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  1. #21
    toomast's Avatar
    Lives in Tartu, Estonia. Last Activity: 14 Hours Ago Has been a member for 3-4 years. Has a total post count of 68.
    Things are moving slow...but at least they are moving forward.
    I wanted to do all steel Z-axis but as I had something thrown together in CAD I changed my mind. Main reason is weight. Steel Z will be at least 7-10kg heavier. As my hole gantry is at the edge of using servos I will go with aluminium Z design.
    I'm not quite finished alu design but it will be from 25 mm thick material.
    Here is a picture of my alu design (left) and steel design (right side).

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  2. #22
    I did all from steel cause i was doing the machine on the cheap and did not have somebody to machine it for me at that moment. Time wise is better to do all from aluminum. cause steel needs straightening, adjusting, first hand primer, second hand paint,...
    project 1 , 2, Dust Shoe ...

  3. #23
    Sven's Avatar
    Lives in a, Netherlands. Last Activity: 16 Hours Ago Has been a member for 2-3 years. Has a total post count of 40. Received thanks 4 times, giving thanks to others 0 times.
    I can't follow the reasoning for not using steel, except for silyavski reasoning. If the gantry is steel, 7-10 kilo'sfor the z-axis will not make things that much different. Except for stiffness!

    If the machine is for wood and this big, acceleration is less of an issue than top speed and for top speed stiffness is paramount. And all is designed to be stiff, except for this z-axis....

    Besides, you can get 20 Nm steppers of the shelf....
    Last edited by Sven; 06-04-2015 at 07:20 AM.

  4. #24
    toomast's Avatar
    Lives in Tartu, Estonia. Last Activity: 14 Hours Ago Has been a member for 3-4 years. Has a total post count of 68.
    Quote Originally Posted by Sven View Post
    I can't follow the reasoning for not using steel, except for silyavski reasoning. If the gantry is steel, 7-10 kilo'sfor the z-axis will not make things that much different. Except for stiffness!

    If the machine is for wood and this big, acceleration is less of an issue than top speed and for top speed stiffness is paramount. And all is designed to be stiff, except for this z-axis....

    Besides, you can get 20 Nm steppers of the shelf....
    I don't have first hand experience for none of them. But I dont think 25mm aluminium stiffness will be worse than 10mm steel.
    Popular belief in this forum is that weight is important and 20 Nm is not always better.
    Anyways I have an opportunity to find out...

  5. #25
    toomast's Avatar
    Lives in Tartu, Estonia. Last Activity: 14 Hours Ago Has been a member for 3-4 years. Has a total post count of 68.
    Quote Originally Posted by JAZZCNC View Post
    I know silyavski told you No but he's Wrong it most definatley could whip at that length at higher rapid feeds. 20mm is much better screw to use at this length and won't cost much more in either Money or performance.
    What would be the maximum length of 20mm ballscrew in this application? What would be critical length that needs rotating ballnuts?
    Im thinking of getting more distance from gantry travel...

  6. #26
    Quote Originally Posted by toomast View Post
    What would be the maximum length of 20mm ballscrew in this application? What would be critical length that needs rotating ballnuts?
    Im thinking of getting more distance from gantry travel...
    Oh very hard to answer because depends on the end fixing etc but I wouldn't go above 2000mm and at that I would have Both ends with Fixed bearings and slight preload on them. When I build 4x4 size machine I always use Fixed/Fixed end bearings as the slight cost difference is worth the extra support it gives.

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  8. To work out the critical speed of a screw there is a calculator on our site.
    http://www.zappautomation.co.uk/inde...lculators.html

    In the lower part of this page is a link for the calculator.

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  10. #28
    To give you an idea I recently built a Slaved axis 4x4 router with 20mm Dia 10mm pitch screws 1700mm length Fixed/Fixed ends with 2 x 3.1Nm steppers running 70Vdc on EM806 digital drives. It safely reached 15mtr/min while testing and probably could have pushed it slightly more but felt no need.
    Machine is actually tuned much lower than this at 10mtr/min rapids to keep a very generous safety margin. 10mtr/min is more than enough speed for machine like this.

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  12. #29
    toomast's Avatar
    Lives in Tartu, Estonia. Last Activity: 14 Hours Ago Has been a member for 3-4 years. Has a total post count of 68.
    I am still amazed by the time it takes to get things organized ;)
    So anyways here is the gantry. Bit on a heavy side - 84 kg. For sure there will be some more fasteners, dust collector, cable-chain etc adding weight. Plan is to use 2 x 2000mm ballscrews 2010 to drive it. Fixed/fixed ends. 3,1Nm steppers.
    Can I make it work?
    I dont want to go servos on first build :)

    Now my frame is looking like a toy :) Will make it 100x100mm tube instead 80x80 and beef it. Also I will ditch the bed design and try making some-kind of hybrid from marine plywood and aluminium surface for smaller parts...
    If you guys approve the design I will order screws and rails and stuff from Fred...


    Thanks for listening!
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  13. #30
    Quote Originally Posted by toomast View Post
    Can I make it work?
    I dont want to go servos on first build :)
    Yes it will work thou with 83Kg + gantry you'll have lower feed rates than I posted lower down. Think the gantry on that machine was 50Kg.

    Now regards 100mm over 80mm then being totally honest I think this kind of building to excess is unnecssary and the little you gain isn't worth the cost and trouble. 80mm is more than enough for a frame this size. You'll gain more thru less resonance using 80mm or even 60mm but buy a thicker walled tube than going with 100mm thin walled (big base drum) tube.!!. 100mm thick wall tube is expensive and complete OTT for machine this size.

    Other than the OTT build strength of the gantry the only thing I see which I'm not sure about is the flimsy looking ballnut supports on gantry ends and Gantry ballscrew.
    Also not sure to how your mounting the motor and fastening to ballscrew across the gantry because pics don't show it very well. But it looks like your planning on connecting ballscrew to rear shaft of stepper and mounting stepper to a plate using normal stepper mounting holes.
    OR doing reverse and mounting stepper to plate using rear holes and having the front hang out in space connecting to ballscrew.? . . . .Both bad idea.!!

    Can you show better pics of what you have in mind.!

    On the Bed idea.! I see you have large distance from bed frame to spindle and no adjustment which will be fine for wood etc but no so good for harder materials. So if you don't need all the bed then think about a 2 tier setup with the alumnium cutting area raised up to bring material closer to cutter.
    The raised section could be made removable for when full bed needed and using a torsion box setup for stabilty.!

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