I'm guessing two pipes might be easier than one.
Would the two tubes try to rotate about each other ? they would need to be connected somehow, it seems the basic strength of one tube has been lost maybe, but you are right that large tube might be expensive unless of course a scrap yard had something.
That gantry you've drawn would be pretty lightweight I imagine.
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.?
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 ?
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...
Last edited by Jonathan; 07-09-2013 at 12:30 PM.
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like I did here, is much less work! I'll be impressed if you can achieve the same accuracy with shims, as it's hard to compensate for twist in the rails.
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)
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.
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.!!
Last edited by JAZZCNC; 07-09-2013 at 12:59 PM.
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