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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > Newb with First Machine Design Build
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  1. #1
    Join Date
    Jun 2013
    Posts
    3

    Newb with First Machine Design Build

    Hi everyone, this is my first machine design as well as my first post and I really need some input my design process. After months of lurking and reading through sufficient build logs to familiarize myself with the general concepts of what it takes to put together a machine, I have decided finally to give it a go. I have already purchased ballscrews and linear slides in order to fully commit myself to the build, so I am set in my overall machine sizing (About 1' X, 2' Y, 4" Z travel).

    I would like the machine to be as accurate as possible over this travel (within .001-.002" is possible). I would like to be able to cut wood, butterboard and aluminum. I would like to have the accuracy and finish quality to make silicone mold masters for robot components (small gear assemblies and assemblies and such), as well as general carving and machining stuff.

    My main question at this point is about the main structural material for the frame. I am leaning toward an overhead gantry type design, so I can stiffen my axis more than would seem possible in a gantry that runs under the cutting surface. I plan on driving dual ballscews with 320oz-in motors. I have currently drawn up my concept with 2" steel tubing with .25" walls. I feel like this will be really stiff, but machining the tubing and joints will be a real pain and will greatly extend the building process. I am considering switching the design to 8020 tubing, or a bolt together steel frame instead of corner fittings.
    Both designs have negligible tip deflection, but I am unsure what type of cutting forces I should be considering when thinking about material to use.

    I have access to a mill, drill press and lathe at work and plan on doing most of the work there, and assembling at home.

    Any help would be greatly appreciated! (below are design picture and components so far)

    Also is it a crazy idea to buy a used surface place and bolt the whole thing down to that?

    Attachment 207998

    Attachment 208000

  2. #2
    Join Date
    Aug 2011
    Posts
    388
    Welcome. At first glance, it often seems like those long-axis linear rails should be at the gantry-tube height. That is a good height if you want to control high accel and decel of the gantry. But now think of what happens with cutting forces: those cause the gantry to tip. The best rail height for high cutting stiffness is at the cutter height (no tipping). So a typical good compromise is about halfway in between each, or about 4" off the table. Same goes for the screw heights. Also, think about the entire load path from cutter to table to frame to gantry to cars and back to cutter: for stiffness, that 'loop' needs to be as short and direct as possible. So those 2 long red tubes on each side are better off being 1 larger tube.

    All that said, a moving-table design is inherently more rigid and accurate than a moving gantry, if you have the floor space. For a 1'x2' machine, making the table move along the 1' axis makes for a relatively compact footprint of about 2'x2'.

    For the gantry cross-members, a single large tube will be much stiffer than 4 small tubes, and simpler. 4 tubes are 4 times stiffer than 1 tube (addition). A single 2x larger tube is 16x stiffer (4th power).
    David Malicky

  3. #3
    Join Date
    May 2005
    Posts
    3920
    Quote Originally Posted by jrnesmith View Post
    Hi everyone, this is my first machine design as well as my first post and I really need some input my design process. After months of lurking and reading through sufficient build logs to familiarize myself with the general concepts of what it takes to put together a machine, I have decided finally to give it a go. I have already purchased ballscrews and linear slides in order to fully commit myself to the build, so I am set in my overall machine sizing (About 1' X, 2' Y, 4" Z travel).
    It isn't usually a good idea to purchase pars before you have a design nailed down. That begin said if this is your first design I'd have to say it is interesting. We will have to keep in mind that this is a very small machine when discussing it below.
    I would like the machine to be as accurate as possible over this travel (within .001-.002" is possible).
    That isn't impossible for a machine this size but it will take some work. Further it will be expensive to achieve because you best bet is likely a welded and stress relieved frame.
    I would like to be able to cut wood, butterboard and aluminum.
    Butterboard?
    I would like to have the accuracy and finish quality to make silicone mold masters for robot components (small gear assemblies and assemblies and such), as well as general carving and machining stuff.
    Depending on you size goals an alternative approach would be a Mill conversion. It might actually be a cheaper solution if you can live with shorter travels like might be seen on a G0704.

    My main question at this point is about the main structural material for the frame. I am leaning toward an overhead gantry type design, so I can stiffen my axis more than would seem possible in a gantry that runs under the cutting surface. I plan on driving dual ballscews with 320oz-in motors. I have currently drawn up my concept with 2" steel tubing with .25" walls. I feel like this will be really stiff, but machining the tubing and joints will be a real pain and will greatly extend the building process.
    2" Square tubing over that distance would be pretty stiff. As far as stiffness goes though you really want a fixed gantry moving table design. Such a design allows for far more stiffness with a minimal of structural design. On a small machine it is very doable and frankly easier to realize, especially in the first crack at a machine build.

    Honestly though I see one big issue with your design, way to many tubes. Realize that you would need to get all those welded up fairly parallel and then final machine any mounting surfaces. Now if you go welded steel you will have to machine the parts anyways but your design looks like a nightmare to machine.
    I am considering switching the design to 8020 tubing, or a bolt together steel frame instead of corner fittings.
    8020 is nice stuff but getting the precision you are asking for would be more work in my estimation. That and I've never really like bolt together primary machine structures.
    Both designs have negligible tip deflection, but I am unsure what type of cutting forces I should be considering when thinking about material to use.
    I had to laugh here because of that butter board mentioned above.
    I have access to a mill, drill press and lathe at work and plan on doing most of the work there, and assembling at home.
    Having access to machine tools puts you well ahead of the rest of the crowd around here. From my perspective that should enable you to do a welded frame without having to compromise. This assuming of course that the mill is big enough to handle your frame.
    Any help would be greatly appreciated! (below are design picture and components so far)
    Well it is interesting! I'm not sure I'd bother with that complexity for a machine that only has one foot of travel.
    Also is it a crazy idea to buy a used surface place and bolt the whole thing down to that?
    Hell for what they charge for imported surface plates you can buy a new one. The idea of building a machine on top of one is not bad at all but I would not use the design you have alluded to. There is the reality of how you go about mounting everything on that plate. Here is where a moving table design ought to be easy (easier?). I seem to remember a thread or two related to this approach, so you might want to search the net a bit. Do realize though that surface plates can be broken, you will have to pay careful attention to just how the parts are fastened to the plate.


    So in a nut shell I think you need to reconsider your design for such a small machine. It is interesting to say the least but just strikes me as excessively complex for a machine with what amounts to very limited travels.

  4. #4
    Join Date
    Jun 2013
    Posts
    3
    Thanks guys for the replies, I will try to digest that feedback and come back with a redesign. I am considering switching to a fixed gantry moving table design, perhaps with 2'x2' travel. The size is mainly due to space considerations, as I have not been blessed with a home workshop at this point in my life.

    I'm going to reduce the complexity and up the stiffness in this next iteration, and focus a little bit on ease of construction. I don't think welding and stress relieving are in the cards, as I lack both the facilities and the skills to make it worthwhile.

    Also "butterboard" is a name for dense urethane foam. It machines fairly well from my limited experience. It has been great for thermoforming, molding silicone, and other prototyping applications. Similar material to Renshape, which is another prototyping board that machines well and I'm hoping to be able to cut with this router.

  5. #5
    Join Date
    Jun 2013
    Posts
    3
    I've got an updated design. It is completely different from the ground up. It is mostly steel and would have a cutting area of about 18"x 22". Does this seem like a good direction for a machine? I am currently most concerned with the attachment point of the rails to the x-axis (the one that moves the spindle carriage). I think this machine should be stiff enough to hold pretty tight tolerances. Let me know if you guys can spot any huge hangups with this situation!

    Attachment 210348
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  6. #6
    Join Date
    Aug 2012
    Posts
    621
    Mounting the rails to the underside of the table, and making the bearing trucks the fixed part will help keep them from getting swarf, chips, etc in them. It usually means making the table a tad oversized, to allow for the travel, but it seems worth it to me. Looks pretty solid!

    Luke
    "All I'm trying to find out is the fellow's name on first base" -- Lou Costello

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