What is the best way to mark out for bearing blocks and linear rails?

[Wal]

Just a word of warning - double check and measure off the print before you punch... I eventually had my plates drilled at a shop but used a 1:1 print out to check the work against. Fair enough - the prints had been kicking about in the damp-ish boot of my car, but that was enough to warp/stretch the paper and introduce some alarming deviations - I thought the machine shop had ruined my plates before I realised what had happened... In theory and as Clive says, the method should work perfectly, just don't take it for granted that the paper's going to remain stable over time. Oh, and don't forget to tape it down..!

+1 on Mike's suggestion of using the rail as a jig and using a transfer punch through the existing mounting holes.

I used M6 threads into 60x60x5 square section to hold my plates.



A fair few of 'em, but I found no need to use backing - as Clive says, it would have been hugely impractical to do so...

Wal.

[routercnc]

For mounting my supported rails I clamped them in place accurately, then used a centre punch to very lightly mark the centre as a best guess. You can't see if it is central until you remove it, so I then looked straight down the hole, and made subtle adjustments with the punch to 'push' the hole around a bit. When happy I gave it a big hit ensuring that the punch was very vertical. Then double checked it and moved on. Finished by drilling the pilot and tapping the hole. Took along time but when it's done it's done.

In terms of thread engagement I've always used up to 1.5D as maximum depth as you get rapidly diminishing returns in terms of thread load for the deepest threads. So M8 has about 12mm of thread. Not detrimental to go over, but not helping either.

In terms of minimum engagement I must admit that I've always used judgement, but out of interest I had a look at Roy's site (DIYCNC) and found the following. Lots of maths to get stuck into but the upshot in minimum thread length appears to be:
M3 = 2.4mm
M5 = 4.0mm
M8 = 6.5mm
These are for bolts and holes of the same material. Steel bolts into aluminium holes are supposed to be deeper.

Link : Screw thread Calculations

Scroll right to the bottom and find the row 'Length of thread' and the symbol Le. There's an approximate calc, and a more accurate calc.

[EddyCurrent]

In terms of minimum engagement I must admit that I've always used judgement . . .

I looked at the dimensions of nuts and used that as a guide.