Please review this router gantry idea

**JAZZCNC** · 07-09-2013

They are connected with the plate in centre that bearings fasten too and they would obviously be fastened at the ends into gantry sides. This was a quick mock up for you to visualise and obviously you'd strengthen has seemed appropriate.

To me there's no point wasting time drawing anything if your not working to dimensions you know are available. Those 2 tubes are standard dimensions for off the shelf steel 100 x 3.2mm. Rest like angle and flat plate is again off the shelf.
I've not done any bending or testing calcs but I'm sure combined strength of 2 tubes will be better than one large tube unless it's wall thickness is much greater.?

**EddyCurrent** · 07-09-2013

Jazz, sorry didn't see central plate but I have now. Is this a design you have considered before ? how does it rate compared to other designs ?

**JAZZCNC** · 07-09-2013

I knocked this together just to show you but yes I've used tube before on a plasma machine.
So Yes I wouldn't hesitate using it router and it would be easy enough to make and very strong.

Like Jonathan suggested I'd either machine the rail surfaces or epoxy them. I'm big fan of epoxy levelling and use it all the time has it's easy for large surfaces and plenty accurate enough for DIY machine.
Weld the plates on, stitch the tubes together then epoxy and you'll have a very stiff gantry with nice flat rail surfaces.
Again like Jonathan I wouldn't use round rail, mainly because I think they are rubbish and secondly they are rubbish.!! Profiled rails make into much better and stronger machine, accurate.

**Jonathan** · 07-09-2013

No, one tube will be much stronger. Second moment of area (J) is proportional to radius to the power 4. The deflection is inversely proportional to J, so this means that for instance one 200mm tube would deflect around 16 times less than a single 100mm tube. The big advantage with a single tube is it's the strongest shape in torsion, so if using two I'd make them rectangular/square section as the forces are now mainly bending not torsional.

So you can't really go wrong if you use a big tube. The same principles apply to box section...

http://www.mycncuk.com/forums/linear...alculator.html

**EddyCurrent** · 07-09-2013

Jonathan, I downloaded that excellent calculator several days ago and it looks like time needs investing to get the best out of it so because I might only have another 10 years to live can you advise on the size of pipe I might need, thanks. (600mm working bed width 2.2kw or 3kw spindle, cutting hardwood but as seems to be the case with others probably light ally too)

**Jonathan** · 07-09-2013

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.

**JAZZCNC** · 07-09-2013

Originally Posted by Jonathan

No, one tube will be much stronger.

I was thinking more in terms of a complete gantry assembly not just the tube and more regards bending not twisting.!

Not got time or inclination to fully draw both and test but I reckon it would be very close and really either would be plenty good enough for routing hard wood.

I also naturally think in terms of what's easily available at sensible prices and 100mm tube will be easy got and cheap 200mm + will be hard and expensive to find.!!

**Jonathan** · 07-09-2013

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.

**EddyCurrent** · 07-09-2013

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.

**mekanik** · 07-09-2013

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 ?