another long rant but,
i also like the idea of carl's rails , but i can see a problem that may be troublesome
that tends to put me off
if the rails are made from aluminium they are going to wear fast , also aluminium in that configuration is going to deform quickly where most working pressure is applied causing kinks in the rails
making them from steel well the running surface is going to be prone to rust that will make the surface uneven after a year or so
and unfortunatly stainless steel is not readily avalable in angle section
i don't fancy having to strip down the machine and sand down the rails every 12 months or keep having to replace them because they wear, should do the job right and with the right materials so this machine should last a long time
maybe using flat 1/2" by 1 1/2" section stainless steel or whatever size is best suited bolted to the frame for the x&y axis and another method for the z , bar or square section may be better in the long term for wear and environmental reasons
grease is only going to get squashed out of the way by the bearings , and add to the friction
skate bearings well ok there readily available and cheep ( but dose not mean there any good )
we could look for a bearing that is as widely available and used in a comman everyday item like a bike or car (of the right size that is
) if skate bearings don't look like there going to do the job
Dave