3D printed driving wheels

[Nick]

Recently the driving wheel centres arrived from the printer. How's that for a very 21st century opening sentence?





I turned steel tyres for them and attached them with 24hr epoxy.



Now in case you think this is another "Because I can ...", I will say that it is a serious experiment in using new technology for G3 wheel centres. I could have asked Mark Wood for iron castings. Except that it would take 12 months at least and, let's face it, investment casting isn't exactly a new technology. Or I could have begged and pleaded with Slaters' (good luck with that one). Or I'm sure Mike (753) would have offered me time on his CNC mill - but time was the issue. There are a lot of spokes and clearing the spaces between them would require a 1.5mm cutter. Even doing it in two passes - the bulk of the material with a larger cutter then finishing with the small cutter - would require more than 12 hours of cutting time, assuming realistic cutting speeds and feeds. And because of the shape of the spokes and rim the cutting would have to be done from both sides, so multiply that by two. I think I'd be camping out in Mike's workshop. If that weren't enough, some details like the bolt heads on the rim (presumably holding the tyre in place) could not be CNCed.

So what were the 3DP options? Ideally metal printing which I've used for some brass boiler fittings, but for a component of this size I was quoted some very silly numbers. Plastic materials are much cheaper but also much softer. Strength isn't really the issue, but flexibility is (see below). The resistance to flexing of the strongest plastic materials is typically 1% of common metals, and the plastics that can be printed by hobby printers is more flexible still, so the DIY option was out.

I selected MJF (multi-jet fusion - me neither) Nylon PA12. I would have liked to use nylon filled with glass beads which is quoted as 40% stronger, but companies printing that material were only interested in industrial customers with large volumes and charged accordingly. I've been told but cannot confirm that is what Slaters' use for their wheel centres, albeit injection moulded rather than printed. I compromised on the unfilled nylon. Interestingly, it is printed on a machine that from the outside looks rather like - a large office paper printer! But comes with a 6-figure price tag, so we won't be seeing many of them in the hobby.

The finish, by the way, is a lot smoother than the photos suggest and what looks like roughness is actually just colouring. In case you are interested, the centres cost about £12 each, including VAT and shipping, and the arrived in just a few days.

So what about the strength? Ah, now we're in the realm of known unknowns. The final answer must wait until the loco is up and running. As a starting point, I did measure the resistance to deflection on the bench in the axial and radial directions by adding weights and measuring the deflection. The pictures show the measuring setups but I cannot show them in action because I needed two hands to hold the weights in place (I expect someone sells a voice-activated camera phone).



 

The radial deflection of the axle, which would be caused by the weight of the engine, was 0.035mm for a 7kg weight. That's probably a typical axle loading for an electrically powered engine, and not something to worry about. The axial deflection was a lot more: 0.13mm for a 2kg load. In service an axial load would occur when the loco goes around a curve. In that case, 2kg on each wheel sounds like a lot but who knows? It depends on the weight and speed of the loco and the radius of curvature. That is all static loading, and the dynamic loading when the train is running is highly uncertain. Still, nothing there seems awful and the experiment is definitely worth continuing. As they say, watch this space.

Nick

[MikeWilliams]

Well, they look superb.  I recognised them straight away by the shape of the spokes and the flare.  Lovely.

Mike

[753]

Nick
Those wheels are very impressive, your mastery of Fusion is paying off. You may have seen my post on an experimental 3D printed driver in PLA where I set up a compression test, the wheel did not disform under several Kgs of weight.
The addition of the trye adds considerably to the strength of the wheel as it distributes the forces around the wheel and distorting laterally. I would have no doubts as to ability of your wheels to perform well. In essence they are no different to Slater products.
Did you consider squaring the centre for quartering or adding a metal centre?

Mike

[MikeWilliams]

Squared axles make it easy to get wheels on and off, always getting the quartering spot on.  But I think Slaters wheels are easy to spot because of their nut in the centre.  Plain axles mean you can shape the axles ends like these I did on a 7mm loco 30 years ago - front axles correctly squared and rear axles correctly slightly domed.  Personally I think for an electric engine its hard to beat split axles with tapered pins ... but beyond my abilities to make them!

[David_V]

Congratulations on this Nick. Those wheels look especially fine!

Can you say who is printing these for you? In the UK, or Shapeways? And how are the tyres attached?

I've been printing G1 wheels in SLS nylon for some time and recently produced this set for a G2 restoration:

(Gauge 2 'Articulated Shunting Locomotive', 1911, stated by Henry Greenly to have been built by him personally, restored with 3D printed electric mechanisms)

These wheels are one piece with the tyre and are adequately strong for this heavy loco, which is approaching the size of a G3 engine.

Best regards, David

[David_V]

Here's a CLOSE UP OF THE WHEEL. mjf DOES SEEM

[David_V]

Oops! something went wrong there! Try again:



David
 

[Doddy]

Thumbs up for both the GNR 0-6-0 + 0-6-0 and both sets of printed wheels (Davids and Nicks). I just love the unusual!

Who are the vendors for printing the wheels Nick?

[Nick]

That's stirred up some interest and valuable feedback. I'll try to respond to the various questions and comments.

I'm glad they were recognisable by an LNWR expert! I found the original wheel drawing in the NRM Library and drew it from that so the geometry should be accurate. More generally, the process should work for just about any wheel centre: H-spokes, Boxpox, whatever, and also means that we don't have to use generic wheels if we choose not to.

I decided not to try the DIY printing route mainly because of the lack of predictability. The mechanical properties of materials used in hobby printers, where I could find them, were distinctly worse than PA12. Often a wide range of properties was quoted, and most suppliers appeared not to know (or care?) what the strength and flexibility of their materials were. Fair enough I suppose if their users are making non-structural parts. Additionally, it became clear to me that the strength of finished parts made by these processes also depended on the control of the printing, such as temperature, humidity, cleanliness, writing speed, layer orientation, and so on and on. Commerical suppliers to industry are expected to provide parts with predictable and guaranteed properties. (I know, I used to be one).

All that said, hobby 3DP technology is continuously developing. For example, a metal-loaded filament is becoming available that may offer considerably more strength, but it does need careful handling. Resin printers are increasingly taking control of the printing including the post-processing, which should ensure a more certain result. I hope that we will be able to print our own wheel centres, but let's remember that the object for most of us is a loco that pulls a train that starts, accelerates, coasts and brakes on a track that will have curves and bumps. So what we do needs to be demonstrated before it will be widely accepted.

Sorry, I'll put my soapbox away and carry on. Fitting to the axle? The centres as made proved to be a very good fit on an axle of stock silver steel, so at present they are a push fit. On the finished model I will help them along with some adhesive, probably the same epoxy as I used to join the tyres to the centres. I didn't think of using squared axles but I agree with Mike W, they make assembly easy but are hard to disguise.

I've used Shapeways in the past but the printing is done in the EU so the last order I had was delayed in customs and came with a demand for duty and import charges which raised the cost considerably. I also note that the cost of some of their materials has shot up, but maybe that isn't their fault. My wheel centres were done by 3DPRINTUK who so far have been helpful and responsive. They offer both SLS and MJF Nylon PA12. The properties of both are very similar and MJF is a little cheaper, so I chose that.

Nick

[753]

Nick
Thanks for your thoughts on printing wheels. Re quartering squares one answer worth considering is using hex stock, where you print a hex centre then insert a brass hex piece with an interference fit. To cut the square you have a jig that has two pins in line with the Y axis one for the centre and the other for the crank pin hole, with your tool centred on the wheel you cut the square, I use a 1.5mm cutter to cut a 3/16in square.
For those who don't like to see the hex hole of a holding screw. A thin covering plate that sits proud of the wheel centre to emulate the prototype axle could be used.

Mike

[David_V]

Thanks for the answers to those questions Nick.

Regarding square or hex axles, I've experimented with this but found that the inherent pliability of the nylon material means that registration is not enough on its own because the wheels will still 'tweak' a few degrees either way. For instance, this 'H' section wheel on a Stephenson Long Boiler engine in G1 is a tight press fit on the (printed) axle, but will still creep either side of 90 deg if forced.



Like Nick I've found that a press fit on a steel axle is a good solution, but a note of caution: different vendor's equipment produce different tolerances on fits like this. For instance, Shapeways wheels where the bore is exact size produce a good tight press fit on Silver Steel axles, wheras wheels printed elsewhere can come in as a loose fit, all while conforming exactly to external dimensions. This is something to do with how they resolve holes like axle bores and need the bore to be maybe 0.1mm undersize. Either that or superglue!

When pressing nylon wheels onto steel axles it's very important to press square because the nylon will happily take up any poor alignment you give it!

Once assembled, wheels like these have run many hours without trouble.

David

[753]

This is a post from the G1 3D Circle, maybe of interest


I'm printing wheels regularly now. The 5 inch gauge wagons have run on them very successfully and more running is planned this season. The wagons weigh about 5 or 6 kg all up so each wheel takes its quarter share. Then there are the bumps and it's difficult to estimate the forces involved. Even with working suspension, the forces will be significantly higher than 1.5 kg. So far, so good.

I print in PETG and push fit onto steel axles. PETG allows some machining of the prints, for example, cleaning out of the bore ID.

The CAD and print is in 27 mm/ft (5 inch standard gauge) for my new Midland goods brake project).

On that latest design with split spokes, the spoke width on the smaller wheel (with the experimental stub axle in the picture) is only 0.4 mm and therefore only one extrusion line width. That is for my 4 inch gauge (21.33 mm/ft) commission work on the Midland standard gauge wagon.

I see no need to use fibre-reinforced filaments. The wheels are very strong. I suppose that is the spokes working as intended.

Some may remember my project to have steel tyres CNC turned for various sizes of wheel. It became clear that the costs would not add up so I shelved that.

In Gauge One, I would print the wheel with a 0.2 nozzle. I might even give it a go shortly because I have some wagon plates to print with a 0.2 nozzle so the machine will be available after that.

Best regards
Graham S

[MikeWilliams]

This is all very positive.  Being a Luddite with a mistrust of all modern materials simply because they are modern, my main concern is not the bumps and bangs of a new wagon in service, but a heavy locomotive sitting on a display track in the same position through hot weather and maybe cold storage, for ten years or more.  I have no reason to think there might be any long term deflection of the spokes, but I remember the wonder materials Plastikard, superglue and others which turned out to have limitations after a few years.  I still have a 4mm carriage built by a professional in Plastikard using the acclaimed Jenkinson method.  It even featured on the cover of a model magazine at the time, but it has warped all over the place and now just scrap.

But I am grateful to the new generation testing these materials because if you don't we will never know!

Mike

[Nick]

Mike,

You're quite right to highlight the uncertainties about long-term life and durability of these materials. I said at the outset that this is an experiment and that is how I see it, because if we don't do the experiment we will never know the answer. In the worst case, if the wheels really do not work, they can be replaced with something more durable.

I have hedged my bets as best I can by researching materials and processes, and choosing a plastic material with the best properties that I can find within what is to me a sensible budget and by using a professional 3D printer and process that produces reliable, repeatable results. I may have been too conservative and I'm very interested to hear about the other examples people here have mentioned. If a "hobbyist" printer and material gives a satisfactory result, that is great for the hobby. But many of us (me included) will still want to see the result demonstrated over a long timescale.

You mention things that turn out to have limitations. With respect to all concerned, many of those limitations were known but not understood, or perhaps ignored, at the time. For example, epoxy resins. Most of us have joints that have failed, but the epoxys that we buy are much the same as those used to stick car bodies or aeroplane wings together. The difference is not in the epoxy, but in the care taken to mix exactly the right proportions, exclude foreign matter from the mix, mechanically clean and thoroughly degrease the surfaces to be glued, use the correct thickness of adhesive and apply the right clamping pressure for the right length of time. All of which are documented, but maybe we don't pay enough attention? After all, a window falling out of one of our model coaches doesn't have the same consequences as a wing delaminating in flight. But because I can't control all the secondary effects as well as a professional printer, I choose to use their services.

I'll tell you how the wheels are working in ten years time!

Nick

[MikeWilliams]

Brilliant Nick.  Can't disagree with any of that!  And well done you for giving it a go.  And if you make a spare wheel to put aside "just in case", then it could always be used as a pattern for investment casting in iron!

Look forward to reading about your carriage painting!

Mike