How to - Push Pull cylindrical cam

[sakkie.coetzee]

Inventor 2020.  I'm stumped.  I've been trying out the "Design" , "Cylindrical Cam" feature, but for the life of me I can't get it to produce a (double sided) cam that push/pulls two cam followers. One on either side of the cam. 

The part attached I "cheated" with "Loft" and "Fillet", but of course that is for looks only.  If manufactured like that it will rip the assembly apart.

[JDMather]

Your sketches are not fully defined?

Where are you getting your information for displacement diagram?  (Particularly the motion laws in transition sections.)

What is the shape, size of the followers (this is also important in the design of cams)?

[sakkie.coetzee]

Thanks for the responds JDMather

Like I said, that is not a real model.

The cam followers are 18mm width and 35mm OD, 55mm PCD

Rotation 150 rpm  120 deg dwell either side of 60 deg sinusoidal transition, and 25mm lift.

At this stage I'm just trying to get to the "how to".

[johnsonshiue]

Hi Sakkie,

Many thanks for sharing the part! If I understood the design intent correctly, I don't believe the right modeling approach was used here. First of all, CAM geometry should be as precise as possible. So, Loft should not even be considered. It is because the Loft geometry is under-defined. You don't have total control on the resultant shape. Imprecision can lead to unstable CAM operation, vibration, and unnecessary wear. Some can even cause the movement to lock up.

Ideally, the geometry needs to be created using Solid Sweep (sweeping a volume along a curve) to be precise. However, Inventor Solid Sweep cannot handle this particular curve transition as robust as it should be. So, 2D profile sweep is an alternative. Please take a look at attached part and see how it was created. It should offer some clue.

Many thanks!

[sakkie.coetzee]

Hi Johnson,

Thanks for the reply.

Please have a look at my 2nd post.  It contains another ipt which is a real cam.  Still not correct due to face angles, but at least a real cam.

Your design has the same flaw as my first one.  If you look at the cam body thickness where it is in transition, you'll see that the linear (along the part centre line) is thicker (21.07mm) than the dwell thickness (20.75).  Because of the cam followers being on a fixed pcd as explained in my 2nd post, the transition section should reduce in thickness.  Wouldn't you agree? 

[JDMather]

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@sakkie.coetzee wrote:

Please have a look at my 2nd post.  It contains another ipt....

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No, it does not. 

[sakkie.coetzee]

Apologies.  I attached the iam instead of the ipt.

The attached assembly maybe explain my problem better.  Crude, but it demonstrates the problem.

In this assembly you'll see a linear cam, with 2 cam followers.

First Bottom position is in dwell, the second position is in transition.

The cam followers are bolted to the same horizontal shaft. The shaft is not attached because of the 3 items limit.

[JDMather]

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@sakkie.coetzee wrote:

The shaft is not attached because of the 3 items limit.

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>>9 months ago<<

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@JDMather wrote:

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@sakkie.coetzee wrote:

Looks like I can only attach 3 items.

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1. Put the entire project into a folder.

2. Right click on the folder and select Send to Compressed (zipped) Folder.

3. Attach the resulting *.zip file here.

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I recommend that you install the Updates for 2020.

[JDMather]

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@sakkie.coetzee wrote:

In this assembly you'll see a linear cam, with 2 cam followers.

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Is your true Design Intent to create a Cylindrical Cam or is it to create a Linear Cam?

I am going to assume a Cylindrical Cam since that is the title of your thread.

[JDMather]

Is something like this what you are after?

Edit:  I just read back through all of your posts in this thread, and now I think I have a better idea what you need.

It will be critically important to have the follower mechanism modeled and positioned in correct location relative the the cam axis in order to design correct cam geometry.  The tangent point between follower and cam is continuously variable and the reason a proper can cannot be modeled without the mating follower.

[sakkie.coetzee]

Clean forgot about zip.  Must be getting old. 😅

Jip.  Your .stp file is the just of it.  Through the course of today I'll see if I can get to a more complete assembly  AND ZIP IT this time.

Two problems that I have with this cam.

Firstly, how to create a working double sided cam profile in Inventor,  and then of course, how to wrap it around a cylinder.  

[sakkie.coetzee]

Okay, here goes.  The attached assembly demonstrates my problem.  You'll see that I only added 2 of the 4 rams.

With the cam follower contact face "constrain", "transitional" to the cam face, no problem in dwell, but of course because it's not a real cam, at the transition point the assembly gets stuck, like it would do in real life.  In real life there's be parts flying.

I made the "cam" to represent what I'm trying to create.

Here is the request.  I don't need somebody to create me the cam, I want to know how to create such a cam in Inventor 2020. 

[JDMather]

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@sakkie.coetzee wrote:

Here is the request.  I don't need somebody to create me the cam, I want to know how to create such a cam in Inventor 2020. 

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I now fully understand the Design Intent.

To go through the process of creating a proper cam would take considerable effort on my part.

Before I would go through such an effort - I would first want to make sure that robust modeling techniques have been used from the start.

I would question virtually every dimension and every feature.

For example - I see multiple unconstrained sketches and when I measure or dimension (I checked in mm and inch units) - I get values that do not make logical sense to me.

I would have to address each and every one of these on my path to the solution.

Are you ready to question everything in the design?

If not, I can move on to my real work and invite someone else to take over on this one.

[sakkie.coetzee]

Thanks for the responds JDMather, and thanks for the effort.  Appreciated.

I created many of the parts by attaching pdf drawings into AutoCAD, scaling it to known part sizes, then tracing the parts, hence the ridiculous dimensions.  Haven't gone through the effort of rationalising it.

I was thinking that there could be a way of using the cylindrical cam feature built into Inventor to create this, but obviously not.

Don't worry about it.  I'll see if I can come up with something that works.  

Maybe something to build into a future version?  This concept is used widely in industry.

[JDMather]

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@sakkie.coetzee wrote:

Don't worry about it.  I'll see if I can come up with something that works.

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Here are a couple of trials that might get you started.

In the first one I do a simple transisiton cut in planar part (I neglected to do both rise and fall).

In the second attempt I tried a Sweep-cut around cylinder but it failed.

I suspect issue of self-intersecting and might try on one half (but be careful, if this works, mirror might not be appropriate for rise/fall, might need to do separately).

If this doesn't work, then surface modeling most definitely will work, but takes longer for me to set up.  I am confident that within 4 hrs I could have a working example verified in Dynamic Simulation. Unfortunately I do not have 4 hrs to spare, but this would make an interesting problem for my second semester Dynamic Simulation class - so at some point in the future I intend to solve this.

[johnsonshiue]

Hi Sakkie,

I am reusing the small assembly you attached here to show Solid Sweep workflow. Please take a look at attached files. Open the assembly and you will see the double cam moves smoothly along the part without any interference.

The rail part geometry was partially done via Solid Sweep. For planar movement along a tangent continuous path, the resultant geometry created by Solid Sweep is the same as Profile Sweep. But, for non-planar  or non-tangent path, the result will be different.

Many thanks!

[sakkie.coetzee]

@JDMather @johnsonshiue 

Again, thank you guys.  I think I solved the cam profile problem.  It measures out perfectly on the flat.

Now to try and wrap that around a cylinder.  I know it's possible in the sheet metal function.  Never used it, but I'll give it a try.

The penny dropped...….eventually.

I need to go build an ipt and test it.

If it works in the iam, I'll post how I did it.

[JDMather]

I don't think you can do this bending the linear cam, but good luck trying.

I think I also figured out a way to do this using the Cam Generator, but it would be done by creating two cams and combining them together.

[sakkie.coetzee]

I've tested an extreme version of the cam.  I'll not build one of these, but the movement is unrestricted.

Unfortunately still linear.

Now, to get that wrapped around a cylinder.

If you open the cam itself, the drawing is pretty much self explanatory.

A whole series of radii, but from predictable centres.

I'm pretty certain one would be able to write formulae that can give your this.

[sakkie.coetzee]

@JDMather 

I'm convinced that creating and then combining two cams should do it.  I tried and failed.  Don't know enough about the cam generator.