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Z-axis counterbalance air balance air cylinder?

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  • Z-axis counterbalance air balance air cylinder?

    The new CNC mill is going to need a counterbalance on the Z. An adjustable air cylinder looks to be the best way. For now, building a simple cylinder is a must since even used ones on ebay are too expensive. We're not talking about a lot of pressure....would DOM steel tubing be smooth enough on the inside to work along with a simple aluminum piston & a couple o-ring seals with an oiler to keeps things lubed?

    This is in the head scratchin' phase now. No need to worry about the tricky regulator & reservoir needed to keep the pressure stable on both the up & down movement; that research will come later.
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    Yes, DOM should be smooth enough, but I would want to run a honer through it to get a smooth enough surface for hydralic pressure.'You don't mention stroke or I would suggest a surplus automobile shock absorber modified . Also use it vertically so the piston rod is pushing UP and using a pulley with one cable on the head and the other anchored to the base, giving you travel that is twice the stroke length of the piston/cylinder.

    Rich
    Green Bay, WI

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    • #3
      Try a gas spring. Cheap, multitude of strokes, forces, mounting. I did a mill conversion using one - worked great.
      https://www.mcmaster.com/#gas-springs/=16yhmj8

      I actually removed the head and weighed it, then selected a gas spring with a force nearest the weight of the head.
      Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
      ~Practitioner of the Electromechanical Arts~

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      • #4
        I agree with Weston that your describing the job of an air spring.

        What force do you need and what is the stroke needed?

        While your noodling solutions learn what a relieving regulator is and how it differs from a non-relieving regulator.

        lg
        no neat sig line
        near Salem OR

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        • #5
          I personally don't think DOM tube is suitably smooth for any moving seal. I think you'll have to bore/hone whatever you use, so if you build your own it might be better to use cheaper, thinner material. Have you considered seamless stainless steel tubing? It can be very thin and still hold all the pressure you'd need for that application.
          Last edited by chipmaker4130; 03-28-2017, 10:38 PM.
          Southwest Utah

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          • #6
            Originally posted by Rich Carlstedt View Post
            Yes, DOM should be smooth enough, but I would want to run a honer through it to get a smooth enough surface for hydralic pressure.'You don't mention stroke or I would suggest a surplus automobile shock absorber modified . Also use it vertically so the piston rod is pushing UP and using a pulley with one cable on the head and the other anchored to the base, giving you travel that is twice the stroke length of the piston/cylinder.
            Around 10 or 11" travel needed. Great idea on the auto shock or strut, thanks Rich! I have access to tons of em for free. I LOVE free! I'll have to take a couple apart & see about how to best convert one. Yes, I had planned to make it with the piston/rod going up into the cylinder under pressure.

            Thanks for the relieving regulator terminology tip Larry. I had no idea what they were called but knew kinda-sorta how they work. I may have to ask for help in 3 or 4 months (hopefully that soon) when the Z-axis part of the machine gets to the top of the list.

            Gas springs were considered but an easily adjustable system would be easier to work with for the different size & weight spindles that are planned.
            Milton

            "Accuracy is the sum total of your compensating mistakes."

            "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

            Comment


            • #7
              I disagree with the gas spring idea. They are too slow and on my mill I found it was causing me problems. If you make your own cylinder make sure there is enough flow from the tank.

              How much does your headstock weight? Mine is around 60kg and I don't need a cylinder to get 6000mm/min with my NEMA 24 motors.

              Comment


              • #8
                Originally posted by DEVILHUNTER View Post
                How much does your headstock weight? Mine is around 60kg and I don't need a cylinder to get 6000mm/min with my NEMA 24 motors.
                The weight at this point is not finalized but should be nowhere near 60kg. Probably less than half that. Are those servos you're using? I have 425 oz. steppers.
                Milton

                "Accuracy is the sum total of your compensating mistakes."

                "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                Comment


                • #9
                  I have made balance cylinders from brass tubing with o-ring piston rings.

                  You need to use a relieving style regulator to keep a constant balance pressure.

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                  • #10
                    Originally posted by DEVILHUNTER View Post
                    I disagree with the gas spring idea. They are too slow and on my mill I found it was causing me problems.
                    You must have used a gas strut, one with damping. Gas springs run as fast as you like, they don't have damping, if you compress one and let it go it'll have your head off ;-)
                    If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

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                    • #11
                      Most of the counterbalances I've seen on CNC mills are a roller chain with one end on the head, going over rollers, and a weight on the other end, the same weight as the head. This is a bullet proof design that moves freely and never wears out.
                      Kansas City area

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                      • #12
                        Is this the type of relieving regulator you fellows are talking about? http://www.ebay.com/itm/SMC-EAR210-R...cAAOSwRLZUKsgS
                        Milton

                        "Accuracy is the sum total of your compensating mistakes."

                        "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                        Comment


                        • #13
                          The downside of it - you just doubled the mass of the head...


                          Originally posted by Toolguy View Post
                          Most of the counterbalances I've seen on CNC mills are a roller chain with one end on the head, going over rollers, and a weight on the other end, the same weight as the head. This is a bullet proof design that moves freely and never wears out.

                          Comment


                          • #14
                            Originally posted by Toolguy View Post
                            Most of the counterbalances I've seen on CNC mills are a roller chain with one end on the head, going over rollers, and a weight on the other end, the same weight as the head. This is a bullet proof design that moves freely and never wears out.
                            Can't be used in a high speed machine, like a Quaser with even 30 m/min rapids, as the chains do not follow fast enough as gravity doesn't work as fast on the counterweight as the servos drive the head up, the mass you are moving has doubled and the chains do wear out, thought it might take 20 years (have changed chains/sprockets to three mills already, one of them had the chains already about 80 mm longer than original and the counterbalance started hitting the machine base casting inside the column during homing.

                            But for a home shop machine the chains and counterweights is an easy, cheap and reliable solution.
                            Amount of experience is in direct proportion to the value of broken equipment.

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                            • #15
                              Yup, I have a stack of brake discs hanging from a couple pulleys & steel cable rigged up on my manual mill's head but I didn't figure it'd work as well with CNC's quicker movements.
                              Milton

                              "Accuracy is the sum total of your compensating mistakes."

                              "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                              Comment

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