"CW" wrote in
:


"Australopithecus scobis" wrote in message
news
On Fri, 14 Jan 2005 22:23:42 -0800, CW wrote:

You can't see 0.0002" with your eye.

Yes, you can, easily.

[This keeps coming out sounding argumentative, and it's not meant to
be]

Can you tell 0.0002 from 0.0003 by eye "easily?"

No, but that was never the issuse.


When you say "easily," do
you mean bright light shows 'twixt straightedge and work? In
astronomy we deal with arcseconds; there are limits to what the eye
can see.

References to astronomy don't make you look good. For various reasons,
I have learned to dismiss the astronomy types.

Really? What types don't you dismiss?

Older eyes
have more trouble. Can old farts here see 0.0002 easily?

I can. YOu would have to live with my eyes for a day to appreciate
that.

Is that amount
small enough that temperature (coefficient of thermal expansion)
matters?

For a guy in his woodshop, no.

Sure it does. Look up the CTE of a good stainless steel. Assuming a
2ft bar, the linear CTE is about 3x the tolerance for every degree F.

Now try the same with the elastic modulus. :-P

You want to pay extra money for a 5um tolerance, go for it. I'll save
my money and buy something useful.