Re: [CAD_CAM_EDM_DRO] Accuracy and heat was Re: re: Re: New file upld to CCED

I got my figures from the Machinery's handbook. Used the figures listed for carbon steel. Naturally this will vary depending on the exact alloy used for the shaft. The only way you could get really accurate figures on something like this would be to contact the manufacturer(s).

-----Original Message-----
From: dave engvall <dengvall@...>
To: CAD_CAM_EDM_DRO@egroups.com <CAD_CAM_EDM_DRO@egroups.com>
Date: Saturday, October 28, 2000 9:50 AM
Subject: Re: [CAD_CAM_EDM_DRO] Accuracy and heat was Re: re: Re: New file upld to CCED

> Excellent post! The trick here is knowing when the numbers can be
> ignored, and when they must be respected. You didn't get into the
> effects of DIFFERENTIAL expansion (steel shaft mounted on aluminum
> structure) or large mass/small mass temp. rise co's. And we put the
> machine in a garage where the temp is constantly changing...
>
> Many beginners will continue to think they can get .0001 because
> that's their step size. We know better :-)
>
> Ballendo
>

Hi all,
Some of this comes under "Murphy was an optimist!"... "Murphy and all his cousins are alive and well".

Somehow, I find different constants for thermal expansion from an older Chem and Physics Hndbk. (1956) . Uh... dates me.

Aluminum 24 E - 6
Iron (cast) 8.6 to 10.5 E - 6
Steel 11 to 13.2 E - 6.

At least they are not an order of magnitude off of Smokes figures. But 2X can still upset calculations. Since the bed of the
machine is going to change temp and there size slowly compared to the ball screw we are still left with getting the preloads 'just
right' to minimize heating.
Running machinery conservatively so it doesn't heat is nice but not always possible.

BTW - plastics eg. PVC have thermal coefficients of about 70 E -6.

Ah, DUH ... of course my numbers are for degrees C. Dang standards. Maybe someday we'll all get on the same wavelength.
Indeed Murphy almost slipped up behind me and clubbed me. :-(

p.s. Ballendo Tnx for adding the info on Dn's. :-)





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