Need advice on gantry design

[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.

[jcarpenter]

An easy change to the configuration of Gantry_3 was to increase the size of the "L" extension to the square 76mm tube used in other parts of the gantry. Called Gantry_4 and shown below.



With this larger "L" the displacement metric goes from 0.040mm to 0.034mm. My gains in stiffness are getting less and less now. The interesting thing here is that there is a more equal distortion level on the top and bottom of the gantry. A well designed mechanism will show a more distributed deformation than one with very high and low distortion areas. This suggests to me that I am approaching the limit of improvements in this configuration. This configuration is probably OK as far as it goes.

I will next try flipping the horizontal and vertical sections of the "L" so that the longer "L" section is positioned is resist bending in the lower part of the gantry.

[routercnc]

Interesting thread John, good work. Your simplified analysis is sound enough to draw conclusions and it is no surprise you are iterating towards the popular L shape !

Watch out if you reverse the L shape then you may gain for the current horizontal load case, but the spindle mass and plunging forces will cause more deflection vertically so I would say run vertical loading on it, and repeat on the initial design.

In general, as per Eddy's comments, putting the rails on the top and bottom, and moving the Z axis close to the gantry might get a little bit more stiffness, so that would make another good one to try out. Plus lots of builders put an additional strip of steel inside the box section where the rail mounts. This is to get good thread engagement but will also add some local stiffness and help spread the load into the gantry, again adding a bit more stiffness.