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To gravity feed, or not to?

Discussion in 'CNC Mills/Routers' started by Slewratesarego, Sep 23, 2014.

  1. Slewratesarego

    Slewratesarego Journeyman
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    Let me preface this by saying that I am still fairly new to CNC machines.
    I have been in the process of designing my own using Open builds parts for the last couple of months. Can't wait to get building it!
    The Ox youtubes really helped!
    Now here is the question, a friend of mine runs two cnc machines and they both use gravity on the Z axis, to pull the bit/end mill in to the stock. What are the advantages and disatvantages of using this method vs having a fixed acme nut block?
    For those of you who aren't following, the lead nut is placed under a block that is attached to the z, so in effect it is free to travel down the rod, but when it goes up the rod it meets the block attached to the z and therefor starts to raise the z.
    Hope this makes sense.
     
  2. Tweakie

    Tweakie OpenBuilds Team
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    It's certainly a very interesting idea and I suppose it all depends on the weight of your Z axis, the type of work you intend to do and the feed-rates being used.
    It would perhaps not be so good for V-Carving as the tool could lift slightly causing chatter marks in the work but I haven't actually tried it so only guessing.

    Tweakie.

    DSC01781.jpg
     
  3. Slewratesarego

    Slewratesarego Journeyman
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    Thanks for the reply Tweakie. Nice work on the signs!
    Yeah, I guess the weight of the spindle/Z would need to be a little heavier than an edge trimmer and a Nema 17.
    Thinking about it now, messing with the plunge feed rate, allowing the end mill/bit to not correctly engage with the stock causing it to skate/chatter surely couldn't be good for tool life. It's all about feeds and speeds hey?
    Mmmmmm, the mind boggles. Guess I'll have to design both possibilities into my build. :)
    Must be a reason both machines have the same set up.
    Out of interest, roughly what sort of weight will a Nema 17 pull with the openbuilds 8mm acme and g540 ~2.5A driving it?
     
    #3 Slewratesarego, Sep 23, 2014
    Last edited: Sep 23, 2014
  4. Tweakie

    Tweakie OpenBuilds Team
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    That depends to a large extent on your Velocity and Acceleration settings but a 2.5 Amp NEMA 17 would be a powerful motor for it's size.

    Tweakie.
     
  5. Slewratesarego

    Slewratesarego Journeyman
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    Velocity and acceleration at minimum allowing it lift maximum capacity using a g540 driver.
    All I want is a ball park figure as I'm planning currently on using a makita 700 series trim router.
    I have allowed for the option of upgrading to a nema 23, but I am wondering if a 17 will do the job just as well?
    The machine is a combination of 3&6mm aluminium and 17mm form ply.
    image.jpg
     

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