Need advice on gantry design

[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

[jcarpenter]

Ran some more math on my triple beam "L" gantry. With a little less simplification. Generally don't see any major problem. I think it is time to add the final details to the design and then start building the thing.

Using Fusion 360 now and its static FEA solver. Not sure I will stick with this although the integration with industrial design capabilities looks pretty cool.




John C

[hanermo2]

Fwiw..

My large cnc mill bed is 240 kg in mass, for the bed alone, and up to 100 kg of vice and workpiece, easily.
So 340 kg++ of mass.

Worked fine with one small nema 23 stepper of 3Nm.
48V, 2M542 drive.
1:3 HTD belt drives.

Heavy is better, not worse.

For non-commercial use the speed was never a concern.
I never needed higher speeds.
Always wanted more mass, more rigidity, more resolution ...
leading to better accuracy.

[jcarpenter]

hanermo2, I have some 2.8 Nm NEMA 23 low inductance steppers and some 60 volt PS and drivers so I will try that with this rig. Will mount the steppers off belts rather than the direct drive I previously planned. Sigh, back to the CAD.

John C