Need advice on gantry design

[EddyCurrent]

I've just noticed you have the top and bottom rails right up to the front edge of the gantry, they would be better set back to the centre of the top edge I think.

[routercnc]

I would say they are a bit more supported on the front edge because they have the vertical face of the section they sit on directly underneath. If they are in the middle of the section they are in the middle of the unsupported membrane of the section. It also minimises the offset from the spindle to the rail (a little bit).

If they are steel they would of course need to be set back a bit as there will be a radius along the edge.

In terms of performance comparisons this confirms what I said earlier in that the vertical stiffness is easier to achieve than torsional stiffness. To get the same deflection in both directions you need a lot more material resisting the torsion/moment than the more straightforward vertical load, hence the horizontal L.

But I would also say that the vertical and horizontal L shapes would both give good performance in practice, and that in the end it will come down to whether the horizontal L leads to unacceptable loss of travel in the X axis for any given design.

[jcarpenter]

To round out this discussion I did a box gantry also. Deflection of 0.02mm is the lowest yet.



Dimensions of the boxed beam are the same as the widest dimension of the horizontal "L" gantry.

So all this playing around with FEA has basically validated the advice given in the sticky threads at the top of the sub forum. That is, use a box or an "L" beam. I think I will. Just gotta decide which one to chose.

Thanks to all.

John C

[routercnc]

Great thread John thanks for posting.

The box gantry will always be a clear winner from the deflection point of view, but then you have to put the ballscrew somewhere. You can put it on the front face but then you have to step the Z axis away from the gantry, loosing some stiffness benefit. Plenty of commerical CNC routers use this layout and they work just fine. Or you can go for the L shape, tuck the Z axis tight against the gantry and put the ballscrew just behind. Plenty of DIY builders on here have gone this route and that also works just fine.

In general section size is key for stiffness as it goes up with the power ^3 for bending and with the power ^2 for torsion. So a little bit deeper / wider goes along way.

In all cases the material needs to all be furthest away from the neutral axis, the bit somewhere in the middle where not a lot is happening. So tubes are good, squares are better. Avoid open sections such as [ or I beams. They are only good in one direction.

Once you have maxed out on the outer size, have a play with wall thickness as this can really boost the stiffness. The limit is then how much weight you are comfortable with.

[jcarpenter]

A Facebook CNC guy I respect asked for another run of the box configuration with the rails mounted top and bottom.



Deflection was 0.024mm. But the Z axis had to be extended downward to account for the increased separation between the rails. So the lever arm is longer and the displacement more. Depending on where the twist axis lies. Too complicated for the simplicity of the model. My personal opinion is the the rail placement is probably not much difference.

OK, now I am done. Gotta start a gantry layout now.

John C

[jcarpenter]

Time goes by so fast when you are having fun. 9 Months since I did the math for the gantry and just now have a new initial design for it.



But there is a problem. My original estimate for weight for the gantry was 40Kg. The weight estimate for this new much stiffer gantry configuration is 100Kg. My question is: Is a 100Kg gantry a problem? The math says I need a stepper that can deliver 1Nm up to 600 rpm to power the two X axis ball screws that move the gantry. That is with a factor of safety of 2. Stepper size looks doable. Does a gantry of this weight pass a reality check?

Thanks,

John C

[routercnc]

Hi John,

Usually up to 80 kg gantry is OK for typical stepper and drivers, above that may be entering servo territory (=money!).

See post #2 here:
http://www.mycncuk.com/threads/6457-...80kg#post48417

I'm intrigued how you ended up with the design in post #22. That thin wall, and very open box section looks like it will resonate in practice, and the intermediate extrusions (to space away for the ballscrew?) are going to flex locally. The ends look fairly tricky to build and maybe a bit complicated.