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  • Building a CNC Indexer

    I have thought about building a CNC indexer for a couple of years. I would like to see some home built CNC indexers for some ideas if anyone has some.

    I plan to use a 8 pitch, 30 tooth worm gear and single lead worm.

    How will I set up the stepper motor? I have setup several steppers on milling machines for "step per distance". But what about rotary motion like an indexer. Is there a setup for "Step per degree"?
    Thanks,
    Jim
    So much to learn, so little time

  • #2
    Thirty tooth worm gear means 12 degrees per tooth. Single start worm means one rotation of worm per 12 degrees.

    First, this is somewhat coarse. Most indexers use more teeth on the worm gear for a finer division: 90 and 40 teeth are more common. But you can use 30.

    OK, now the stepper. One of the most common number of steps that is available on steppers is 200. That's 200 steps per revolution. If you connect a 200 step stepper directly to the worm you will have 30 x 200 or 6000 steps resolution per 360 degrees or 16.66666... steps per degree. This is not an even number and I would not recommend it.

    From this point one possibility would be to add some additional gears between the stepper and the worm. One possibility would be a 12:72 step down ratio: a 20 tooth gear on the stepper and a 72 tooth on the worm. This would increase the number of steps per degree to 60 or one minute per step. If I was stuck with the 30 tooth worm, I would go for this.

    A better solution would be to get a worm gear with a multiple of 36 teeth and again, 72 comes to mind. With a 72 tooth worm gear, single start worm, and 200 step stepper connected directly to the worm, you would have a total of 14400 steps per revolution or 40 steps per degree. That's 1.5 minutes per step: not ideal, but much better than 16.66666...

    Many other numbers are possible. If you could find a stepper with 90 or 180 steps instead of 200, you could use your 30 tooth worm gear for 2700 or 5400 steps per 360 degrees. These are somewhat coarse numbers, but the 5400 figure would provide 15 steps per degree or 4 minutes per step. If you want finer resolution then the additional gearing would be simpler: a 1:4 reduction would provide one minute per step.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

    Comment


    • #3
      Okay, is there any reason not to use a store bought rotary table and attach the stepper motor to that? It has been done numerous times succesfully and is an easy way of getting CNC indexing.
      Amount of experience is in direct proportion to the value of broken equipment.

      Comment


      • #4
        Take a look here.

        http://www.divisionmaster.com/divisionmaster.html

        Go to the examples of use page for some various ideas.
        .

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



        Comment


        • #5
          First, I'm not married to the 30 tooth worm gear. I found the worm gear and worm on Ebay. When I recieved them I realized they were a tad coarse. My first choice would have been a 40 tooth wormgear since a standard rotary table used 40:1.

          The physical size of the 30 tooth wormgear was the attraction. Not sure where I will find a 36 tooth wormgear.

          I still do not understand how to control the indexer. Say I want to index
          30 degrees. I'm not aware of a G-code instruction for angles. Will I need to convert a linear distance to a 30 degree angle? I will be using MachIII.

          I'm aware I could make a CNC indexer from a standard rotary table. I want to make one from scratch that can be mounted verticle or horizontal and be of a small physical size. The CNC indexer may end up being a toolchanger for my Denford lathe project.

          Thanks for the replys,
          Jim
          So much to learn, so little time

          Comment


          • #6
            Nosing around in Mach3 Mill & Turn I see that if you click the Config/General Config pull-down you will see "Angular Properties" and make sure the A (or B or C) are set to Angular and that axis should respond to rotary G-Code commands in degrees.

            I haven't done this myself or slept in a Holiday Inn Express but I'm sure Mach will handle it easily. There's a ton of people doing 4th axis work and they sure make it look easy.

            ORAC toolchanger....awesome; you got my attention with that one!
            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
              Originally posted by outback

              I still do not understand how to control the indexer. Say I want to index
              30 degrees. I'm not aware of a G-code instruction for angles. Will I need to convert a linear distance to a 30 degree angle? I will be using MachIII.


              Thanks for the replys,
              Jim
              Jim.
              You work out how many steps are needed to move 1 degree and enter that under the A axis in Mach as steps per.
              As Milton says you change the A axis to Angular in General config.

              In the case of your 30 tooth wheel that's 200 motor steps x 10? microsteps = 2000

              Then 360 / 30 = 12

              So steps per = 2000 / 12 = 166.66667

              Then when you command A30.00 it will move 30 degrees.

              Simples ?
              .

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



              Comment


              • #8
                Fantastic!!! that is what I was looking for. I got it now.

                Thanks again for all the help.
                Jim
                So much to learn, so little time

                Comment


                • #9
                  wI'm looking a building a CNC indexer using timing belts and pulleys.

                  Base on the math you guys supplied us with it looks like a 36 tooth pulley and a 16 tooth pulleys would be an ideal combination. I did some hit & miss calculations. The 36 tooth and 16 tooth would be a 2.25:1 ratio

                  360* / 2.25 = 160

                  2000/160 = 12.5 micro steps per degree???? Do I have this right? Is the 2.25:1 a nice even ratio to use with stepper motors?

                  philbur: This is for a slightly different project from the indexer. I want to setup my CNC lathe to cut threads by powering the spindle with a stepper motor. Since I'll need to purchase the pulleys anyway I wanted to buy a ratio that would work well with the stepper motor. Maybe I'm over complicating the project. Maybe any ratio will work as well as any other. The 2.25 : 1 ratio breaks down to 12.5 even steps intead of 16 and 2/3 step or 16 and 1/3 step.

                  Maybe I should rephrase the question: If you had to buy any two pulleys to use with a stepper motor, which ones would you buy?
                  Jim
                  Last edited by outback; 05-11-2012, 06:35 PM.
                  So much to learn, so little time

                  Comment


                  • #10
                    If you need compact size then using pulleys is counter productive. You need to identify the necessary resolution before you include pulleys. High resolution means slow and large. Lower resolution means compact and fast. You should seriously look at direct drive with a driver/motor of 200 steps per rev and 10 micro-steps per step and see if this resolution is acceptable.

                    with a 30:1 worm that's 0.0333 of a revolution per turn of the worm. So on a 4" diameter that a linear movement of 0.4189 inches per turn of the worm. So with 200 steps and 10 micro-steps that gives a resolution of 0.000209" at 2" from ther center of rotation. Good enough for most work I think. You might want to double check the maths.

                    Phil

                    Comment


                    • #11
                      Originally posted by outback
                      I'm looking a building a CNC indexer using timing belts and pulleys.
                      I wouldn't do it. Belts drives by themselves are "mushy". My final would always be a gearbox. Ultimately a harmonic drive. Zero backlash on those guys.

                      Comment


                      • #12
                        I just read a LONG thread on CNCZone or Mach3 forum (can't find it now.) where dude makes a CNC combo servo drive/indexer that uses an air cyl/caliper/brake disc to positively lock down the spindle once it has reached the commanded position from Mach3. No mushy/backlashey stuff allowed!

                        I can't remember what he calls them but he does make them for sale. Someone will remember I'm sure.
                        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
                          harmonic drive

                          I see a bunch of harmonic drives on Ebay. How do these things work. Is there a particular harminic drive I should look for? How do they hold position?

                          Thanks,
                          Jim
                          So much to learn, so little time

                          Comment


                          • #14
                            Originally posted by John Stevenson
                            Take a look here.

                            http://www.divisionmaster.com/divisionmaster.html

                            Go to the examples of use page for some various ideas.
                            +1

                            Read:

                            http://divisionmaster.co.uk/divisionmaster-manual.pdf

                            Comment


                            • #15
                              Finally found the awesome indexer thread I mentioned above. It's on the Mach3 forum and is amazing and well worth wading through all 3 bazillion posts. http://www.machsupport.com/forum/ind...c,11422.0.html
                              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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