Z-axis design - help sizing materials please!

[ecat]

Cost is the killer and it all comes down to where are you willing to make the compromise. From what I've picked up so far you chose your target tolerance and design so the inherent flex etc of the machine is better than 1/10 of you target. So, for 0.1mm tolerance you design for better than 0.01mm deflection, or the other way around, if you suspect an assembly will flex by 0.1mm then you should expect no better than 1mm tolerance.

Unsupported rail will flex but the bearings should be consistent in all directions (?). Is the flex significant to you?

Supported rail suffers from much less flexing but what about the opening in the base of the bearing? How much extra movement does this allow?

Profiled rail + blocks cost a lot more than the equivalent round rail set up but minimises the issues with both unsupported rail and open blocks and does so in a smaller footprint. Are the benefits worth while and the cost acceptable for your design?

Second hand parts? Just how second hand are and why were they removed? What would the cost be of replacing a single worn part? The true cost of second hand = price + testing + reassembly + replacement_part_costs - how_much_you_enjoy_the_process

It wouldn't be so bad if there where a cheap and easy upgrade path but that's not the way the critical parts are designed. Going from the £70 round rail set up to a £200 profiled set up involves changing all the widths and lengths and offsets and leaves you with a pile of used parts to sell or the idea that a second machine could be built someday.

Your compromise, your budget, your choice :)