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How to draw Cam lobe


henrynguyen

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Your "solution" does not appear to be even close to correct.

Before all else, you should draw and dimension the angled lines - I do not see these in your solution. (lengths of the lines don't matter for the solution - make them extra long so that they can be used for trimming the arcs as needed)

It makes sense to place the vertex of the lines at the origin.

As I noted previously - NONE of the arc centers are on ANY of the given lines (nor are they at the origin).

That makes it an interesting geometry problem.

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Rather obvious that it is a circle at the vertex of the angled lines. (keyway is also trivial).
Yes, that's what I said. But the center of it is not trivial, it is key, that is, it is whence the angled lines originate. Without it, they no longer have an origin in the original problem. Or am I missing something? The reason I say this is that, the way I understand this, these angles are not the angles of the arcs, but of the endpoints to the center of the shaft, that is, to the point of rotation.

 

I would have saved down myself, except that parametric constraints are critical to the way I found the solution.
Yes, I can see that the parametric constraints are critical. I'm working in 2012, but save down to 2007 format due to work environment constraints. I think that the 2012 parametric constraints are retained when saving, though they may not actually work in 2007. Does anyone know if there are new parametric constraints in 2013 compared to 2012? Edited by neophoible
added "The reason..."
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AFAIK AutoCAD 2009 (OP's version) doesn't have constraints.
That would make this a very difficult problem for him indeed!
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So JD, can you tell me if I got it correct? I took me less than 10 minutes as well using AutoCAD.

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So JD, can you tell me if I got it correct? I took me less than 10 minutes as well using AutoCAD.
Bill, you did it the way I initially interpreted it, which is not correct. If you look at the endpoints of your arcs, they do not lie on the angled lines you drew. As I mentioned to JD, and as he apparently drew it, these angles represent how much rotation each arc sees about the center of rotation of the cam, not the angles of the arcs themselves.

 

BTW, it looks like AARi's microstation solution may be correct and match JD's, but it is very hard to tell.

Edited by neophoible
added note about AARi's solution
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Bill, you did it the way I initially interpreted it, which is not correct. ...

 

I didn't think it would have been that easy. But what I see when I trim the arcs to the angled lines, I'm left short on the left side with the arc which is 47.3R so if I extend it to the angled line to meet the other arc, the surface does not follow a smooth transistion. Again, I'm no ME and never work with cams, but that would seem to me that your push rod would have a rough time hitting this surface imperfection. Am I off-base on this too?

 

BTW all my arcs end on the angled lines with the exception of the 47.3R line in the upper right portion of the cam.

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So JD, can you tell me if I got it correct? I took me less than 10 minutes as well using AutoCAD.

 

I don't see an attachment?

 

Did you look at my solution? Every arc is tangent on both ends. Every arc begins and ends on the lines. (but the centers are not on the lines)

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Ok no reply on an amateurs post (POST #11) on page 2 It was done in Autocad LT 2000.

 

Attach dwg file here. It is impossible to analyze from a picture alone.

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Attach dwg file here. It is impossible to analyze from a picture alone.

 

big mistake i did not save the original dwg and this is the only way that i found to get it back into autocad.

 

if you can not use this let me know and i will do it all over again

 

will not upload BUT i will do it all over again and post it when it is completed it may look different because i have to start from scratch.

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If we assume all of the arcs are tangent and they stop/start at the angles given - there is one solution. (none of the arc centers are on any of the construction lines, but it is fully defined with the given dimensions) (BTW - solution was found in less than 10 minutes.)

 

[ATTACH=CONFIG]41871[/ATTACH]

 

JD...I guess there are some assumptions I made which differ from yours. For example, the arc on the right side of the cam which appears to begin at the horizontal construction line and has an angle of 90°. In your solution this arc's center is offset from the construction line by a distance of 6.08 units. I'm not sure I follow how you made this the location of the center of this arc. But I guess it's also a good thing I don't design cams...I'm only throwing my limited experience at this. Still it's been an interesting exercise and judging by the number of responses it peaked the interest of some others as well. And that's what keeps your mind young.

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First off I'm not a CAM designer nor a mechanical engineer, and no offense to anyone but cams do not have a radius as I was taught many years ago by a mechianial engineer that turned instructor. When taught on the old board it was drawn with french curves and not a compass. 30 some years later in a CAD system I was taught to draw it with a spline not an arc. IMHO with out knowing the rise and fall of the cam as it rotates you can not create a profile of it (think graph) that is used to draw the face.

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JD... In your solution this arc's center is offset from the construction line by a distance of 6.08 units. I'm not sure I follow how you made this the location of the center of this arc. .....

 

The geometry constraints located this for me. Doesn't have anything to do with cams or anything other than geometry. I would like to see your solution, but don't see an attachment above. If I get a chance I will make an actual cam (3D) and run Dynamic Simulation on it to find the Acceleration and Displacement Curves (although that is dependant on the follow shape/size which we don't know).

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First off I'm not a CAM designer nor a mechanical engineer, and no offense to anyone but cams do not have a radius ....

 

You are absolutely correct. If I had a displacement diagram and motion law I would use Autodesk Inventor - Cam Generator Design Accelerator to design the cam and it would be a spline curve profile. I was looking at this as a pure geometry problem. At some point I will run Dyamic Simulation on this and see how well it would actually perform as a cam.

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First off I'm not a CAM designer nor a mechanical engineer, and no offense to anyone but cams do not have a radius as I was taught many years ago by a mechianial engineer that turned instructor. When taught on the old board it was drawn with french curves and not a compass. 30 some years later in a CAD system I was taught to draw it with a spline not an arc. IMHO with out knowing the rise and fall of the cam as it rotates you can not create a profile of it (think graph) that is used to draw the face.
I haven't done anything significant with cams in thirty years, and this problem isn't the kind of thing I did back then. I basically agree with JD, including the point that the problem at hand is really one for geometry. There was no inquiry about the output of the cam. There wasn't even a cam follower. While certain portions of a cam may have a radius, e.g., when you don't want any motion (stay concentric about point of rotation), in design you would be looking for an output. That output is what generates the profile. Of course, one may be interested in what sort of output a certain profile would yield. There's nothing wrong with investigating.
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BTW all my arcs end on the angled lines with the exception of the 47.3R line in the upper right portion of the cam.
Well, that's one way of looking at it. Like I said, your solution looks the same as my original misinterpretation. Here's another way of looking at it: You have a concentric arc for the 45 radius, which does end at the 0 & 90 deg marks. The other endpoints miss. So, one out of four arcs meets the angle criteria. Or, four out of eight endpoints meet the angle criteria. All of the arc pairs are cotangent. But overall, to meet all of the intended criteria, the profile is incorrect, the endpoints do not hit where they must, the arc centers are wrongly placed. However, as a comparison, I think it might be even more interesting if JD also ran this profile through his simulator.

 

BTW, I'm guessing that solving this correctly without parametric constraints might take a very long time indeed.

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Attached is r2007 version (student license)

I have no idea what 2007 will do to the parametric constraints.

The radius dimensions point to the centers of the arcs.

Again, thanks, JD. The constraints seem to have come through just fine when opening in 2012. I don't know what earlier versions will experience when they open it.
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