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  1. #11
    I'm considering 90 x 150mm RSJ over 120 x 80mm 4mm Box; partly as I have 4 meters of RSJ and no box but mainly as my brain tells me 90x150 RSJ wouldn't flex like Box Section would ... but the spreadsheet says otherwise
    Me to, I didn't see a RSJ option in the sheet, did you use the I beam one? :whistling:

    RSJ's have deeper flanges :naughty: than the webbs and therefore the majority of the mass is at the extremities of the section. Changing the values will change the Ixx value and therefore the deflection. If you stand back and think about it the rsj is slightly deeper and nearly a third (box 10kg, rsj 16kg) heavier in weight so it will deflect less. But the weight is more likley to be a problem for CNC

    For a purely vertical load then an RSJ is the most efficient beam type there is, It just for a gantry most of the load is on the side and its poor torsion qualities let it down.

    Without knowing the size or intended use of your machine it is pure speculation as the best options and problems you face.:whistling:

    Best start that thred and let us know........

  2. #12
    Well put Ross. I knew someone would want to have different flange and web thicknesses ! , but when putting the spreadsheet together I couldn't be bothered with the extra calcs. Maybe I'll add it.

    Anyway, since most of the work in bending and torsion is done on the outer surfaces, then you could cheat and enter the flange thickness value as the thickness everywhere. This would probably give close to the correct answer, with a slight overestimation.

    Since you have so much RSJ lying around and for economic reasons want to use it (fair enough !), assuming a conventional moving gantry router, RSJ could be made to work on:
    1. Gantry sides: if one flange is mounted to the main beam and the other faces directly away from it
    2. Baseframe long sides carrying the bearings: if used with the flanges facing up and down, with plenty of braces across to the RSJ on the other side to resist the torsion (you mentioned using profile rails and carraige which will resist a moment if the RSJ can be restrained)

    Don't use it on the the gantry beam, or if you do consider plating over one open section to create a closed section on one face. Having said that if your gantry is short, or your requirements are modest, the deflection due to twisting may still be acceptable. Depends on your intended application.

    Perhaps Ross knows more, but how true and straight are RSJs? Especially if it was intended for buildings. Aluminium profile is extruded through a die, and is pretty accurate. Ideal for a CNC machine. Maybe another option would be to sell the RSJs and put the money towards some Ali . . . ?

    Bring on that new post so others can get back to the videos . . .
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  3. #13
    You guys are great!

    Thank you so much for the feedback.Will get the designs up asap for feedback.


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