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UPS Backups, Limit Switches, and Magnetic Contactor

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  • scmodelworks
    Hi Guys, I m in the midst of spec ing out a CNC upgrade for my 2000 mill. I ve been going through the archives and have found some great info. There are a
    Message 1 of 4 , Mar 9, 2008
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      Hi Guys,

      I'm in the midst of spec'ing out a CNC upgrade for my 2000 mill. I've been going through the archives and have found some great info. There are a couple of things that I still need help on.

      1. A while back someone brought up the subject of dirty power. I believe that the solution was to use a UPS system. I'm thinking that I should probably include something to protect me in this manner as there will be other computers and motors running in the same room and sharing electrical circuits. How does one go about deciding what power is needed for a UPS system? Are there any brands that I should stay away from?

      2. What benefit will I get using a limit switch? Do they really work or is it a lazy guy's back up? Do full size CNC mills utilize these?

      3. Back in October Jerry J mentioned using a magnetic contactor on the front end. Is this a type of limit/home switch or am I out of the park?

      Thanks for your help,
      Terry Wellman

      [Non-text portions of this message have been removed]
    • Jerry Jankura
      ... A lot depends on the motors that you re using and whether or not they can cause damage to the machine when you run them into the hard stop (note - I said
      Message 2 of 4 , Mar 9, 2008
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        scmodelworks wrote:
        > 2. What benefit will I get using a limit switch? Do they really work or is it a lazy guy's back up? Do full size CNC mills utilize these?
        >
        A lot depends on the motors that you're using and whether or not they
        can cause damage to the machine when you run them into the hard stop
        (note - I said "when," not "if."). Large machines DO use them because
        you can cause a lot of damage fast. Personally, I don't think you need
        them for a Sherline, but that's me. Others will tell you that they're a
        necessity. They definitely should be thought of as a backup; they're
        there to protect the machinery from self damage when all hades breaks
        loose. It's not just poor G-Coding that will cause a problem. Dropped
        steps can also make an error free program drive the motor where it
        shouldn't be.
        > 3. Back in October Jerry J mentioned using a magnetic contactor on the front end. Is this a type of limit/home switch or am I out of the park?
        >
        No, it's not a limit or a home switch. It's a kind of power switch.
        Consider two push button switches. One is normally open, the other
        normally closed. Consider a Relay with a normally open contact. in
        addition to any that actually have to switch power to your machine. In
        the AC line, wire one terminal of the NO switch to your power main's
        "hot" side and the other to the coil on the relay. Wire the other end of
        the coil to the NC switch and the other terminal of the NC switch to the
        power main's neutral. Now, you've got a relay that will be energized
        whenever you push the NO button. Take it one step further. Wire one set
        of NO contacts from the relay across the NO switch. Now, when you push
        the NO button, the relay coil will energize to close its NO contact.
        Once that happens, you can release the NO switch and the relay coil
        remains energized. Now, push the NC button and the relay drops out. When
        you release the NC button, the relay stays dropped out until you push
        the NO button. That's a rudimentary contactor. The relay will, of
        course, have additional contacts that will be used to switch power from
        the mains to the circuit. The advantage to this scheme is that, if the
        powe fails and the coil deenergizes (turning the machine off), the coil
        will not reenergize by itself when power comes back until you manually
        push the NO button so the machine won't simply start up by itself. It's
        common procedure for industrial machinery to be wired that way for
        safety purposes.

        -- Jerry Jankura
        So many toys.... So little time....
      • scmodelworks
        Gotcha, like the magnetic starter switch on my table saw. OK, had not thought of it that way. I m thinking metal and magnetic attraction to that. Duh! So the
        Message 3 of 4 , Mar 9, 2008
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          Gotcha, like the magnetic starter switch on my table saw. OK, had not thought of it that way. I'm thinking metal and magnetic attraction to that. Duh!

          So the home/limit switch may be a good idea then. I'm looking forward to more info on this.

          Thanks,
          Terry


          [Non-text portions of this message have been removed]
        • Alison and Jim Gregg.
          H Terry et al. Answers interposed next to questions below. ... UPS is possibly a good idea - specially in a workshop environmet. Typical PC uses around 350 -
          Message 4 of 4 , Mar 9, 2008
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            H Terry et al.

            Answers interposed next to questions below.

            At 11:31 AM 10/03/2008, you wrote:
            > Hi Guys,
            >
            >I'm in the midst of spec'ing out a CNC upgrade for my 2000 mill.
            >I've been going through the archives and have found some great info.
            >There are a couple of things that I still need help on.
            >
            >1. A while back someone brought up the subject of dirty power. I
            >believe that the solution was to use a UPS system. I'm thinking that
            >I should probably include something to protect me in this manner as
            >there will be other computers and motors running in the same room
            >and sharing electrical circuits. How does one go about deciding what
            >power is needed for a UPS system? Are there any brands that I should
            >stay away from?

            UPS is possibly a good idea - specially in a workshop
            environmet. Typical PC uses around 350 - 400 watts. so at least a
            500 watt UPS for that alone.

            If you are going to run the whole CNC machine off it you will
            obviously need to add the power for the rest of the machine. There
            may be a problem if the breaker trips or the supply company goes down
            and the UPS tries to drive the computer if the machine power is
            down. Depends on your software.

            >2. What benefit will I get using a limit switch? Do they really work
            >or is it a lazy guy's back up? Do full size CNC mills utilize these?

            Yes they do save a few embarrasing smashes, but they will NOT save
            you from every mistake - the limit does not know the tool extension!
            Yes full size machines do use them - a Mori Seki I ran (about 12 tons
            of it!) used both limit switches and pre-limit switches. If you got
            into pre-limits you could back out, but if you tripped the actual
            limits it was much more of an excersise to get it back up.

            >3. Back in October Jerry J mentioned using a magnetic contactor on
            >the front end. Is this a type of limit/home switch or am I out of the park?


            I think that what Jerry meant was what is sometimes referred to as a
            safety switch or zero power switch. This is energised when the main
            switch is actuated, and supplies power to the machine. If you get a
            brief power outage it will drop out, so when the power comes back on
            the machine does not restart by itself. The safety advantages of
            this are obvious. These are compulsory in many countries under
            Health and Safety legislation

            Jim Gregg
            Western Australia.


            >Thanks for your help,
            >Terry Wellman
            >
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