Reverse Engineering with Rhino and a Digitizer

I purchased a used Faro arm to digitize our automotive parts & jigs, so we could get them into Alibre Design, and forward them for manufacturing. I purchased Rhino3 (and now 4 SR1 candidate1) after the demo, and have been using it to try to reverse engineering a front spindle, which is a pretty odd shaped forging (nothing very planar, other than the snout the hub goes over). The Solidworks/Dezignworks/Microscribe digitizer & modelling demos at trade shows make things look so easy, for a $10k combination of software.

The surfaces created from the digitize planar section curves have these very weird flares, twists, waves, and wrinkles. Rhino tech support suggested doing a Fair or FitCurve on the curves to simplify them. It helped quite a bit, but didn't eliminate them. How do you make a smooth, simple surface, that's still accurate? I'd offset the curves .125", but they're not always normal to the probe, so I'd introduce errors there. Once the surface is made, I then have a hard time averaging/blending/trimming the overlapping surfaces. What's the best strategy for that? Should I be using loft, or sweep2? If you guys don't mind sharing a workflow on how you digitize & create the part, I'd be grateful, since I'm sure I'm doing things the hardest way possible.

So far, my work flow has been:

  1. Digitize multiple points on all the flat surfaces, create a plane through the points, and offset the surface .125" to compensate for the probe diameter.
  2. Digitize multiple points for a few heights for bolt holes, and create "circle from multiple points". Create points at the centers of these circles, and draw a line for the hole axis. I usually draw a line connecting two quadrants of a top and bottom circle, and revolve that to make the hole ID or cylinder OD, and offset .125" for the probe dia.
  3. Use digitize planar sections to make a curves with point clouds. Often the curves don't quite "get it", and have these crazy fly-outs, which I then need to split, hit Point, click on the line, enter, click on the line again, and delete. Settings have been .05" between planes, and .02" point spacing, to help capture the detail.
  4. Create a surface with loft or sweep2, and offset it .125".
  5. Try to trim the surfaces and blend them, where I always get into trouble.
Andris, frustrated in CT...


One trouble is, the shapes you have are not best represented, in terms of surfacing, by a series of parallel section curves. It might be better to look at the object and decide what surfaces you need, bearing in mind that there may be some useful 'underlying geometry' (like parts of cylinders or spheres etc that might not be obvious at first glance) that you can build and then trim back. Digitize points that will help you to get those surfaces. Also, transition surfaces that are obviously filleted edges say, I would not build into the initial surfaces, I would try to add these as edge fillets between the base surfaces- things like that. -Pascal Golay

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