Thread: Leadscrew

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  1. #21
    For the turntable, I have found a complete direct drive "pro" turntable it has 5kg of torque which beats anything else available these days. Technics 1210s have been used successfully but at 1.5kg torque wow/flutter does manifest itself. What i have found should suit my purpose, but if i get to the stage of building my own i would look at using an precision lapped thrust bearing, oil bearings/drives have been used but they prob. too complicated or expensive for my purpose. The use of heavy platter reduces wow/flutter too. Whatever - i will be looking at pro turntable designs old and new, i don't feel it's necessary at this time to personally further development in this field! Most of the lathe parts should be off the shelf.

    Cutting heads - There are a couple available, i intend to build my own, there have been successfull heads made from a pair of tweeter (speakers)

    On the feedscrew again, i would prefer as many off the shelf parts as possible including the nut, i am now going to look again at what's available

    Neumann lathes start at around 10 000 these days for a VMS66 upto maybe 45 000 for a VMS80 (DMM lathe) setup. It's extremely unlikely that any studio would put a lathe in the skip, they're too rare and valuable. An pro audio broker would bite your hand off, the last few i've seen were sold rapidly!

    I haven't seen a lathe driven by gravity! But i know one of the first portable disc recorders (during WW2) had a turntable driven by a wound spring like a gramaphone, and a battery operated electrical cutting head, they were used in the trenches!

  2. #22
    Quote Originally Posted by BillTodd View Post
    I would use one or perhaps two stages of plain rubber belt speed reduction between the motor and lead-screw (stepper motors, gears and notch belts are noisy)
    So if i was to use an 0.050" lead leadscrew thats 1.27mm per rev so my 2mm feedrate will require 1.575 revs of the leadscrew per minute. (correct me if i'm wrong!)
    What would be a sensible gearing ratio? I'm thinking a single stage might be better, to reduce losses/ slip ?? What kind of speed would a servo motor prefer to run at whilst giving me some leeway? This is all new to me, does anyone have links regarding servo motors or tacho generators?
    If i was to have 2 or more stages of speed reduction surely i would be better to have the tacho generator driven by the leadscrew itself? Is this ever done?
    Many Thanks

  3. If i was to have 2 or more stages of speed reduction surely i would be better to have the tacho generator driven by the leadscrew itself? Is this ever done?
    It is possible (I have made a speed controller with a feedback from the a 90:1 gearbox) but, because of backlash in the gearbox, or flex in the belts in your case, motor speed can be hard to stabilise (this didn't really matter in the above example).

    It is far better to control the motor speed with a close coupled tacho. There are many different type of tacho feedback; some have DC generator coupled to the motor shaft, most PC fans have a magnetic sensor that's part of the brush-less motor driver but, most modern servo motors will have an optical encoder wheel.

    There are plenty of tacho feedback motor speed controller chips and designs around.

    Old cassette machine motors used to use a centrifugal switch that modulated the windings directly, which looked crude, but worked incredibly well.

    You might be able to get away with a simple back emf speed (sometimes referred to negative resistance generator) since your motor load will not vary very much. All you need is a stability (low wow and flutter) , the actual speed is not critical

    to be continued...

  4. #24
    So i've been looking at leadscrews and found a sales pdf with some useful notes about leadscrew design considerations

    But i seek more information........

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