I do not understand cams

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tornitore45
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I do not understand cams

Post by tornitore45 » Mon Jul 08, 2019 4:56 am

Think of a round flank cam with a flat poppet.

Where is the contact point tappet-to-lobe?
Is not on the poppet axis, a simple look at the geometry deny that.
If the cam turns CCW the lift point is somewhat to the right of the poppet axis. (poppet is above the cam)
How far to the right?
That depends from the cam rotation angle.
How large the poppet to ride on the tangent, instead of digging in?
If the poppet does not ride on the tangent it is approached at an angle and may dig into the cam.
If the poppet radius is larger than the distance cam-center-to-cam-nose then there is no dig in for sure but the larger the nose radius the smaller the poppet need to be.

Since the poppet does not touch the cam at his center but slightly off center, we can not say that the lift profile follows the machining table because machining cuts on the spindle axis positioned just like the poppet.

Are all mi statement correct?
Mauro Gaetano
in Austin TX

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Bill Shields
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Re: I do not understand cams

Post by Bill Shields » Mon Jul 08, 2019 6:46 am

Is there a question here besides 'are all my statements correct'?....the answer to which is 'no'. they are not all correct.

when the round cam is at maximum and minimum lift, the intersection point / tangent of the cam to the poppet is on the center line of both.

your question of 'how far to the right' can only be answered by knowing the geometry of the camshaft itself. From there, it is an easy calculation.
Too many things going on to bother listing them.

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tornitore45
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Re: I do not understand cams

Post by tornitore45 » Mon Jul 08, 2019 7:43 am

The top of the lobe and the base circle are the obvious trivial exception I did no consider to include. The interesting part of the discussion is during the active portion of the cam profile.
There is no specific question but knowing which statement is correct and which is not help understand the kinematic of the whole system.

Consider one of the way to machine a cam:
Horizontal RT with axis intercepting the spindle + rotate & cut according to a provided lift table.

If one plots the machining profile table z=f(theta) and the lift of the poppet y=q(theta) the two plots do not match because of the point of contact offset from the poppet axis.
Does the machining table account for the difference?

A small diameter Ball End Mill will replicate the profile defined by the table but the poppet lift profile will be different
A square End Mill the same diameter of the poppet will guarantee that the lift table profile matches the poppet lift profile
An much larger square end mill may cut metal where is not supposed to. I am not real clear on this, I can surmise it but not demonstrate it.
Is the popped diameter used a compromise or is actually the minimum that prevent digging?

How do the poppet lift, its diameter, the machining table and the type and size of end mill interplay?

As I said, I do not understand cams.
Mauro Gaetano
in Austin TX

John Hasler
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Re: I do not understand cams

Post by John Hasler » Mon Jul 08, 2019 8:51 am

Any flat cam follower large enough not to dig in will follow the same profile. The excess diameter simply never touches the cam. Visualize this and then replace the cam follower with an end mill.

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tornitore45
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Re: I do not understand cams

Post by tornitore45 » Mon Jul 08, 2019 9:59 am

I broke down, analyzing the geometry right now.
Mauro Gaetano
in Austin TX

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