James, you're absolutely right when you say designers need to think about their output formats.

In my CAM Engineer days I occasionally saw some designs where the gerber artworks had been exported in either 2.3 or 2.4 inch format but the drill file was 3.3 metric which was always "A Very Bad Idea".  I don't know why the designer had done this, I suspect that perhaps they didn't even know they had, nor had any idea what they were doing when they pressed the output button.  Fortunately thought the CAM software had a function that would "snap" the drills back to the centres of the artwork pads.

I also saw designs where the tracks and pads had been laid out on either an imperial or metric grid, but the gerber outputs had been done in the opposite system.  The result was the designer had done their DRC checks and passed the board but the subtleties of rounding errors in their outputs in a different system and different precision meant that DRC violations to our rules existed.  They were usually very small and easily dealt with by us but occasionally needed contact with the designer whilst the design was on hold.

Using metric I can design an "imperial" part easily using measurements like 1.27 mm, 2.54 mm, very easily whilst still outputting my drill and gerber data in 3.3 metric.

I totally appreciate you're restricted to imperial paper sizes and can't get hold of ISO 216 A series sizes in the US.  When I passed my CID I received my certificate in the post from the US printed on a US paper size and it took me two weeks to find a picture frame to put it in!

Regards,

James

James Head BEng CID+ MIIE MIET
Senior PCB CAD Engineer
Crowcon Detection Instruments Limited
2 Blacklands Way
Abingdon Business Park
ABINGDON
OX14 1DY
Telephone: 01235 557700 extension 289
Fax: 07092024504
E-mail: [log in to unmask]
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