Bespoke, not re-profiled, tyres - commission offered

andrewb

Western Thunderer
Dear Andrew,

I think this is an incredibly interesting project as I’d like to do something similar for a Schools Class loco, as no directly correct wheel exists in the Slaters range for the drivers (I discovered on WT - informed by very knowledgable WTers).

I’ve spoken to Alan Buttler of Modelu about this and he’s already provided wheel centres in 4mm scale that have worked successfully. Also since then he’s got hold of tougher/stronger materials that would hopefully be even better suited to this task of creating wheel centres.

I just need a drawing of a Schools Class driving wheel now!

best wishes,

Neil.
Thanks Neil - good to hear from you. It's proving to be a fascinating project, but not without its frustrations, as my design aspirations currently outstrip my CAD skills by quite a margin! I'm getting there slowly, however!

I'm don't have an in-depth knowledge of Southern locos and where you might find details about the wheels, although I did a quick scan of the NRM archive of SR drawings. The listing can be found at this link:


Word-searching under 'Schools', 'Class V', and just plain old 'Wheels' threw up some finds that might be useful. Of the findings, the following two seemed to have best potential:

Box 13, Roll 64 Diagram 7376 titled 6' 7" express engine cast steel driving & trailing wheels, dated 1910/1932 - might help if they used standard wheels for the Schools;
Box 44, Roll 164 Diagram 50039 titled Sketch to show arrangement of wheels & crank axle, Lord Nelson class - which is the same 6' 7" diameter with 22 spokes, although I don't know if they are, or even look, the same.
Sorry - probably teaching you to suck eggs here, but your message tickled the Sherlock Holmes in me! :))
Looking more broadly at actually producing these wheels in model form, I have also been contacted by a WT-er in New Zealand who shares our interest, and has been making wheels for some time. Let's stay in contact so we can share what we know and what we learn. :)
 
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andrewb

Western Thunderer
This is the aforementioned trial print by Shapeways (other bureaux are available), in an eye-watering enlargement! It uses their MJF Plastic PA12 material, which is printed by a sintering process, chosen for being both tough and strong, and having one of the better printing accuracy figures.

IMG_0501.jpeg

Pros and cons:

Pros: tough and resilient - bearing in mind the outside rim is scarcely 1mm thick, likewise the narrow end of the spokes, there is no appreciable yield in the material. It has printed very flat, and the accuracy is pretty good although at this magnification some artefacts can be spotted (which were mainly I think shortcomings in the design file - note the joggle in the 12 o'clock spoke, which I suspect was caused by me leaving tiny slivers at a join). Good shade of black. Appears easily strong enough for 7mm use, and proves a design concept of using prototypically profiled spokes while maintaining a flat print.

Cons: surface texture way too coarse. A smoother version of this plastic is available but demands thicker 'minima' that may present challenges maintaining prototypical accuracy (I think the smoothing processes is chemically subtractive, but I've yet to confirm). Not as good as I think I can get it, and not comparable to injection moulded products from the likes of Slaters and Ultrascale (4mm)

Conclusion: at actual size, it looks a lot better and would almost 'do', but I think I can 'do' better! So the search goes on. If anyone can recommend another plastic or print bureau (or has an 'in' to small-order plastic injection moulding), I'm all ears!
 

Brian McKenzie

Western Thunderer
An impressive initial result, Andrew.

I'm wondering if this material can provide a good finish, when turned in a lathe (or simply be smoothed with wet 'n dry), for which a slight increase in the thickness of the hub and counterweight might be beneficial.
How did the outside diameter compare to your CAD file size, and is it truly round and concentric to the bore? Probably best to leave the axle hole undersized and bore it out after tyres are fitted.

-Brian
 

Podartist79

Western Thunderer
Hi again Andrew,

Once more, it's such an interesting path that you are following here!

Thank you for all the information; you're not teaching me to suck eggs, but thankfully I am lucky to have a couple of friends in the know (plus I've used the NRM Search Engine) and found what I'm looking for... I think, on the Schools wheels. Yet to purchase said drawings.

I will keep you updated with anything I manage to push on with, but I think it'll be a very slow burn. I'm also lucky in that I have a 3D CAD designer friend who is also a terrific modeller (loco builder) and he will undertake the CAD for the wheels. I'll then pass the files onto Modelu - as arranged - and see what can be achieved.

I am aware that Alan B seems to be able to print a far finer quality than others I've seen. Whether he can in tougher/stronger material waits to be seen.

Best wishes,

Neil.
 

simond

Western Thunderer
My guess is that the Modelu prints will not be tough enough to use, however, I expect that they could be suitable as lost “wax” masters. Interested to know how it works out.

Atb
Simon
 

andrewb

Western Thunderer
An impressive initial result, Andrew.

I'm wondering if this material can provide a good finish, when turned in a lathe (or simply be smoothed with wet 'n dry), for which a slight increase in the thickness of the hub and counterweight might be beneficial.
How did the outside diameter compare to your CAD file size, and is it truly round and concentric to the bore? Probably best to leave the axle hole undersized and bore it out after tyres are fitted.

-Brian
Hi Brian.

I think this project's challenge was always going to be the fact that 3D printing comes with a ± accuracy figure, which can sometimes be pretty scarily high for work in our scale. For example, the quoted tolerance for this plastic is ±0.25 mm + 0.1% of the length, meaning that - in theory - this wheel could have been up to about 0.3mm bigger or smaller in any direction. In reality, the rim - 6' 3" giving 43.75mm - measures exactly that (at least as far as I can determine from my 'analogue' vernier calipers). Was I just lucky, or is a 3D printer that's set up for circles - perhaps using the axis as datum - self-cancelling of its own errors? Dunno! As for concentricity, it also looks spot on, but I haven't spun it for a definitive test. I made the mistake of reaming out the axle bore to full size by hand before checking the print, and may have introduced my own inaccuracies by doing so.

Indications so far are that wheel centres can be printed with sufficient accuracy, but your post triggered another thought that I didn't realise could be open to me - can plastics be cleanly lathed? Given the possibility that one could reasonably expect a slight decrease in rim diameter, is it wise/feasible to print a fraction oversize and lathe to the finished dimensions? I don't think I would want to have a massive cleaning-up job for each and every wheel centre produced, so my search for a smoother finish continues, but the idea of being better able to ensure a tight, concentric fit in the tyres is one I should probably pursue.

VBW

Andrew
 
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