Which is better, round or square rails?

[digits]

Hi guys, sorry if this is a FAQ. I am thinking of building a bridge/gantry style mill and I would like to know if I can use fully-supported round rails.

I had planned to just extend the X-axis of my current machine, so I bought some T-slot table, a longer screw and bearings and a pair of 700mm long round fully supported rails and 4 bearing blocks for them.



I am beginning to think though that I could do with some extra Y and Z too, as well as some more spindle power so I am considering using my first machine to help me build my second! Is there any advantage to using those square rails e.g. Hiwin, or should I just buy more of the same http://www.marchantdice.com/linear/SBR/index.htm - which might come in handy if I need to re-arrange things in the future.

Cheers.

[Halfnutz]

Square rails offer the best performance characteristics, but supported round rails are completely sufficiant for a wood/light metal gantry machine. The difference in accuracies between the two are too slight to notice if your going to do wood work and non critical signage type stuff. They can offer slightly less resistance and give greater speeds, but not anything that merits the cost in my oppinion. I use them because they are available on eBay for next to nothing, and long supported round rails are rarely found on eBay. Maybe for retail value there is an advantage also, because of thier value they give the impression of a higher quality machine.

Ive read that the Hiwins costs are similar though, if that is the case I would go with the square Hiwins, but not if I had to pay THK prices. They do offer much higher load ratings for thier size, greater linear accuracies, and less friction than round rails, if you really need the difference its worth the additional cost of course.

[Switcher]

I prefer square rails!


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[digits]

Thanks guys, I will get a quote for square rails for Y and Z - I hope I the round ones will do for X though - they weren't exactly free!

My tentative plan is to build something for cutting aluminium with about a 500x300x300mm work envelope out of that 40x40 or larger alu extrusion and custom brackets CNC'ed out of alu on my current machine.

You wouldn't believe how much everything costs over here in the UK - it really is as though $1=£1 - an X2 is about £450/~ $830!

[ViperTX]

Well square rails are better, round rails if supported will work. Look at some of the specifications of the parts.

[digits]

Looking at the UK prices, the rail costs are similar but 4 square rail blocks would set me back about £150 vs £80 for round rail blocks. Are they really twice as good for 600mm lengths and <50kg loads?