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  1. #41
    I did write a reply but I think the mod's must have it ??
    James

  2. #42
    Quote Originally Posted by luke11cnc View Post
    I did write a reply but I think the mod's must have it ??
    James
    So what did it say.?????

  3. #43
    Ok, still struggling a little with sketchup. This is my latest attempt and i have put the ballscrew on top of the i-beam. Would this work, i'm struggling with ideas on this.




    Ignore this
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by deannos; 01-08-2012 at 03:45 PM. Reason: ,no good

  4. #44
    Ok, i think i'm finally getting there. All plate is 20mm thick ali and also using I-beams. I'm not sure if the screw mounts are ok doing it this way. Would the bearing blocks need spreading out more. All the rails are 20mm with 1610 screws. Some pictures to show my progress and please tell me where i have gone wrong and where improvement can be madeClick image for larger version. 

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    I will have the ballscrew end machining increased from 15 to 30mm as i want to use belts and pulleys.
    The RSJ's are 178x102x19mm, would it be better to use a smaller size on the gantry, 127x76x13mm.
    Also just realised i haven't taken into account the thickness of the beams
    Last edited by deannos; 01-08-2012 at 04:12 PM.

  5. #45
    Ok have redone the RSJ's and added steppers.Click image for larger version. 

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  6. #46
    Quote Originally Posted by deannos View Post
    The RSJ's are 178x102x19mm, would it be better to use a smaller size on the gantry, 127x76x13mm.
    \

    Although it's convenient to use RSJ on X and Y, you'd be better off using a different cross section. The torsional stiffness of RSJ is pathetic, hardly much better than a sheet, so especially on the gantry where the cutting forces will tend to twist the beam you would get a lot of deflection. Similarly RSJ is poor under shear stress, which is the case on the X-axis. To an extent you have compensated for that by using a large thickness, but with less material you could use box section and get much greater stiffness.
    One reason not to use smaller RSJ is it reduces the vertical spacing of the Y-axis bearings, which is something you want to maximise.

    What you've drawn is still fairly strong, so it would work - but that is purely because of the large thickness.

    Try folding an RSJ cross section and a box section out of card then try and twist them and you will see how big the difference is. This spreadsheet lets you compare them.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  7. #47
    Quote Originally Posted by Jonathan View Post
    Although it's convenient to use RSJ on X and Y, you'd be better off using a different cross section. The torsional stiffness of RSJ is pathetic, hardly much better than a sheet, so especially on the gantry where the cutting forces will tend to twist the beam you would get a lot of deflection. Similarly RSJ is poor under shear stress, which is the case on the X-axis. To an extent you have compensated for that by using a large thickness, but with less material you could use box section and get much greater stiffness.
    Ok, would i be better using Ali extrusion, or are you talking about steel box section. What size would you suggest

    Quote Originally Posted by Jonathan View Post
    One reason not to use smaller RSJ is it reduces the vertical spacing of the Y-axis bearings, which is something you want to maximise.
    I'm still confused on the axis direction, by "Y" are you referring to the bearings the longer gantry is riding on

    I want to get it right the first time, or as best as can be


    How do you use the multi quote option, i clicked it and nothing happens

  8. #48
    Quote Originally Posted by deannos View Post
    Ok, would i be better using Ali extrusion, or are you talking about steel box section. What size would you suggest


    I was thinking steel box section, but aluminium extrusion is pretty good too. If you're happy to do some welding then steel box section is the cheapest and strongest option for making a frame. The standard length is 7.5m, so it would be prudent to design the frame to use an integer multiple of 7.5m! I used 60x60mm, 3mm wall thickness box section for most of the frame and 100x60mm, 3.6mm wall thickness for the two lengths the X-axis rails mount on.

    How long is the piece of RSJ you've drawn for the gantry?

    Quote Originally Posted by deannos View Post
    I'm still confused on the axis direction, by "Y" are you referring to the bearings the longer gantry is riding on


    I always call the axes on the gantry Y and Z and the other axis X. So by that definition in your case X is shorter than Y.


    Quote Originally Posted by deannos View Post
    How do you use the multi quote option, i clicked it and nothing happens
    The what.. never seen that! I just copy and paste the [QUOTE] tags in the relevant locations. I've put them in on your post for you, so if you click 'Edit Post' you can see how...
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  9. #49
    Quote Originally Posted by Jonathan View Post
    [/COLOR]

    I was thinking steel box section, but aluminium extrusion is pretty good too. If you're happy to do some welding then steel box section is the cheapest and strongest option for making a frame. The standard length is 7.5m, so it would be prudent to design the frame to use an integer multiple of 7.5m! I used 60x60mm, 3mm wall thickness box section for most of the frame and 100x60mm, 3.6mm wall thickness for the two lengths the X-axis rails mount on.
    I can weld , it wouldn't win any contests though, i would like to keep the RSJ for the X axis

    How long is the piece of RSJ you've drawn for the gantry?
    It measures out at 1715mm, but could be slightly longer.

  10. #50
    Instead of using a rsj for the gantry, could i use 100mm x 60mm x 3mm steel box section. I can get my hands on a piece long enough for a tenner, and its very local.

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