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  1. #11
    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.

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    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.

  2. #12
    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.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  3. #13
    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.

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    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

  4. #14
    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.
    Last edited by EddyCurrent; 27-11-2017 at 09:43 PM.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  5. #15
    Quote Originally Posted by EddyCurrent View Post
    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 . . . . .
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  6. #16
    Agree on the Z axis distance from the gantry. Z axis design is probably next. The Z axis plate will be as wide as I can make it with my already purchased ballscrews and the cutting area needed. I bought some components early when I mistakenly thought the design was done.



    Did vertical loadcase on the Gantry_0 design (twin separated beams) and the Horizontal "L". Used 500N line loaded distribution in vertical direction.

    Twin separated beams: 0.1mm vertical deflection which will be my vertical comparison metric

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    And the horizontal "L": 0.03mm vertical deflection.

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    Still happy with the horizontal "L" configuration. Also like that again the beam has a fairly nice distortion distribution with the horizontal and vertical deflections similar under similar loading. Not sure I need to increase the vertical height of the horizontal "L" configuration or not. Deflections are well matched in both directions and adding height will be a little complex.

    This discussion is really helping me to think this out.

    John C

  7. #17
    I've just noticed you have the top and bottom rails right up to the front edge of the gantry, they would be better set back to the centre of the top edge I think.
    Last edited by EddyCurrent; 29-11-2017 at 09:18 PM.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  8. #18
    I would say they are a bit more supported on the front edge because they have the vertical face of the section they sit on directly underneath. If they are in the middle of the section they are in the middle of the unsupported membrane of the section. It also minimises the offset from the spindle to the rail (a little bit).

    If they are steel they would of course need to be set back a bit as there will be a radius along the edge.

    In terms of performance comparisons this confirms what I said earlier in that the vertical stiffness is easier to achieve than torsional stiffness. To get the same deflection in both directions you need a lot more material resisting the torsion/moment than the more straightforward vertical load, hence the horizontal L.

    But I would also say that the vertical and horizontal L shapes would both give good performance in practice, and that in the end it will come down to whether the horizontal L leads to unacceptable loss of travel in the X axis for any given design.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  9. #19
    To round out this discussion I did a box gantry also. Deflection of 0.02mm is the lowest yet.

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    Dimensions of the boxed beam are the same as the widest dimension of the horizontal "L" gantry.

    So all this playing around with FEA has basically validated the advice given in the sticky threads at the top of the sub forum. That is, use a box or an "L" beam. I think I will. Just gotta decide which one to chose.

    Thanks to all.

    John C

  10. #20
    Great thread John thanks for posting.

    The box gantry will always be a clear winner from the deflection point of view, but then you have to put the ballscrew somewhere. You can put it on the front face but then you have to step the Z axis away from the gantry, loosing some stiffness benefit. Plenty of commerical CNC routers use this layout and they work just fine. Or you can go for the L shape, tuck the Z axis tight against the gantry and put the ballscrew just behind. Plenty of DIY builders on here have gone this route and that also works just fine.

    In general section size is key for stiffness as it goes up with the power ^3 for bending and with the power ^2 for torsion. So a little bit deeper / wider goes along way.

    In all cases the material needs to all be furthest away from the neutral axis, the bit somewhere in the middle where not a lot is happening. So tubes are good, squares are better. Avoid open sections such as [ or I beams. They are only good in one direction.

    Once you have maxed out on the outer size, have a play with wall thickness as this can really boost the stiffness. The limit is then how much weight you are comfortable with.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

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