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Rotary 4th axis design

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  • Rotary 4th axis design

    This guy took a Mini-lathe spindle and used a belt reduction to turn it into a 4th axis.
    I question his design with the belt. I would think the belt would flex too much for milling operations. However he did give me one great idea. I think I will take my spare Taig spindle and connect a small harmonic drive to the back of it which is driven by a stepper for a miniature 4th axis on my Taig mill.

  • #2
    Had mixed emotions watching that. The moment he held the headstock so you could see inside- yikes, plastic gears- and they don't even look good for plastic. But not to distract from an interesting conversion.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #3
      Nice, his mill runs faster then some Fanuc small mill/drills.
      https://www.youtube.com/watch?v=SeWBK_u9siQ

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      • #4
        I have a direct drive Yaskawa servo motor I am planning on using to make a 4th from: https://youtu.be/fNexwHbrKbs

        I think it will be stiff enough for light machining, 10Nm continuous torque and 30Nm peak. It has more poles so it runs slower and has more torque and has a million count encoder.

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        • #5
          Originally posted by darryl View Post
          and they don't even look good for plastic
          Trust me, they arent...

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          • #6
            I built a 4th using a 9x20 lathe head. I did it because I only paid $250 for the lathe a few years back and didn't really have a use for it. The deciding factor was I had the 3-jaw, 4-jaw, faceplate, tail stock and a set of morse 2 collets all of which would be real handy with a 4th axis. My mill runs off linuxcnc for control using servo motors so I used a brushless servo motor with a timing belt drive to the spindle (4:1 as I recall). The belt drive was mostly to make the 4th more compact which allowed mounting the servo beside the spindle head. It worked out very well, possibly there is some belt flex but only if taking pretty heavy cuts, nothing really objectionable. The slight flex might also be that nature of a servo drive, the larger the following error the harder the motor/drive fights to correct it, so only a few encoder counts can make it just a bit springy, again, nothing objectionable, A stepper might actually work better in this application because it develops it highest holding torque at zero rpm's to very slow rpm's, it "locks up" more solid.

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            • #7
              I had one of the 7x12 lathes that use that headstock and I never had any issues with the plastic gears and I actually did some decently heavy turning with it, 4' 4140 to make a collet chuck for it, I had to get creative on how I attached it to the spindle. There were a few reports of gears breaking but it is pretty rare.

              The belt should be fine, they really dont have any stretch, If a timing belt starts stretching the fibers inside are breaking.

              My 10ee is servo for the spindle and that thing is solid, it does have an overly large motor (5kw) though, and decently high encoder count, I think 16384 so that helps.

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              • #8
                you guys helped me with plastic change gears on my 9" lathe, they hold up surprisingly well

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                • #9
                  My Fourth Axis is a Harbor Freight lathe stripped of everything but the head stock and tail stock. The spindle is driven by a timing belt, but is HTD 8mm pitch x 30mm wide and the sprocket is 60 tooth or about 10" in diameter. So the belt drive is quite stiff. The stepper motor is 980in/oz holding torque and driven by microstepping drive fed by 68vdc so the top speed is pretty good too, 500 rpm or so.

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                  • #10
                    What about some sort of a solenoid powered lock for heavy cuts with no rotation?
                    21" Royersford Excelsior CamelBack Drillpress Restoration
                    1943 Sidney 16x54 Lathe Restoration

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                    • #11
                      You wouldn't want that, look at a braking mechanism. electromagnetic, big commercial systems use hydraulic brakes to lock and conical pins at hard index locations.

                      Though a lot of generic electromagnetic brakes have the brake sitting on a spline so it can self adjust for wear, these often have some lash that would have to be worked out.

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                      • #12
                        Originally posted by The Metal Butcher View Post
                        What about some sort of a solenoid powered lock for heavy cuts with no rotation?
                        I've also been thinking about something like this, I guess really more of a digital indexer than 4th axis. As I have never seen or used a dividing that you didn't want to lock to make a cut, and with a lock you could use a much lighter drive train, I think its something that needs to included. Original idea was a disk brake, but am now thinking a split cotter tighted with a stepper might be cheap and easy. Split cotters have a fair bit of holding power, you could just set the stepper to advance so many steps, maybe even gear it down.

                        Personally I wouldn't made such a device with such a headstock, as I think the bolt on tooling would spindle would just be too frustrating. I'll either go 10 or 12mm to use existing tool, or maybe something like an ER25
                        in Toronto Ontario - where are you?

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                        • #13
                          Great job!! Not only can you fly a 747 but you can do this! We have some great people here on this board.

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                          • #14
                            Originally posted by Tungsten dipper View Post
                            Great job!! Not only can you fly a 747 but you can do this! We have some great people here on this board.
                            Technically, it's a 777 now, no more 747.
                            I wish I was home to play with these ideas, still in training on the 777.
                            The stepper driver in my cabinet has a spare port for a rotary axis. The spare Taig spindle is ER16. Perhaps I can get away with a belt drive for it... Harmonic drives are pretty neat though!

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