Why not use unsupported rails?

[Jonathan]

The purpose of this simulation is purely to compare rails, not to say one type of rail is better than another in all circumstances, clearly each of the 3 main types of rail has its place.
What the supported rails are mounted to has to be included, as nobody would mount them without any support, however including other factors such a bed strength is unnecessary complication and so should be considered separately. Clearly when designing a real machine you must consider how all the parts interact, so ideally one would simulate the machine as a whole. However since the rails are a major source of error it is reasonable to look at them independently to help with selection.

The main advantage with profile rails is they can tolerate high preload, so the stiffness is much higher and there is no play (so long as the preload is not exceeded), which is why I will soon replace my round rails. Round rails have much less, if any, preload so the deflection of the bearing block relative to the rail is non linear and greater. In the case of just supported rails the load rating/stiffness varies depending on the direction of the force which is a significant disadvantage if not mounted 'opposing'. The accuracy of a profile rail is not much different to a round rail - it depends how you mount them as how many of us correct the bend in a profile rail by comparing to an accurate straight edge reference? Not many as for the vast majority of machines it's excessive.

Your right john, it would be interesting to see what the equivalent would be of unsupported against...for example would 50mm unsupported be the same as 25mm supported?

Depends what you mount them to...I've run simulations to find the equivalent in supported rail of 30mm unsupported. Since you suggested comparing 40mm extrusion I have used 40mm *light* aluminium extrusion. Same forces as before.

Table, Ratio colum shows deflection relative to the equivalent loading on 30mm unsupported.



Pretty pictures attached:

Simulations.zip

So the 30mm unsupported rail is similar in bending to the SBR16 rail on weak 40mm extrusion, yet currently using the cheapest prices for each I can find the 30mm system costs 70% more. That's for the rails, bearings and end supports, or supported rails, bearings and extrusion. That implies that now the prices of SBR (and TBR) rails have decreased significantly since 2eopoz bought his it is no longer makes economic sense to use unsupported. The exception of course is a small machine (Jazz's 300mm estimate sounds reasonable) with lower forces, or laser and plasma cutters.

The thing to do now would be to run the simulation with two rails as that is more realistic, however I don't think it will tell us anything new.