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  1. #1
    I will start by saying, that this is my first ever CNC design and build, so any critics and suggestions are much appreciated. General idea from here.
    Before I start welding, I want to make sure my desired design actually is any good.

    So overall description:
    Steel frame contains from 100x50mm box sections, 40x40mm intermediate supports; 50x50mm for granty and another 100x50mm in the bottom for locknut. Z axis is built from 20mm aluminium.
    Y axis - 900mm SBR20 supported rails (distance between rail blocks outside-to-outside 250mm)
    X axis - 700mm SBR20 supported rails (distance between rail blocks outside-to-outside 200mm)
    Z axis - 400mm SBR16 supported rails (distance between rail blocks outside-to-outside 200mm)
    RM1605 ballscrew for all three axis. Buying complete set from Chai.

    For electronics, Nema23 3Nm (425oz.in) should be enough to move this beast around. Buying complete set from here.
    Chinese water cooled 2.2kW spindle from ebay as well.

    This CNC will be mainly used to mill/cut/drill aluminium. Preferably with air coolant, to get rid of chips.
    Like I said above, please fill free to comment on any mistakes in design.

    Thank you.
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    Last edited by ivars211; 14-04-2014 at 11:08 PM.

  2. Quote Originally Posted by ivars211 View Post
    ...
    For electronics, Nema23 3Nm (425oz.in) should be enough to move this beast around. Buying complete set from here.
    ...
    Don't... They are crap steppers with 9mH inductance... On 36v they'll perform like treacle. Don't buy any unless they are 3.5mH or lower and even those really need 60v supply on 80v drivers
    Last edited by irving2008; 14-04-2014 at 10:57 PM.

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  4. #3
    Thank you for your reply, I think I messed up links, the set I was actually talking about was this.
    Last edited by ivars211; 14-04-2014 at 11:10 PM.

  5. #4
    Quote Originally Posted by ivars211 View Post
    Thank you for your reply, I think I messed up links, the set I was actually talking about was this.
    Better, but still a bad choice - go with Irving's advice. e.g these

    Whilst it is good to have a large spacing between the linear bearings for better stiffness, you need to consider the 'aspect ratio' (distance between two bearings on rail divided by distance between rails), as if this is too small you'll effectively get lower stiffness due to racking. On what you've designated the Y-axis, this is compensated for by using two ballscrews. However on your X-axis (gantry), the spacing of the rails is large with a small bearing spacing, so when a radial force is applied to the cutter parallel to X, the Z-axis will rotate (or rack/whatever you want to call it).

    I'd go with bigger rails if you want to cut aluminium regularly - certainly not less than 20mm. Ideally not round rails, but I realise that's a big increase in cost. Also attach the rails on the Z-axis to the plate which holds the spindle (i.e. the other way round), it's generally stronger due to reducing the overhang.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

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  7. #5
    Thanks Jonathan for your response. The question now is, what's the optimum 'aspect ratio'?
    For Y axis - distance between rails 700mm, distance between rail blocks 250mm, so that's aspect ratio of 0.3571
    For X axis - distance between rails 320mm, distance between rail blocks 200mm, so that's aspect ratio of 0.625
    For Z axis - distance between rails 120mm, distance between rail blocks 200mm, so that's aspect ratio of 1.667

    I might consider going double ballscrew design, but that's also increase in cost. I have also read that there is a possibility of racking and damaging machine, if one of the steppers stops while other one is still going. So is the gain really noticeable in relatively small machine? Isn't steel frame rigid enough to prevent racking?
    Last edited by ivars211; 15-04-2014 at 02:04 PM.

  8. #6
    If you want to mainly cut aluminium on this machine then you really need twin ballscrews on your Y axis (X in my world). Even with using steel the gantry will still rack when cutting aluminium or harder materials. Also like Jonathan mentioned the distance between rails on your X axis is too great so either increase clearence or lower the gantry height to bring closer together.

    Regards the twin ballscrews then use single motor and join them together with timing belts and pulleys. Use 10mm pitch and gear 2:1 ratio to give same resolution as 5mm pitch but at same time increase torque.
    This way it's Win win you get around the racking issues give flexabilty to machine because you can fine tune with differant ratios or increase speed if required with simple pulley change.
    Less electronics to go wrong and slight saving on cost's. Motors perform smoother and better due to less resonance affects and with a All steel machine resonance can affect motors quite a lot, esp if using cheap drives.! . . . . This brings me onto Drives.?
    Don't buy cheap drives with a steel machine because of resonance, paying that bit more for digital drives will pay back big time in the long run. You'll get much smoother performance from motors but more the point they deal with resonance much better which will drive you crazy and give a very rough running machine if you do encounter it.!

    If your dealing with Chai ask him for a price for Profiled linear rails and you may get a pleasant surprise.?
    Last edited by JAZZCNC; 15-04-2014 at 07:34 PM.

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  10. #7
    It might be stupid question, but what do you mean with "digital drives"?
    I think you convinced me, so I am going to redesign it for twin ballscrew drive then.
    Any advice where to buy timing belt and pulleys?
    Last edited by ivars211; 15-04-2014 at 08:14 PM.

  11. #8
    ]edited so you did
    Last edited by deisel; 15-04-2014 at 09:18 PM.

  12. #9
    Yes, I mentioned that in first post as well. Thanks for suggestion, but it seems like he hasn't been logged in for a while.
    Last edited by ivars211; 15-04-2014 at 08:18 PM.

  13. #10
    Quote Originally Posted by ivars211 View Post
    Yes, I mentioned that in first post as well. Thanks for suggestion, but it seems like he hasn't been logged in for a while.
    so you did I only remember it because of the cracking job he did on that Harley sign,but that was wood.
    like said 2 screws are must for the size of it,i reckon you,d have been buying a second screw inside a week of using it cutting alu ,look forward to seeing how it performs if you get a video up.

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