Surfacing question

Hello All,
I recently made models of large parabolic radio telescope reflector panels with stiffener ribs on the backside. Then I machined RenShape
patterns and had aluminum castings made. Then I machined the parabolic reflector surfaces on a big CNC machining center. Then I measured them on a CMM and made topographical maps of the surface inaccuracies. I need to increase the accuracy of the final product by compensating for small discrepancies that are on the order of .0001" to .0002". Now for the question:
My current method to tweak the accuracy is to add or subtract numerous, very thin patches, based upon the reverse of the topo map, ie, just remove material where the map shows too much. This sort of works, but ends up with abrupt terraced edges that cause their own problems during machining. I have made the terraces thinner and thinner until I have them down to about 25 millionths thickness. What I really want is to have 'curvature continuous' patches that are perfectly tangent and smooth where they blend into the parabolic, solid model surface. I need absolute mathematical control of these patches, just like what is available when working with solids. I am a long time Solidworks user with limited surfacing skills. I would hate to have to go the route of complex sweeps and lofts - the ideal solution I envision seems to call out for surfaces. Do I need Rhino? What are some suggestions to get started? Can this be done in Solidworks? - I hope so.
If this is not clear, please contact me.
Sincerely, Jerry forcier snipped-for-privacy@comcast.net
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Do you really need the patches or wouldn't it be easier to somehow regenerate the whole surface from the measured map ? Why did you say "I would hate to have to go the route of complex sweeps and lofts" ? How about creating curves from points calculated from formulas and topomaps, then sweep, with a macro ? That's basically the approach in our MathSurf add-in (www.solidplus.com/mathsurf/ ) which might help.
--
Philippe Guglielmetti - www.dynabits.com




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Hello Philippe,
The problem I see with going directly from a single topo map, which is developed with MathCad, is that we need to look at several maps to be sure we find a reliable, repeating pattern before we try to implement any corrections. Besides, the topo maps show hundreds of small errors that there is no need to correct. We are looking for the large scale, smooth, gross errors. How could formulas be generated from such a complex bitmap? Send me an address that I can upload a typical topo bitmap to - for your appraisal. I can also send a SWX model of one of our panels with the thin patches. Like I said, these do correct for the main errors we are concerned with, but the abrupt edges of these patches cause other problems.
I was thinking of something like a surface with zero offset to the paraboloid that had 'handles' I could 'pull' and 'push' portions of, up or down a few microns, in a controllable way.
Maybe sweeping the whole panel at one time could work as you suggest - its not something I thought of.
Sincerely, Jerry Forcier Forcier Machine Design snipped-for-privacy@comcast.net
Philippe Guglielmetti wrote:

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I find that using insert>surface>fill is a pretty good way to get a smooth patch.
You can do this by using a split line then delete face to get rid of the bump in question, and then surface fill with tangent edges.
If you do end up surfacing I suggest using splines as the quality of the surfaces seems much better.
hope some of this helps!
Lee

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Hello Lee,
This idea sounds promising. I have a million questions, not having messed with surfaces much. I guess I should experiment a bit to reduce the confusion.
I currently use split line to generate the boundary of the patches followed by offset surface of a tenth or so and then thicken back. Do you mean to delete the top surface of my current patch. If it is necessary to also delete the vertical wall of the patch, I could be in trouble, because it is so small that I can't zoom in far enough to be able to select it!
Sincerely, Jerry Forcier
Lee Bazalgette wrote:

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alternatively, you could select the entire boundry of the panel and surface fill that. You can add control lines if you find that it isn't filling as you want. Sometimes this can be great, and other times it creates things that I suppose could only be considered as ART since they are of no use to a product designer!

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Lee,
How would you select the vertical wall if it is only .0001" tall? I zoom in but still can't 'see' it to select.
Jerry Forcier
Lee Bazalgette wrote:

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Did the 'surface fill' option work in the end?

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Get Unigraphics or CATIA .....

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.0002" seems like a small amount to me-maybe its going to distort/warp by more than that when its assembled and 'hung',and also change shape with temperate-are you sure you aren't being overly concerned with accuracy-this maybe is due to variations in the cnc itself and therefore patching is not going to help?? I understand large scientific bowls have many automated adjustment points on the back to compensate for this...dunno just a thought...
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How large?

Did they get stress relieved? Aged?

Not on a lathe as the dieties intended?
How repeatable is it?
How much does it vary with thermal drift?
Did you rough and then remove all clamping and then do another setup to finish (thus removing clamping & stock stresses)?

How repeatable is the CMM? How did you calibrate the CMM to your process?
What temp are you holding for machining and measurement?

How accurate do you need to be? What is the maximum *accuracy* of your machining process?
Most CNC machines are programmable in a smallest increment of .0001: ... so the actual expected accuracy can be no greater than that. It might be a little better if you programmed in metric as .001 MM is smaller than .0001:

Can you just create your surface with many knot/control points and probe at those points? Then just adjust them by the error ...
BTW, What degree is the surface? A parabolic surface should be of degree 2 X 2 IIRC ... and so should any patches I suspect.
--
Cliff Huprich

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Jerry, What frequencies are you dealing with here? Is this in the terrahertz? Smaller?
Anyways, SW can draw it. Can you find a machine shop that can make it? If so, I want their phone number cuz I have some jobs for them.

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Jerry, I sent you an e-mail.

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Sorry George,
I didn't get it. Or I may have trashed it, thinking it was spam. Please resend.
Jerry Forcier snipped-for-privacy@comcast.net
George wrote:

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Hello Mark,
Of course the errors are not in Solidworks. They are a combination of many things; Mostly panel flex from tool pressure and stress relaxation during the cut, (even though the panels were thoroughly stress relieved, there is still some stress change). Tool galling is not the issue as the cutting edge contact is natural crystal diamond running at 4000 surface feet per minute and the metal slides off the diamond cleanly. The dishes I work on are between 12 feet and 100 feet in diameter. There are many individual panels (mostly trapezoidal) that make up a complete dish. I machine the panels (about 36" size), one at a time, in a tilted orientation so that the spindle is never normal to the workpiece surface. The machine is running constantly, in a temperature controlled room, so machine growth is not a big problem. Everything, and more, that both you and I have mentioned affects the measured accuracy. What I try to do is find repeatability in the sum of all these errors and carefully apply a correction factor. These panels are currently very good, but there is a repeatable error, and that is the .0001" to .0002" I speak of. I want to change my machining model to compensate for this. If I couldn't see this error in a repeatable way, it would be folly to go further, but I CAN see it and it calls me to fix it.
Sincerely, Jerry Forcier
Mark Mossberg wrote:

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Does the machine's control have an internal errormap? If so calibrate it ..... and switch to metric programming.
--
Cliff Huprich

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Jerry,
How are you getting the topo mapped errors back into Solidworks? Also, the suggestion that splines are more accurate drive curves in SW it true. Any surfacing geometry is goning to be much much cleaner when based on splines. Give me a call if you still need some help with this.
clay 916.687.7784
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Jerry,
Now I understand, your trying to correct a repeatable machine related phenomenon in geometry. SW doesn't really give you that kind of control over surface curvature. What you need is to locally deform an (irregular ??) area, without loseing the mathematical accuracy of the surrounding surface ? The only program I (personally) ever used that's capable of that is Catia. Somebody else may want to chime in about Rhino.
Sounds like a hell of a set up. I'd be very interested in seeing it. I've seen micron finishes done on a lathe with diamonds, but never on a VMC. Does the surface come out finished ? Or does it require polishing ? Even with the enhancments to your machine, it's amazing to me that you can come as close as you do on a part that big.
These are trapaziodial "radial" sections ? Like a slice of pie with the pointy end cut off ? How thick are they, and how do you dampen them ? Is the back side conssistent enough to use a poured urethane vacuume nest, or something similar ?
Allot of what Cliff said about arc fitting is true. It can actually make the surface less accurate, but smoother. However, with proper tolerance settings this shouldn't be a problem. I use arc fitting all the time in Mastercam (Basically the Northwood codes, but integrated).
Chasing tenths on a VMC is kinda like chasing your tail sometimes, I know. About all you can do is try to eliminate variables up front, untill you're only dealing with machine accuracy. problem is, at that scale, somebody farting in the next room seems to affect it.
Regards
Mark

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Jerry,
Now that I think of it, you may be able to use "fill surface" if the error boundary is regular.
You would need to use parabolic curves as constraints and fuss with it a bit. You could use 3D sketch to create a grid of points on both surfaces, and constrain them in the X-Y to match your CMM points. Then you could tweak the curves untill the point to point numbers equaled the error. May work,,, dunno
Mark

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You should use the "grid points" as the points at which the CMM measures the surface IMHO if you do things this way.
Then you need a *lot* of faith in what happens between & beyond them <G>.
As a test I'd try (on scrap) measuring outside the points to be modified as well and compare those locations to the nominal math surface and then modify & recut & remeasure to see how the areas were affected by the modifications. Lots of measurements. And I'd NOT trust the CMM without a lot of proof .... you need the standard deviation of the CMM process (on blind data sets of similar geometry) too .....
--
Cliff Huprich

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