LNWR Precedent

[Nick]

While the T3 class loco is in the paint shop, this is the next loco project. I chose it partly because there is plenty of information available. I got copies of the LNWR drawings from the NRM and visited the preserved Hardwicke at Shildon to take lots of photos (great cooperation from the staff at both places). From that I was able to do a very detailed solid model. I intend to use commercial laser cutting and 3D printing services, and possibly some CNC machining, partly to speed up the project but also because I'm interested to explore how we can use modern manufacturing methods.

Laser cut steel for the frames and other flat components.



The frames and stretchers were designed with slots and tabs for assembly. The slots were so well cut that I could just put it together for the photo. It will, of course, be soldered up later.



I had the driving wheel centres 3D printed in PA-12 nylon some time ago to see if the combination of printed centre and turned steel tyre was viable. There's another thread on this forum about that, but here is the finished product.



Other components 3D printed.



There is also a growing collection of machined parts. These were the ones I judged not feasible or too expensive to have made.



And this is the Mark 2 version of my spur gear drivetrain. It comprises a Polulu 60W motor which comes with an integrated gearbox to drop the rotational speed to 500 RPM, then a spur gear to the driving axle. The Mark 1 version went into the T3 loco. The only problem with that was that I underestimated the axial force exerted by the spur gears - they try to push each other out of mesh. Not only was it hard to stop the driving gear sliding along the motor shaft, but I was concerned about durability because, when I dismantled the inline gearbox, I discovered that the thrust bearing was quite crude (but at least it had one).



The Mark 2 version has a ball thrust bearing between the spur gear and the motor casing to take the load. There wasn't enough room on the driving axle for anything similar to that, so I added a brass sleeve to fit between the spur gear and the gearbox frame. At least that will be quite accessible for regular oiling. The T3 drivetrain has been retrofitted with similar bearings and so far it works well.

Nick

[MikeWilliams]

I think I'm going to enjoy this build!

Mike

[Nick]

Doh! For spur gear, read bevel gear. The curse of old age strikes again.

Nick

[John Branch]

Regarding the accuracy of the laser cut parts, this might have had something to do with the accuracy of the drawings.

John

[Nick]

Hi John,

The laser cutter follows the instructions it's given, so for sure the accuracy depends on that. Using 3D CAD makes it easy to ensure that the slot and tab are both exactly the same size and in the right positions. From the CAD model I can generate inputs for both laser cut and 3D printed parts.

Laser cutting like this was new to me, so I was pleased to discover that the process gave sharp corners and edges with no significant roughness or burring.

Nick

[Nick]

Coupling, connecting and eccentric rods made by traditional drilling, sawing and milling.



While making them, it occurred to me that I could have shortened the process.

When I was deciding what to make by laser cutting, I was concentrating on flat parts like frames, and the rods struck me as too 3D. Not so! The traditional way of making them is to start by cutting the outline and then machining the width as necessary. Since laser cutting can be done in a wide range of material thicknesses, I could have had the outlines (side or top, depending on the complexity of the part) done that way, leaving me with the 3D shaping to do on my mill. That would have been a significant saving in time. Oh well, I'll remember that for the next project.

Next up, the remainder of the valve gear parts, then assembly can start.

Nick

[753]

Excellent work Nick.

On the coupling rods do you fit them to the wheel centres to check fit before opening out for the bearings?

Mike

[Nick]

Hello Mike,

It's all done by direct measurement. I checked the laser cut frames and they are as accurate as I can measure. The coupling rod centres are drilled directly (my mill has a quill and a DRO - the latter means I hardly have to mark out anything), and I set the quartering of the driving wheels using a jig. They may not be exactly 90° but it's an electrically powered model so that doesn't matter, but they are all the same, which does matter.

Of course nothing is perfect and I often have to open up one or more of the coupling rod bushes to give free running, but not usually by more than 0.1 or 0.2 mm.

Nick

[Nick]

Here are the slidebars, crossheads and all the remaining valve gear parts, all made by traditional methods. As previously mentioned, I'll consider making more use of laser cutting next time around, but sadly not 3D printing, which in metal is way outside my budget.



When assembled, it should look like this. The valve gear is a form of Stephenson's, but straight link and balanced, so the valve rod goes up when the link goes down (and vice versa). From the modelling point of view, it just means more components and pivots (groan).



That's everything done for the chassis, bar a few little bits and pieces. Ready to start building!

Nick

[richardgreen]

Wow, Thats really impressive!

[Nick]

The crank axle.



In case anyone is in doubt, it is built up, not machined from solid - I'm not clever enough to do that. All the joints were glued and pinned, then the axle was cut away between the cranks and cleaned up. The eccentrics are secured with grub screws so as to get the valve timing right. I know it's an electric drive, but I might as well get it right.

When I checked my photos of the preserved Hardwicke, I was surprised to discover that the left crank was leading. On delving further, I discovered that this was common practice for the LNWR, unlike many other British railways and builders where the right crank led. That meant I've built several locos in smaller scales the wrong way round. Whew, nobody noticed (unless they were too polite to mention it).

Nick

[Nick]

"There's this crazy guy in G3 who assembles his model locos the same way as the real ones were ..."



Let me explain. Usually I'd do features like this using simulated studs and nuts, but my usual suppliers could not provide them in the right size. Why not, I thought, use scale nuts and bolts? M1.0 was exactly right, and I bought enough of them from Prime Miniatures for less than I would have paid for the simulated fittings. Win, win! Okay, they were a little more fiddly, but the result was worth it.



Nick

[753]

Impressive work Nick, the more crazy the better.

Mike

[Nick]

This project was always an experiment into how far I could take new technologies, and of course you don't know the limits until you try to push too far. Herewith, the story of the motion plate. The real thing wasn't a plate, it was a cast component with flanges, strengthening ribs, and of course brackets to hold the slidebars. An obvious candidate for 3DP.



Unfortunately the print in nylon wasn't up to the job. It was just too flexible. Mike Palmer very kindly offered to make one in brass on his CNC mill, but the many narrow corners and pathways would have been a serious struggle. Miniature CNC machines do exist, but they come with anything but miniature price tags. So it was back to the old ways, starting with a steel plate (done by laser cutting, FWIW). To this I silver soldered the brackets for the slidebars.



The tedious bit was cutting out the ribs along the top and round the openings from brass with a piercing saw, then soft soldering them into position. You will see I have simplified the detail somewhat, but I don't think it will be too obvious. The plate will be attached to the frames using tabs and slots instead of rivetted flanges. The flanges on the original will be dummies, riveted to the frames.



How else could I do it? 3DP in metal would be nice, but brass has become very expensive and for various reasons mild steel still isn't very printable. Stainless steel is probably the best option at present (as long as you can be sure you will never need to solder anything to it), but for this component it is still about £40 for a one-off. No, until metal printing becomes a lot cheaper and better, for some things the old methods will still be used.

Nick

[John Candy]

Nick,
A pity about the nylon motion plate since it looks to have a very nice "finish"/patina and would have saved a lot of time. I have resevations about using non-metallic 3D printing for mechanical/stressed  parts and your testing the limits has been a useful exercise in that respect.
The intricate work involved in fabrication of the replacement part has certainly produced a worthy replacement.
I am working with PETG to produce coach bogies and it is extremely strong in thicker sections but is still at the proving stage.
Who would you use for printing in brass or steel?
Regards,
John.