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PuppyWoofie
12-11-2012, 05:47 PM
Hello, Sorry if this has been asked on here before or there's a right place to put this... But I'm building a 3x4 CNC and I'm buying the aluminium extrusions for the machine in the next few days... The problem is this is my first machine, So I'm in a whole different world to what I usually do, So I have been wondering, for the 3 foot side, (which will be over 3 foot... About 4 in total but working area 3) Will 60x120 "80/20" be suitable for the shorter side? Stability and rigidity-wise?

I'm using 20mm Supported Rails too so I'm hoping they will add some stability.

Thanks in advance!

martin54
12-11-2012, 07:28 PM
Best bet would be to put up a design so people can advise you properly, you could end up saving yourself lots of time & money in the long run.

PuppyWoofie
12-11-2012, 07:47 PM
My design will be extremely similar to this: http://zedomax.com/blog/wp-content/uploads/2007/02/cnc2.jpg

So I don't have the gantry sides to be too concerned with, the 3 foot section will be mounted directly to the 4 foot rails and the 4 foot rails raised off the bed to allow for height. Just my machine will be made from aluminium extrusions since it's a simpler construction process.

Hopefully that helps!

mocha
12-11-2012, 10:44 PM
hi PuppyWoofie

There is a spreadsheet on the site somewhere, sorry cant recall the link to it, which allows you to enter various materials and sizes and works out the amount of "flex" you can expect to see. It will of course depend on what you are going to do with it as to if that extrusion will be suitable.

My gantry is about the same length but I decided I needed something stronger than extrusion for the materials I want to work. I also seem to recall that reinforcing the extrusion was a viable option. Have a dig around with the search tool, I'm sure I've seen that question discussed before.

Jonathan
13-11-2012, 11:05 AM
The design you have linked to is a good start. For the X-rails (4-5' ?) you should put some more supports in the middle to avoid having a long unsupported section

The spreadsheet Mocha referred to can be found by searching 'gantry stiffness calculation':
http://www.mycncuk.com/forums/linear-rotary-motion/2214-cnc-machine-stiffness-calculator.html

However it doesn't support extrusion, unless you draw the extrusion in a CAD program (e.g. Solid Edge, Solid Works, Pro-engineer) which will tell you the area moment of inertia for the cross section, which can then be entered into the spreadsheet. You can also sometimes find the numbers on the manufacturer's website.

PuppyWoofie
13-11-2012, 01:19 PM
The design you have linked to is a good start. For the X-rails (4-5' ?) you should put some more supports in the middle to avoid having a long unsupported section

The spreadsheet Mocha referred to can be found by searching 'gantry stiffness calculation':
http://www.mycncuk.com/forums/linear-rotary-motion/2214-cnc-machine-stiffness-calculator.html

However it doesn't support extrusion, unless you draw the extrusion in a CAD program (e.g. Solid Edge, Solid Works, Pro-engineer) which will tell you the area moment of inertia for the cross section, which can then be entered into the spreadsheet. You can also sometimes find the numbers on the manufacturer's website.

Don't worry, on the X axis I wasn't going to have it exactly as shown there, that is just what I'm going for as far as the gantry goes, so I can eliminate the gantry sides as a point of deflection. I'll most likely have the X supported with a post every half foot to foot with 60x60 extrusion, plus the brackets which will be 30x30 or 60x60 if I can find them, reducing the un-supported sections even more.

It's mainly the gantry I'm worried about, since it will hold the router, 2 rails and the Z axis.

But since my supported rails have a hole spacing of 30mm, the only extrusion I can bolt it to directly are multiples of 30... 60x120 being the largest I can find.

The calculator now seems to support the aluminium extrusion and if I've done it correctly there should be less than a mm deflection, 0.19 on the Z and 0.016 on the X.