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

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 (www.pct.edu). He has created a
number of tutorials that reside at:

http://home.pct.edu/~jmather/content/DSG322/SolidWorks_surface_tutorials.htm

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
www.h2omarkdesign.com

 

Nope . . . I think I was wrong -- false alarm. SHI_! Looking at the
cut from a view normal to the outside of a cylinder I can tell that the
cut is not precisely normal to the surface or perhaps (yes I think) the
cut tapers toward the center of the cylinder. Feature Wrap with Deboss
apparently does NOT create a cut that is like an endmill cut. The
rather extreme complexity of Dr. Mather's technique is apparently
necessary to create accurate geometry. And I'd have to say that just
should not be.

 

did you look at the model I sent you Mark?

 

......... the cut tapers toward the center of the cylinder. .........

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?)

 

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.

 

5b is the one you want. Very enlightening!

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

Muggs
Change "home" to"comcast" to reply

 

Yes indeed, sir, and I thank you. I don't think that technique can give
me this result, however.

 

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 think is accurate (enough) geometry using
Jeff Howard's / Dr. Mather's technique of creating offset surfaces from
a Wrapped surface, and that requires a good bit of subsequent surfacing
work (trimming and knitting and etc). Your's does also, but isn't quite
as complex. It looks good and I'll look at it further.

 

you will need to show all 8 decimal places to see the discrepancies in

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.

 

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.

---------------

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.
================================

 

it doesn't appear to model what and endmill would cut.

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)

 

I posted a couple of avi files of the motion.

http://home.pct.edu/~jmather/content/DSG322/SolidWorks Tutorials/end.zip

http://home.pct.edu/~jmather/content/DSG322/SolidWorks Tutorials/side.zip

 

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.

--------------------

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.

===========================

 

Am I smoking somthing or does this work?

http://home.comcast.net/~muggs828/solidworks/3-inch_Dia-2.SLDPRT

Muggs
Change "home" to"comcast" to reply

 

..... Sorry for the distraction.

Not to worry. "Distracted" defines my normal state of existance. 8~)

 

yes post it if you like

 

I'd VERY MUCH like to look at that model, John. I'll send you an email,
but a valid email address for me can be found on the main page of my Web
site.

Thanks very much,
Mark Stapleton
Watermark Design, LLC
www.h2omarkdesign.com

 

Unfortunately I don't believe it is accurate, Matt. The surface offset
to the inside is what I tried with my cylindrical cam. It doesn't seem
to want to offset accurately radial to the axis (or normal to the
surface). See the pic I've got up on my Web site:

http://www.h2omarkdesign.com/img/screenshot_cam.jpg

It doesn't really work. Although I think it SHOULD work.

'Sporky'

 

......

Thanks a million, John. The message is on the Earthlink server (~1 MB).
You probably got a spam trap message back (how I long for the "good ole
days" when such things weren't necessary). I'll retrieve it and get back
to you if it didn't make it thru intact.

Thanks again and sorry to put you thru the trouble.

========================

 

Anyone else besides me think that the Offset Surface oughta work
(although it doesn't)? A radial surface offset radially really SHOULD
produce the exact geometry we're talking about with an end mill tool
path. The fact that it doesn't says something about what's going on
under the hood with SolidWorks. In other words, something smells really
fishy here.

'Sporky'