round rails

[wilfy]

hey guys, i'm starting to think about making a small cnc machine possibly around the 15" x 15" size, my original plans had me doing a 4ft x 4ft but i can justify the costs until i make and learn a smaller machine.


so my question is at what point should i steer away from a straightforward round rail and got to supported round rails. my hope is that with a machine as small as 15 x 15 i will get by with round rails and keep the cost down.

[motoxy]

Hi Wilfy. I have learnt on this site that strength is very important. For accurate cutting you need to be able to control the cutter and that means that the frame must not flex. A weak point can be the rails. It all depends on what you intend to produce but the excepted wisdom is supported rails are always superior to unsupported as the deflection is greatly reduced.

Bruce

[wilfy]

i'm thinking more about engraving and cutting small pieces of mdf and acrylic. i'd be happy to sacrifice speed, if it reduces the cost

[JAZZCNC]

i'm thinking more about engraving and cutting small pieces of mdf and acrylic. i'd be happy to sacrifice speed, if it reduces the cost

Ok this is were people get it wrong regards speed and machine strength in relationship to materials, or more importantly what they think are soft materials.?

When cutting things like MDF and plastics like acrylic then you need certain amount of speed other wise the finish is rubbish and tool life is drasticly reduced. These speeds or feed rates are higher than most realise, so the small sacrifice in favor of costs becomes the Achilles heel of the machine.! . . . Normally not worth it. .. BUT.? . . . If decent sized rails are used with good end support then on a small machine like this then they can be ok.

Personally I wouldn't use them because the difference between supported and unsupported isn't great enough and the performance difference is substantial enough to warrant. Also if your thinking to keep short term then it makes the machine far more valuable and sell-able than one that doesn't.? Put it this way if someone says to me "what you think to this".? . . I'd say stay clear it uses unsupported rails.!!

[Jonathan]

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.

[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.

[wilfy]

brilliant info as always guys thank you very much, and yes i had planned steel box section, it isn't that expensive and i've bought myself a mig welder so although a small machine it will be well built and now on 16mm supported rails.

i have a question about the spindle but that can wait for a while, i want to get some of the frame built first and see how money works out.

thanks again

[TrickyCNC]

Whilst I agree steel is stronger than card board , and supported rails are stronger than unsupported, there are plenty of wooden/MDF machines out there working very well. Also there are ways to support 'unsupported' rails.

[JAZZCNC]

there are plenty of wooden/MDF machines out there working very well.

Yes they do work very well.???. . . . . Has Spoil boards and Fire wood. . .