I have a box with internal pillars fairly near the corners. Ideally I would like to make a rectangular PCB with 6mm diameter holes leaving just 2mm between the hole and the edges of the board. Is this practical or should I go for a milled corner cutout avoiding the pillar?
With only 2 mm of material left on the board edge, I would go for the routed corners and allow a bit less than a mm (25 to 35 mils) of clearance around the posts. (If the board edge clearance was greater - say, 3 mm or more - I’d probably favor the holes.)
In my experience with molded plastic cases, the combination of size tolerance plus locational tolerances makes the case assembly pillars shift around with respect to the PCB mounting bosses. If you use the case assembly pillars to position the board, then you need some oversize holes for the mounting screws; if you use the mounting screws to define the board’s position (my preference), you need a little extra elbow room around the case pillars.
Also, during assembly, it’s easier to drop a board in place when it has corner cutouts rather the getting all the pillars aligned to slide through their respective holes. If you’re using the pillars to set the board’s position inside the case, you WANT the closer fit around the pillars but in most situations it’s easier on the assemblers to have more clearance around the pillars.
I think you are right. This means that I may have to flip the board to sit between the pillars at the top of the lid. It’s a cheap flanged box, almost ideal for the job, but not quite right
Look carefully at the case drawings (if you have them), and give your case a good eyeball inspection if possible. Sometimes the pillars are different diameters on top versus bottom sides. And, the PCB mounting bosses may be in different locations on top and bottom. (I had a recent situation with a Polycase enclosure that started with the comment, "Well, if we move the PCB assembly from the bottom of the case to the top . . . . ". )
No drawing unfortunately. Pillars and sidewalls always taper slightly, the draw angle to allow the part to be removed from the mold.
And, there is a significant fillet in most corners which prevents you from mounting anything that wants to touch two intersecting surfaces.