Need advice on gantry 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.

[jcarpenter]

To get to this design you gotta remember that as much as possible stock on hand is being used. Decided on a box beam as analyzed in post #19. Turned sides inside out to get the offset for the Y ballscrew to fit between the rails. Side plates don't cover the complete length of extrusions since 48 inch wide thin plate (5mm) stock is available but the rails (and extrusions) are 60 inch. Double plate gantry side supports have been recommended here so 16mm plate was used for those and the rest of the end plates.

As you can guess there is aluminum plate, sheet, and extrusions in abundance to use. All sunk costs. Below is what the structure inside the gantry looks like before adding the end plates.



Ends are complicated but the CNC knee mill in the garage will sort that out if necessary. Could use some advice on how to simplfy them.

I am worried about resonance. If people here think that resonance will be a problem with a manufactured box beam like this I will probably abandon it and try an "L" configuration. Looks like a lot of people here are having success with less overkill than this. How do the chinese big box beam gantries avoid resonance?

Thanks,

John C

[routercnc]

If you have a mill then things are much easier. Are you going to weld this all together then skim flat?

If this is a fabrication then I would add a horizontal web inside the section joining front and rear faces. Easiest way would be to mill short occasional slots in the front and rear face plates and puddle weld down onto the edge of the web. Maybe even add 2 webs close in line with the unsupported edge of the extrusions. This makes several smaller boxes and will tie the extrusions into the section and reduce resonance. Some people fill open sections with all sorts of stuff but I would prefer adding a few webs (before considering filling).

Commercial machines I've seen using box section gantry are steel with thick walls. They sometimes weld 2 thick strips to the front face, skim them, and mount the rails to those. The drive is often on the top face, or sometimes they squeeze it on the front face as well.

For the end pieces you could go with RHS section trimmed into a triangle shape. Then crop the extrusions flush with the fabrication, butt up the triangular fabrication and insert short extrusion pieces inside the triangles so that when you bolt right through it will prevent them crushing and allow you to torque the bolts. See my mk3 build to see how I did it.