[Texasbowhunter]

Thanks guys
Appreciate this info
Paul

[joeavaerage]

Hi,

Linear rail usable travel = rail length minus the bearing spacing (outside face of one bearing car to outside face of the other bearing car)
So this depends on your design because you choose the spacing.

pippin is 100% correct, although I phrase it differently:
The length of the rail is the travel distance PLUS the distance (between centres) between the cars PLUS the length of one car PLUS 10mm so you don't run off the end.
maximizing the distance between the cars improves the stability of the axis, but also therefore requires longer rails and the machined surface to mount the rails to.
If you draw a force diagram there will be commonly one axis (at least) where the applied machining forces is as great or even exceeds the distance between cars, resulting
in instability of that axis and potential loss of accuracy.

The attached pic shows my Y axis. The cars are 200mm between centres and the travel is 350mm, the length of the cars is 80mm, and the rails 650.

With the Y axis centered the Z axis is right in the middle of the Y axis saddle, stable and accurate. Note that when the Y axis is at the extreme of the travel the Z axis
is now 'outside' the footprint formed by the cars, which will sort of cause the Y axis saddle to 'tumble'. This is not ideal, but there is a balance to be struck. The rails available
to me were 650mm long and I could not make the beds longer than 700mm or they would not fit in the heat treatment oven. Thus I had to make the best compromise
that I could. I wanted as much travel as I could, but did not want to introduce a situation where the axis was too unstable.

So when pippin says 'So this depends on your design because you choose the spacing.', he somewhat understates it. This is one of the most critical design decisions you'll
have to make.......space the cars widely for best stability but reduced travel or increased final size....OR....space the cars closer together for maximum travel for a given size but at the
expense of stability.

It is a very VERY good idea to draw some diagrams to establish which, if any, of your axes do this, and then make a well thought choice, maybe supported by calculation,
on what arrangement suits you best.

Craig