A couple of general design questions - ball-screw thread length, stub-shaft etc.

[Wal]

I have several questions that I need to ask and a few of 'em may be pretty elementary, but some of the details are holding me back and I could really do with clearing them up before I start buying potentially expensive paper-weights...

I'm about to build a couple of axis akin to this example:

http://cfnewsads.thomasnet.com/image...804/804510.jpg

...but I'll be using TBR20 supported rail.

(I'm aware that I'll be spending money on motor-mounts/ballscrew brackets etc. that many of you could knock up in a matter of minutes on your gear - but I have no such tools to hand and would rather not make a pain in the arse of myself asking for favours/to-ing and fro-ing with dimensions, so I'm just gonna bite the bullet, buy the hardware I need and design the axis based on the available measurements)

So, the questions:

1. When you design your machines, do you buy your ball-screws at a ball-park length and then design the machine around that, or do you work everything out in advance and then make the purchase?

In this instance I have done the latter and am pondering this -

If I know that the distance between my floating and my fixed bearings is 400mm - do I specify my ball-screw thread at 400mm + the machined ends - or would I go for slightly under (say 399mm) just to be sure that I'm not 'squeezing it in' between my bearings?

2. End machining of the ball-screw. This looks reasonably straightforward to specify, but how long would you spec the 'F' length (stub-shaft) as a rule? Is standard + an extra 30mm overkill? Would standard give me enough length to reach the coupling housed in an MBA12C motor-mount like this one?



3. TBR20 supported rail. Am I right in thinking that the mounting holes are drilled 20cm apart with the first hole 10cm from the starting end - ie. mounting holes on a one metre length rail would be drilled at

10cm-30cm-50cm-70cm-90cm

How accurately are these mounting holes drilled (from the end) and what happens when a rail length such as 49cm is used - are you left with 19cm of unfixed rail at one end?

I'm sure that there are other questions I've omitted here, but this is a start for now!

Thanks for any help/advice.

Wal.