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Custom CNC build ........... how to workout work area

Discussion in 'CNC Mills/Routers' started by T4Concepts, Dec 11, 2017.

  1. T4Concepts

    T4Concepts Veteran
    Builder

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    Hi all,

    It seems I was trying to run before I could walk !

    I was placing an order for my parts earlier today, when I got to the second item on my list, the linear actuators, it dawned on me that I didn't actually know the resulting work area I'll have if I ordered the standard 500mm linear actuator kits. One thing I do know however ............. 1000mm is way too long.

    So I spent the remainder of the day researching the concepts of CNC design.

    One word that Rick 2.0 mentioned in another post that I didn't pay particular attention to was 'Envelope' ! My mistake, I didn't know what it was so just breezed passed it. Am I to understand that that really is what a cnc is capable of. the amount of work area available ?

    I've searched everywhere and still can't find the answer, is there some sort of formula for working out the 'envelope' for any given linear actuators you intend on buying ? So, I almost bought x3 500mm linear actuators ( they were already in my basket ! ), then I realised maybe there wasn't enough travel on my Y axis. Would I be right in saying that combination would give me an 'envelope' of 330mm x 330mm ?

    Below is a very crude diagram of what I need to do. So if I increase the length of my Y axis actuators to 600mm would that automatically increase my Y axis on my work area ? or is there something else to consider ? >>

    Envelope.png

    The blue section is the part I need to machine .................. as you can see it's quite high, 180mm to be exact, so I'll also raise the height of the gantry. And Rick 2.0 also mentioned having a cut-out in my work bed to lower my part which is a great idea, which is the reason why I've had to move away from my idea of the minimill construction as that has a single central linear actuator, so a cut-out in the work bed wouldn't be possible.

    I'm thinking in terms of what I need to do to complete my headlights parts, my original questions were related to finishing off the parts, drilling holes in the tops of them for instance. Now I'm also thinking of what else I can do with a cnc machine.

    As I make moulds ( custom headlight assembly parts ), using Polyurethane foam blocks that have a density of just 96kg/m┬│ there are no great stresses involved. The size and dimensions of the PU foam blocks are an issue though, as can be seen from the image below, here's a set I'm currently forming by hand >>

    2031_IMG_3873.jpg


    So, am I going down the right track ? with the wealth of knowledge on this forum, would you say that x2 Y axis linear actuators of 600mm and a 500mm gantry would give me sufficient travel to cover my work area ?



    TURK
     
    #1 T4Concepts, Dec 11, 2017
    Last edited: Dec 11, 2017
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
    Staff Member Moderator Resident Builder Builder

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    Usable Envelope

    For CNC Routers:

    X axis envelope equals lesser of:

    Usable beam width - carriage width
    Table width - router width

    Router X Envelope.jpg

    Y axis envelope equals usable beam width - carriage plate width

    Router Y Envelope.jpg

    For CNC Mills:

    X axis envelope equals usable beam width - plate width

    Mill X Envelope.jpg

    Note range of motion can be increased slightly by raising the spoil board 3mm so it clears the C-beam end mounts. Range of motion is then defined by the width of the metal carriage plates or the spoil board hitting the motor whichever happens first.

    Y axis envelope equals throat depth

    Mill Y Envelope.jpg

    For both CNC Routers and CNC Mills:

    Z axis for systems with routers fixed to moving axis beams:

    Effective envelope equals lesser of:

    Range of motion
    Effective throat
    Bit extension

    Mill Z Moving.jpg

    Z axis for systems with moving routers on fixed axis beams:

    Effective envelope equals lesser of:

    Range of motion
    Effective throat
    Bit extension

    Mill Z Fixed.jpg

    Note bit extension in this case equals the amount of the bit length that can extend into the effective throat. Effective throat is the either the gap below the bit or the gap below the Z-axis bottom plate whichever is less.

    There are a couple of caveats to all this though. First off you never intentionally run the gantry to where it smacks either end so you'll inherently lose 5mm or so on both ends. Also on the C-Beam gantry plates with the inset mini-V wheels, you'll need to stop motion before the wheels hit the lock collars on the screw to prevent damaging the wheels so you'll lose about 10mm each end.

    Additionally, based on the nature of what you are cutting, may not need to extend the cutting envelope all the way down to the board and this may open up some possibilities in the design.
     
    GrayUK and T4Concepts like this.
  3. T4Concepts

    T4Concepts Veteran
    Builder

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    That's absolutely brilliant Rick 2.0 and many many thanks for taking the time to explain things that even us 'Newbies' can understand ! :thumbsup: :thumbsup:

    I've already read through that a few times, now I'm going to save it and print it :D


    I'm pretty sure that that post will now become the go to place for newbies wanting to design and assemble their own cnc machines. And I received a quote from Ryan @ 'Ooznest.co.uk' for some 600mm linear actuators with Nema23 stepper motors ................ I'm already halfway through my budget and haven't ordered everything yet ! :ROFL: All in all, today turned out to be a very productive day, after a lot of head scratching at first.

    ............................ off to my workshop to build a mock-up ;)





    TURK
     
    GrayUK likes this.

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