Sunday, 3 February 2019

A turntable for Tumbarumba - pt 2

At the finish of the previous blog post, I had installed a hard-disk bearing as the pivot, constructed the shell of a NSW 60 foot table out of brass, made the 60’ track section, and installed the concrete ring rail support
3026 turns on Wagga's 60 foot turntable in 1983 (easter) on an LVR tour.  Tumbarumba's turntable was the same design as Wagga's

Continuing from part 1:

Please note that Step 1, 2 and 3 were done simultaneously. 

Step 1 – add magnets
Step 2 – add track supports for the entry and stopping exit tracks
Step 3 - secure the track to the turntable, and the entry and stopping exit tracks

Rare earth button magnets off ebay.   Cost was around 25c each.  These are seriously strong

I made up a balsa block to secure the magnets in the pit.  This way I could hold them down, whilst doing the fine adjustments on the alignment.  Once the alignment was confirmed, then I screwed the balsa blocks to the pit.  

Both magnets are fitted at equadistant locations in the turntable bridge.  If this step is mucked up, then the magnet solution will not work.  Note, I marked the magnets with a felt tipped marker, as the polarity had to also match.

Track locations were marked on the wooden blocks, so the ends could be aligned

Testing the table showed up a problem.  The magnets were VERY powerful, and the turntable oscillated whilst eventually achieving, and holding  the alignment.  I thought reduction of the oscillation would be an easy fix by  simply remove one of the magnets, but that threw out the alignment by just under 1mm.  Advice on options to fix from the NMRA guys in Canberra was encouraging, but I will tackle the problem in another way – see later.

Step 4 – adding the ring rail
Track has been added to the turntable bridge, and the entry and exit tracks.  The latter is not going to be permanent, as longer rail will be installed once the turntable is positioned on the layout

There is no easy way to do this.  I painted the ring rail support ring concrete.  I then drew a scale 56’ diameter circle on the ring. I took two sections of rail, and slowly started curving them by hand, steadily reducing the radius of the curve, whilst checking the curve against the drawn 56’ circle.  Once both sides were complete. I cut out the centre curved section of the rail, discarding the harder to bend bits on the ends.  Attachment of 2 Peco insulated joiners to complete the ring.

step 5 – adding the ring rail sleepers

As I had a lot of left over Peco track base, why not use it.  The trackbase was broken down into 2 sleeper sections, and the NSWL chopper made this task easy.  The sections were simply slid back onto the ring rail

step 7 – secure the ring rail to the support

Glued with white glue - in case I need to remove it?  After taking the shot, I realised that one of the support sleepers had moved, and this was fixed.  Some more sleepers were slid under the insulated joiner "gap" after the sleepers had been reduced in width

Step 7 – add pickups
2 pickups soldered, ready to install.  Printed circuit board sleepers, and phosphor-bronze out of old cassettes - I needed a flat surface on the pickup to bridge the gap in the ring rails at the insulated joiner

Pickups glued into place with 2 part epoxy.  It was easier to do this on the underside of the turntable bridge

Step 8 – add handrails

I find it easier to solder straight bits of wire onto the edge, before bending them to shape.  The wire was precut to 17mm.

After bending them, a top handrail was fitted, cutting off any surplus brass uprights to size.  I have chosen a completely straight handrail, as I was not confident I could match the prototype's curve at the ends

Step 9 – Add weight. 

The weight is something I felt would assist in providing some inertia to the turntable, to resist the savage kick that the lightweight table did on encountering the magnets.  Whilst having a loco on top of the table will also add some inertia, more weight was desirable.  After testing I added 15 grams to each side.  Note the 2 different types of car wheel weights.  The more modern type is physically larger, and is also partially magnetic – so my guess is that they have a percentage of weight made of iron.  The older type is not magnetic, and most likely full of lead.  Whilst removing lead out of the environment is a healthy choice, I am going to quarantine my lead weights in models.  The smaller weights fitted into the hollow of the turntable bridge without modification.  Oh, yes, adding weight did help.

Step 10 – deck plates. 

The prototype steel tables have a solid deck plates.  I cut 3 lengths of styrene to fit – and on the edge plates, attached a strip of evergreen 0.060 square strip to hide the PCB sleeper ends  The side plates are glued into position, but the centre plate is secured with blu-tac, as access to the pivot screws may be necessary for maintenance tasks in the future

Step 11 – filling in the pit and Step 12 - the abutments

The Sculpt-it was still drying when I took this picture.  Although the screws I had used to secure the balsa block with the magnet had started to rust

View showing the weights, and the arrangement of the wire from the pickups.  The magnets are vitually hidden.  

I used Sculpt-it for this task.  I thought I made a small amount, but I goofed, and had to use the surplus on the outside of the ring rail.  The sloping of the sculpt-it for the pit was also rather dramatic – as I put too much in there too.  Removal of the extra was achieved whilst the sculpt-it was still drying – if I waited until it dried, much more sanding would have been necessary

The abutments were made with balsa, and backed with polystyrene.  I have added some aldi brand filler to the join, and once smoothed will give a bit of a curved surface.

Well, the turntable is now awaiting painting, painting or grassing the pit, fitting the dummy ring rail rollers, and final positioning on the layout.  I will not kid you in saying my turntable  is fully detailed – it isn’t.  But the bigger question is “Will it work?”  A multimeter shows that power gets from the ring rail to the turntable bridge, and turning a loco on the bridge works fine, although the alignment kick did make the tender rock.  That can be minimised by carefully anticipating the kick, and holding the table appropriately.  Running a loco onto the bridge from the entrance road will have to wait for now.  I already know that sound equipped DCC engines will lose their sound, as the turntable ring rail gaps are crossed.  But I don’t think this will be a big issue.  However, the “caution” sign at the end of my previous blog post sort of indicates that people will need some training in its use. 

That is enough turntables for now, it is time to move to another project.  Until then, build a model.

Driver side.  The loco is an ancient Bergs brass - upgraded with a can motor, but not yet DCCed.  It is typical of the standard goods locos that run on the Tumbarumba branch.  Yes, it does fit

Turntable pivoted 180 degrees to display the fireman side.  There is not much clearance between the turntable bridge and the pit

The lack of ring rollers is evident in this view.  The ring rollers will be cosmetic,  It is difficult to see the pickup wiper resting on the ring rail. 

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