Need advice on gantry design

[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.

[jcarpenter]

No welding will be involved. Everything will be bolted together. Built a machine once that had terrible resonance. Don't want to do that again. I think I will back up again and layout what a "L" shaped gantry with three 3x3 inch extrusions will look like. And single plate gantry supports. More deflection but less chance of resonance.

[jcarpenter]

The box beam gantry configuration is the stiffest design. But having fought resonance before I do not want to do it again. Scared of those large thin sides of the box. And the 100kg weight of the model. So tried another iteration. This time its a simple "L" beam made of the three 3in x 3in 80/20 extrusions. Ran the math and while it is half the stiffness of the boxed beam it is still over twice as stiff as the original twin separated beam gantry design. Comparison deflection metric is 0.05mm. Going with this.



Be back for more comments after getting this style gantry modeled.

Thanks,

John C

[jcarpenter]

New gantry layout using three 76mm x 76mm T-Slot extruded aluminum beams in an "L" configuration. Based on the FEA above.




This is a concept layout not a design yet but please give me comments and criticisms. What have I missed? Gantry side plates are 25mm thick, Z axis plates are 19mm, and the extruded T-Slot beams are 1520mm long. Y axis rails are 25mm HiWin, Z axis are 20mm HiWin. Light preload.

Thanks,

John C

[routercnc]

Looks good John. Some thoughts are:
You could centralise the end plates on the horizontal plates so they are directly above the X bearing carriages and bolt from underneath and not include the angle supports. The bolts into the X bearing carriages may just about straddle this end plate but if the bolt access holes to the bearing carriages underneath is obscured then add a second horizontal plate to bolt to the carriages and then bolt these to each other.

One end plate could be extended rearward slightly to accommodate the stepper which then belt drives to the ballscrew.

Make sure the extrusions and joined to each other to get the section benefits.

Check how you are going to drive the gantry and connect the X ballscrew nut before finalising the exact layout otherwise you might need complicated joining brackets either on the gantry ends or the bed ends where the stepper and bearing blocks are fixed.

[jcarpenter]

routercnc,

Looks good John. Some thoughts are:
You could centralise the end plates on the horizontal plates so they are directly above the X bearing carriages and bolt from underneath and not include the angle supports. The bolts into the X bearing carriages may just about straddle this end plate but if the bolt access holes to the bearing carriages underneath is obscured then add a second horizontal plate to bolt to the carriages and then bolt these to each other.

Yes, I could use two 12mm horizontal plates instead of the one 25mm one. That might make for easier alignment. Could mill an alignment rabbet on the bottom of the carriage plate to square the two carriages to each other.

One end plate could be extended rearward slightly to accommodate the stepper which then belt drives to the ballscrew.

Ok. Was going to direct drive everything but belts and sprockets might let me move a little faster with the 5mm ball screw pitch. What are the pros and cons of direct drive vs belt?

Make sure the extrusions and joined to each other to get the section benefits.

I have ideas for both internal and external fastening methods. Will try them and see what works better.

Check how you are going to drive the gantry and connect the X ballscrew nut before finalising the exact layout otherwise you might need complicated joining brackets either on the gantry ends or the bed ends where the stepper and bearing blocks are fixed.

Yes this was just a layout, not enough detail yet for a design. Looks like it might be time to put in the details.

Thanks,

John C