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  1. #31
    Quote Originally Posted by JonnyFive View Post
    The gantry will be the same 160x80 ali extrusion whichever way I go with the frame.

    Is the 50N loading a fair number
    The 50N loading in which direction.? However, to be honest, no matter the direction the correlation between simulation in SW and real-world when it comes to cutting and vibrations etc won't match.

    What I can tell you from experience of building god knows how many steel-framed machines is that the extra cost of using thicker steel pays big time regards vibrations. The loads going into the machine don't come close to stressing the steel so it's not something worth fretting over.

    Regards the Gantry then again most of my machines use HD ITEM Profile. If you want a much stiffer gantry then I'd suggest you change design slightly and go with L shape setup like I use. Over the years I've used both setups and the L shape is far far stiffer and two pieces of 120 x 80 arranged in an L shape gives much stiffer gantry with lower vibrations than single 160x80.

  2. #32
    Quote Originally Posted by JAZZCNC View Post
    The 50N loading in which direction.?
    I'm referring to the reaction force of the machine pushing the cutter through the material so could be in any direction depending on which way the router is moving. My only reference point is using a hand router and it doesn't fell like it needs 50N to push the router along. I'm talking about the cutting force as shown on this diagram:

    Click image for larger version. 

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    Quote Originally Posted by JAZZCNC View Post
    two pieces of 120 x 80 arranged in an L shape gives much stiffer gantry with lower vibrations than single 160x80
    Do you mean 120 x 60? I can't find 120 x 80. I have run the calcs on the L section and I'm amazed how much difference it makes - you've convinced me! Do you have any examples of the machines you've made, I'd love to see them.

    Cheers

  3. #33
    Quote Originally Posted by JonnyFive View Post
    Do you mean 120 x 60? I can't find 120 x 80
    Don't worry, I've found it - I was looking in the Profile 10/12 sections, it's in the Profile 8.

  4. #34
    Quote Originally Posted by JonnyFive View Post
    Don't worry, I've found it - I was looking in the Profile 10/12 sections, it's in the Profile 8.
    Yes, the 8mm slot is better because gives more area for profile rail to sit onto.

  5. #35
    I've found the 50 N force in X and Y directions to be a good guide but it is only the start. If under this simple load condition if you are <0.05 mm then I would call that a fair start point, and you would expect a fair amount of accuracy on the finished dimension.

    But wait, this 50 N is a simple static force, and does not account for the repeated impacts of the cutting action in the material which will cause a ripple on top of this average force, say from 45 N to 65 N (making the numbers up to show the point) with every rotation. As the cutter rotates very fast this will cause vibrations in the machine, not only at the main impact frequency but across a wide frequency range with amplifications where the machine frame resonances are and this may show up in the finished part.

    This is where the wall thickness will help you as Jazz is advising, and this is money well spent. It adds significant stiffness (more than might be suggested is required by the simple static load analysis), which will lower the vibration levels as higher frequencies decay quicker and as they are stiffer they also deflect less initially with each impact. It also adds mass which will also lower the vibration (acceleration) levels as shown by re-arrangement of the famous F=ma equation ; a = F/m). So it is a win-win up to the limit of what your steppers can accelerate around.
    Technically you would not have added much damping (energy absorption through friction, e.g. damping pads or sand) but you will still have reduced the vibration amplitude through the mass and stiffness which is all that matters.

    If it helps I went through the same thought process on my mk4 machine for the upper box section members on the bed. I went with 5 mm wall in the end, as every mm costs £, but I know it makes a difference.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

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  7. #36
    RouterCNC - thanks for taking the time to explain all that, I appreciate it. I've had a bit of a play with your stiffness calculator spreadsheet and I'm getting gantry deflection of <0.05mm as I would expect following Jazz's advice.

    I think I'm going to redo my frame design using 60x60x5 for the legs, 120x60x5 for the rail supports and maybe use 50x50x4 for the bed and triangulation / stiffeners.

  8. #37
    Quote Originally Posted by JonnyFive View Post
    I think I'm going to redo my frame design using 60x60x5 for the legs, 120x60x5 for the rail supports and maybe use 50x50x4 for the bed and triangulation / stiffeners.
    I wouldn't take that route as the difference in steel price won't be massive. It will make welding more difficult because of your mixing different thicknesses and tube sizes.
    At this width, the bed will be much less resonant with the thicker, wider tube size giving a better finish on parts. Esp when cutting harder materials that will resonate into the bed and frame.

    Keep in mind it's the foundation of the whole machine. It really isn't worth spoiling the broth for apeth of salt.

  9. #38
    Quote Originally Posted by JAZZCNC View Post
    It really isn't worth spoiling the broth for apeth of salt.
    Where's yer grammar lad? "It in't worth spoilin' t' broth for an ha'p'orth o' salt"
    .
    An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.

  10. #39
    Quote Originally Posted by JAZZCNC View Post
    Keep in mind it's the foundation of the whole machine. It really isn't worth spoiling the broth for apeth of salt.
    Understood, you're right, if it's worth doing it's worth doing right. I'll crack on with 60x60x5 & 60x120x5.

    Cheers!

  11. #40
    Quote Originally Posted by Kitwn View Post
    Where's yer grammar lad? "It in't worth spoilin' t' broth for an ha'p'orth o' salt"
    .
    Tha's reight old lad, cummin from't God's own country a shuda known bet'ta, but think tha'll fi'nd it's spelt ha'peth.!!. . . . cos we'll av non that posh "orth" rarynd here si'thi.

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