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  1. Hi guys im in the middle of building my machine but i would like to know your thoughts on what thickness ballscrew i should be looking at to span over 1829mm ( 6ft ) without none to much "whip" and another thing is the pitch as on zapps website they do a 5 or 20mm pitch in the 20mmOD screw, the machine im building is for precision work mainly but with it being a 6x4 bed i was thinking that a 5mm pitch would take all day get to the other end? :confused:

  2. #2
    I used 25mm OD ballscrew with a 25mm pitch for mine, and with the motors I've got, I get 18m/min, which for about a 1.4m run gives ~4.7 seconds for total travel. However, you will need to worry about whip.

    Gary (from Zapp) gave me a website which allows you to calculate the whip speed based on the diameter and so forth - unfortunately, I've only got the link at work, but Gary may come on here before I get the chance on Monday to get it.

  3. thanks tribbles for that info, 25mm OD is some fair chunkey stuff!

  4. #4
    Use this link.

    http://www.nookindustries.com/ball/BallCalculators.cfm

    use the root diameter that is normally about 2-3 mm lower than the outside diameter and select the mounting type you are using, and it will calculate the max speed before it will whip.
    If you have any question, or need help give me a ring on monday.

    Gary

    Quote Originally Posted by tribbles View Post
    I used 25mm OD ballscrew with a 25mm pitch for mine, and with the motors I've got, I get 18m/min, which for about a 1.4m run gives ~4.7 seconds for total travel. However, you will need to worry about whip.

    Gary (from Zapp) gave me a website which allows you to calculate the whip speed based on the diameter and so forth - unfortunately, I've only got the link at work, but Gary may come on here before I get the chance on Monday to get it.

  5. Thanks Gary much appreciated ;)

  6. #6
    Gary (from Zapp) gave me a website which allows you to calculate the whip speed based on the diameter and so forth
    As Tribbles said the diameter required to reduce whip is fairly easy to work out. The pitch is not so. You mentioned speed and accuraccy..... I dont think the two are easily achived without lots of money.......

    You havent said what you are cutting or what accuraccy you require. But if you are using steppers when 200 steps per rev at 5mm pitch gives 0.025mm resolution. With a 25mm pitch you get 0.125mm resolution.

    Also given that for your 1829mm travel you will probably need 20mm + dia. then motor torque will be an issue, lots of mass and inetia to control.
    I may be corrected by others but you are probably in Servo Motor terratory for the distances and speeds you require.

    See the attached pdf that I got from Cnc-zone.( hope its ok to repost it. cant remember who its from. {Lee any probs just remove it}

    Goes in to detail about designing motors, screws and feeds.
    Attached Thumbnails Attached Thumbnails CNCMechanics%20v1.2[1].pdf  
    Last edited by Ross77; 16-08-2009 at 01:22 AM. Reason: Bad speeelaing as usual

  7. #7
    Sorry. Gary and your self posted whilst I was writting...
    30mins to post. Must be time to go to bed....:heehee:

  8. #8
    Quote Originally Posted by AdCNC View Post
    thanks tribbles for that info, 25mm OD is some fair chunkey stuff!
    I was looking at 40mm before realising the motors wouldn't go as fast as the speed I was anticipating, so Gary suggested using the 25mm pitch stuff instead - and it's worked out great (thanks!)

  9. Quote Originally Posted by Ross77 View Post
    You havent said what you are cutting or what accuraccy you require.
    Well i was planning on cutting woods and none ferrous metals!

    Ive been looking at the 25mmOD ballscrew and im leaning towards that! as for the motors being not power full enough i can gear them to something like a 5:1 or more and get a 20-25 pitch ball screw!

    What do you guys think!

  10. #10
    Well i was planning on cutting woods and none ferrous metals!

    Ive been looking at the 25mmOD ballscrew and im leaning towards that! as for the motors being not power full enough i can gear them to something like a 5:1 or more and get a 20-25 pitch ball screw!

    What do you guys think!
    Did you mean none or some? Gearing will definatley help with the motor power and accuracy (200 steps at 5:1 is 0.025 resolution) but will also slow the travel time down. Max stepper RPM is 350 ish I think

    If your using belts then you might also want to check the 5:1 ratio, sounds a bit high. the small pulley will not have enough teeth in mesh if the bigger pulley is to large. Ive got a calculator at work so if its any help I'll post it tommorow.

    Hope you dont think I'm being negative but I had to make the same decisions for my build. I chose Slow and accurate because Im only using it to build the fast machine :heehee:

    Have you concidered rack and pinon? its cheaper and very fast.

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