# Thread: Please review this router gantry idea

1. My first machine uses a 910mm long piece of 80x80x3mm aluminium box section for the gantry, and it's strong enough for hardwood aluminium (but not especially fast), so I'd advise getting something bigger. Your design lends itself nicely to making the rail spacing the same as the diameter of the tube, so if using profile rails you wouldn't want to go below around 160mm spacing. A round tube that diameter would fit well and be plenty strong enough. Edit: However something a bit smaller would be strong enough, just less convenient.

Also, you don't necessarily need a high wall thickness. Use the formula for the moment of area to compare them:
J=k*(od^4-id^4)
Where od is the outer diameter is the inner diameter, and k is some constant we don't care about as we're just comparing. For example suppose you find a 180mm diameter tube with 3mm wall thickness and a 150mm diameter tube with 5mm wall thickness, J for the large tube is 1.3*10^8 compared to 1.4*10^8 for the smaller one, so a tube that's a little larger can still be stronger even though the wall thickness is small. In this example I'd go for the 180mm tube as it's about 40% lighter than the smaller tube.
Last edited by Jonathan; 07-09-2013 at 02:10 PM.

2. I agree both designs could be made strong enough, but the basic formulas imply that you can make it stronger with less material if just one tube is used. It's just a matter now of looking at what material is available, and seeing which design ends up with the best strength per pound (or lb!) ratio.
Last edited by Jonathan; 07-09-2013 at 02:12 PM.

3. Just had a quick look at pipe prices ! so what about making a fibreglass pipe with the mounting points incorporated ? and I've just remembered there's bendy plywood now so a pipe of any wall thickness could be laminated up. Now I'm not descending into the MDF gang but I think there may be enough strength in the plywood idea.
Last edited by EddyCurrent; 07-09-2013 at 02:56 PM.

4. Fibreglass rules OK, I was contemplating this when i first started thinking about making a machine, i had a try @ Vacuum Infusion on a little project a couple of years ago and the casting was very rigid and i had the feeling it could be a good material for reducing vibration ?
Last edited by mekanik; 07-09-2013 at 02:55 PM.

5. Originally Posted by mekanik
But no big diameters

6. Time to ruminate this lot over a pint I think.

7. We actually used to make make carbon fibre gantry mills before i left that industry

8. Originally Posted by compositepro
We actually used to make make carbon fibre gantry mills before i left that industry
I can imagine that, it's marvelous stuff. I was looking at carbon fibre matting earlier as an alternative to fibreglass.

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