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  1. #1
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    Fixed Gantry CNC Mill for cutting Aluminium and softer materials

    Hi everybody,

    So, this is literally my first post on this wonderful forum, so please be nice to me (English is also not my first language)

    This post is intended as a build log (even if it's going to take a year to complete) and to gather experience from you, adjusting my design based on your input.

    I am planning on building a small fixed gantry/moving table Mill for milling aluminium with reasonable precision, between 0.01 and 0.005 mm would be ideal if I manage with my low budget.
    Work area will be 500x500x150mm, made with aluminium T-slot extrusions (I plan on filling them with sand or epoxy granite later to improve vibration damping).
    I have already purchased a complete set of 20 mm square linear rails and ball screws (the chinese set, 1000, 700, 400mm , just like this one). No other purchases were made.
    I am planning on using plain nema 23 motors 1,8Nm/254 Oz-in with TB6560 driver.

    Gantry is 700mm wide, I plan on using a 80x160mm extrusion. Base is will be 1000mm long, made from 80x80 and 40x160mm extrusions, and 40x80mm traverses.
    Z Axis design is still pending. Please let me know what you think.

    Thanks in advance!

  2. #2
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    Re: Fixed Gantry CNC Mill for cutting Aluminium and softer materials

    If you haven't bought that TB6560 driver board, don't. If you have, throw it out and start looking for something better. Look up the inductance on those motors - less is better. Too much, and you'll need to spend more money on drivers that can handle enough voltage to run them at top speed.

    Your design is pretty basic, but I think it will be easier to align if all the Y-axis rails were in the same plane, instead of one on top and the other on the front.
    Andrew Werby
    Website

  3. #3
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    Re: Fixed Gantry CNC Mill for cutting Aluminium and softer materials

    You will not reach your desired tolerance (0.01mm) with your design and intended hardware.

    That level of tolerance requires a lot to achieve. Linear scales, servos with high encoder counts, ground screws, very rigid frame, temperature controlled environment.
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

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    Re: Fixed Gantry CNC Mill for cutting Aluminium and softer materials

    Quote Originally Posted by pippin88 View Post
    You will not reach your desired tolerance (0.01mm) with your design and intended hardware.

    That level of tolerance requires a lot to achieve. Linear scales, servos with high encoder counts, ground screws, very rigid frame, temperature controlled environment.
    Yes, you are absolutely right, I don't know what I was thinking. 5-10 Microns accuracy .. I wanted to rival a 300k machine
    Back with our feet on the ground, If I will be able to get 0.05-0.08 accuracy for a hobby machine, it will still satisfy my needs. Heck, I am able to achieve 0.1 mm accuracy with my home made FDM 3d printer, why shouldn't be possible with a home made mill?

    I was first thinking about building the base out of UHPC but the thought that I could never be able to move that thing from place to place and out of the basement without help or specialized tools kind of scares me. Plus, building it from heavy duty aluminium extrusions and then filling them for better dampening, will give me more flexibility in adjustments and later on for transport if needed. I hope i will not regret this decision.

    Now, I have another question for those that have already built such machines using aluminium t-slot. I want to bolt them together directly using rounded head hex screws like in the picture attached. Basically just drill a hole through the profile just to reach the screw with a allen wrench and bolt them together every 10 cm or so. Would this cause any issues ? I don't see how else could I bolt those 1m long extrusion together lengthwise.



    awerby: no, I haven't bought anything else except the linear rails and the ballscrews. I still have buy and build the aluminium structure until I will purchase the motors. But I am looking to see what else is there at the basic level, and all I can see are 3.5 mH phase inductance motors, and a cheap driver would be DQ542MA, DM556 or EMA2-070D56. I honestly don't know which one would be better, they all look like black boxes made in china, so I always suppose that the internals will be almost identical. Should I try to invest more in closed loop steppers ? The price difference is not that astronomically high.. maybe 30-40% more.

    Thanks again for your input.

  5. #5
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    Re: Fixed Gantry CNC Mill for cutting Aluminium and softer materials

    Yes, you can put extrusions together that way, with bolts and nuts. Make sure to drill out the holes on the nut side big enough to clear the socket, and use a thread-locker compound to discourage them from working loose with vibration. But there's no particular advantage to aluminum extrusion for a fixed bridge. It's not a moving part, so it can be as heavy as it needs to be - and heavier is usually better. If you got a section of heavy-section steel box tube Blanchard ground, it would be flatter than the extrusion and stiffer as well. It's a good idea to build the frame before starting to purchase electronics and motors; this gives you a more realistic idea of the forces required, and also gives you more time to research the various alternatives. You might look into an lightly-used automation actuator for your Z axis - they often have a couple of high-quality linear rails and ball screw, mounted solidly in an extrusion, for no more than the price of the slides.

    I usually recommend Geckodrives, since they're proven to work well and come with good support - something that's lacking in most Chinese offerings - but I'm not sure what's available in Germany. Closed-loop stepper systems are worth considering, but they aren't the same as true servo systems. They can save your part in a catastrophe by shutting down, but when they wake up they'll be lost, and need to be re-zeroed.
    Andrew Werby
    Website

  6. #6
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    Re: Fixed Gantry CNC Mill for cutting Aluminium and softer materials

    awerby: I don't think that I will easily find those ground steel tubes in my area, but your advice made me rethink my design. I have a supplier of heavy duty steel tubes in walking distance from my home, and also a CNC manufacturing business where I could mill the grooves for mounting the rails at a relatively low cost. I will bolt everything together with screws, at least 3 for every joint, contact surface should be enough. I don't have a welder, and even if I had, my welding skills are non existent. I hope this will be rigid enough.

    Regarding the Z axis, having already bought the entire set which has the linear rails and ball screws for the Z axis as well, I am going to use those. I am trying to keep costs down (I have mouths to feed ).

    So, I came up with a second design. The gantry is made from a 200x100 mm 8mm thick stell box tube with 200x100x5mm legs. The base is made of 80x80x5mm where the rails sit and with 100x40x4mm traverses.
    A static stress test shows almost half of the deflection for steel tubes with a 5mm thickness in comparison to a standard aluminium extrusion of the same external dimension. So I get a lot more stiffness and with less money as well.

    One more thing: regarding the closed loop steppers. I knew that if they loose steps, they will compensate and recover, but how does this "shut down" work. They will give feedback to the motion controller when the lost steps reach a certain amount and cannot be recovered in time?
    Re-zeroing with high precision to be able to recover a part isn't that complicated if done manually with dial indicators.. am I right?

    Please let me know what you think.


  7. #7
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    Re: Fixed Gantry CNC Mill for cutting Aluminium and softer materials

    Quote Originally Posted by unromeo21 View Post
    awerby: I don't think that I will easily find those ground steel tubes in my area, but your advice made me rethink my design. I have a supplier of heavy duty steel tubes in walking distance from my home, and also a CNC manufacturing business where I could mill the grooves for mounting the rails at a relatively low cost. I will bolt everything together with screws, at least 3 for every joint, contact surface should be enough. I don't have a welder, and even if I had, my welding skills are non existent. I hope this will be rigid enough.

    [Bolts will work, if you use enough of them and seal the threads so they don't loosen up. In some ways they're better than welding, since there's no heat to distort the steel. Milled channels will work for mounting rails if there's enough thickness for them; you don't want to have the steel warp from uneven stress relief. See if your CNC shop offers grinding services as well. The other way to go would be to bolt some steel onto the tubes and mill the channels in it for attachment to your rails.]


    Regarding the Z axis, having already bought the entire set which has the linear rails and ball screws for the Z axis as well, I am going to use those. I am trying to keep costs down (I have mouths to feed ).

    [It sounded like you hadn't decided what to do, but sure, that should work.]

    So, I came up with a second design. The gantry is made from a 200x100 mm 8mm thick stell box tube with 200x100x5mm legs. The base is made of 80x80x5mm where the rails sit and with 100x40x4mm traverses.
    A static stress test shows almost half of the deflection for steel tubes with a 5mm thickness in comparison to a standard aluminium extrusion of the same external dimension. So I get a lot more stiffness and with less money as well.

    [Right; steel is a lot stiffer and cheaper, if weight isn't a factor. You can also beef it up by filling it, either with epoxy-granite or fitted spacers bolted in place.]

    One more thing: regarding the closed loop steppers. I knew that if they loose steps, they will compensate and recover, but how does this "shut down" work. They will give feedback to the motion controller when the lost steps reach a certain amount and cannot be recovered in time?
    Re-zeroing with high precision to be able to recover a part isn't that complicated if done manually with dial indicators.. am I right?

    Please let me know what you think.
    IMG]https://www.cnczone.com/forums/attachment.php?attachmentid=405596&stc=1[/IMG]
    [I've had experience with some hybrid systems, but each one is somewhat different in how it works, so you'd need to specify which particular one you're going with. In general, there's a band of steps within which the system can adjust discrepancies between actual and commanded position. But once that's exceeded, a shut-down error will be triggered. If you've got extremely accurate home switches, you can re-zero with them, otherwise you need to have a previously recorded home position you can re-etablish with your dial indicator, and an accurate idea of where it was in the program when the fault occurred to resume milling without discontinuity.]

    [
    Andrew Werby
    Website

  8. #8
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    Re: Fixed Gantry CNC Mill for cutting Aluminium and softer materials

    Been working on finishing the CAD design (well, not quite finished, the steppers are missing). I made some extra support for the gantry legs, I don't know if this will improve rigidity much, but like this I can bolt the legs from both sides.

    I am not 100% sure about the Z axis design, with the fixed rails versus moving rails and fixed carriages. What would be the most rigid setup? My Z travel is 160mm.



    What do you guys think?

    Thanks!

  9. #9
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    Re: Fixed Gantry CNC Mill for cutting Aluminium and softer materials

    you probably want to bring your Y rails in a little closer, you don't want the trucks all the way at the edge of the bed. The stiffness of the table is going to be greatest right where the it is bolted to the truck, so by putting them on the edge you make a longer weaker span in the middle, and throw away half of your stiffness off the edge where the table ends. You shouldn't be seeing so much load leveraging on the table that the wider stance is necessary.

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