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

[jcarpenter]

Got some math working (I think) to compare different Gantry cross sections. Would appreciate any criticisms or comments on my methodology.

I have gotten my static FEA capabilities up and running and will attempt to analyze some different gantry configurations with it. My FEA refuses to deal with complicated aluminum extrusion CAD so I will simplify the extrusions as a hollow rectangular beams with internal cavity adjusted so that the hollow beam and the 80/20 extrusions have the same moments of inertia. Not looking for absolute displacement numbers, only comparisons between configurations.

First image is my starting CAD design for the gantry. Called Gantry_0
Second image is the FEA geometry used.
Third image is the loadcase used. I am fixing the beams ends in space and applying a line load of 500N at the bottom of the Z axis block.
Fourth image is the result. About 0.125mm. Means nothing except as a comparison to results from other configurations.



Next is my first configuration change where I box the beams with a plate underneath the rails. Gantry_1. Again the images are Geometry, Loadcase, and results. This time the displacement is about 0.086mm compared to the original of 0.125. So the plate helped some.



My next configuration (Gantry_2) is to box the other side of the gantry and see what I get.



Image progression is the same as above. Resulting displacement is 0.056mm for further improvement in the stiffness of the beam. Hard to see in the results image but the majority of the flex is in the lower beam at the Z axis attachment. See below.



So I will add a half size beam along the lower beam to form an "L" shape to the gantry. Just like the sticky above recommends, haha. More to come.

John C

[EddyCurrent]

A circular tube or L shape are among the best ideas I think. In my own build I used an L shape with a flat plate attached to the front, that seemed to work well.

[jcarpenter]

I added an "L" shaped 1/2 size extrusion to the lower edge of the gantry and ran the deflection calculations again. Got another 20% reduction in the displacement metric. 0.056 to 0.040mm. Not as much reduction as I expected and the gantry is getting pretty heavy. I will try a full size extrusion soon and see how that works as an "L" beam. After that I might try 4 half size extrusions to form the frame of a 250mm square box tube.



John C

[EddyCurrent]

I don't think the weight will be an issue but are you also taking account of the spindle weight ?
Try a different L arrangement, http://www.mycncuk.com/threads/6565-...5076#post55076
Also try the Y rails on the top and bottom faces rather than the front face. This makes the rails further apart which is good and also allows the Z axis to be closer to the gantry.