Smallest Possible Feature ?

I can't sketch anything smaller than about 100 nanometers. Is it
normal ?
As I wrote in an old thread "Modelling space size" :

So I thought I could "easily" design sub-micron features, especially
when setting microns, nanometers or angstroms in the document options,
but it seems the workspace is "fixed" to a few cubic kilometers,
limiting the resolution at the bottom of the scale...
What limits sketching to 100 nanos while numbers would allow to go
down to about 0.2 nanos ? What's the use of using angstroms as units
if you can't use them ? Did I miss something ?
For those who ask why I care : some swiss watchmakers model gears
with a sub-atomic precision. The fact that atoms can't be cut in two
flat pieces is considered as a machining error...
Philippe Guglielmetti - www.e-systems.ch

 

When SW goes 64 bit you will have that precision if it ever does.

Corey

 

Philippe,

There are upper and lower limits. Even working at the size of a pin head SW
can be a bit awkward.

It isn't just the ratio inherent in C doubles. SW has certain tolerances
built in to help it decide when elements are or aren't coincident, etc.
These would seem to be much larger than a few nanos. Take the .005 inch
minimum involved in certain sweep geometry.

 

08.10.2004 17:58:48

no Corey, sorry, it won't change anything. Doubles are already 64
bits (see http://babbage.cs.qc.edu/courses/cs341/IEEE-754references.
html ) but with 52 bits mantissa. The only way to extend precsion
would be to move to "extended" formats. AFAIK the 80 bits format with
64 bits mantissa is supported by Intel hardware, but it would push
the limit "only" by a factor 1000.
Don' expect too much from 64bits procs : few geometry or CAD related
algorithm (if any) require manipulating intergers larger than 32 bits
(about 4'000'000'000)
Philippe Guglielmetti - www.goulu.net

 

Where does that one show up? I haven't seen it yet (that I know of) and we
use sweeps a lot.


Jerry Steiger
Tripod Data Systems
"take the garbage out, dear"

 

If you search back on the newsgroup there was a discussion long ago
(99, 2000).

When you do a sweep that just skins an existing face you want to embed
the sweep at least .005 into the face. You can find some discussion in
the KB. Also if your generatrix has a construction line to a directrix
I think there is a minimum length for that construction line of about
..005".

There are other examples of tolerances, for example the 3 face / 1
face behavior on an extrude protruding from a flat face. If the
extrude protrudes just a very, very small amount and approaches being
coplanar with the existing face, then it depends on which side you
approach the existing face whether you will get one face or three when
coplanarity is achieved. I would probably have to post the example to
show this one.

 

Ah, that brings back some very old, (and, unfortunately, very vague)
memories. Thanks!

Jerry Steiger
Tripod Data Systems
"take the garbage out, dear"

 

Don't "know" a thing about it (fuzzy stuff to me); just some general bs and
curiosity questions:

I'm sorta surprised you can get something as small as 1 E-7. Somewhere in
there there's an absolute tolerance below which two points are considered
to be coincident. I'd have figured this is 1 E-5 to -6.

(What were model units? Does SW scale absolute resolution
or set it to match model units; e.g. 1 E-x meters, 1 E-x mm
or is it a static value?)

Probably can't get the full 15 significant digits of accuracy. ACIS uses
10 digits, lopping off 4 to 5 digits as for "noise", round off errors, etc.
and 1 more to provide a buffer over absolute resolution. This range can
supposedly be shifted, but not expanded. You can, in fact, specifiy values
outside of the default 1 E-5 to 1 E4 range, but shouldn't be surprised if
problems are encountered.

Somewhere up above someone mentioned .005 for swept features (?).
Polynomial surface calculations are never as accurate as analytic surface
calcs, and .005 is probably a good buffer. Surface modeling functions are
sometimes more appropriate vs. a "solid" function that's trying to make a
swoopy face coincident with another one.

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