without debating the merits or demerits of this approach 
(specifying drill size rather than finished size) 
i must say that the industry standard is to specify finished size

one proto shop (AP) is the only exception to this i have seen over many
years of many shops

also examining the title blocks of many high end customers fab dwgs
without exception they always say
'all sizes are finished sizes'
often the min/max plating thickness inside the hole is called out

i guess i would worry that someone would not follow the instructions and
compensate the drills anyway
of course you could reject the boards but then time is lost
(ok so i did debate)

Dennis Saputelli

[EMAIL PROTECTED] wrote:
> 
> In a message dated 5/17/2002 1:43:29 PM Eastern Daylight Time,
> [EMAIL PROTECTED] writes:
> 
> > For
> > example, I may specify an 0.028 hole and that is what will be in the PCB
> > file, but the fabricator will probably drill the hole at 0.031 to allow for
> >
> > the thickness of the plating.
> 
> I used to do this too, but I've found that there's a very good reason to make
> the plating allowance when you choose the hole size, and consider the nominal
> hole size to be the exact drill size. All Protel's clearance checks are based
> on the outside diameter of the barrel (the named hole size) being exactly as
> given. If you let the fabricator increase the drill size to allow for
> plating, you don't have the clearance you think you do. Maybe you get away
> with it this time, maybe the next time too, but sooner or later it's sure to
> bite you. Thus I routinely go up a few mils on hole sizes, to ensure that
> plating plus all the worst case tolerances can't make the hole smaller that
> the pin which goes in it. That way, all the design rules are really
> guaranteeing the clearances I asked for. And I make a point of telling the
> fabricator that my drill file is real drill sizes, that they aren't permitted
> to increase drill sizes to allow for plating.
> 
> Steve Hendrix

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