LNWR Precedent

[NickB]

More experiences of using 3DP components. I drew and had printed driving wheel hornguides pretty much to scale with very little compromise, and they looked good.



The problem was the narrow crossbeam at the top. On the prototype it doesn't matter because it doesn't take any significant load, but it's a different matter for the model. The prototype axleboxes have underhung springs that transmit the weight of the engine directly to the frames. I could not use that on the model (now someone is going to tell me how to make working leaf springs of exactly the right strength) but chose the usual modeller's arrangement of coil springs above the axleboxes bearing on that there crossbeam. The crossbeams would be taking most of the weight of the loco, they would flex too much and ultimately fail.

My first attempt was a little piece of angle above the crossbeam to strengthen it.



It wasn't terribly successful and I didn't like it because it looked like the bodge it was. The photo also shows another problem that arose in rivetting the hornguide to the frame. I didn't want to rely on epoxy alone considering the shear load there would be and anyway, rivets are prototypical. The softness of the plastic meant that it was difficult to set the rivets well, and if you look carefully you can see some of them bent this way and that.

So I gave up on the printed components and my friend Mike Palmer very kindly made me a set in brass on his CNC mill. In doing so we had to loose some of the detail but the result is much more robust and could be soldered in place. The rivets are now cosmetic.



The photo also shows the new motion plate and the rear covers of the cylinders which I had to redesign and replace because it proved impossible to attach the slidebars to the original one without a 1:22.6 scale spanner and 1:22.6 scale fingers to hold it. F360 does a lot of clever things but fails to warn you about that.

The bottom line: 3DP technology (as we amateurs know it) isn't ready for components that take any sort of load.

Thanks to Mike Williams' good offices, I have recently acquired a copy of the Bill Finch Portfolio of these locos. What a wonderful resource - full of detail sketches of all the fittings which clarify the works drawings immensely. Oh, but all those extra fittings to model. When did I say this would be finished?

Nick

 

[simond]

The bottom line: 3DP technology (as we amateurs know it) isn't ready for components that take any sort of load.

I have only a few weeks’ experience, but I’m not sure I’d agree with that.

clearly printing a prototype metal object in plastic will to lead to a reduction in strength and stiffness, but equally, scaling down by 1/43.5 will lead to a mass reduction of 1/82313, so I suspect it might still be adequate - and that doesn’t account for the density differences…

joking aside, the resin I’m using seems to be adequate for wagon chassis including W irons, and the sneaky hidden extensions from the back of the axlebox up to the guitar-string spring.

what I am interested in knowing is how long does it last, particularly under load. Does it deform & sag or simply turn to dust?


turning back to your horn guides, I understand that it is possible to 3DP in metal, though that’s not an amateur operation, alater natively, it’s possible to 3DP a sprue, and have it cast, though that might be pricy, and take a while.

in any case it’s a fascinating technology, and I’m thoroughly enjoying your loco build
cheers
Simon

 

[michael080]

Nick and Simon,

I have to agree with Nick that I don't feel comfortable to design 3d-parts that would be permanently stressed with a mechanical load. But then, it is a matter of the desing process to combine the superior appearance of a 3d-printed part with the mechanical rigitiy of a metal part.
In your case above, you could have inegrated that little angle into the body of the 3d-part, so that the spring is in direct contact with the angle. The 3d-part would have to have a cavity that houses the angle.

From my personal point of view, the hype about 3d-printing ignores the fact that designing parts is not as easy as it appears. You need to have some knowledge about material sciences and about mechanical load distribution. And of course, it requires some knowledge of chemistry.

Michael

 

[NickB]

Simon,

Your scaling is absolutely correct and applies to static loads. However, nylon has much lower flexural strength than brass and in like-for-like comparisons deflects more. When you place the loco on the track, brass deflects immeasurably but nylon deflects visibly. That in itself isn't a problem as long as the loco stays still. The problem is in the dynamic loads - the bumps and shocks that occur when the loco is moving over rail joints and round curves. Our curves are sharper than scale so the sideways loads are greater and if the track is less than perfect, the other bumps are larger than scale (particularly in the garden - remember this is G3).

Plastics are particularly poor in fatigue, that is, repetitive loading. Remember how you can break a moulded plastic part off its sprue with just a few twists? That is a fatigue failure. Nylon is better in that respect but still miles below common metals, and that was the main reason for my concern. This is intended to be a working loco, not a glass case model.

My summary was, in retrospect, a bit glib and was meant to apply to locomotives. I'm aware that the technology is becoming widely used for rolling stock. You pose the question how long will it last? I think the main problem is long term embrittlement, which is exacerbated by UV light - so use a barrier coat of paint and keep them out of direct sunlight as much as possible. Apart from that, we will learn as we go on.

3DP in metal is possible but must be professionally done and is expensive. The least expensive materials at present appear to be stainless steel and aluminium, which are both fine for us as long as you never want to solder anything to them. Mild steel, for technical reasons, is rather tricky and probably best avoided. Brass can be printed directly but it's more common to print masters for conventional investment casting. Technology developments in this area are driven by industrial requirements, and there isn't much demand for the modeller's favourite material, brass, so that may not change quickly.

Nick

 

[NickB]

Michael,

I agree completely about designing. In both instances I've mentioned here, the hornguides and the motion plate, I started with designs that were almost exact scales of the prototype (because that is my objective), encountered problems that weren't easily fixed using 3DP and had to change the design and the material. It's been sometimes frustrating but I've learned things that I can apply in the future. Maybe my experiences will help others in similar circumstances.

Nick

 

[NickB]

Slidebars in place, and checking clearances with crossheads:



Just like the prototype, the slidebars are held in place with bolts, except that mine are M1.0. I was nervous about cutting a dozen tiny threads in 2mm thick steel (visions of a bin full of broken taps), but using a brand new tap, cutting oil and taking it slowly and carefully, all went well. I can pull the bolts up suprisingly tight, but if they do come loose in service I'll use a drop of threadlock.

Then disaster, or so it seemed. On one driving wheel the tyre had come detached from the centre over a segment of the circumference.



The rest of it was still secure and I could not see how to get the tyre off completely without a high risk of damage. Close inspection showed epoxy still attached to both surfaces, so most likely the failure was due to poor mixing of the glue and hardener (another lesson learned). I used a few drops of the thinnest Loctite readily available in the hope that it would penetrate into the gap before going off, and it seems to have done. Phew.

Nick

 

[NickB]

Next the assembly of the wheelsets. The wheel centres were 3D printed so the bore was "dead-on", but nylon is an elastomer and in practice they grabbed the axle a bit more than was convenient for final adjustments, so I very carefully eased them out with wet and dry wrapped around a mandrel of suitable diameter. These are the tools to set the wheels in place and hold them while the adhesive takes.



The H-shaped part carries the axle and the two cutouts are sized to hold the axleboxes. Quartering is done by resting the crankpins on the two pillars.



Once you know the height of the axle from the baseplate, the size to make the pillars is a matter of geometry (or if the thought of square roots makes you weak at the knees, draw it in CAD). The remaining parts are to check the back to back dimension, but I leave them in place during hardening just in case anything tries to move.



And there's the finished wheelset. This is the driven axle so it has the motor-gearbox unit attached, but the crank axle is similar.



Nick

 

[NickB]

Here's the valve gear assembled. You can't really say it works for an electrically powered model, but it goes round as it should. Still some temporary fittings to be replaced with the permanent ones, but it feels like a milestone reached.

As always, the assembly was fiddly and frustrating, with the usual problems of building order: can't fit A before B, but need A in place in order to fit B. 1:22.6 scale fingers would have helped a lot. Or a third hand. Or both.





Nick

 

[simond]

Oh, that is so very nice.

 

[Jon Nazareth]

I agree with Simond, excellent, Nick.

 

[NickB]

The brakes are between the driving wheels, with a single actuating lever. This appears to be common for 4-coupled LNWR locos of this era, and it is a characteristic feature. The parts are the usual mixture of laser cut, 3D printed and fabricated.



Of course the assembly hides much of it. At least I didn't have to model all the linkage behind that cover.



This is the brake cylinder located beneath the cab and the various actuating bits and bobs. Notice the pull rod - as an experiment I had it 3D printed.



The eyes are fine, but the rod itself looks rather coarse and is not straight. The material is sufficiently elastic that it won't straighten of itself so I might have to tension it when I assemble it enough to pull it straight. If that doesn't work I will replace it with a metal rod and reuse the ends if possible.

Nick

 

[NickB]

I didn't like the pull rod, but I did manage to cut off the forked ends and drill them for a steel rod so it was not wasted. This is a trial assembly. Hats off to Crewe, it is an ingenious mechanism that pulls the engine and tender brakes all from the one cylinder. I won't try to replicate that completely, even in a non-working form.

Nick

 

[NickB]

Thinking ahead, I shall need a number of brass fittings in the cab. For the T3 I had similar fittings 3D printed in brass and was very pleased with the results. However, this process has become a lot more expensive and that's made me think about alternatives. Mike Williams suggested I have several sets cast and take advantage of the economies of scale, because the same fittings were used in different classes of loco. But at my rate of building I doubt that I will last long enough to make it worth doing.

I wondered about metallic paint on 3D print in a plastic material. It is a long time since I last used such paint and maybe it was worth another try. The motion plate printed in PA-11 MJF that I had rejected as too flexible was now spare and served as a test piece. The paint is Humbrol #54.

The left hand half of the plate in the photo is out of the tin. For comparison and because your screen will probably render colours differently from mine, I included a couple of scrap pieces of brass, one newly polished and the other somewhat tarnished. The paint colour is much too red, more like bronze than brass. The right hand half is the brass paint with a bit of yellow added. That brings the shade closer to the tarnished piece but also loses some of the metallic lustre. I think that's because the brass paint is actually tiny flakes of brass in a clear paint medium, and adding a non-metallic colour has the effect of diluting them.

So I'm really not sure at present. Fine-tuning the mix will probably get the shade more accurate but I'm not hopeful about getting that lovely polished brass finish this way.

Nick

 

[michael mott]

Here's the valve gear assembled. You can't really say it works for an electrically powered model, but it goes round as it should. Still some temporary fittings to be replaced with the permanent ones, but it feels like a milestone reached.

As always, the assembly was fiddly and frustrating, with the usual problems of building order: can't fit A before B, but need A in place in order to fit B. 1:22.6 scale fingers would have helped a lot. Or a third hand. Or both.

View attachment 182341

View attachment 182342

Nick

I have to agree with Simon, really looks great. Your comment about assembling order made me smile, thinking about an apprentice back in the day on the full size loco.
Michael

 

[Rob R]

Thinking ahead, I shall need a number of brass fittings in the cab. For the T3 I had similar fittings 3D printed in brass and was very pleased with the results. However, this process has become a lot more expensive and that's made me think about alternatives. Mike Williams suggested I have several sets cast and take advantage of the economies of scale, because the same fittings were used in different classes of loco. But at my rate of building I doubt that I will last long enough to make it worth doing.

I wondered about metallic paint on 3D print in a plastic material. It is a long time since I last used such paint and maybe it was worth another try. The motion plate printed in PA-11 MJF that I had rejected as too flexible was now spare and served as a test piece. The paint is Humbrol #54.

The left hand half of the plate in the photo is out of the tin. For comparison and because your screen will probably render colours differently from mine, I included a couple of scrap pieces of brass, one newly polished and the other somewhat tarnished. The paint colour is much too red, more like bronze than brass. The right hand half is the brass paint with a bit of yellow added. That brings the shade closer to the tarnished piece but also loses some of the metallic lustre. I think that's because the brass paint is actually tiny flakes of brass in a clear paint medium, and adding a non-metallic colour has the effect of diluting them.

So I'm really not sure at present. Fine-tuning the mix will probably get the shade more accurate but I'm not hopeful about getting that lovely polished brass finish this way.

Nick

View attachment 185222

In the long and distant past I have used Hubrol Gold paint for polished brass. It has that nice "just finished with the Brasso" look.
Sorry, can't remenber the Humbrol number

 

[Paul Tomlinson]

FWIW I used Sharpie pens from Wilko in a pack of three (Gold, Silver, Copper) to detail these cast backheads. There might also be something in the Molotow range, used to good effect by Yorkshire Dave : https://www.westernthunder.co.uk/threads/whats-on-your-workbench.3804/page-113#post-195737

 

[john lewsey]

Beautiful work Nick, Just beautiful
John

 

[Mike W]

Nick, its probably too late for this engine but I often cast parts for others myself in resin or w/m, and get them cast professionally in brass or nickel, and I stand the cost. I can then add them to the range and make available to anyone. Some scale societies would do that anyway - the G3 Society don't (yet), so I do. But for brass it would mean a delay of two or three months which I guess might not be acceptable. Back in the '70s Ks used to do something similar - hence their boxes of strange small parts like 7mm loco shed window frames made by Laurie Ward and 4mm coach bogie sides by Philip Millard!

 

[NickB]

Rob and Paul,

Thank you for your suggestions. Gold, in one form or another, sounds like an option worth trying. Whatever they use in gold paint, at the price it certainly isn't gold, so brass maybe?

Mike,

I haven't written off casting and 2-3 months is an acceptable timescale. But I'll pursue the paint options a bit further to see where they go.

Nick

 

[David Waite]

Hi Nick
It’s a joy to follow your build wonderful workmanship.
David.