Hobbyhorse GNR Stirling Single No 1

[Hobbyhorse]

With the three Great Western locos nearing completion on WR action, thoughts are turning to the next project.
Like a lot of us I’ve got a pile of kits and a few scratch builds planned, but I’m going to return to a project that started in 1994. It’s a bit strange to start a build with photos of the completed loco, this loco was commissioned by a friend. It was my first full scratch build that was inspired by the late Tony Reynolds work. Key to these models was the purchase of a pantograph milling/engraving machine and upgrading of some of the other workshop tooling. I originally cut out parts for three locos, two assembled one for my friend and the other for me.
So returning to it after 30 years with a bit more experience and some better tooling I’d like to change a few bits, luckily I also purchased a set of Harris wheels for the third model.
A the time I didn’t document the build with photos, the only one I have is one the Railway Modeller did, so this build will be fully documented.

Simon

 

[P A D]

Superb!

 

[freelance7]

I am looking forward to this build Simon.

Paul

 

[Hobbyhorse]

Machining the wheels is always an interesting exercise, which over the years I’ve completed a good many sets. I always have a bit of trepidation before starting especially now as the casting are no longer available, so great care is needed insuring all measurements and machining stages are noted.
After all the preparations the first wheel didn’t go as planned, having taken some cuts to reduce the diameter to near size, looking at the DRO, horror its set to Metric with my measurements are imperial, not to be disheartened I carried on with all the other castings without any issues.
At the stage I needed the undersized wheel, I machined off the tyre and reduced the front face to near the spokes, next was to machine a new tyre front some fine grade cast iron bar, luckily I had some in stock which was very near to the outside diameter. Fixing the new tyre on wasn’t done until the Tufnell insulating bushes have been glued in, this was done with loctite. On these wheels both sides are insulated which should reduce the chance of any shorting, on the original two builds with only one side insulated this is sometimes an issue, gluing in the bushes with 24 hour Araldite and cooking for an hour at 100 degrees makes them a permanent fit. Occasionally it comes up on the forums that axles come loose on Harris wheels, I’ve not in countered that as I use a press fit of the axles into the tufnul bushes, the key being to use reamers that allow this press fit, which are metric and are slightly undersize. To prove the press fit method I did a test on a spare casting years ago by trying to move the wheel on the axle, what happened was snapping the spokes so I’ve been confident ever since. I used to use the brown Tufnul but changed to the white version as it’s got a small amount of give in it, you do have to be cautious when pressing the wheels on as if it’s to tight a fit you can split the bush and the hub.
Setting the back to back is done at the pressing on stage. Drilling the crank pin location is done slightly differently using a jig with a axle hole and lots of peg holes to fit pegs between the spokes, once setup on the mill and position set with the DRO all holes are drilled easily. Both crankpins are insulated with tufnul and are pressed into the holes.
The last job is to position the locking tapered on the axle, again this uses a jig that holds the wheel accurately, sets the quartering and sets the back to back, once drilled a small tapered reamer that matches the pin is carefully powered into the axle.
Well that’s the wheels completed, with a few challenges along the way, but part of what we do is overcoming problems which adds to the fun, so the next job is to get the drawings out and go through the machined parts to see what I’ve got and what needs making.

Simon




Machining the front and side of undersize wheel.

Replacement tyre.

 

[adrian]

I have a couple of sets of Mark Wood's wheel castings I really need to get around to machining up at some point so thanks for posting. I'm still debating whether to go for rim insulation or axle bushes. I have some brown Tufnol bar, paper and fabric impregnated resin but I'm struggling to find a source for "white Tufnol" or is this PTFE bar?

Second - with respect to the crankpin insulation, the photos indicate you're accurately marking and drilling the castings for the crankpin. So what is the order of operation? On the the axles you seem to be fitting a solid bush and then drilling afterwards for the axles, whereas you seem to be taking care for marking out and drilling the crankpin position in the casting. So for the crankpins are you filling with a solid insulationg bush and drilling out afterwards or are you bushing the crankpins first and then inserting them into the wheels?

 

[Hobbyhorse]

I have a couple of sets of Mark Wood's wheel castings I really need to get around to machining up at some point so thanks for posting. I'm still debating whether to go for rim insulation or axle bushes. I have some brown Tufnol bar, paper and fabric impregnated resin but I'm struggling to find a source for "white Tufnol" or is this PTFE bar?

Second - with respect to the crankpin insulation, the photos indicate you're accurately marking and drilling the castings for the crankpin. So what is the order of operation? On the the axles you seem to be fitting a solid bush and then drilling afterwards for the axles, whereas you seem to be taking care for marking out and drilling the crankpin position in the casting. So for the crankpins are you filling with a solid insulationg bush and drilling out afterwards or are you bushing the crankpins first and then inserting them into the wheels?

Adrian, can't really help with the white tufnol as it was acquired a long time ago through Tony Reynolds, I still have a stock of the brown stuff Alan used to sell and use it occasionally. PTFE is a different beast that I've not used. I think either method of insulation has its merits, personally I've always used axle insulation.
Sorry about the limited info on the crank pins, this is the method.
I mount the tufnol in a collet and face and drill to accept the crank pin, it doesn't need to be a tight fit as when reducing the tufnol to fit the hole in the wheel it could split. When I drill the hole in the casting for the pins, I tend to used the same drill which is a stub verity, to reduce any flexing when drilling, obviously the hole is started with a centre drill. I have made a plug gauge to check hole diameter, which has what I call a cheesy fit into the hole( slight resistance ) and measure off this for the diameter of the tufnol, which is machined to 2 thou over the hole diameter. It's worth measuring the depth required accurately to enable it to be flush on the rear and the bottom of the pin flange level with the boss. A small taper is put on to start of the tufnol, helps to keep centred. I use a drill press to press the pins home, the rear of the casting needs packing to protect it from splitting. With the slightly oversize tufnol and the bit of give it's been a reliable fit without any problems over the years.

Simon

 

[adrian]

Thanks for the update. I have the brown Tufnol bar, I was wondering if there was much difference in the turning for the white bar vs the brown bar stock.

 

[simond]

I have never heard of, or seen “white” Tufnol, and unfortunately their website doesn’t really suggest the colour.

What engineers need to know about Tufnol laminated bearings

Tufnol produce a range of Laminated bearings. while the materials may also be used for some forms of spiral gears and racks.

tufnol.com

I guess it’s an epoxy grade rather than the more familiar brown phenolics.

I was lucky enough to buy a fairly large, thick, slab of cotton phenolic for the princely sum of a fiver at a steam fair about ten years back. I don’t doubt it’ll see me out.

 

[Brian McKenzie]

Should there be any reservations about using Tufnol in this application, can recommend PEEK -as being an exceptionally strong plastic, that also usefully offers good heat resistance.

PEEK Rod | Direct Plastics Limited (many other suppliers available)

It is quite expensive, but a short length of small diameter is worth having. Holes are best drilled using a sharp drill of the final hole diameter required. It will resist further enlargement by using e.g. the next drill size up, and it will close down tightly on reamers.

But after getting used to these foibles, it provides an insulated joint as good as if it was made from metal.

-Brian McK.

 

[Overseer]

I have a couple of sets of Mark Wood's wheel castings I really need to get around to machining up at some point so thanks for posting. I'm still debating whether to go for rim insulation or axle bushes.

Have you ruled out split axles? The late Peter Turville of the Scale7 Group used to make lovely telescopic split axles with the split towards one end and the telescopic part with taper pin towards the other end. He usually made 3/16" but made a couple of 1/8th inch diameter for me which inspire me to be as precise as I can whenever I look at them.

 

[Brian McKenzie]

​

Photo shows 5mm dia axles - with stub ends pressed (at right) - into sleeves turned from PEEK plastic rod.

The interference fit in total amounted to 0.001" - which proved excessive, as it caused the thin wall of the stainless to grow in diameter!
Later axles were made having a thicker wall for the centre piece.

(1 thou per 1 inch of diameter is a good interference fit allowance for rigid materials.)

- Brian McK.

 

[Hobbyhorse]

Should there be any reservations about using Tufnol in this application, can recommend PEEK -as being an exceptionally strong plastic, that also usefully offers good heat resistance.

PEEK Rod | Direct Plastics Limited (many other suppliers available)

It is quite expensive, but a short length of small diameter is worth having. Holes are best drilled using a sharp drill of the final hole diameter required. It will resist further enlargement by using e.g. the next drill size up, and it will close down tightly on reamers.

But after getting used to these foibles, it provides an insulated joint as good as if it was made from metal.

-Brian McK.

Thanks Brian,

I’ll have a look at this materia.

Simon

 

[Hobbyhorse]

View attachment 247030
​

Photo shows 5mm dia axles - with stub ends pressed (at right) - into sleeves turned from PEEK plastic rod.

The interference fit in total amounted to 0.001" - which proved excessive, as it caused the thin wall of the stainless to grow in diameter!
Later axles were made having a thicker wall for the centre piece.

(1 thou per 1 inch of diameter is a good interference fit allowance for rigid materials.)

- Brian McK.

Thanks Brian,

I’ve seen Peter work before and like other have admired his work.
Having not done any split chassis in 7mm it’s something I’m looking for a future project.
It would certainly make for an interesting job with an inside crank, not insurmountable but a good challenge .

Simon

 

[Hobbyhorse]

Thanks for the update. I have the brown Tufnol bar, I was wondering if there was much difference in the turning for the white bar vs the brown bar stock.

It machines the same as brown, just my preference.
The Peek rod could be a good substitute.

Simon

 

[bambuko]

Worth trying (they will send you free sample on request) :

Shop Plastic Rods at igus® UK | iglidur® Bar Stock

Plastic rods by igus® are made with high performance polymers to provide them with properties including self-lubrication, maintenance-free, FDA-compliant and wear-resistant. Shop iglidur® round rods online.

www.igus.co.uk

 

[simond]

good stuff, but for bearings, rather than static bushes, surely?

Specifications of iglidur materials

What are iglidur materials made of? What is behind the self-lubrication effect? Learn more about the functionality of iglidur plain bearings.

www.igus.co.uk

 

[adrian]

Have you ruled out split axles? The late Peter Turville of the Scale7 Group used to make lovely telescopic split axles with the split towards one end and the telescopic part with taper pin towards the other end. He usually made 3/16" but made a couple of 1/8th inch diameter for me which inspire me to be as precise as I can whenever I look at them.

I probably will be making them with split axles but not a split chassis so I will still need insulation in the rim with an axle bush. The attraction of the rim insulation is that I don't need to insulate the crankpins.

 

[Hobbyhorse]

Having dug out the Patterns, Drawings, Machined parts and dismantled the first built model, which is great as a reference its time make a start. The chassis first on the list, with most of the bits found I made up some bearing to suit the cutouts in the main frames. I’d previously made some patterns for the horns which with the keeper plate screwed onto the bottom. The small change I’ve made with this build is to allow 40 thou upwards movement of the driver bearings. The first build didn’t have this movement being only downward movement on all axles, which worked ok but this should allow for not so good track. The small sprung plungers transfer weight to the driver.

Quick plug for this Thursdays On Line Modellers Meeting, all O gauge guild members welcome from 18:00, 25th September.

With log in details on the Guilds Events Diary page.

Simon