On Thu, 16 May 2002, "David W. Gulley" wrote:

> 
> I just spent a morning trying to resolve why my Drill Drawing Gerber 
> plot was showing a whole bunch of holes as size 0 mil (99SE-SP6).
> 
> It seems that since there are only 16 symbols defined for the drill 
> drawing, once you have more than 16 different drill sizes the CAM Drill 
> drawing lumps drill sizes together and assigns a hole size of 0 mil.
> Apparently the NC drill file does contain the correct information.
> 
> It is a Knowledge Base Item (# 1472), however when something like this 
> sneaks up on you it is very disconcerting, as I was assuming lots of 
> possibilities from corrupted design files to invalid design rules. I 
> guess I would have preferred some indication that a limit had been 
> reached and the system was compensating, rather than just arbitrarily 
> assigning several different holes to 0 mil.
> 
> Just a reminder to any of you who might see something funny in your 
> Drill Drawing regarding hole sizes...
> 
> David W. Gulley
> Destiny Designs
> * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *


Unless pcb designer I'm a pcb fabricator as well and I'd like to inform everybody for 
the following:

1. Drill bit manufacturers normally make tools with diameters in the range 0.3 - 6.5mm 
(0.012 - 0.256") in step of 0.05mm (0.02"). So although you have in your design 20, 
50, 100 or even more different diameters, they will be converted and groupped into: a) 
standard sizes; b) available bits in the workshop (except for very large orders when 
the appropriate sizes will be purchased).

2. Drilling/milling machines have positioning acuracy of about +/- 0.025mm (0.001") so 
if your design is so precise that very small differences in diameters are critical, 
they will be overridden by above mentioned facts and you will have rubbish as a result.

3. Pcb fabricators do not require drill drawings, just NC drill files. There are 
various softwares to do what they need. The of number of hole sizes is limited by 
drilling machines. Most of them i.e. "Excellon" can handle 140 tools at a time which 
is far enough.

4. It is good practice to provide fabricators with your whole design or at least the 
.pcb file because often slight corrections are necessary to be made just to adapt the 
pcb to their technologies, and let them generate files and reports they need.

My advice is: Always consult pcb fabricators prior to order pcb's!

Best regards to all
Emil Strezov
Pcb Designer and Technologist
ON BOARD ELECTRONICS Ltd.


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