole pieces across the length is 10 microns. Our original idea was to join all the magnet plates together using dowels, controlling the location of the dowel holes to the pole surface very accurately. The flatness from pole to pole would depend on the hole location tolerance and larger scale errors would be taken out using shims. However, we've not been able to achieve the necessary tolerances on the plates; there just seems to be too much tolerance build up with the location and straightness of the hole, flatness of the poles surface etc. The best pole to pole flatness we've achieved is about 30 microns.
To me, the obvious solution is to assemble the magnet, then do a post-assembly machining operation i.e. surface grinding or lapping. However I'm having trouble finding out whether such an operation is possible at the lengths and tolerances in question. Obviously surface plates can be made to the sub-10 micron flatness level but would that process translate to this application?
Apologies if the above is poorly conveyed, it's a Friday afternoon...
Thanks,
Steve
To me, the obvious solution is to assemble the magnet, then do a post-assembly machining operation i.e. surface grinding or lapping. However I'm having trouble finding out whether such an operation is possible at the lengths and tolerances in question. Obviously surface plates can be made to the sub-10 micron flatness level but would that process translate to this application?
Apologies if the above is poorly conveyed, it's a Friday afternoon...
Thanks,
Steve





RE: Surface grinding/lapping
My experience is that lapping is as much an art as it is a science so good luck
RE: Surface grinding/lapping
There are some large gantry polishing machines out there that could diamond lap to your requirements.
h
RE: Surface grinding/lapping
h
I'm not saying they can do something as big as you list but maybe you can learn something.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Surface grinding/lapping
RE: Surface grinding/lapping
Now, invert the magnet, still on its normal mounting means, on some theoretical infinitely rigid support.
Are the pole piece faces still going to be aligned within 10 microns?
I.e., how much is the assembly going to deflect under the influence of its own weight?
Mike Halloran
Pembroke Pines, FL, USA
RE: Surface grinding/lapping
I was thinking the same thing concerning the high flux and keep the field aligned. Using a Blanchard and Mattison grinders he should be able to get the faces parallel and flatness to 0.0005"
ksv86241
If it is possible to get to the 0.0005" flatness and parallel with a the appropriate setup diamond lapping should get you the required precision.
I would contact someone with the National Magnetic Lab at Florida State University.
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I would also contact someone at the Mirror Lab at the University of Arizona
http://mirrorlab.as.arizona.edu/LBT.php?navi=1cast
RE: Surface grinding/lapping
BillPSU: That's a really good point about the material. We'll be using as close to pure iron as possible, so transfer of particles may be a problem. I'll have to follow that one up.
MikeHalloran: The magnet will be supported at regular intervals from a massive beam. Any deflection under it's own weight should be able to be corrected with shims. I think it's really the flatness from each pole to its neighbour that is critical?
Once again, thanks for the contacts and links!
Steve
RE: Surface grinding/lapping
RE: Surface grinding/lapping
Mike Halloran
Pembroke Pines, FL, USA
RE: Surface grinding/lapping
Can you comeback with the overall dimension's with the require precision.
What is the allowed radius on the periphery of the point of interest surface?
what is the procedure for measuring the flatness of the surface?
RE: Surface grinding/lapping
Obviously a drawing would be ideal in the circumstances, but I'm only a graduate, so a little wary of giving away anything I shouldn't...
There are two magnets facing each other, 2m long and the surfaces in question are 80mm wide. The magnets are made up of alternating poles and coils and it is the height of the poles that is critical to the field profile.
I'm not sure I understand what you mean by the peripheral radius? I believe the outer edge of the surface is largely irrelevant to the field profile, so hopefully it shouldn't matter.
I'm afraid I don't know the name of the system we'll be using to measure the flatness, but it works by using a pendulum to measure the direction of gravity with respect to the surface in question. I believe it's accurate to less than a micron per metre. Apologies for my ignorance on the subject.
Our other solution, asides dowel location and post-assembly operation, is to lay the plates on a flat surface and insert a dowel into a clearance hole, which will then be filled with resin. I'm not entirely convinced by the long term stability of resin under the loads in question, hence my preference for post-assembly machining.
RE: Surface grinding/lapping
From your description of two opposing magnets brings into play that the face and bottom of both magnets would involve some pretty high precision. This would require both magnets to be machined an ground together.
I talked to a colleague of mine and he stated that he could constantly grind to 0.0002.5" on parallel sides and lap to quarter light band on one surface. We also discussed the situation where you are using ARMCO iron as we work with hardened tool steel. We used a 60" Blanchard and a 48" diamond lapping machine for our work. We also discussed the Mattison grinder and our consensus was that with care you could beat 0.0005"
RE: Surface grinding/lapping
I've thought about several potential approaches to your problem. The first would be to talk to someone like Standridge Granite with your requirements. As they produce Laboratory Grade Granite Surface plates they may be able to point you to someone who makes Cast Iron Surface Plates with similar tolerances.
Another approach might be to contact someone who makes CI work holding tombstones.
The last one is to contact Lafayette Grinding as they can handle large objects and finish with good precision.
http://www.standridgegranite.com/surface.htm#8
http://www.lafayettegrinding.com/index.htm
Off the wall:
If you could get your part close to size by grinding one of the Granite Finishers might be able to bring it to size. The only problem I see is that you would be finishing an edge.