New Zealand A Class 4-6-2

[suctionhose]

Two years ago, I completed a 3” scale Fowler Ploughing Engine which had me thinking: “Is this it?” meaning “Is this my last model? There have been several and at some point, one would expect to declare “I’m over it”. Nothing left to dabble with…

Not so. A couple of months ago, with relish born of abstinence, I started a new locomotive. I don’t propose to do the blow by blow build diary – I did with the plougher on TT – but for general interest, I thought I’d post the progress here now and again.

The prototype is a New Zealand Railways 4-6-2 made famous on the early days of North Island Main Trunk service from Auckland to Wellington in 19 hours or thereabouts. The journey involved many high viaducts, tunnels and the Rimutaka Incline which was essentially a rack railway (technical differences) with grades of 1 in 15.

New Zealand has wild, spectacular topography and the rail connection of two major cities at each end of the North Island was a major achievement in 1908. These loco’s, the A Class, were a response to a desperate need for motive power for the job and as a “balanced 4 cylinder compound with 225lbs WP saturated boilers”, they represented the best technology in the world at that time.

A decade later, superheated, 2 cylinder Ab’s ruled but the A’s still saw 50 years of service all over the Country. A428 still operates in preservation and displays many of the numerous modifications that were implemented over their long service lives, including conversion to superheated 2 cylinder simples. I have chosen to model an early version with HP piston valves prominent, the best-looking chimney barely visible behind the giant oil headlight, 1700 gal tender etc. Best summed up in this photo with the Express enroute to Wellington.




I've only been at it for a couple of months. First the Cowcatcher followed by the main wheels which were manually milled from 170mm diameter slabs of Continuously Cast Iron... At this point, I want to acknowledge the willing assistance provided by various Kiwis whom have freely shared information from their various collections of NZR loco drawings etc! 

[tenor]

I think I am going to enjoy this thread!
Beautiful work as usual.
Martin

[simon6200]

In case anyone missed it, those wheels were “manually milled”. Let that sink in for a while….

[mugbuilder]

If it is built to your usual standard Ross,[and I am sure that it will be], it will be a beautiful model of a handsome machine. Ideal for the heavy West Ryde traffic.

[suctionhose]

To add to the opening remarks above, I will say a few words about “Scope” (of the model).

The A was significant in NZ because it was locally designed and built by the Railway Workshops in 1906. They incorporated a variation on the de Glenn Compound principle, popular in Europe pre superheater days and had wide firebox boilers on a 4-6-2 wheel arrangement– a concept that had been tried only a few years earlier for the first time in the world, in New Zealand in 1901. (Search NZR Q class)

The Compounds had HP’s between the frames driving the leading axle. The first four engines had HP valves driven by Stephenson’s Gear between the frames with eccentrics on the second axle. The LP’s on the outside were conventional with Walschaert's gear ignoring the mass of pipework in the smokebox and interceptor valve for working double high on starting. The 5th engine was simplified by adopting two more sets of Walschaert’s in lieu of the Stephenson’s. The Walschaert’s between the frames derived its motion via rockers from the outside expansion link trunnions. HP & LP valve gears had separate reversers.

I have to tell you I’ve struggled to suppress the urge to do the mixed valve gear version! Wot fun! However, this is where “Scope” enters the equation; more importantly, “Sensible Scope”. So many grand endeavours never make it because the builder cannot resource the dream with adequate time, money or energy. Also overly complex engines can be a bit of pain to own (as the NZR found out...)

Therefore, in view of present circumstances, the Scope of my A Class is this:
1. Build time 2-3 years – and it must get finished.
2. Conventional 2 cylinder superheated model on 5”g at 1.5” to the foot scale (3'6" prototype).
3. External detail and appearance as per A&G Price built Compound (circa 1910)
4. Welded 16mm thick frame for weight, rigidity and simplicity.
5. Minimum complexity for reliability and ease of maintenance.

He’s one from today with the wheels mounted on axles, quartered and ready for tyres:

[runner42]

Hi Ross,

it's going to be a great model locomotive. I am wondering how you are going to fit the tyres, assume heat shrinking them on? If so will this produce any problems on a completed axle arrangement? You have already made the decision on the axle boxes, complete and fitted. The use of 16mm thick material for the frames has me intrigued, I assume that this thickness is occupying the width of the axle box cut out so axle box horns are not fitted.

A more patient obsever will await the subsequent postings to have these questions answered.

Brian    

[David]

I'm really looking forward to this! Even though it goes without saying, those wheels are awesome.

[suctionhose]

Apr 6, 2024 2:04:56 GMT runner42 said:

Hi Ross,

it's going to be a great model locomotive. I am wondering how you are going to fit the tyres, assume heat shrinking them on? If so will this produce any problems on a completed axle arrangement? You have already made the decision on the axle boxes, complete and fitted. The use of 16mm thick material for the frames has me intrigued, I assume that this thickness is occupying the width of the axle box cut out so axle box horns are not fitted.

A more patient obsever will await the subsequent postings to have these questions answered.

Brian    

Hi Brian, yes, no horn blocks. The exact details of the design are still forming in my mind so I'm yet to find all the answers myself. But that's the journey!

Re tyres: 8 thou shrink allowance. They don't need a lot of heat - just barely blue like a temper. Final profile done between centres which is one of the reasons the main crankpin is not yet fitted - gets in the way...

[steamer5]

Hi Ross,
Really looking forward to this build... my dad has a NZR WW, would love to see your setups for your wheel making if you have photos.... they are truly excellent!

Cheers Kerrin

[suctionhose]

Apr 6, 2024 23:28:58 GMT steamer5 said:

Hi Ross,
Really looking forward to this build... my dad has a NZR WW, would love to see your setups for your wheel making if you have photos.... they are truly excellent!

Cheers Kerrin

Hi Kerrin,
Not much to show really unless you consider a big bucket of swarf to be photogenic??

A couple pics showing the basic process of mounting turned blanks on rotary table with spigot to centre for a 5 step process to remove metal by drilling, connecting the holes and gradually getting down to finishing.

I've deliberately shaped them to suit tooling that I have - only an approximation of the actual shape of an A wheel (but then they had a few variations) - but they'll do (see "scope" above)

More telling I suppose would be my 'running sheets' showing the DRO positions and rotary table angles for each step. The finishing cuts were using a tapered end mill, which are a rare legacy from mould making pre cnc days, but can be found online. The taper just gives the appearance of 'draft' like a casting - I was thinking I might double side the draft but decided nothing to be gained by doing so.

The first wheel took a while - maybe 12 hours or so - to settle on the shape and note down the DRO co-ords. With that prep, the other 5 were just under 6 hrs each. More time to bore and key for quartering and final crankpin location (which is on a spigot using the key way as the reference)

[steamer5]

Hi Ross,
Thanks for the pictures & words..... we could of traded buckets of swarf, i ended up with over 30 liters make the stretches for K1.

Wonder how close your wheels are to Dads WW, must have a look when I'm up there this week

Cheers Kerrin

[suctionhose]

I have the 16mm frame welded up and machined now. Not something you could ever do in LBSC's day with hacksaw and file but I have the capacity to remove metal when needed and that has enabled the frame to be cut from 100 x 16 flat bar and welded together in a day or so. Machining a datum surface and thence the horn openings took but a few hours to accomplish.





The rationale here was to keep it simple and heavy for adhesive weight. It weighs 20kg at the moment with another 400mm frame extension still to go on the back to carry the wide firebox, footplate and cab. Interestingly, plate frames are very reliant on running boards and/or side tanks to provide lateral stiffness especially passing the firebox where stretchers are not possible. Curiously, this prototype, though plate framed, had the running boards attached to the boiler. I'm not sure how the frames were held in alignment - possibly with intermediate attachments to the boiler(?) - but in simplifying matters for a model, I felt thick frames were necessary.

There are some other methods to the madness which I will discuss when I have the pictures to help explain my thinking.