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Wire tension on a CNC Hot Wire set-up

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  • Wire tension on a CNC Hot Wire set-up

    OK the subject line was pretty vague. I am new to the CNC community, but I have some exposure locally with friends and their equipment.

    I am building a dual purpose CNC machine that will have both an X, Y, Z, router table (28" X50") and an Y, Z, U, V, Hot-Wire cutting set-up, all in one. I have most of the design figured out, and many of the parts machined. I have the main table structure completed and the gantries for the Y, Z and U, V, axis built. I have finally settled on the Linear Bearings after a small hick up with the original concept (too much flex). I am in the final stages of machining the chassis for the Y, Z, and U, V, movements, and have the mounting brackets for the router motor (Z axis). I am using a Dewalt hand router motor for now, but eventually I want to go with a water cooled spindle motor, much quieter and less vibration.
    I have one hitch in my project and that is the wire tension method for the hot-wire rig. It looks like most of the commercial rigs out there use a stepper motor and gearbox to provide wire tension. I am a little concerned about issues with that set-up, mainly the heat build up of a stalled motor. I am thinking about a static weight stack and pulley system. My machine has a 60" X 32" main table, one movable gantry (X, Y, Z) and a fixed gantry (U, V) on one end. I plan to payout the hot-wire from the fixed gantry end. The wire attaches to the Z axis on the movable gantry, The X axis is set and fixed to what ever distance is needed for the material being cut.
    My vision is to have a dual sized pulley, wire take-up reel and weight tension spool,on a common axle mounted to the end of the fixed gantry. The weight will hang off the side and rise and fall as wire is needed for the job. The Hot-wire will feed from the side of the gantry, to the V axis chassis via pulleys and run through a brass eyelet to be energized. The weight will only have to move a few inches during most cuts and I envision the weight sliding inside an acrylic tube to keep it from being bumped. The pulleys will be locked together with a series of holes and a spring loaded pin to allow for paying out wire needed for the task. I will need 20 LBS pull on the wire while cutting, but I plan to use a ten pound weight (more compact) and use large pulley for the weight line and a smaller wire pulley to multiply the pull.

    My questions are: What do folks think of this plan? Does anyone have experience with CNC hot-wire? How did they handle the wire tension?

    Any ideas/suggestions are welcome.

    Peace, Wolf

  • #2
    Have you tried the CNC hot-wire forum here http://www.cnczone.com/forums/cnc-wi...tter-machines/ ?
    Max.

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    • #3
      The only hot wire stuff I did was to cut styrofoam. I made a frame out of wood, put an eyebolt in each end and hooked the wire to one side, an extension spring to the other side and the wire to the extension spring. The spring kept the slack out of the wire when it heated up. Haven't seen a CNC version of this.
      Kansas City area

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      • #4
        I don't know if one would work for your application but there are motors called "torque motors". They are pretty much like normal AC induction motors but are designed to operate stalled or at very low speed. A "Variac" supplies a variable AC voltage and that determines the torque. I salvaged some from a movie film duplicator where they controlled the film tension.

        I suspect many AC and DC motors could be used by supplying a suitable voltage if the available torque was suitable.
        Last edited by Don Young; 01-23-2015, 09:16 PM.
        Don Young

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        • #5
          Thanks for the quick replies everyone, keep em coming.

          Thanks for the link Max... I will definitely utilize that site as well.

          Toolguy- I currently have a Feathercut rig (non-CNC) that I have had since the early 90's. It has served me long and hard, and is still going. I just want to expand my abilities. Go to YouTube and search CNC Hotwire

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          • #6
            Wingnut,
            Torque on a DC motor is proportional to current. I used to build a large hose reel that fed hose to a moving machine. I tensioned the reel using a DC motor and a KB Penta four quadrant drive and simply set the torque pot (current) to give me the tension I wanted. When the machine moved faster the reel would spin faster and when the machine moved very slow the reel would turn slowly. When it move towards the reel the reel wound the hose back up without changing the drive direction, which was always inwards.

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            • #7
              Gary-

              I see most of the commercial rigs use a motor, inside a housing usually. Most have fans associated with the housings and I just did not want to try and reverse engineer from an image. I am no EE, but I do know from past experience that when you stall a DC motor, it will get hot in a big hurry. Servo motors have drivers to handle current and I am just trying to keep it simple AND reliable.

              That said, working out the logistics of a weight and pulley system does come with its own challenges...

              Thanks, Wolf

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              • #8
                A stalled DC motor connected to its normal voltage will quickly overheat. However, if you reduce the voltage enough it will still develop torque without overheating.
                Don Young

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                • #9
                  I may have to revisit the motor/wire tension problem.

                  Thanx, Wolf

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