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motorizing the nut instead of the screw

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  • motorizing the nut instead of the screw

    For a long axis travel it would seem to me to stop some of the whipping of say a ballscrew why couldn't one drive the nut instead of the screw? I am thinking of this in relation to my upcoming build of a cnc plasma table. I could see on a mill it would be not so practical but on a gantry type machine I think it would have less tendency to cause vibration.

    Anyone know of someone that built something like tha?
    How to become a millionaire: Start out with 10 million and take up machining as a hobby!

  • #2
    The first thing that occurs to me is the possible backlash of any "final drive"
    to the nut. Possible a anti-backlash worm drive???
    Interesting idea. Why not for a mill table, packing the drive in a pretty limited
    space is the only drawback I can see.
    ...lew...

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    • #3
      I did.
      I machined the Delrin gear and tapped for 1/4-20 threaded rod and sandwiched between two ball bearings. A gear of identical diameter but fitted to the shaft of a stepper motor engages this one.



      I use the "leadnuts" to position the X- and Y-axes on my little CNC table. the screw is fairly short, but I would be comfortable using it in applications with a longer screw, especially if I could hold the screw in tension.


      <edit> The Delrin gears are not perfect, but are tightly meshed so that they deform slightly in normal operation and have no backlash. they will work a long time that way.
      Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
      ~Practitioner of the Electromechanical Arts~

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      • #4
        It has been done before, one method attaching a pulley to the nut housing and mounting the motor above (or below it), this assembly is then firmly mounted to the traveling gantry, the other way is a right-angle planetary reduction with a through-hole through the final drive type, Bayside Gear make them.
        Max.

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        • #5
          The Bridgeport BOSS did this for years, X axis screw was fixed and the motor drove the ballnut, supported in bearings and used a belt drive.

          A lot of laser cutters also use driven nuts but many are now on rack and pinion due to the whipping problems
          .

          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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          • #6
            Originally posted by Weston Bye View Post
            I did.
            I machined the Delrin gear and tapped for 1/4-20 threaded rod and sandwiched between two ball bearings. A gear of identical diameter but fitted to the shaft of a stepper motor engages this one.



            I use the "leadnuts" to position the X- and Y-axes on my little CNC table. the screw is fairly short, but I would be comfortable using it in applications with a longer screw, especially if I could hold the screw in tension.


            <edit> The Delrin gears are not perfect, but are tightly meshed so that they deform slightly in normal operation and have no backlash. they will work a long time that way.
            That table makes me think if one could build something like that to mount on the table of an exsisting manual mill. You would then have a 2d cnc mill wouldn't you? Keep it low profile so as to not take up very much headroom. Drop the knee on the mill down put the CNC table on and have at it! It wouldn't be any different than a CNC'd rotary table would it?
            How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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            • #7
              Originally posted by Black Forest View Post
              That table makes me think if one could build something like that to mount on the table of an exsisting manual mill. You would then have a 2d cnc mill wouldn't you? Keep it low profile so as to not take up very much headroom. Drop the knee on the mill down put the CNC table on and have at it! It wouldn't be any different than a CNC'd rotary table would it?
              That indeed was my original intention and I used it that way until I built a frame with a z-axis to become a stand alone mill.
              Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
              ~Practitioner of the Electromechanical Arts~

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              • #8
                These are pretty slick http://bell-everman.com/products/act...t-power-module. Basically a servo motor built around a ball nut.


                bob

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                • #9
                  Ball screw the screw turns, a “jack" screw the nut turns from what I was taught. Some of the flight controls I worked on had both. Ball screws were used for normal highspeed movement, jack screws slower but more torque. Spring applied brakes and hyd motors. The jack screw is a turned by a gear or by a gear/chain set up. The largest one I worked on was hollow and pressurized. If the PSI dropped, it was time to start looking for something wrong. If I remember the numbers the screw was 6” in diameter and 11 foot tall.

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                  • #10
                    Originally posted by rowbare View Post
                    These are pretty slick http://bell-everman.com/products/act...t-power-module. Basically a servo motor built around a ball nut.


                    bob

                    Some of the current laser cutters have what is called Spindle drive. This is the same idea. Btstronic fir them on thier 7.5w laser.
                    It's basically a fixed screw in this case about 4 to 5" diameter held between two vey heavy fabrications and the screw is under tension of something like 40/50 tonnes to keep it straight and prevent any whipping.

                    The nut is powered by a massive but shallow pancake servo motor about 7.5Kw in power. Pancake design is chosen so they can keep travel to a maximum. That one in the quote by Bob above has very limited travel because of the motor nut design. In the Bystronic one the nut is the rotor of the motor.

                    Because laser cutting is non contact they can run these at high Kw power and insane speeds, they can cut at 25m/ min on thin materials which is about 984.251968503937 in/min [ approx ]
                    .

                    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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                    • #11
                      There is a reason 99.9 % of plasma table use rack and gears. Plasma is a dirty environment and all that dust and smoke is pulverized metal that is abrasive and conductive which is not condusive with ball screws. Plasma tables can be capable of holding tight tolerances but the process does not need the precision of ball screws. Sure you can build one that way - but I would not. I can use the scriber on my table to mark a center punch mark on a plate, move the gantry all around and it will still come back and drop the scribe point in the same exact mark it made previously with racks and gears.
                      Plain ol Bill

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