4mm Lynton & Lynmouth - 4mm scale, 8mm gauge.

Tim Watson

Western Thunderer
It’s quite important to get weight, or springing, or both over a pony truck. I therefore took two lumps of brass, filed a common face, sweated them together and then made a rectangular block with a joint in the middle. This was then milled further to shape and drilled twice at 1.9mm diameter for the 2mm axle.

The block was further milled to give a rebate at each end that would accept the A frame of the pony - to be made from 1mm thick brass. The midline solder joint is obvious in this image.

The as-milled block for the rear pony is sitting in front of the front pony in this image, with the A frame soldered in to place. This follows a lazy z shape to allow the ride height to be correct. It also means that the pony ‘works’ from ‘bottom up’ rather than ‘top down’, which I think helps stability.

The front of the brass block was lightly milled and an indication of the springs filed into place - not that they will ever be visible. Also of note is that the A frame was filed with a bevelled edge to make it look thinner: it is quite thin strap material on the prototype. The Heljan original shows the clear difference in mass.

With the pony in place it is clear that there is plenty of room for side play and the correctly place pivot point will guide the engine into curves. If a guide spring proves to be required then that can be added easily at a later stage.

The rear pony will very probably have a compression spring built into it to take some of the rear overhanging weight of the engine. The pivot bolt will be replaced with a correctly shouldered fitting and the pivot hole then reamed to an x shape to allow movement in all axes.

Tim
 

simond

Western Thunderer
Nice.

my milling exploits today have led to three broken cutters, and no useful parts :(

maybe I’ll get it right tomorrow...

Atb
Simon
 

Giles

Western Thunderer
For brass and NS profiling on the Stepcraft, I tend to use 1.6mm D bits with a pass depth of around 0.3mm and a rate of around 0.25 - 0.3mm/sec. Thereabouts....... i had very little luck with conventional milling cutters at the sizes. I still get the occasional breakage - but the mortality rate is just about acceptable!

This was done with a 0.8mm D cutter, at similar parameters, and took about 50 minutes!


CNC cylinder end cover
by giles favell, on Flickr
 

Tim Watson

Western Thunderer
That is really impressive, Giles. My mill / drill is handraulically powered, but very accurate, being made by John Stevens - who makes watchmaking equipment.

Tim
 

simond

Western Thunderer
thanks both,

I'm trying to make somemething really simple, the lifting lugs for the Garratt's tanks, they're 0.4 brass (fret waste, so at least that part is cheap...) and I think I have got down to some settings that might work, using Chinese 1mm endmills.

I think it was these 10pcs 0.6-3.175mm PCB Carbide End Mill TiN Coating CNC Engraving Milling Cutter | eBay

I think the final failure was that my DOC was a bit ambitious, I should go for around 0.1 to 0.2. My milling machine is another ebay special, and I suspect, not that accurate at all...

the cylinder cover is brilliant.

it's all learning
atb
Simon
 

Tim Watson

Western Thunderer
Certainly will be, they’re the obvious choice - I have some also. How they will work is anyone’s guess, especially with any automatic uncoupling, but the fine scale standards should help with vehicle alignment. It will be much better not to have the couplings fixed to the pony trucks.

Tim
 

Giles

Western Thunderer
Tim, a thought - I suspect that the PWM setting on the receiver probably isn't set to the optimum for your motor. It is easy enough (certainly with stick transmitters) to change the PWM frequencies, play around and find what your motor is happiest with, which may improve slow speed running.

Best

Giles
 

Tim Watson

Western Thunderer
Thanks for that observation Giles. I think you are correct. I would seriously value your programming input when we are allowed to meet-up more easily. This project has taken a bit of a backseat at the moment as the YR tube has taken a wedge of time. I am quite looking forward to starting some proper engineering again, especially with Tom Mallard’s build underway.

Tim
 
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