Need advice on gantry design

[jcarpenter]

First real post! Looking for advice on moving gantry design.

Proposed machine cutting area: 1220mm x 1220mm cutting area (1/2 plywood sheet)
Materials to be cut: Foam, Hardwood, Plywood, Aluminum to 6mm thick.

Proposed machine parameters:
Floor standing
Moving gantry with high wall frame
Gantry beam length about 1500mm
Twin X axis ballscrews, single Y and Z
Non-production environment
Need decent but not perfect finish on edges of aluminum plate.
Would like to hold plus minus 0.1mm on dimensions

Materials available for the router build:
80/20 extrusion 3030 (76mm x 76mm)
80/20 extrusion 1530 (38mm x 76mm)
6mm aluminum plate
15mm plate
20mm plate

Shop tools available:
20 inch bandsaw
Full size CNC knee mill
Small CNC lathe
300mm x 300mm CNC router
Small shear, small pan break, air compressor, die filer, etc.

Experience:
Amateur CNC machinist. Built a couple of small CNC routers and some custom automated machinery for pool cue
builders. Just enough knowledge to be really dangerous.


First picture is what my design looked like before searching around this forum for a while. Looks like I hit all the usual problems
such as:
Separated twin beam gantry
Ballscrew between gantry beams
Thin Z axis plate
Z axis rails not on Z axis plate
Single gusset plate to hold end extrusions to gantry carriage plate

Probably a lot more problems lurking in that design. So I am starting over. And the first thing I want to do is to settle what the cross section of the
gantry should look like using the materials available to me. The second picture is of some gantry and Z axis plate cross
section views with different configurations of extrusions. Trying to come up with some ideas that combine the recommended
box or L box beam construction with things I have on hand.

Do the drawings make sense? Any of my ideas decent? Got any suggestions?

Thanks,

John C

[EddyCurrent]

Consider the information here; http://www.cncroutersource.com/do-it...NC-router.html
That should be a guide to the best arrangement.

There was some discussion here about "stiffness", it's a pity more people did not contribute at the time. http://www.mycncuk.com/threads/7155-...stiffness+test
There may be some merit at looking at the numbers to see what each design offered.

[jcarpenter]

EddyCurrent, your first reference seems to recommend widely separated unsupported rails for the gantry. That might be better for a smaller CNC than I plan. The second reference concerns measuring the stiffness of the CNC machine after it is built. A great idea but I am far from even starting my build.

What I really was hoping that someone could advise me on what would be a good gantry design (cross section anyway) using square and 1/2 of square extrusions with using thin plate to box in the beams. Without advice from someone with experience I will probably need to do some simplistic FEA on configurations of hollow rectangular tubes to get a comparison of configurations. Even if I had an FEA program that analysis if done accurately will be somewhat of a task. How about a gut feel comparison of the cross section sketches in my original post?

Thanks,

John C

[EddyCurrent]

I think you missed the point in both cases.
The first link was not to show which components to use but rather the best geometry for success.
In the second instance, the stiffness, which you want loads of, was measured after the build, as you say, but that gives us the benefit of looking at the designs to see why those figures turned out as they were.
So now we can either use or avoid those ideas in our own designs.
Somewhere in this forum is a spreadsheet that calculates various gantry designs, you should find that and test your own ideas out with it.

[routercnc]

Here is the calculator I wrote some time ago if you want to compare sections
http://www.mycncuk.com/threads/2214-...ess-calculator

Of the designs you have drawn they all have the ballscrew next to the YZ axis which is compromising the gantry stiffness.

A popular design here is the L shaped gantry with ballscrew tucked behind. Away from pc so will post example later or someone might post it.
L

[jcarpenter]

Ok, thanks to both. Sounds like I need to go back to the math for a while.

John C