People are often surprised to learn that I don’t use Proper Real CAD Software, or even simple 3D software like Google SketchUp, when designing enclosures. Coming from a graphic design background and a bit of drafting, my preferred instrument is actually Adobe Illustrator…a 2D program. But even before that, I usually sketch things out “old school” on index cards or the back of a receipt.
As an American, for most things my brain is attuned to imperial units like inches. And circuit board designs usually use 0.1" hole spacing and mills for placement. But when it comes to laser-cut parts, I’ve always found metric units…millimeters…quicker to work with and more “natural” for the medium. Dimensions for most of the materials and hardware can be expressed in whole units and not fussy little fractions. And decimal values carry over more easily from related tools like calculators, engineering scales and calipers. Most of these tools come in fractional versions too, if that’s your bag, but I’ve always been at ease with decimal figures.
Whatever your units, indulge in a nice set of calipers. You need these for properly fitting a case around real-world things. The Mitutoyo digital calipers are a joy to use. Robust, precise, selectable between millimeters and decimal inches.