Material Choice for CMM Fixture 1

[TheBradish]

I'm designing a fixture to elevate the qualification sphere on our CMM table by 6". The fixture does not need to repeat position with high accuracy, but it does need to be dimensionally stable during the probe qualification routine (up to about 6 hours). The temperature in the lab may vary within a range of about 5 degrees F. I need the position of the sphere, once located, to vary by less than .0001" due to environmental factors.

What material would you choose to be both stable and affordable?

 

[KENAT]

Stable & affordable.

Hmm, when we care about thermal stability (drift as we normally call it) and can't somehow control the temperature of the piece then we use materials like Invar or Super Invar but they aint cheap. Zerodur is even fancier.

I'd calculate the max rate you can handle based on desired max total drift & the temperature delta then look at a table like this and see what might work:

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

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[TheBradish]

Thanks for the link, KENAT. According to those resources, I'm looking for a material with a CTE of about 3 (10-6 in/(in R)). Looks like I've been overthinking the problem; I can just place my qualification sphere on top of a brick!

 

[KENAT]

You may get away with granite, while it's nominal thermal coefficient is a bit higher than you'd like, reality is being a big monolithic mass it will have a fairly low time constant.

So if you're worried about relatively short term fluctuations it may be fine, if you have a long term thermal gradient though you may have an issue.

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