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  1. #21
    Hi Monk,

    Firstly a quick check on Irving's first calc from the calalogue giving 0.00084mm deflection per Newton. I think this value is OK, as I'll show below.

    1. For simplicity work out the equivalent RHS:
    size Ixx Iyy
    Profile: 40x80 614000 170000
    RHS: 40x80x3 558332 208049
    RHS: 40x80x3.5 636869 184292

    Choose 40x80x3 to be on the safe side

    2. Vertical bending, and deflection due to twisting:

    20kg z axis (router, motor, metal brkts etc.)
    length 1200mm
    150mm to tool
    40x80x3 alum RHS

    Z bending deflection is 0.183mm (Irving got 0.164mm for 20kg using catalogue profile data, so think Irving's calc is OK)
    X bending deflection is 0.028mm
    twisting deflection is 0.003mm

    3. You have an 'L' which is part way between the following results:
    40x120x3 Z bending is 0.06mm, X bending is 0.020mm, twisting is 0.0019mm
    80x120x3 Z bending is 0.043mm, X bending is 0.004mm, twisting is 0.0005mm

    I need to create a spreadsheet to work out the true value, but it is between these. For a wood router it's looking OK to me.

    If your z axis is less then 20kg, then you can ratio the Z bending deflection down accordingly (but not the twisting, or X bending).

    If you go for just 80x80 profile:
    Z bending 0.112mm
    X bending 0.006mm
    twisting 0.001mm

    So you are better with what you have. Make sure they are well joined to each other, otherwise you will get back much less than this as they will work on their own.

    4. Final note:
    If you stack the profiles to get 40x160:
    Z bending 0.031mm
    X bending 0.015mm
    twisting 0.001mm
    which is the best of the lot (in bringing down the highest value) for no weight gain

    You also asked about ballscrew location etc. I'll get back to you . . .

    EDIT: added X bending results for completeness
    Last edited by routercnc; 12-08-2010 at 09:06 AM. Reason: added X bending data for completeness
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  2. #22
    Monk's Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 12-04-2015 Has been a member for 6-7 years. Has a total post count of 6.
    Quote Originally Posted by routercnc View Post
    Hi Monk,

    Firstly a quick check on Irving's first calc from the calalogue giving 0.00084mm deflection per Newton. I think this value is OK, as I'll show below.

    1. For simplicity work out the equivalent RHS:
    size Ixx Iyy
    Profile: 40x80 614000 170000
    RHS: 40x80x3 558332 208049
    RHS: 40x80x3.5 636869 184292

    Choose 40x80x3 to be on the safe side

    2. Vertical bending, and deflection due to twisting:

    20kg z axis (router, motor, metal brkts etc.)
    length 1200mm
    150mm to tool
    40x80x3 alum RHS

    bending deflection is 0.183mm (Irving got 0.164mm for 20kg using catalogue profile data)
    twisting deflection is 0.003mm

    3. You have an 'L' which is part way between the following results:
    40x120x3 bending is 0.06mm, twisting is 0.0019mm
    80x120x3 bending is 0.043mm, twisting is 0.0005mm

    I need to create a spreadsheet to work out the true value, but it is between these. For a wood router it's looking OK to me.

    If your z axis is less then 20kg, then you can ratio the deflection down accordingly (but not the twisting).

    If you go for just 80x80 profile:
    bending 0.112mm
    twisting 0.001mm

    So you are better with what you have. Make sure they are well joined to each other, otherwise you will get back much less than this as they work on their own.

    4. Final note:
    If you stack the profiles to get 40x160:
    bending 0.031mm
    twisting 0.001mm
    which is the best of the lot for no weight gain

    You asked about ballscrew location etc. I'll get back to you . . .
    That's great thanks, I've just been sat looking at the graph on the Hepco Catalogue doing a few sums. With either option (although the stacked 40x160 looks my best option) I would assume with the Hiwin Rails attached it becomes even stronger?

    I think I have worked the ballscrew mech out. Just working out how to mount the servo/stepper motor and connect them to the ballscrew.

  3. #23
    Hi Monk,

    I've created a spreadsheet to work out simple arbitrary shapes I values - will post it sometime.

    Anyway, to recap, your L shape was somewhere between:
    40x120x3 Z bending is 0.066mm, X bending is 0.020mm, twisting is 0.0019mm
    80x120x3 Z bending is 0.043mm, X bending is 0.004mm, twisting is 0.0005mm


    Actual value for L shape is:
    40/80x120x3 Z bending is 0.052mm
    which is closer to the 80x120 than the 40x120

    But oddly this is the same deflection as:
    60x120x3 (0.052mm) which is half way between width 40 and 80. I wasn't expecting that, and neither was Irving with his guess of 55. Double checked it. Can't find anything wrong.

    Enough maths, looking forward to seeing the ballscrew options. . . .

    Ah, yes, adding rails will increase stiffness but don't bother working it out, get the structure right underneath and you're away.

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

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