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

 

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.

 

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

 

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

 

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

 

..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...

 

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

 

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.

 

Jerry,
I sent you an e-mail.

 

Sorry George,

I didn't get it. Or I may have trashed it, thinking it was spam. Please
resend.

Jerry Forcier

 

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

 

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

 

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

 

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

 

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