Need advice on gantry design

[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.

[jcarpenter]

I inverted the legs of the "L" gantry configuration and reran the loads. As before the images are respectively, Geometry, Loadcase, Results, and another view of results. Deflection decreased from configuration Gantry_4 of 0.034mm to 0.030mm for Gantry_5.



Better than the decrease of about 10% in deflection is the more uniform deflection distribution over the gantry. In addition the most distorted part is the Z Axis block which is fake anyway. Pretty happy with this configuration but as mentioned above I will need to do some comparisons with the base gantry before committing to the Gantry_5 design.

John C

[EddyCurrent]

1. Top and bottom rails need to be further apart, i.e. vertical part of the "L" needs to be taller.
2. Z axis is way too far away from the gantry, it needs to be as close as possible to it.
3. The Z axis plate should be wider, when looking at the front face of it. It needs 2 bearings on top and 2 on the bottom, these should be as far apart as practical to resist sideways deflection at the tool.

[routercnc]

1. Top and bottom rails need to be further apart, i.e. vertical part of the "L" needs to be taller.
2. Z axis is way too far away from the gantry, it needs to be as close as possible to it.
3. The Z axis plate should be wider, when looking at the front face of it. It needs 2 bearings on top and 2 on the bottom, these should be as far apart as practical to resist sideways deflection at the tool.

Agree, #2 and #3 will make any of the options looked at so far a little bit stiffer.

#1 Though is an interesting one. The results for torsional stiffness show that the horizontal L (0.030 mm deflection) is better than the vertical L (0.034 mm deflection). This is easy to understand as that load case has a combination of bending in the X direction (for which the section is now very deep), and twisting (for which the section is the same as before). The sum of these gives an overall benefit for the horizontal L.
But the vertical loading needs to be checked as this will now be less stiff for the horizontal L. This is important to resist sagging due to the spindle/Y axis mass and causing machined surfaces to be bowed.

From a practical point of view the vertical L is always going to be a good choice as it allows more fore/aft gantry travel for any given bed length. But from an analysis point of view it will be interesting to see which will win overall . . . . .