LM Rail

[Jonathan]

yes i should have waited,too impatient ,im not even sure it will be up to the job, i bought a little over 400mm for the z and about 1200mm for the y with the mind to cutting it down if need be as i still cant committ to a size of machine or design.

Don't cut it - use the extra length to space the bearings out further. Normally the force on each bearing is inversely proportional to the spacing, so you want to space them out as far as you can (within reason, assuming what they're mounted on is stiff) to reduce the load on each and thus reduce the deflection. I got two 1150mm (hiwin HGH15) rails for my Y-axis, which is only 760mm travel but I wont be cutting the rails.

ive only managed to find 1 working machine using it on his z and y axis,great little machine but he has 4 lengths of sr15 on his y axis alone.

He's probably used 4 as unlike most of the good rails the SR series does not have an equal load rating in all 4 directions - http://www.thk.ru/cataloguespdf/sr-300e_cc.pdf ... but since you've already bought them I wouldn't worry about it - any profile rail is going to be a huge amount more rigid than the SBR20 rails you currently use.