Midland Railway Centre

Discussion in 'Line Side Buildings' started by Toto, Aug 18, 2017.

  1. Rob Pulham

    Rob Pulham Staff Member Moderator Feature Contributor

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    Mmm... :scratchchin:

    This has reminded me that I have a part built 70' Cowans Sheldon version in my shelf queens (not that it would fit on a shelf of course.....Bought from Dave Smith when he was trading as Metalsmiths.


    I haven't got any sort of motorising kit so I will be interested in what others are using.
     
  2. Toto

    Toto Staff Member Founder Administrator

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    Hi Rob,
    Keep an eye open as his nibs may do a blow by blow instruction. Could be amusing ...... Er...... I mean interesting. :avatar:the dark side beckons once again.
    Do you have your deck made up as yet?

    Toto
     
  3. paul_l

    paul_l Staff Member Administrator

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    Ok Gents a few questions

    1. What is the turntable shaft diameter, and how far does it protrude below the base ?

    2. What is the length of the turntable deck ?

    3. How deep is the well ?

    4. What is the external diameter of the well ?

    Here's why -

    Based on a 70ft turntable
    Deck (diameter) = 70 x 7 = 490mm
    Circumference = 3.142 x 490 = 1540 mm
    Nema 17 stepper motor 1.8 degrees per step = 200 steps per rev.

    Direct drive (motor connected to turntable shaft) with no gear reduction

    Each full step would move 1540/200 = 7.7mm
    Half stepping would move 3.85mm
    Quarter stepping would move 1.92mm
    16th stepping would move 0.48mm

    This only gives us approx 1/2 rail width resolution.

    Thinking of something like the belt drives on record turntables, using a 3D printed 120T GT2 gear on the turntable shaft, and a 20T GT2 gear on the motor connected with a GT2 toothed belt.

    This gives a 6:1 reduction, so applied above would give (7.7/6)

    Full Step 1.3mm
    Half step 0.65mm
    Quarter step 0.32mm
    16th step 0.08mm

    Depending on the gear box Toto has purchased (I'll assume 5:1 - until he tells me otherwise)
    ((7.7/6)/5) - giving a total of 30:1 reduction

    Full Step 0.26mm
    Half step 0.13mm
    Quarter step 0.07mm
    16th step 0.02mm

    Measuring a piece of O gauge rail the width is approx 1.6mm wide, giving approx 6 full steps to cover the rail width.

    Why full steps ?, when 1/16th stepping direct drive would take 3 steps to cover the rail width and the tolerances would cope with the small misalignment.

    If you rotate a stepper motor you feel it notching from full step position to the next full step position, and providing no turning force is applied to shaft it will hold its position.

    Micro-stepping works by energising two coils to hold the motor between positions. As the turntable will spend long periods of time stationary, the coils will need to be permanently energised to maintain the position - which is going to get hot - very hot.

    By using the Full step, once the move is completed the motor coils can be de-energised.

    The gear box solution also will increase the torque available to move the turntable.

    Paul
     
  4. Rob Pulham

    Rob Pulham Staff Member Moderator Feature Contributor

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    Toto wrote:
    Hi Toto,

    By deck do you mean the bridge? if so yes, almost.

    This is as far as I have got, I haven't done anything with the base/well.

    [​IMG]
     
  5. Rob Pulham

    Rob Pulham Staff Member Moderator Feature Contributor

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    Wow Paul, that is a comprehensive reply,

    With regard to your questions the only measurement that I can recall is the the square base that the well sits in is 600 x 600mm. I would need to dig out the other bits to measure them.

    [​IMG]
     
  6. Toto

    Toto Staff Member Founder Administrator

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    Oh man ..... You have made a great job of the bridge. Very neat. If I remember correctly it also comes with timber sleepers. May be wrong there. I'll need to check my own.

    I have bought more of the brass stanchions which I thought about using around the outside of the turntable well. Just leaving gaps at each of the track openings. It will depend how many entry / exits I end up having. Just a thought ....... Might not happen.

    Cheers

    Toto
     
  7. Rob Pulham

    Rob Pulham Staff Member Moderator Feature Contributor

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    Toto wrote:
    Thanks Toto,
    It is a nice kit to build although there are a lot of rivets to punch out for the framing.
    I do recall making an error in that I cut the slot for the operating lever in the wrong place. The error will be covered by the planking (it is supplied) I have stained mine with Rustins dark oak stain and I haven't got as far as fitting it yet (I have cut all the pieces to fit so I have that jigsaw to work out again when I get back to it.
     
  8. paul_l

    paul_l Staff Member Administrator

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    I've used Teak stain for sleepers to give that slightly faded effect on timber, and then rub talc or plaster into the grain again to flatten it a bit more.

    Paul
     
  9. Rob Pulham

    Rob Pulham Staff Member Moderator Feature Contributor

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    paul_l wrote:
    Thanks Paul,
    That's one that I haven't tried.
     
  10. Toto

    Toto Staff Member Founder Administrator

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    My Stepper motor and gear box arrived yesterday. Finally got it today as it was left with the neighbour. Anyway, I'll post a picture or two later.
    Cheers
    Toto
     
  11. Toto

    Toto Staff Member Founder Administrator

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    and here it is ........
    [​IMG]

    and another for posterity .......
    [​IMG]

    cheers for now......
    toto
     

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