1. #1
    Hi all, i would like to hear your opinions for my gantry sides. Do you think is better to use two plates from 5mm like on the picture or its better to use only one from 10mm?!
    The another question is: Is it better to use welded frame from steel profiles 40x20x3mm and bolt down the supported rails on it or use a plate 5mm thick like on this picture? Looking forward for your answer.
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  2. #2
    Gantry sides:
    2 spaced apart 5mm plates will be stiffer than a single 10mm plate, but they need to be well connected to each other to make it work, otherwise you are wasting all the effort in creating it. In a simple RHS section they are well connected to each other by the other 2 sides so they can transfer the load from one side to the other and give the stiffness benefit.
    In your design they are only connected periodically with spacer posts so it will not be a stiff as a RHS section the same overall outer dimensions as your spaced out design, although it will be better than a single 10mm plate.
    Are these steel plates? If aluminium I think they would not be stiff enough. You could add a layer of plywood bonded in between (as well as the spacers) to try and transfer the load better, plus maybe add a bit of damping. Better still would be just use 20mm aluminium plates as this is simple and is proven to give reasonable results on a raised gantry router.
    Gantry beam:
    Do not use 5mm flat plate for the beam - it will be very poor for stiffness. You might be thinking that the rails would add a bit of stiffness, and they will do a bit of that but the overall performance will be poor as they can twist easily about their own longest axis.
    The 40x20x3 steel profiles are much better than plate, although depending on what you want to cut that is still on the small side. Much better and suitable for cutting more materials would be 50x50 as a minimum.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  3. #3
    So what should i use as selection for gantry top and gantry side in your calculator if i use 2 spaced 5mm plates and i use the welded frame(650mm lenght,160mm height) from steel profiles 40x20x3mm?

  4. #4
    The stiffness spreadsheet calculator is only for simple shapes. It will not calculate 2 spaced plates or a complex gantry shape. If you really want to do that then you will need to purchase some finite element analysis (FEA) software. In general this is not cheap.

    In the spreadsheet all you can do is make some approximations:
    Gantry sides - model an RHS section with 5mm wall thickness and the same overall outside dimensions as your gantry sides. Your design will be a little bit less stiff than this analysis because you have posts and gaps instead of continuous walls on the front and rear faces.

    Gantry top - Run one analysis on a single RHS profile of 40x20x3, write down the numbers. Run another analysis on a single RHS profile of 160x20x3, write down the numbers. Your design will be somewhere between the 2 analysis results.

    Do you actually want to explore theoretical designs, or do you just want guidance on proven designs? If you want to just get on with something which works then post up what you want to cut and you will get feedback and design improvements from the forum members.

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

  5. #5
    For what cutting speed are the results i get with your calculator?

  6. #6
    The deflections are only based on generic cutting forces for each material, not any particular feed rate. Doing it this way allows you to make simple comparisons and see relative performance from different designs, and that is all the spreadsheet was design to do.

    There is lots of information on the internet relating to feedrate, number of flutes, depth of cut and chip load but this was not the objective of the spreadsheet as chip load is a topic in itself.
    Last edited by routercnc; 21-06-2015 at 05:49 PM.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

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