Re: [CAD_CAM_EDM_DRO] Re: Timing Pulley Accruacy and Servo shaft runout

Polaraligned wrote:

> >>I am struggling to see if that
>
>
>>>runout can cause any significant error.
>>>
>>>
>>>
>
>
>
>>It certainly will not.
>>No, the tooth engagement doesn't affect accuracy much. And, the .001"
>>eccentricity
>>will have so tiny an effect on your system accuracy as to be only
>>measurable in a
>>metrology lab (6.33 x 10^-5 ")
>>
>>
>
>Thanks Jon. I will not worry about it then.
>I only bring this up because I have seen posts
>about reboring the pulleys because they are
>almost always off a few thousands.
>So there is no need to rebore the pulleys.
>There is no need to return the shafts.
>Life is grand.....
>
>
>
>

If it was only a few thousandths, it would be no concern except to the
nut case.
But, I've seen some pulleys that were about .010" eccentric (.020" total
indicator
reading)! These were brand-new pulleys from McMaster-Carr, I think. I was
apalled, but it didn't matter in this case, as I had to bore them up
quite a bit
anyway. But, I made some centering rings on the lathe so I could spin them
very accurately, and was astonished when I put them in and saw the holes
wobbling by that much!

>>How it would eliminate runout as a source of error, I have no idea.
>>
>>
>
>I do not really understand how an eccentric shaft will cause error.
>I see it as the belt just getting a bit tighter, and a bit looser
>as the pulley goes around. I was thinking it has something to do
>with the tooth engagement. Now I think I understand: The pulley
>is seen as being a larger dia when the error moves the motor pulley
>away from the opposite pulley, and as a smaller diameter when
>the motor pulley is closer to the opposite pulley. Correct me if
>I am wrong.
>
>
>
>

Yes, that is exactly the case. The teeth just keep it from creeping slowly
out of alignment, but otherwise they work just like plain belts. If the
effective
diameter of the pulley is changing, that will introduce errors in the
transmitted
motion. They will work out to a cyclical error every rotation of the
motor, in
this case.

>
>Like I said, I only have brought this up because many, including
>yourself, have said you bore out new pulleys to center the shaft
>hole.
>
>

In almost all cases, I need to bore it out, anyway, to fit a shaft or
leadscrew
that is a larger diameter. But, if given the chance to buy a pulley
with exactly
the right hub diameter, i might get a smaller hole anyway, because of
the errors.
When they call these parts "precision" they sure don't have machine tool
axes
in mind.

Jon