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  1. #1
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    Default Z axis desig

    Hello,
    I need your advice about z axis design: screw for z axis movement is 500mm 1605 ball screw, so after C7 mounting with BK,BF screw useful length is about 440mm; aluminum plate for z axis base is not problem, I have aluminum plates and so on. As rails I will use THK 20 block rails. Most important question is about z axis design, what is advantages and disadvantages of:
    A design: rail blocks is fixed to stationary plate and rails is fixed to moving plate: example pic:http://www.mycncuk.com/attachment.ph...6&d=1451863790

    B design: rail blocks is fixed to moving plate and rails is fixed to stationary plate: example pic:http://www.cnczone.com/forums/attach...1&d=1389527580

    which design is more accurate and rigid?

    Sorry for bad English

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  2. #2
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    Default Re: Z axis desig

    http://www.mycncuk.com/attachment.ph...6&d=1451863790

    The one where you can get the bearings the furthest distance apart.

    So this one is.

    However, in that photo the bearings are closer together than they need to be. You want two bearings as low as they can go and two as high as you can get them. If the top of the plate the Z axis bearings were attached to was extended upwards you could even get the top two higher.

    Plus with this design if you are cutting things close to the bottom of your gantry, you don't have the long moment to your first two bearings. The other way, there is always a long moment to your first two bearings. Obviously the further apart the bearings the less movement you will have OR the longer your Z axis assembly will be. So the rails need to be longer this way with the bearings spaced further apart.

    440mm is alot of travel. That's over 17 Inches. So the gantry and every other part will need to be very sturdy. Are you thinking for a 5 Axis machine?

    Also, for this kind of distance, flat plate for the Z isn't good. Flat plate is easy to bend / deflect. Square box tube or at least stiffeners attached at 90 degrees to the plate are required.

    Are you planning on a pneumatic counterbalance?

    So this video doesn't belong to me, it's not my machine, I'm just posting a link. You can see that he has mounted his Z axis rails like this. Probably one of the best successful high performing DIY CNC machines made IMO.





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    Default Re: Z axis desig

    Hello, Nic,
    thanks for quick replay yes, I planing to build 5th axis machine. about aluminum plate: yes, just plate can bend, I will use 40 mm thickness aluminum "construction". I have not heard about pneumatic counterbalance, for what is it?

    So z axis design will be http://www.mycncuk.com/attachment.ph...6&d=1451863790



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    Default Re: Z axis desig

    Quote Originally Posted by TadasVoka View Post
    Hello, Nic,
    thanks for quick replay yes, I planing to build 5th axis machine. about aluminum plate: yes, just plate can bend, I will use 40 mm thickness aluminum "construction". I have not heard about pneumatic counterbalance, for what is it?

    So z axis design will be http://www.mycncuk.com/attachment.ph...6&d=1451863790
    A pneumatic counterbalance is to offset the weight of the Z axis so your Z axis motor doesn't have to work as hard, and your Z axis ballscrew isn't wrecked by too much weigh constantly pressing against the balls.

    A typical spring does not output a constant force. Force = K x Distance, so it changes considerably over the length of elongation or compression.

    Gas struts (the kind you might find to help raise or lower the back door of a minivan) typically have alot of dampening, which isn't good for a CNC machine with many quick movements. Also the pressure isn't adjustable, some are adjustable, but then you can only let air out, not put it back in, so not fully adjustable in the same sense. Also they work at much higher pressures.

    A pneumatic counterbalance is basically an air cylinder or two that are attached to an air reservoir under pressure. These can deliver a very constant force depending on the size of the reservoir and the working pressure. The first step to size one of these is to figure out the weight of your Z axis.

    I have made a thread about it here, and there are some equations in there you can use for sizing your cylinder bore(s) and reservoir size:

    http://www.cnczone.com/forums/mechan...44106-cnc.html

    I have not done it yet, but have ordered some parts for my own build. So, not yet an expert on the subject.

    In this thread here by linux_fan, if you read through it, you will see an example of one:

    http://www.cnczone.com/forums/uncate...aluminium.html

    As for the Z axis, yes 40mm is very thick plate, but box structure is more efficient per weight to get more stiffness. If you are using plate, regardless of the thickness, you can bolt structure at 90 degrees to the plate, it will make it stiffer. Even thinner plate would be stiffer with attachments at 90 degrees than stiff plate.

    There are some engineering calculators online, you can test it for yourself using a plate or beam of constant cross section with a cantilevered load applied at one end.

    Good luck, 5 axis machines can be hard to design and build. I think a good used industrial robot arm and a turn table would be good for a 5 axis machine and much simpler to implement. Of course, finding the appropriate robot arm at a reasonable price would be the hard part there.

    Please post a link here to your design and or build log should you choose to do one. Good luck.



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