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Variable Speed on lathe lead screw

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  • #31
    Did it to my 9x20, not because it didn't have a QCGB, but because to set the machine up for the most commonly used threads meant the feed rates were then way too fast. Changing back and forth means swapping idler gears every time both ways. That sucks, so I bought a cheap mill power feed off ebay and extended the leadscrew a bit.
    You may only view thumbnails in this gallery. This gallery has 1 photos.
    I just need one more tool,just one!

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    • #32
      The treadmill motor would be overkill, power wise, but that doesn't mean you can't use it. You have a couple ways of working it out- find a worm gearbox and calculate your feed rate using a motor rpm of say 3000- about 60% or so of what it would spin with full voltage. If that gets you within the ballpark for maximum desired feed rate, you're 3/4 of the way there. If that doesn't get you close enough, then you need a different ratio gearbox. You might be able to use what you might have on hand, which would mean that you then define what motor voltages represent the maximum and minimum desired feed rates. I think you'll always find that the torque from the motor would be enough, especially since the lead screw represents a step down ratio already. The other way of working it out would be simple math, where the outcome defines what gearbox ratio you need- and then you try to find a gear box with that ratio. You're unlikely to find that perfect match, but if your math says you can get your slowest desired feed rate with no less than say 300 rpm at the motor, you could call that good. I seriously doubt you'd run out of torque.

      If you used a smaller type of motor, it becomes more important that you close in on an optimum ratio.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #33
        Originally posted by wierdscience View Post
        bought a cheap mill power feed off ebay and extended the leadscrew a bit.
        Pretty neat idea. Are you running it with a controller, or are you setting the speeds manually?

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        • #34
          Originally posted by wierdscience View Post
          Did it to my 9x20, not because it didn't have a QCGB, but because to set the machine up for the most commonly used threads meant the feed rates were then way too fast. Changing back and forth means swapping idler gears every time both ways. That sucks, so I bought a cheap mill power feed off ebay and extended the leadscrew a bit.
          How did you tie the spindle drive to the lead screw drive?

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          • #35
            Originally posted by wierdscience View Post
            Did it to my 9x20, not because it didn't have a QCGB, but because to set the machine up for the most commonly used threads meant the feed rates were then way too fast. Changing back and forth means swapping idler gears every time both ways. That sucks, so I bought a cheap mill power feed off ebay and extended the leadscrew a bit.
            This is the best / easy option. All in one box and $ 150.00 on Ebay. Just have to link it up. Have the same idea, you just beat me to the lathe!! Nice work.

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            • #36
              I put a 80W maxon DCX with a zero backlash three stage planetary gear head on my 7x10 lead screw before I bought my new lathe. You will want very low speed, medium-low torque.

              A 2.25hp treadmill motor will slam your carriage off the end of the bed and put your tailstock in the next zip code without significant gear reduction. Even with reduction, you'll be shearing drive pins or half nut teeth regularly. Far too big and fast of a motor clr this application.

              I wholeheartedly endorse a motor driven carriage, though. Having used the vari-servo hardinge units before, they're great. It would require very good synchronization between the leadscrew and spindle motors to cut accurate pitch threads, though. I plan on doing it and have the encoder and a controller that is precise enough, but that's like 40 projects deep in the stack today...
              -paul

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              • #37
                Originally posted by Bented View Post

                How did you tie the spindle drive to the lead screw drive?
                Extend the leadscrew and hang off the right end I'd guess. If your motor doesn't have crazy high inertia, you can just unplug it and use the qc gears as usual.

                If it is gear reduced, you might need to have a coupler that disconnects so you're not always backpowering the passive motor which is just dead weight on the shaft.
                -paul

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                • #38
                  Originally posted by thin-woodsman View Post

                  Pretty neat idea. Are you running it with a controller, or are you setting the speeds manually?
                  It has a DC controller built in to the power feed. Something like 0-140 rpm output, plus the lathe carriage has a reduction built in also.
                  I just need one more tool,just one!

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                  • #39
                    Originally posted by Bented View Post

                    How did you tie the spindle drive to the lead screw drive?
                    I just made a shaft to mimic the end of a Bridgeport mill table leadscrew and then sleeved and pinned it to the leadscrew. That way the power feed bearing supports the end of the leadscrew.
                    I just need one more tool,just one!

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                    • #40
                      Originally posted by psomero View Post

                      Extend the leadscrew and hang off the right end I'd guess. If your motor doesn't have crazy high inertia, you can just unplug it and use the qc gears as usual.

                      If it is gear reduced, you might need to have a coupler that disconnects so you're not always backpowering the passive motor which is just dead weight on the shaft.
                      The power feed has fwd/rev/neutral built in via gearing and a jaw coupling. When in neutral it just freewheels, no drag at all.
                      I just need one more tool,just one!

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                      • #41


                        Originally posted by wierdscience View Post
                        Did it to my 9x20, not because it didn't have a QCGB, but because to set the machine up for the most commonly used threads meant the feed rates were then way too fast. Changing back and forth means swapping idler gears every time both ways. That sucks, so I bought a cheap mill power feed off ebay and extended the leadscrew a bit.
                        Thanks for that post. I've been wanting to do that modification to my South Bend for a few years. Just haven't got around to it. Are there any drawbacks to this setup? Anything you would change if you could?

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                        • #42
                          Originally posted by Stu View Post
                          This is on my 12" Atlas lathe, windshield wiper motor, with a PWM variable speed controller and 12v power supply. It has plenty of power and can easily be disconnected for threading. Click image for larger version

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                          Do you have any specifics about that motor, like what vehicle it came off of. What about the output speed when it sees its rated voltage? Amperage? And the sprocket tooth count?

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                          • #43
                            Originally posted by tom_d View Post



                            Thanks for that post. I've been wanting to do that modification to my South Bend for a few years. Just haven't got around to it. Are there any drawbacks to this setup? Anything you would change if you could?
                            Nothing that I can think of, it's a pretty good improvement over what it was before.
                            I just need one more tool,just one!

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                            • #44
                              [QUOTE=tom_d;n1941116]

                              Do you have any specifics about that motor, like what vehicle it came off of. What about the output speed when it sees its rated voltage? Amperage? And the sprocket tooth count?[/QUOTE

                              It's 12V, drive 20t - driven 32t, I don't know what kind of vehicle it came from or output speed. It is a two speed unit (the switch you see controls that). I just sorta winged it with the ratios but either speed works well with my setup.


                              I just remembered I made a chart when I first made it. 1 through 7 are just arbitrary numbers I put on the pot dial. Anyway I highly recommend it as I can change feed anytime during the cut (real nice for power facing) and It is quiet in that the QCGB is not engaged along with the change gears. When I single point it reminds me how loud the gear driven drivetrain is.
                              feed inch per min
                              low hi
                              1 0.321 0.23
                              2 1.271 1.255
                              3 2.027 2.5
                              4 2.636 3.045
                              5 3.048 4.515
                              6 3.44 5.154
                              7 3.547 5.428
                              Last edited by Stu; 05-03-2021, 03:43 PM.

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                              • #45
                                Clough42 on the tube has an excellent series of videos on DIYing an electronic lead screw.

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