At 06:18 PM 2/13/01 -0600, Jon Elson wrote:

>Abd ul-Rahman Lomax wrote:
>
> > At 10:46 AM 2/13/01 -0800, Dennis Saputelli wrote:
> >
> > >on the other side I once had a shop put a board on hold for a week
> > >because the fab print said 8.000" but they said the board Gerbers
> > >measured 7.999"
> > Especially since it is reasonably unlikely that they could measure the
> > Gerbers with that accuracy.

>No, they can measure the RS-274 data that CREATES the gerber
>photoplots to rediculous accuracy, .00001" at least.  Depending on
>the photoplotter actually used, they might get pretty close to that level,
>too!
>
>Jon

Yes, the *data* can be measured with *perfect* accuracy. But unless you are 
paying very big bucks, the resolution of that plotter (pixel size, I think) 
is not going to be better than .25 mil.

However, I would prefer to call what one would derive from the Gerber a 
"calculated" distance, and use "measure" for measurement using physical 
tools on the film. I'd be very surprised to find that they could measure 
eight inches to a true 1 mil accuracy. Remember, also, that the film 
expansion and contraction with varied temperature, assuming it isn't glass, 
is also going to swamp this number; and I'd, again, be surprised if the 
absolute positional accuracy of the plotter was in the mil range. The 
*resolution* may be higher (not necessarily, by the way, there are plenty 
of photoplotters with 1 mil resolution)m but not the accuracy.

The board gerbers are not a definition of board outline, if they really 
wanted to be correct. The fab drawing is. There was no ambiguity, the 
dimension was fully specified as 8 inches. A board outline is only drawn as 
a convenience; if the deviation were large enough to raise suspicion of an 
error, then a prudent shop might well make a call. But not for 1 mil. As I 
said, sheesh....

[EMAIL PROTECTED]
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433



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