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half sheet heavy duty cnc

Discussion in 'CNC Mills/Routers' started by tparsons, Dec 2, 2016.

  1. tparsons

    tparsons New
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    Hi All

    I am planning to build a CNC machine. The cutting area will be around over 1300x1300, to cut half sheets of ply for a construction job I am doing- including a staircase multiple bedroom cupboards and a kitchen. In the future I intend to move it into a different location (I am currently limited by space) and to extend the Y axis rails to be longer than would be needed for a full sheet. This is the primary reason I am not sticking with the OX. I also have some concerns about it wearing out. It will be based to some extent on the american 'Joe's Evolution' ( ) and on the OX in other aspects. I am a paid up member of Joes but am reluctant to source parts from the states/ in imperial. and this is likely to result in a large amount of deviation from his design (it would be nothing at all like his hybrid design). In fact the way its going it looks like it will nearly be a whole redesign from scratch, near enough just be my own design.

    I intend to use entirely pieces from the UK and without too much modification. It will be rack and pinion for X and Y and I will use an off the shelf ballscrew Z axis. The X axis beam will be a 160x100 extrusion, and I am in the process of considering options for the Y axis. The intention is that it will cost less than 3 times more than an OX of the same size and be over three times more sturdy. It will all be mounted to a heavy timber base- largely because I am not planning to get into welding. Plates will be 15mm aluminium.

    I am currently trying to get my head around constructing a rack and pinion gearing system similar to this; PRO Rack and Pinion Drive, NEMA 23 | CNCRouterParts but for a metric rack to provide 3:1 gear reduction. Where the design is starting to differ from the 'joes' is to do with making this from off the shelf parts (excluding the plates). I am thinking of mounting the rack onto the bottom of the Y axis extrusion onto the inside of the angle iron that will also act as the rail for V shaped metal wheels. The X axis one will probably be mounted on to the top of the beam, spaced off the beam using the angle iron

    I have spent a long time reading forums here and elsewhere but if anyone has any advice or has attempted anything like this then please fire away.

    I am also interested in vacuum beds but haven't worked out how big the pump/ if this is achievable cost wise yet.

    x axis;
    [​IMG]
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    The only real "gotcha" that I can think of is that bolts are sized nominally and the claimed size is not the actual size. Where this becomes relevant is the pivoting drive assemblies. The undersized nature of bolts when fitted into bearings or sleeves may create too much slop in the system and this is something best worked through before you get to that point.

    As far as the remainder of the system, resist the temptation to jack the X-axis way up and reach back down with a long Z like the guy did in the video. This just introduces unnecessary leverage into the system which leads to inaccuracy in the cuts and unnecessary strain and wear on the bearings. Keep the system as low and lean as possible, building it for what you intend to use it for not what you "might someday" use it for as you'll get better long term results. If you do feel the need to build the might factor in, just leave the vertical parts of the assembly a bit longer and add a set of high and low stops to the side posts to where you can still easily raise the gantry should the need arise.

    While a wood base is a perfectly workable solution, remember wood changes dimensionally with variation In moisture content so account for it. The big issue is going to be the spoils board due to it's size so make sure any size changes don't affect machine alignment.

    Looks like a nice project. Please keep us informed on the progress.
     
    Anthony Bolgar likes this.
  3. tparsons

    tparsons New
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    Hi Rick, thanks for the encouragement.
    I'm aware of the movement of timber and that it is something to take into account. Will probably have square section steel across the timber bit-spaced ~ 200 apart for the bed that the rest of it will mount from. Hopefully that will ensure that the bed stays in alignment with the gantry, even if the table itself does move. An idea I also had was that these pieces of metal could form the void for the suction bed. Might leave that as a possible future addition.

    Yes I agree his is extravagantly high. I will want to cut thick polystyrene/high density foam for moulds and architectural terrain models after I am done with the ply furniture. So will probably leave allowance for that but anything larger isn't needed like you say, i can move up the gantry.

    His design with a vertical face is quite nice to be able to cut the ends of long pieces. I might overhang the Yaxis beams slightly, 50mm, at the 0 end to be able to do that. Could do some intricate carpentry joints on the furniture with that. Doubt that would affect performance really

    The bolts for the rack and pinion will be 'precision ground shoulder bolts' i think that is what they are called. Hopefully not nominal with that name? I'm still working that part through- it's the main bit I am uncertain about. At the moment I can't see a way of making it work without needing to do some machining on the lathe to get the pinion and pulley to interface properly. So changing the bore hole too won't be much extra work if I do need to. Of course I would rather not. Would be good to make it more accessible for others to copy too. Hmm...
     
  4. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    If It weren't for the size of the system, I'd recommend a two plate system with the pulley sandwiched between two plates rotating on a simple dowel and bearing axis but I'm not sure set screws tightened into ground flats on dowels will have sufficient strength on a larger system. (If you didn't quite follow that here is a similar example, one that I have actually morphed to use on a R&P system in another drawing but I'm a hundred miles from the computer that the drawing is on at the moment so I can't offer it up.) If you do think you can get set screws to hold, it provides a much simpler solution that the average person can copy without the need for a lathe.
     

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