round rails

[wilfy]

To save repeating myself, please refer to this thread:

http://www.mycncuk.com/forums/faqs-p...ted-rails.html

Bear in mind the rails I modelled there are substantially longer than what you will require, so as Jazz said so long as you stick to a large diameter (=>25mm) short rail you'll get away with it. However if you compare that to the price of say SBR16 supported rails from linearmotionbearings2008 on eBay the difference in price is not that great compared to the difference in performance.

acctually looking at the prices of 25mm unsupported vs 16mm supported it works our cheaper using rough prices from zapp...

so my question then would using 16mm supported be better/as good as 25mm unsupported for a 15" x15" machine?

[Jonathan]

acctually looking at the prices of 25mm unsupported vs 16mm supported it works our cheaper using rough prices from zapp...

Yes, which is why you don't use Zapp:

linearmotionbearings2008 | eBay
4 SBR16-500mm rails+8 bearing blocks | eBay

etc... clearly buy as big rails as you can afford and also email linearmotionbearings2008 to get the best price for the exact sizes you need. For 15" travel a 500mm rail is about right. I wouldn't like to see much smaller as it's best to have a decent spacing to lower the bearing forces.

so my question then would using 16mm supported be better/as good as 25mm unsupported for a 15" x15" machine?

Better so long as you mount them to something substantial - i.e. not MDF. Once you use supported rail the force is transferred to the material it is mounted on, so if you mount them to a piece of cardboard you don't gain much! Use steel box section (cheap) or aluminium extrusion (expensive) and the stiffness will be significantly greater. Another advantage with the rail being supported is the resonant frequency is much greater. Think of the unsupported rail like a string on a musical instrument - you apply a force to it and it will vibrate. This motion is transferred to the cut resulting in a poorer surface finish. Of course this is a very general statement, with the right diameter rails, bearing spacing and mount you can make it work with either type of rail. It's just a lot easier with supported rails.