Building an MMP RMB - a box of delights

[Heather Kay]

I've just confirmed a research trip to explore and photograph a real RMB over at the Kent & East Sussex Railway at Tenterden next week. Unfortunately, it's in the shed and being stripped for repair/refurb before the new season begins, but I'll still hope to get some useful information and images.

So, from all the line societies I approached for a research visit to see their RMBs, only one major one was unhelpful. I won't name them, unless provoked. The Colne Valley, Chinnor & Princes Risborough, Watercress and K&ESR have all been forthcoming and helpful.

It's just a pity all the preserved ones I can get to are steam heat and vacuum brake only.

 

[Heather Kay]

Rather than get engrossed in more workbench activities today, as I wait to go for an appointment with my osteopath shortly, I've been researching dual brake and dual heat layouts for Mk1 coaches. The latter is really down to the various dangly bits on the buffer beams. I'll have to have a good think about how to fit the extra brake pipes and ETH connections around the "standard" vac and steam fittings.

As for the air brake bits elsewhere on the coach, my searches turned up a page by Mr Jim S-W of P4 New Street fame. A very useful page, with useful detail photos of air brake fittings on what appears to be a catering vehicle of some kind. I think I almost have enough information to make a good impression of how everything was attached. All I really need is a clearer idea of the air brake pipe runs, if they're even visible.

 

[alcazar]

Hi Heather, lovely work.

I've PM'd you.

 

[SteadyRed]

This is a fantastic build, so temped to order. Must resist must resist.

 

[Heather Kay]

Blimey! Nearly a week since the last update!

My excuse is I've been rebuilding the workbench area, and working on the 57xx for a spell. Today, I spent the morning investigating a real live RMB at the Kent & East Sussex Railway. You can see the result of my labours in a Gallery thread.

On the bench, I've been working on completing the Commonwealth bogies, which are the JLTRT cast ones that Richard had mostly built. There's an event this coming weekend, and I'd like to be able to take the frames so far built with bogies just to show what I've been up to.



This is the state of play. I'm cleaning up and blackening the wheelsets, and also attacking the castings with chemicals to darken things down before a coat or three of paint. You can just make out the suspension more or less operates just like the real thing.

I'll add some brake yokes, possibly nicked from the unbuilt etched bogies in the kit. Seems a shame to do that, but unless Richard wants them for another project, the bits will be going spare.

 

[Heather Kay]

Before I fix the wheelsets in the bogies, I want to see what the team thinks about ride height.

My take is things look a little too high, but not outlandishly so.







The castings have had a dose of chemical blackening. As I type, I've slapped a coat of grot black/brown over them, as I can access parts with the brush more easily before the bogies are completed.

So, do they look a little high? The fix may well be just to file down the central bearer a little on the casting. No mods are necessary for the underframe that I can see.

 

[OzzyO]

Before you start filing bits off, weigh the rest of the kit and put that on the frames and see they look then. The only way that you can really know is to check the centre hight of the buffers (24.5mm from rail head comes to mind). Doing it this way at least you should know how much to remove.

OzzyO.

 

[Heather Kay]

Before you start filing bits off

Ah, yes, I realise the springs aren't being compressed at this stage. I was looking at the gap between the top of the bogie and the bottom of the solebar in general. Comparing it with an image I took on Wednesday, the model seemed a tiny bit high.

 

[Pugsley]

Have you considered winding your own springs from brass or copper wire of a larger diameter? I guess those springs are included in the kit, but they look awfully weedy compared to the real things.

 

[Heather Kay]

I guess those springs are included in the kit, but they look awfully weedy compared to the real things.

Agreed, but they are functional springs in the JLTRT kit. My mechanical knowledge isn't really up to winding suitable replacements that will look better and still function. I think I'll leave that to better practitioners.

I've done as Ozzy suggested, and loaded the frames with all the remaining components to gauge the buffer centreline height. It's not very far off the 24.5mm he mentioned, so I am content that the riding height is not a million miles off.

Pleasingly, it seems there is no major surgery required to fit the JLTRT bogies to this kit, so I am aiming to get the wheelsets and keeper plates fitted, and some temporary bolts, so I can take the rolling chassis to the client this weekend.

 

[Dan Randall]

Have you considered winding your own springs from brass or copper wire of a larger diameter? I guess those springs are included in the kit, but they look awfully weedy compared to the real things.

They look very feeble to me too. There ought to be 6 or 7 coils visible, with not much daylight between them. Perhaps some kindly soul on here could 3d print some suitable replacements, that would look the part, but also be functional?

I was thinking about doing some printed springs for my MMP BCK's bogies when the time comes, but there are a few 3D projects in the queue ahead of them at the moment!


Regards

Dan

 

[john lewsey]

Just think chaps I get to see this on Sunday

 

[djparkins]

Just think chaps I get to see this on Sunday

Yeah! - then order a coach kit!!!

 

[SimonT]

The springs can be printed if sufficient information is available for the 3D drawing. What we don't know is the physical properties of the cured resin other than being described as having the same properties as ABS plastic. Anyone got the info? I've got the printer and drawing package. It has to be worth a play.

Simon

 

[OzzyO]

They look very feeble to me too. There ought to be 6 or 7 coils visible, with not much daylight between them. Perhaps some kindly soul on here could 3d print some suitable replacements, that would look the part, but also be functional?

I was thinking about doing some printed springs for my MMP BCK's bogies when the time comes, but there are a few 3D projects in the queue ahead of them at the moment!


Regards

Dan

Some things just do not scale and work, without a lot of work.

OzzyO.

 

[Pugsley]

Agreed, but they are functional springs in the JLTRT kit. My mechanical knowledge isn't really up to winding suitable replacements that will look better and still function. I think I'll leave that to better practitioners.

If I can do it, so can you! Do you have a lathe, or pillar drill, that will run at very slow speed?

 

[Heather Kay]

If I can do it, so can you! Do you have a lathe, or pillar drill, that will run at very slow speed?

I do, but I don't plan on spending any more time on these bogies. I've already had to repair one set of brake blocks, and the others appear to be cast from butter or something. The less fiddling about I do the more likely the bogies will continue to work.

 

[Pugsley]

Fair enough.

 

[alcazar]

Cast from butter? muahahahahahahaha!

 

[Bob Reid]

It may not be of any use to Heather however, here's the gen on the Commonwealth Bogie Springs....

Equaliser Springs (right wound) - Bar Diameter 1.4" - Height of spring at Tare Load 12" Outside Dia. of Spring 8.988"

Bolster Springs (Double Nest) - Outside Spring (right wound) - Bar Diameter 1.177" Outside Dia of Spring 9.979". Inside Spring (left wound) - Bar Diameter 0.865" Outside Dia of Spring 7.355" Height of Double Nest at Tare Load 9 5/8"

Note the dimensions above are for the springs on the bogie at the non-toilet end. There were different springs used at the toilet end however the differences in dimensions are in the overall scheme of things pretty "nominal"!