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Modeling a globoidal cam


Re: Modeling a globoidal cam

We do have motion envelopes which is what SW does. They seems to still be very crude. It also doesn't use the "paper part" method of trace curves. If the two were united, then you might have something.

Overall, this is indeed an interesting subject. I overlayed a swept solid to a thickened solid and ended up with wildly different shapes. One of the major issues here is one that is compounded by the significant difference between the normals of the inside and outside curves. Sweeps can only utilize one. With the reversal, other sweep methods have to be broken up or it will fail. It is yet another mobius problem.

Re: Modeling a globoidal cam

This is the motion envelope for the cam. 112,000 polygons and nearly 30Mb.

Enough to bring my computer to its knees.

In order to get the motion, you have to plan on being able to move the entire mechanism.


And talk about mobius,,,


I suddenly have a hankerin' for a softserve

Re: Modeling a globoidal cam

Oh man, nice!

I actually had some thoughts on that this weekend. When I get some time I'm going to play with it. Reminds me of playing with AotuCAD in the early/mid '90's and patterning boolean cuts like that. Ouch.....

Re: Modeling a globoidal cam

Here it is my files.

I hope that they could explain well what I've done.

Re: Modeling a globoidal cam

With the envelope generation, I can create a very close surface on the inner and outer faces. The problem is the face that counts; where the bearing rolls. It is an involute face.

I will have a look at your file to see what may be resolving inherent issues. It is sort of amazing that it requires a study to solve what is actually simple in real life. But with rapid prototyping on the rise, our software should be able to "easily" recreate real world machining operations.

Below is what I know is accurate, but "facetted". And with Creo 2 M040, patterns are still buggy.

This has been my long time solution to such problems.


Re: Modeling a globoidal cam

Giulio, thanks for being so persistent with me and providing this file.

I confirmed your findings against the known validation model above.

Indeed, the surface offset method for the walls where the rollers ride creates an accurate roller surface.

The initial sweep based on the two trace curves are all that were required. This appears the only valid sweep as all others seem to deviate due to a twisting action in the trajectory.

I used the offset on the file above and it came out identical (within the graphic limits, obviously).

I then went back to the envelope model I was working with for validation. The offset surfaces from a center sweep did much better than all the sweeps and blends I was able to throw at it.

I also compared the offset to the thicken and when it does not fail, they are also identical. This means that asm0066 above should also be correct (I will review again).

The only difference I found with the envelope model was the bottom of the milling operation. The boundary blend is an approximation. This is no big deal as this is not a functional feature. Just be aware that if milled, it will be somewhat different. I conformed this with a revolve of the mill in the bottom of your cam.

This is the overlay. Very good conformance on the cam surfaces. I could not achieve this with sweeps or boundary blends.


Boundary blend approximation of actual bottom milled surface...


This is the difference with the shaded envelope...


This is the mill shape in the bottom of your cam..

What is nice is that this image shows the tangency between the mill bit cylinder and the offset walls.


I will see if I can make a motion envelope of you device.

Great work, Giulio! Thank you again for sharing.

Re: Modeling a globoidal cam

I reworked your file to get the rollers to generate an envelope. Since I cannot work with skeletons, and since PTC found it "useful" to lock the 1st placed components locked to "default"... I had to replace the skeleton part with a pin-constrained reference part. Fortunately, all the motion features were preserved. I did not have to redefine any of the motors. I simple applied a gear connection to my replacement "skeleton" and the action was "reversed".

The envelope model was generated at the max resolution of "10".


Re: Modeling a globoidal cam

Wow, your PC must HATE you for this one!

Re: Modeling a globoidal cam

It is actually not too bad on the CPU. But I did run into a bug in the selection process.

There doesn't seem to be any way to tighten up the coarseness.

Nice thing is that selecting two opposite ends on this mechanism will create a closed loop.

By making it an assembly with 3 envelopes, it removed some of the artifacts.


Re: Modeling a globoidal cam

Giulio; I found GRAPH_1 in your assembly model. What was that used for?