On 11/26/2013 11:11 AM, Edward A. Falk wrote:
In article ,
jim wrote:


I'm saying that any CAD system can produce geometry that
is far more accurate than any manufacturing process.

Fun fact: when they built the difference engines from Babbage's
designs, they needed to compensate for the fact that the CNC
machine they were building it on was accurate to 1/10,000
inch but the manufacturing processes of Babbage's time were
only accurate to 1/2000 inch.

They not only wanted to know if the machine would work as Babbage designed
it, but they wanted to know if Babbage could really have built it.
So they added 1/2000" of random noise to the CNC data before giving the
data to the machines.


A program like Sketchup caters to people who want to
model something that looks good without paying much attention
precise numbers. But that doesn't mean it is sloppy. It just
means it is not as easy to hit the exact numbers you may want as
in other programs that cater to people who want models driven
by precise numerical inputs instead of mouse actions.

I've had a lot of trouble with Sketchup working in sub-1/4"
sizes. I'm making some toys for a friend now with parts down
to 1mm.

Try it yourself at home: make a 1/4" sphere by sweeping a circle
over 180 degrees, or any other way you want.

What I normally do is scale my model up 1000x, work on it, and
then scale it back down.

It shouldn't be like that, and I'm not entirely sure why it is;
precise or not, it shouldn't be so affected by working scale factor.

I see this a lot in models downloaded from the warehouse. You download
a chess set or something, and find out all the pieces are ten feet tall.


If you design something large, and shrink it, it will probably look
better. Compare to the pixel resolution of digital images.