Re: Elastic belt stretch, lead screw twist, and far end encoders

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <jmelson@a...> wrote:

>
>
> "Heuver, Brad (B.R.)" wrote:
>
> > I finally got around to doing the math on the differences

between

> > timing belt stretch, and shaft twist.
>
> <snip>
>
> > From this whole math lesson we learned the following:
> > With the CNC described above, the belt stretch causes an error of

0.0065",

> > and the screw twist 0.0001" or simply put, the belt error is 65

times the

> > size of the lead screw twist, and ball screws are not

frictionless so

> > therefore, the belt stretch, being upstream of the friction, will

be even

> > greater than this.
>
> Thanks much for doing this exercise! Very useful info. It does

look like

> it is important to put the encoder on the leadscrew, not the motor,

as I

> suspected. (Of course, there are other sources of error in

addition to

> stretch, too.) I'm surprised at how little twist there is on the

screw.

>
> Jon

It helps me understand better to put the torque on the lead screw in
ftlbs. 100inlb torque is 8 1/3 ftlbs. If we condiser that we
routinely torque 7/16" car wheel lug nuts to 75+ftlb, vs the 8.3 on
the much larger lead screw, it seems more reasonable. In fact, we
probably put that much torque into a 1/4" tap.
Now that tap really has some twist!
Brad