Gantry Options/advice

[robe_uk]

Hi Lads,
Am still planning my latest cnc router and need some advice.
I have a pair of 1100mm supported round rails and a pair of 1600mm nsk type rails, I have two options, use the round rails and have a gantry length shorter than if I used the nsk rails.
My question is would 60x120mm aluminium extrusion by strong enough over a 6ft span to use the nsk rails without sagging or would I be better using the short round rails for the gantry. The longer gantry option would suit the router location better.
Also I have attached the extrusion I was thinking of, about GB£59 ($91)/meter for the heavier.
The lighter is about GB£48/meter.

Any advise on this welcome,

Rob
ps dont expect any builds to follow as it takes me an age to think about things lol

[hemsworthlad]

Hi robe,

I have some of that value frame profile but it's the 160x80 light and I can tell you for sure it's well strong enough so i think the slightly smaller will be plenty ok.
I use a lot of the 90x45(not the light) from KJN and even that is very strong and would do it if supported from below with triangulated braces, it's only £25mtr for HD and £16 for light, the light would work ok for braceing.

Word of warning about Value frame, their cutting service is rubbish and they charge for it, if you need 2 piece's same length then cut it your self.
I've only had a 5 or 6 pieces from them and only 1 of them was exactly size I asked for.!
KJN on the other hand are perfect and dont charge and i've had hundreds of meters off them now and all cut exactly has requested.
(No I dont work for them or involved)

Edit: Just re read and realise (well think you do) mean for the gantry not the X axis rails to sit on.? IF so this piece of 160 x 80 is 1500mm long and for a gantry and this wont sag.!

[robe_uk]

Originally Posted by hemsworthlad Hi robe, Edit: Just re read and realise (well think you do) mean for the gantry not the X axis rails to sit on.? IF so this piece of 160 x 80 is 1500mm long and for a gantry and this wont sag.!

Thanks for the reply, yes i do mean for the gantry although the size i've shown is 60x120 so are you recommending using 160x80?

Just had a look at an old thread of mine about gantry design before, (yes long time planning lol) and the thinking then was to run with the round rails to give stability, I wonder if i would loss much if using the nsk rails for the gantry instead. (oh I did make the mdf gantry shown in the thread, but thats as far as i got )

Rob

[robe_uk]

Ok, change of plans again and am interested in feedback, as the cost of aluminium extrusion is quite high I decided to go with box steel for the gantry.
The size available was 100x50mm which is very strong over the length of 1300mm(~4ft) so this is what I've bought. As I have no fancy ways to make gantry end supports I have thought about using a piece of 80x80mm to sit the gantry on and use 80x80mm brackets to hold in in place and stop it twisting.
Now I was thinking of using one in front and one behind, what do you think?

(forgive the basic drawing, supported round rails not show)

[hemsworthlad]

Hi robe,

Nothing wrong with the stregth of the box but it does have some issue's.?

Big problem with box section is the resonance it produces.! This can have a few affects and none of them good.
Resonance can affect motors causing all sorts of issue's and while some drives can help deal with this it's always better for it not to be there in the first place.!

It can and does show in the finish of the cut on harder materials, cause's a chatter affect due to the resonance vibrations.

Often the surface of the box section is never truely flat and can make lining and setting the linear rails a proper bear.!

It's also very noisy.!

I recommend you fill it with something to help dampen the resonance.! Kiln dried sand is a very good and cheap way to do it, does mean sealing the ends etc but well worth the work.
Dont be put off by the extra mass of the gantry, mass helps with a cnc machine not hinders and will give better results in harder materials.

If you dont do anything I think you'll be surprised how noisey and the vibrations that come from it.

The mounting looks ok.
Hope this helps.

[robe_uk]

Hi Thanks for the reply.
Yes noise is a concern, I had thought to use extrusion but apart from cost I was able to get a free cut off of 80x80mm x600mm long and it rings like a bell when tapped so using this would be no better. I have seen sand mentioned before, in fact I think it was by you

ok thinks this is my final design lol, just have to get on with things, time limited with 2 kids and a wife

Rob

Oh, after-thought, what about using a liquid rubber or something like that? more hassle than its worth?

[hemsworthlad]

Cant say about liq rub but i can tell you expanding foam doesn't work.!

I've found kiln dried sand to be the best and most cost affective, also if done with drain bungs in right places it can be easily removed when needed.
The extra mass of the sand also helps considerably with the quality of cut with only a slight loss of acc/deleration and speed. (Obviously this will depend on other variables, motors, screws etc) . . . .But it works good.


Edit: The Extrusion I use doesn't resonate so it's strange that your offcut does.?

[Flateric]

I second the suggestion of filling with sand and designing in fill and drain ports into the boxes. I even filled the bed and gantry of my machine and the reduction in noise and damping is substantial to say the least.

I would even go as far as saying the any feed speed reductions required due to weight are no worse then the feed reductions required to combat the resonating when empty. If anything I would have to guess the machine is faster heavy.

Cut quality is immensely better though.

I used thick walled box aluminium for my build.

[harryn]

Locally we can get ground up used tire rubber pretty cheap, and in different sizes. I am toying around with using that for a filler.

[hemsworthlad]

Would be intersting to see what affect this has.? . . Density is the key I,ve found so not sure.? . . . but worth a try if cheap enough.!

[robe_uk]

Well am liking to idea of sand as it can be easily removed if needed, some of my frame is also box section so this may get the sand treatment too. Right I really need to pull my finger out and get started lol...