Midland Railway Centre

Mmm...

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.

 

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

 

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.

 

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.

 

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.

 

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

 

paul_l wrote:

Thanks Paul,
That's one that I haven't tried.

 

Building the deck was fairly straightforward and as you suggest much easier than building a loco.

 

And now I'm worried

" Here we go again, off down the road again ..........."

And here was me thinking he'd forgotten about this ......

Paul