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Motor required for spin casting, 10-100rpm

  1. Feb 2, 2012 #1
    I'm attempting to spin cast a parabola, and aim to hang my spinning vessel directly from the driveshaft of a motor to get it to spin as cleanly as possible.
    Are there any PWM (12v ideally) 10-100rpm dc motors than could support 10kg thrust load?
    I would strongly prefer to not use gear systems, but I suspect it may be ineviatable.
    Also, could AC motors be used?

    Thank you, Tom
     
  2. jcsd
  3. Feb 3, 2012 #2

    jim hardy

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    my old standby - a Ford windshield wiper motor.

    available from junkyard or roadside derelict.
     
  4. Feb 4, 2012 #3
    possible to make a pwm controller? forgive my ignorance
     
  5. Feb 4, 2012 #4

    jim hardy

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    We're all ignorant just on different subjects. What's spin casting ?


    """possible to make a pwm controller?""



    that is very do-able, though i never have..
    here's a kit
    https://www.amazon.com/Motor-Speed-Controller-PWM-Kit/dp/B006L7SWJ2
    i've used Ramsey kits before and been pleased.

    and a DIY hobby article
    http://www.dprg.org/tutorials/2005-11a/index.html

    to experiment,
    A car battery charger will run it, and on 6 volt setting it runs slow. Put a headlamp in series and it'll slow down ..

    if you use a battery charger to power the PWM control add a huge capacitor in parallel.
     
    Last edited by a moderator: May 5, 2017
  6. Feb 5, 2012 #5
    Can a windshield wiper motor be changed from ten to a hundred rpm? I'm guessing 5A PWM will be good enough for a wiper motor?

    I want to get form a mold of a parabolic shape, and spinning liquids form parabolas. So, spinning plaster should set into a parabola. Tests look good, I just need to get it all smooth.
     
  7. Feb 5, 2012 #6

    jim hardy

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    i used them for rotating fishing poles while the clear coat set, about four RPM. I had then a 4 amp battery charger and Variac adjustable transformer.

    What's highest speed on your car windshield wiper? Time a few sweeps - seems to me it's at least a sweep per second.

    I'll check my car today...

    sounds like a neat project - what's end product? (if not a trade secret)
     
  8. Feb 5, 2012 #7
    Reflector for a spotlight. Still working out how to apply reflective materials.
     
  9. Feb 5, 2012 #8

    jim hardy

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    sounds neat.

    might ask on the math forum whether you'll get a parabola or a catenary, though....
    just to be safe.

    probably you already know.
     
  10. Feb 5, 2012 #9
    It will make a parabola. I know that at least, ha.
    How does the motor perform at 4rpm? I didn't know wiper motors could do that.
     
  11. Feb 5, 2012 #10

    jim hardy

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    well - i was just trying for real slow rotation, which it did very well.

    as i recall it had good torque and that was with low voltage applied. all i had for a supply was my battery charger and Variac - this was like 1972.

    They're just a DC motor with worm gear speed reduction.. If it has a shunt field (i think so) you can set field field current and apply PWM to armature for very stable speed and better torque.
     
  12. Feb 6, 2012 #11
    Im afraid I dont understand what you mean. Two inputs into the motor, one going through the PWM?
    Also, is there some way to check if it is shunt field? I'll probably get two, so I can open one.
     
  13. Feb 6, 2012 #12

    jim hardy

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  14. Feb 6, 2012 #13

    sophiecentaur

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    If this process lasts only a few minutes then I'd be inclined to think in terms of a variable speed electric drill, or even a pillar drill with the pulleys set to minimum speed. Perhaps a flexible / bungee coupling could help - or a further belt drive if the speed is too high. Either way, the messy electric / mechanical bit would be straightforward and you could get on with the interesting casting bit.
    What material are you going to be casting?
     
  15. Feb 6, 2012 #14

    jim hardy

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    that's a good thought Sophie.

    i just assumed he was making a sorta permanent shop fixture.

    anyhow - reason i logged back on is i just looked under hood of my 83 Ranger.
    Windshield wiper motor has its earth(ground) wire of course,
    and going into motor are two small black and one large white wire.
    I expect large is armature and small ones are field. Field is brought out with both leads separate so motor can be reversed to "Park" the wipers, and control speed.

    Last one i opened (decades ago) was a three brush design - has better speed regulation.
     
  16. Feb 7, 2012 #15
    Aim is to cast plaster of paris as bulk filler. if it doesnt set evenly then maybe a thin layer of epoxy resin on top. I can only assume this will be good enough.
    I considered a drill but I will be using this more than once, and also plan to make a 2m wide rotating version where a decent motor will come in handy.
    As regards the 'messy electric/mechanical bit' its gonna have to be very messy anyway, Im going to have to make some sorta individual screw adjustment system for each of the container support struts for nice spinnage.

    I assembled a 20A PWM kit today and got a wiper motor. The motor has red, black, yellow, brown and green wires. With R/B to +/-, touching yellow and brown makes the motor run for about 2 seconds, stop, then go for another 2 seconds in the same direction. No other combos seem to do anything, however I just touched y/b/g and its not working anymore so thats something, ha.
    Maybe I might need an older-style motor?
     
  17. Feb 7, 2012 #16

    sophiecentaur

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    How about a gramophone turntable? It may take some time to get up to speed but it will be steady and you can always tinker with the capstan diameter. 78rpm could be just what you wanted!

    Just read the "2 metre"bit - so forget it haha.

    If you plan to go so big then surely your main problem is with the turntable / suspension. A real crude solution could be a potter's wheel mechanism. I think the speed is controlled by sliding a belt on a tapered shaft.

    I must say, I love the idea of producing a large parabaloid by spinning. You may find bubbles in the plaster are a problem because they will affect the density of the liquid mix at different levels and that would spoil the parabola shape, I think. It all depends on the accuracy you need.
     
    Last edited: Feb 7, 2012
  18. Feb 7, 2012 #17
    Bubbles are a big problem, they accumulate right at the very bottom, where it has to be the most precise. Im sure theres something better than paris plaster.

    I built a potters wheel with a pulley attatched to a high rpm sewing machine motor. There were all sorts of problems with levelling, the thing has to spin invisibly to get a smooth surface. Thats why Im hoping hanging it will make this easier.
    With hanging, tho, the vessel has to be constructed evenly, else I suspect it will spin with a wobble. I plan to get a potter to make me a container that is roughly a paraboloid shape (maybe I dont need the plaster, and could use resin right away . .?), as 'throwing' looks like it would give consistent thickness.
     
  19. Feb 7, 2012 #18

    jim hardy

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    Last edited by a moderator: May 5, 2017
  20. Feb 7, 2012 #19
    ideally there should be no axial load. I was looking at this thread. I think it needs to be supported from the top. 3am, sleep time.
     
  21. Feb 7, 2012 #20

    sophiecentaur

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    I think you have to make the support (underneath) as massive and rigid as possible. Hanging your spinning drum from above is to invite instability. When speeding up, there is always the possibility of a wave forming in the mix and a serious wobble to develop.

    I reckon a good solid (concrete) base with a spindle on two, well-spaced, bearings should allow the spinning to establish a good parabola even if the centre of the drum is not on the axis. A 'live' back axle from an old scrapyard vehicle would do fine.

    On the subject of bubbles, why not use a very wet mix of mud plus cement, vibrate for some while, to let the bubbles out, and then spin it up gradually? If the water in the mix is allowed to drain out slowly, the shape can establish itself and then the cement can set. Possibly not a strong casting but it could easily take a coat of resin as a female mould for copying.

    I reckon you need to overkill a bit on the mass and rigidity of your machine and be sure your investment is not wasted, were it to fail. Good luck with it.
     
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