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  1. #385
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    Default Re: David A's New 2x3 Bench Top CNC

    Quote Originally Posted by davida1234 View Post
    0.00004".......

    Well, Outlawtaz, yours capabilities are definitely way beyond my capabilities.

    As you said, well outside the precision of a homebuilt, let alone some semi-commercial machines.

    I do like your different perspectives, though. Keeps me aligned, so to say.
    Precision test equipment for a precision part. I always liked the challenge that was posed in helping the customer make something dead nuts so to speak. Not to mention that customer did all kinds of R&D and experimental stuff that was really cool to see. The above mentioned part was a friction stir welding stylus the had a rounded triangle (like the rotor in a Mazda rotary engine) at the business end. On the faces of the triangle were the vanes that resembled those tiny threads. He actually deburred these "threads" with an exacto knife and a magnifying glass.

    Now that I don't do that work anymore, I have entered the world of home built machines. I kinda keep tabs on what other people are doing, collecting ideas and trying to see how precise a machine can be made without spending tons of money. Just a personal mental exercise I am in the middle of that hopefully will turn into something tangible in the near future



  2. #386
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    Default Re: David A's New 2x3 Bench Top CNC

    In case somebody is interested.....

    The cart under the machine was built out of two sheets
    of 3/4" Baltic Birch. Total height of cart is 29 1/2". The
    cross braces are at the bottom of the 4x6 or 6x6
    (preferable, just didn't have any at hand) corner posts.
    Takes about a day or two to built. Total cost about $250
    with wheels. If you want to save with smaller wheels,
    just make the body taller.

    It uses the Harbor Freight large wheels, $25 each.
    Very sturdy wheels with turn/swivel lock; easy to roll.
    Carries the 350 pound machine with ease but will
    probably vibrate when cutting hard.

    Here is the concept of it:
    David A's New 2x3 Bench Top CNC-cart-1-jpg

    David A's New 2x3 Bench Top CNC-cart-2-jpg



  3. #387
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    Default Re: David A's New 2x3 Bench Top CNC

    Quote Originally Posted by Outlawtaz View Post
    I acknowledged that he had adjustable feet, that's why I asked about twist. It is possible to level a machine and take the twist out of it. The only one that I ever had problems doing both was on Haas MiniMills as they are a boxed welded base. Its really hard to reduce twist in something that is made out of 1/4" steel plate welded into a roughly 3 foot cube...
    Even the Haas MiniMill which I have some of them you can take the twist out if you have that problem it's how you approach it that gets the job done, the machine it's self only mounts on 3 points to the base, so it normally does not have a problem with twist

    Mactec54


  4. #388
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    Default Re: David A's New 2x3 Bench Top CNC

    Btw, the rotational mass of the power ball nut is just
    4.4 ounces compared to several pounds of a ball screw:
    Wow, sometimes it's hard to get a sense of scale in a photo.
    To put that in perspective the AC bearings I bought for a 'someday' driven nut weigh 7.4 oz each. Most of that weight wouldn't be in rotation but it gives a sense of the overkill in my design.
    In your design I see the 'light'. (There's a pun in there somewhere)

    In a long span would it make sense to hold the screw in tension ?

    Anyone who says "It only goes together one way" has no imagination.


  5. #389
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    Default Re: David A's New 2x3 Bench Top CNC

    cyclestart, I am amazed at the interest I am getting for the power ball nut system.

    The longest screw on my machine is 46" and there is no sagging.

    I don't know how it would be with 8' lengths but I think that really should
    not make a difference. The screw is guided by the balls in the ball
    nut so, even if there is a bit of sagging on the free portion of the screw,
    the positioning of the screw inside the ball nut would always be inline.

    Besides, I am not sure if the tensioning forces we would be able to apply
    here would help much for the little bit of sag. Just look at cable cranes
    lifting many tons of loads. Their cables still sag a bit.

    Last edited by davida1234; Yesterday at 01:16 PM.


  6. #390
    Community Moderator ger21's Avatar
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    Default Re: David A's New 2x3 Bench Top CNC

    I've always read that you can still get "whipping" even when the screw is not spinning. Tension can help there.

    If it works fine as is, then there's no need to tension it. But i you see the screws flexing under load, you may need to.

    We have a 12' router with a spinning nut on a 40-50mm ballscrew on the long axis. The mount for the screw actually stretches it about .03".

    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  7. #391
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    Default Re: David A's New 2x3 Bench Top CNC

    ger21, valid points.

    So far, I did not observe any "whipping" but the screw is just 46" long.
    Any possible "whipping" is probably induced by harmonics vibrations.

    I guess your machine is one of those $200k + machines. All steel and HD.

    There, you might have solid anchoring points. But on a 80/20 machine, how
    much could you really tension a steel rod before the extrusion itself gets distorted?



  8. #392
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    Default Re: David A's New 2x3 Bench Top CNC

    There, you might have solid anchoring points. But on a 80/20 machine, how
    much could you really tension a steel rod before the extrusion itself gets distorted?
    .
    I think even a small amount of tension might make a noticeable difference, if you needed it.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  9. #393
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    Default Re: David A's New 2x3 Bench Top CNC

    I will keep that in mind and watch for any "whipping".

    Thank you.



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