SUCCESS!! A toolpath cut normal to a cylinder (cylindrical cam)

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Finally I have found the proper way to create the CORRECT geometry for a toolpath cut normal to a cylindrical surface, and have been able to create my cylindrical cam accurately as a result. The thanks go to Dr. J. D. Mather, Assistant Professor in CAD & Product Design at the Pennsylvania College of Technology

He has created a number of tutorials that reside at:

One of his tutorials is for a cam that apparently represents a sine wave accurately. It's a VERY complex creation he's built -- perhaps more complex than absolutely necessary, but it's hard for me to say. The tutorial is definitely worth downloading and looking at, but the core of the technique that he (and I) used is the Feature Wrap function. Create a plane tangent to the surface of the cylinder and create a CLOSED sketch on it. You can create a curve apparently accurately representing the outline of an endmill cut by offsetting both ways a construction spline or series of construction elements (controlling the ends of a spline with tangency to line segments is useful) and closing the ends with an arc tangent to both curves. Exit the sketch, select it, and use Feature > Wrap, with the Deboss option selected. This will wrap the curve onto the cylindrical (or whatever) surface and make a true cut normal to the surface. The distance value is the amount the projected cut penetrates. Emboss (obviously) creates just the opposite of a cut. Magic!! Extremely powerful function that I did not realize existed.

Kudos and thanks, Dr. Mather. And "way to go" SolidWorks Corporation. And I was cussin' 'em under my breath for not providing that easy functionality.

Mark 'Sporky' Stapleton Watermark Design, LLC

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Which of Dr. Mather's tutorials includes the example you describe? I have been looking through several, but each page loads fairly slowly due to the interesting background.

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did you look at the model I sent you Mark?

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Mark, I'm not sure if I'm following exactly how you did it, but an embossed groove will taper toward the center of the cylinder, just as will a wrapped sheet metal representation. If you emboss a cut that is parallel to the cylinder axis and emboss to a depth of 1/2 diameter you should end up with a wedge cut out of the cylinder (it's all working on surface normal offsets).

If you emboss to create a surface that represents the cutter axis path (should be a ruled surface; e.g. from any point on the surface draw a line perpendicular to the cylinder axis and that line should line on the surface) and then offset that surface cutter rad both ways you should have a good set of groove walls. (zat make sense?)

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The kudos go to Jeff Howard. He posted the solution back in OCT 04 or

03 (I'll find the printout of the post when I get back to the office.) The key is describing the just the centerline path of the cutter with a surface. And then offset that surface for the slot walls. When cutting a curve with an endmill the tangency is continuously variable. Check the results with a follower pin in an assembly in motion. Let me know if you find any problem as we are about to begin the semester and I don't want to introduce an inaccurate technique. I need to be able to add different types of motion ie. parabolic, cycloidic and dwell. There is an example on the SWX website that I have not had time to analyze for accuracy.
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5b is the one you want. Very enlightening!

Yes, thank you Dr. Mathers and/or Jeff Howard.

Muggs Change "home" to"comcast" to reply

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Thanks. Now I recognize that image. For some reason I didn't connect the subject with that particular image previously. I seem to remember it being a roll of toilet paper and never really noticed the blue cutting tool.

I've been working too hard.

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For what its worth, the pdf documents load much faster if you go ahead and save them to your desktop to open them directly in Acrobat. I had been viewing them in Internet Explorer which causes the pages to load much slower.

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Well wait a minute . . . umm . . . I may not have looked at your technique close enough. That looks like it might have a lot to recommend itself. Did you say I could post this model on my Web host?

I managed to create what I th>

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I want to srart by thanking Dr. Mather for making this set of tutorials available to us. This sort of thing is the most productive part of comp.cad.solidworks and one of the best aspects of the internet.

This is a very interesting tutorial, but it doesn't produce a normal cut as an endmill would. Prior to deleting faces and creating offset surfaces, the deboss wrap feature creates something close but only an approximation. If you create sketches on planes intersecting the primary axis of the original cylinder and the deboss wrap feature you can see this very clearly. Use the Intersection Curve tool to see how the various faces relate. Each face will produce a straight line, but they are not quite perpendicular. You can see this using the Measure tool or by adding Driven angular dimensions. Either way, you will need to show all 8 decimal places to see the discrepancies in the angular measurement. You can also try forcing the lines to be perpendicular by adding relations.

If you continue on with the tutorial and perform the offset surface operations, the result gets even more interesting. On the same planes you created to explore the cut previously, create additional sketches and use the Intersection Curve tool again. Now, the cut surfaces created by offseting .25 to each side of the original deboss wrap cut face will no longer be straight lines, but splines. Of course, an ideal end mill wouldn't produce a spline like this. I don't yet understand why the offset surfaces result in splines, but they seem to splines with only two control points. Regardless, you can't make the splines perpendicular to both of the adjacent straight lines.

I would be happy to send a SW 2005 model containing sketches as I describe above to anyone interested.

By the way, has anyone been able to use the Wrap, Scribe feature to do anything useful? When I do this on the original cylinder, I get a result that can't be used for subsequent feature generation. The Help is extremely brief and provides no examples.

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the angular measurement

I make the tutorials available to all so that I might learn something. I am basically an over-educated machinist who got tired of dealing with designers who didn't understand geometry. The good ones were not afraid to come back to the shop and discuss why I marked up their drawings. The others never set foot in the shop again - at least not during my shift. I encourage physical (well now virtual) models to analyze geometry. If you can provide me with an alternative model I would appreciate it. In trying to do a more comples motion I became aware that I hadn't found the altimate solution yet. I think what we really need is a way to sweep one solid (endmill) along a path on another solid resulting in the intersecting body being removed - just like the actual machining operation.

I really like to get perfect mathematically geometry in the CAD data set because then I know (or think I know) I am doing something right - but I am not too worried about 8 decimal places in the shop.

J.D.

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That's it in a nutshell.

Something that I think is pertinent: we are dealing shapes defined using NURBS math and 8 place accuracy isn't likely to be attained under a variety of circumstances.

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I think that a proof of groove geometery might involve looking at a section normal to the cutter path. If I remember correctly the problem is cutter path normal direction for any given ray off the cylinder axis changes as the radius from cylinder axis changes. It's been a while since I've looked at and tried to understand what's going on. (Makes me wonder about the swept groove I sent you, J.D. Could be inaccuracies in it; maybe small enough to ignore (??).)

Other guestimate check methods involve running a pin in the groove and checking interference and doing radial hole cuts to compare with groove geometry. These are at least 3D checks vs. the 2D section cut.

I'd love to see Neil's (thicken?) solution and any other possibles. Maybe we could get away with posting pics and STEPs (I don't have SW) on Sean's site (mcadforums.com).

At any rate the cutter axis ruled surface offset is about the most accurate way I know to describe the walls. Groove bottom surface for an endmill cut is another subject.

A question for somebody that might be involved in coding the cut; what's necessary to define the cut? Seems a curve that describes the cutter's end face / axis interection path and cutter diameter would be sufficient. Is the solid model topology really necessary or useful?

Regards to all. ================================

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I now have something closer to ideal based upon the original Deboss Wrap feature. Although you can't mirror the Wrap feature, you can mirror the body containing the feature to produce the complete path around the cylinder.

I will be happy to send the resulting model to anyone that asks.

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Stepping through the tutorial, it doesn't appear to model what and endmill would cut. The cam path generated is the shape left by a flat rectangle swept around the path. Since the endmill has width, there will be parts of the profile that do not appear to form a smooth surface.If you made an SLI cam from this model, a cylindrical cam follower would bind in it.

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Put it in an assembly and animate it. The follower (or endmill) constrained to linear motion in one axis and the cam constrained to rotate only about the cylindrical axis. I'll fire up the RP machine on Monday and produce a physical prototype. But I can zoom in on the virtual assembly much closer than the tolerance of machining tolerances and it looks good to me. I'll let you know.

John Eric Voltin indicated he had a better method. I don't have version 2005 to see his technique but I inserted a STEP model he sent me into an assembly with my model and they appear to be essentially identical. (not to imply that means the model is correct - I would be interested in seeing other solutions)

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I posted a couple of avi files of the motion.

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Likewise. John, could you email me a STEP (reply to sender - ignore the spam trap message - I'll fish it out) and (or post) a brief description of the method? Twould be appreciated.

If Neil wouldn't mind going to the trouble; same same.

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Dale, I've run pin in groove interference checks in grooves created using the surface offset method and it seems to resolve to about 1e-4 inch; e.g.

1e-4 under (don't remember; dia or rad?) clears and 0 interferes. ... If I remember correctly, could have been a larger value but still in the range of xE-4. Could also be that the groove will grow oversize, but I don't believe that should happen.

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Am I smoking somthing or does this work?