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Thread: A3 Router Build

  1. #1
    Hello, I am hoping that with the input of the members of this forum that I will be able to develop a design of a smallish router that is functional and reasonably simple to construct.My original goal was for a working area of 1300mmx1300mm, but for this machine it has been scaled down to a working area slightly larger than A3.This will be a fixed gantry 3 axis cnc, made mainly of steel with the exceptions of the Y axis extrusion and table.I hope that we can not only use this thread to discuss the design but also the methods of construction as I wish to learn how you would approach some of the problems I stumble across.I want this to be able to machine aluminium and wood so it needs to be rigid. I have already made a start on the side plates which are 12mm steel and very heavy (maybe a bit over the top).Here are a couple of sketches to start with.
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    Last edited by GTJim; 09-02-2012 at 01:53 PM.

  2. #2
    Jim the main thing I see with this design I dont like is the position of the Y Axis ballscrew. Because of the profile (looks like 90mm) It's too far back.
    I would think about using either two pieces 90x45 (Or cheaper still 80x40 box) laid flat one above other with a gap between so the screw can go between and fasten directly onto Z axis backplate.

    Or if like me you prefer the Ballscrew out the way round the back then use 90x45 (Or 80x40 box) arranged in an L shape and bolted together.
    The slight difference of 45(40)mm doesn't have a big effect and I've built a few machines using this approach and it works ok with no visual difference in performance to when connected directly to back plate.
    Doing it this way means you eliminate the back plate on Y axis and if you use 90x45 the Bk/BF12 blocks fit the slots and can be bolted to extrusion then the motor can bolt directly onto the gantry side or be inverted round the back if you want to belt drive.
    This way use's a small drop plate to connect to Y nut but because like in your case your using the FK blocks then you could mount them higher and mount directly to the under side of the top bearing plate.

    See Cad I did recently did to show some one and also pic of an actualy small machine done the same way, thou it doesn't show the drop down bracket.
    Last edited by JAZZCNC; 09-02-2012 at 06:49 PM.

  3. #3
    Hi Jazz, I completely understand the issue with the position of the Y axis ballscrew, but these side plates have been cut. It was done in haste as a friend was leaving his job and offered, if I could get a sketch to him quickly.
    Initially these side plates would have been 20mm aluminium but 12mm steel was available so the design is evolving already. So I think I’m stuck with this configuration until its made, and hopefully then make a better Y axis.
    I am hoping to use 80x80 aluminium extrusion with 20mm rails.

    My first question is regarding mounting of the extrusion to the side plates. Currently the design relies on 4off M16 cap head screws with 4 location pins/studs on each side, will this work or are there better ways, such as threaded rod going through the extrusion. See Sketch.
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    Last edited by GTJim; 10-02-2012 at 02:10 PM.

  4. #4
    Ah I see Ok Jim, The bolts/pin setup will be plenty good enough.

  5. #5
    Another question. What methods do you use when mounting your supported rails to RHS to ensure they are straight, level and parallel to each other and the ballscrew.
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  6. #6
    Quote Originally Posted by GTJim View Post
    Another question. What methods do you use when mounting your supported rails to RHS to ensure they are straight, level and parallel to each other and the ballscrew.
    Since we can't machine the mounts for rails/ballscrews etc accurately enough for them to be fixed you need to build in adjustment. For instance to get the rails parallel you can start by fixing one down parallel to the frame, then attach the linear bearings/gantry to it and move the gantry back and fourth such that the other rail self-aligns to the first rail. Then put one bolt in it, move the gantry and check it's still smooth .. keep adding more bolts checking each time that there's no binding. That will get the distance between the rails constant, but it wont correct for any bend/bow in the rails. To do that you need a precision straight edge, or tensioned wire to act as an accurate reference from which you can measure to set the first rail. For the majority of DIY machines that's taking it too far.

    You set the ballscrew in a similar way. First fix the motor end and ballnut, then run the ballnut to the other and fix the end bearing so that the ballnut aligns. Check it's smooth by spinning the screw by hand and if not adjust. If it's tighter towards the ends of travel that implies the centre height of the ballnut is slightly off, so adjust it...

    You can of course do things in a slightly different order so long as there is sufficient clearance or adjustment in the mounts to obtain perfect alignment.

    Also have you considered increasing the spacing of the X-rails? That should reduce deflection parallel to X when the tool is near each end of the gantry.

  7. #7
    Thanks for the reply Jonathan, and I will space the X axis rails further apart.
    So if I’ve got this right, this process should work.
    1. Position one of the X axis rails to the frame as straight and parallel to the frame as possible, using a taught wire or other method.
    2. Using a transfer punch mark the fixing positions of the rail support to frame.
    3. Drill, tap and mount rail, and recheck straightness.
    4. Roughly position second rail and mount a jig/fixture to the bearing mounting blocks on both rails, and slide back and forth adjusting the second rail until motion is smooth.
    5. Mark positions of fixing positions onto frame and drill, tap and mount second rail.
    Up to this point I think I’m Ok. The problem I see is due to the FK and FF supports for the ballscrew. These don’t seem to allow for any adjustment as the fixing positions would need to be made in the frame prior to fitting?
    Maybe I could fix the FK support directly to the frame and amend the design to allow for an adjustable plate at the FF end.
    Would I need to have the table made and fitted at this point to attach everything to it?
    How would you ensure the rails were level vertically in the Z axis as this could result in the rails being like a wave. Should you shims or use some sort of leveling compound and how would you measure it?


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