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Lathe project -- About DC drives

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  • #16
    A 4 inch cube- a modern brushless rc motor of that volume can be capable of more than 10 hp. Because of that, it can be run at a fairly slow speed and still have lots of torque, even though the continuous power would be down to 1 horse or so. If you figure out the voltage required to run at say 300 rpm, then factor in the continuous current it can take without overheating- this could be the motor that would drive your lathe to any speed you'd want with only one initial speed reduction stage. Make this a belt drive to the spindle, with a ratio of about 2.4 to 1. Since typical motors like this are rated at around 6500 rpm, you can easily get a spindle speed of 2000 rpm, and you can go down to perhaps 75 rpm and still get more than 1/2 horse out. In fact, because you wouldn't normally need much power at high rpms, you'd want a control that would alter the current limit with rpm. It would have a maximum current (not interrupt-able) and would automatically lower that setting as the motor rpm increases. That way you neither need nor want to supply more than a certain amount of power to the motor. No need to have 8 kilowatts of available power to satisfy the motors maximum thirst- something around 1200 watts would give a fairly impressive performance.

    This is a good concept I think. The right power supply and controller would be key- and of course none of this is going to be cheap.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #17
      Originally posted by elf View Post
      Not including the pulley and shaft, the motor is approximately a 4" cube. I'd have to uninstall it to get the weight. It's been running on my rose engine for over a year. There are quite a few accounts of internet posters using them on lathes.
      Wow, that's a lot smaller than I thought! That would be perfect for a project on my bucket list: upgrading my little Denford/Sherline CNC toy mill. If I get around to doing the Moglice leadscrew nut backlash fix, a motor upgrade would be icing on the cake.
      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

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      • #18
        You are describing a typical CNC type drive.

        And don't be so sure about lower power use at speed. Running carbide, you might want lots of high speed power to use the carbide in the range where you get the good finish. There is not so much need for the low end speeds when the power and cutter exist to get the job done at higher speed, and there is a control able to handle things like high speed threading, etc. Maybe for use with powered tooling the slower speeds are needed, but there spindle power is less important and positioning capability is more so.
        3751 6193 2700 3517

        Keep eye on ball.
        Hashim Khan

        If you look closely at a digital signal, you find out it is really analog......

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        • #19
          Originally posted by J Tiers View Post
          You are describing a typical CNC type drive.

          And don't be so sure about lower power use at speed. Running carbide, you might want lots of high speed power to use the carbide in the range where you get the good finish. There is not so much need for the low end speeds when the power and cutter exist to get the job done at higher speed, and there is a control able to handle things like high speed threading, etc. Maybe for use with powered tooling the slower speeds are needed, but there spindle power is less important and positioning capability is more so.
          Good torque at low speed is important on a cnc too. Not everything is done with carbide ! Drilling is obviously a very common operation and HSS bits are the norm. For example, drilling a 1/2" or bigger starter hole for a boring operation that follows. Reaming is another common operation where good low speed performance is required. If the control is equipped, rigid tapping is yet another case.

          My factory cnc lathe goes down to 35rpm !

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          • #20
            Ok. About a month later now. I fixed several other problems on the lathe while rebuilding it to install a 3/4 hp 3 phase motor (with a type 48 frame) and a VFD. Initial testing looks positive. I'll know more once I get the right belt as I found out why I don't want motor mounts on this machine. (They compress and cause the belt to jump.) The belts come in tomorrow.

            I wish I would have known sooner that a VFD and motor are a lot cheaper than a DC motor drive solution.

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            • #21
              Originally posted by Tundra Twin Track View Post
              I think they would work great,I have a older style brush type still in box that can be reversed easily.I thought someone posted about the brushless style was a little complicated to reverse direction.
              BLDC (brushless) have sensors for commutation and for this you need a drive that is capable of responding to these, unlike the DC brushed where the commutation can be reversed easily.
              RC motors run extremely hot and can only run for short periods, or have some kind of cooling in place.
              Max.

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              • #22
                Nah, what you need is a giant DC drive with a seperate field winding, a 7.5hp 3phase AC motor that drives a DC generator, and another generator for the field winding, have a bunch of relays and giant rheostats that control both the voltage to the armature and field windings to the giant DC drive motor.

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                • #23
                  Originally posted by RB211 View Post
                  Nah, what you need is a giant DC drive with a seperate field winding, a 7.5hp 3phase AC motor that drives a DC generator, and another generator for the field winding, have a bunch of relays and giant rheostats that control both the voltage to the armature and field windings to the giant DC drive motor.
                  Sounds like an old motor generator Monarch. Wonder why?

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                  • #24
                    Originally posted by RB211 View Post
                    Nah, what you need is a giant DC drive with a seperate field winding, a 7.5hp 3phase AC motor that drives a DC generator, and another generator for the field winding, have a bunch of relays and giant rheostats that control both the voltage to the armature and field windings to the giant DC drive motor.
                    Sounds like the drive on the CPS6-B radar in 1951-3 . :-)
                    ...lew...

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                    • #25
                      It's been a while since I posted here. I first got a Weg VFD and only long afterwards realized I couldn't use it for threading as the 110 VAC model wouldn't support a braking resistor. So I ended up picking up another VFD from Automation Direct and it works perfectly.

                      Also shown is the Centroid Acorn and Geckdrive stepper motor drivers that control the lathe....


                      Click image for larger version

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                      • #26
                        Here is a picture of the front of the machine. I just added the parts catcher over the last week. The parts catcher is on the 3rd revision of the mechanical parts and 2nd revision of the circuit board.

                        Click image for larger version

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                        • #27
                          that's super cool! I love the collet closer mechanism too, very elegant. Out of curiosity, why'd you go for round linear rails rather than the square ones (can't recall their proper name right now)?

                          Comment


                          • #28
                            Originally posted by Gary Paine View Post
                            On a small machine with proportionately sized chips, a 1/4 horsepower would work. But unless you are using gearing to change motor speed to work speed, you will only see a fraction of the 1/4 horsepower. 1/2 speed is 1/2 horsepower, for example.
                            That's not true. When you change gearing both torque and RPM change, but horsepower does not (discounting friction, etc. losses).

                            Horsepower = Torque(LB-Feet) X RPM/5252

                            Mike

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                            • #29
                              I originally went for the round rails as they are cheap and I'd seen that other people used them on small lathes. They do work as I just ran off a couple parts for a customer on this machine. However, testing shows that the rails are an aspect of this machine that could be improved. Coincidentally, I am looking at the dimensions today to see exactly what needs changed to go from the current SB12S rails to HR25 rails. I believe that the HR25 rails would be a nice improvement but will add at least $500 to the materials cost of the machine. (I also have an idea for improving the X Axis plate as well.) Either way, the current machine, that I refer to as prototype 3 or SN1, is completely functional.

                              The collet closer works very well and, with a mini bar puller, would allow this machine to make a bunch of parts without user intervention. (The goal of this machine is to have many features only seen on a $100k machine but in a small, portable, size.) I also have buttons programmed on the Centroid Acorn screen to set up and operate the collet closer.

                              The 3rd prototype of the parts chute was finished this weekend as well. This parts chute has the second revision of the circuit board and there is a plan to make one more revision to the circuit board yet.

                              While drip coolant could easily be added to this machine, the next version of this machine will also have better accommodations for flood coolant as it's a plan to make it possible. (I don't know how practical flood coolant would be on a machine this size but it's a goal to support it.)

                              The second prototype was also fully functional but had the left-over wiring mess of the first prototype and didn't have the ability to add other options such as a parts chute or tool setter. Below is a video of the second prototype in action.



                              Also, I got really tired of taking forever to setup the tooling and added a tool setter to the machine as well. The video below shows it in action with the old interface.



                              Here are some pictures of the prototype parts chute. I still need to make a cover for the electronics.

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                              Last edited by briuz; 01-13-2021, 02:39 PM.

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                              • #30
                                Originally posted by darryl View Post
                                A 4 inch cube- a modern brushless rc motor of that volume can be capable of more than 10 hp. Because of that, it can be run at a fairly slow speed and still have lots of torque, even though the continuous power would be down to 1 horse or so. If you figure out the voltage required to run at say 300 rpm, then factor in the continuous current it can take without overheating- this could be the motor that would drive your lathe to any speed you'd want with only one initial speed reduction stage. Make this a belt drive to the spindle, with a ratio of about 2.4 to 1. Since typical motors like this are rated at around 6500 rpm, you can easily get a spindle speed of 2000 rpm, and you can go down to perhaps 75 rpm and still get more than 1/2 horse out. In fact, because you wouldn't normally need much power at high rpms, you'd want a control that would alter the current limit with rpm. It would have a maximum current (not interrupt-able) and would automatically lower that setting as the motor rpm increases. That way you neither need nor want to supply more than a certain amount of power to the motor. No need to have 8 kilowatts of available power to satisfy the motors maximum thirst- something around 1200 watts would give a fairly impressive performance.

                                This is a good concept I think. The right power supply and controller would be key- and of course none of this is going to be cheap.
                                do you have a particular one in mind? such motors tend to be $500 w /o drive.

                                Comment

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