Little & Large - The Socially-Distanced Challenge

andrewnummelin
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Little & Large - The Socially-Distanced Challenge

Postby andrewnummelin » Sun Apr 12, 2020 12:14 pm

Well here goes.
The "little" one is a Taff Vale Railway wagon with iron boxes for carrying coal, the drawing being dated 1851 was numbered 4845 in the old BR/OPC collection and I also have a copy of drawing 4846 that shows a slightly different design box.
TVR coal box wagon trimmed.jpg


The "large" one is the GWR armour plate and girder wagon with three bogies that I first became aware of when looking at the back of the dust jacket of "A History of GWR Goods Wagons Vol 1" by Atkins, Beard, Hyde and Tourret from 1975. For this I have drawing 1207 from the BR/OPC collection.
totem.jpg


I suppose in both cases there is a strong arguement for starting a CAD drawing and then having much etched, but with businesses closed at present because of the pandemic I'll use a more traditional approach and try to hone my scratch building skills. However, along the way I'll try to note what would be better made as an etching, especially as the TVR wagon would look odd on its own as they almost certainly operated in rakes.

First job was a study of the drawings to identify what materials and parts I will need before checking whether I need to go shopping or not. But, a first quick perusal suggests the following are likely to cause problems. For the TVR wagon it will be wheels (7 spoke according to the drawing), suspension (it will be very light) and couplings (lug & shackle not hook & chain). For the GWR wagon it will be the suspension of the bogies (compensated, sprung or rigid?), suspension of the wagon on the bogies (simple springing, "flexi-chass" or prototype arrangement?)

LITTLE

Close examination of the drawing showed significant distortion in places so simply tracing for a CAD programme will not be possible: drawing from scratch will be needed. The boxes very well dimensioned but there are almost no dimensions on the underframe so these will need to be estimated. Scaling from the drawing gave the following for the model:
wheels: 11mm diameter
wheelbase: 19.5mm
length over headstocks: 39.3mm
width between solebars: 21mm
track gauge: EM (18.1mm !!!!)

So, I'm going to assume:
wheels: 12mm
wheelbase: 20mm
width between solebars: a bit more than 21mm, but how much? (P4 wagon wheels measured at 22mm over the tyres.)

The real problem here will be the width between the solebars. I know some people think that such a short wheelbase wagon does not need any form of suspension, but I'd like to spring it but achieving that will be a challenge.

When I first looked at the drawing I thought the wagon would not have a floor: this would have made construction more "fun", but it does appear that there was one. So instead of thinking of solid metal underframe members I can use my usual wood or plastic and there should be room to add a sheet of lead under the floor to give a bit of weight. I might even try lead for the flooring if I can finish it to look like unpainted wood.

The coal boxes should be fairly straightforward, just flat metalwork. The pieces are of varying thicknesses, scaling at 2, 3, 4, 5, 9 & 11 thou. I think I'll go for a compromise and use just 5 & 10 thou, but to get the right impression of the overlapping plates these may have to be made of 2 or 3 thou brass shim.

As I've got good selection of metal, plastic, wood and wire I should not need to go out and buy anything other than the wheels. Perhaps the solution will be 3D printing of centres to fit thinned down wagon wheel tyres that I have in the spares box.

LARGE

As far as materials go, the problem I envisaged was of channel solebars of a size not commercially available. However examination of the drawing showed the solebars having been fabricated from plain sheet plus angle irons top and bottom so making these should not be too difficult. The rest of the body should be simple metalwork without too much rivetting (the floor had flush rivets so I assume these may be ignored on the model).

I should be able to buy the wheels needed but buffers may be a problem being standard wagon bodies but with oval heads. Will I have to accept the challenge of the axelboxes, or are there commercial castings available?

The final questions today are:
- can I turn the stanchions? I have doubts about my abilities.
- how to arrange the bogie suspension? This is the real challenge of this model!
Regards,

Andrew Nummelin

Terry Bendall
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Re: Little & Large - The Socially-Distanced Challenge

Postby Terry Bendall » Mon Apr 13, 2020 8:51 am

andrewnummelin wrote: can I turn the stanchions? I have doubts about my abilities.


Looking at the drawing Andrew, the stanchions look to be parallel above the solebar so they may have just slide into the "socket". Perhaps a plain length of rod/wire of the appropriate diameter and a piece of tube to suit. This of course may have to be solid rod drilled out. It looks like there is a plate top and bottom of the socket which is triangular so this might be thin sheet filed to shape, bent into a squared off U shape over a square or rectangular bar of the appropriate size with the tube soldered between. Then drill through to form the socket for the rod.

andrewnummelin wrote: buffers may be a problem


Perhaps find some round head buffers that are close, remove the heads and fit oval ones instead? Alternatively turn them up on the lathe which is a challenge. A very interesting prototype however.

Terry Bendall

andrewnummelin
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Re: Little & Large - The Socially-Distanced Challenge

Postby andrewnummelin » Tue Apr 14, 2020 6:09 pm

Terry Bendall wrote:
andrewnummelin wrote: can I turn the stanchions? I have doubts about my abilities.


Looking at the drawing Andrew, the stanchions look to be parallel above the solebar so they may have just slide into the "socket". Perhaps a plain length of rod/wire of the appropriate diameter and a piece of tube to suit. This of course may have to be solid rod drilled out.

Stanchions were 2.5" diameter with 4" diameter bit that sits on top of the socket forging (2.5" hole in a 4" wide socket). So the answer may be to make a tube of 1.3mm diameter and cut a washer to solder to the stanchion rod (0.83mm diameter) with a generous solder fillet at the top of the washer. I wonder, will I be able to turn a rod 0.83mm diameter wide and 15mm long?

Terry Bendall wrote:
andrewnummelin wrote: buffers may be a problem


Perhaps find some round head buffers that are close, remove the heads and fit oval ones instead? Alternatively turn them up on the lathe which is a challenge. ...

Terry Bendall

Turning an oval needs a very special lathe! I may have a go at turning a buffer with large head and filing it to shape; but it might be possible to use start with a commercial coach buffer head.

First lathe job may however be turning down some wagon tyres for the little wagon, the drawing scales to a tyre width of around 3.5". (Will have to be thinner than the P4 standard to fit the narrow frame.

Thanks for the encouragement.
Regards,

Andrew Nummelin

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Penrhos1920
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Re: Little & Large - The Socially-Distanced Challenge

Postby Penrhos1920 » Wed Apr 15, 2020 9:47 pm

Cambrian do the axleboxes on their GWR bogies, the last proprietor sold them separately. Rhymney Models also offer them as castings.

andrewnummelin
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Re: Little & Large - The Socially-Distanced Challenge

Postby andrewnummelin » Thu Apr 16, 2020 10:05 am

Penrhos1920 wrote:Cambrian do the axleboxes on their GWR bogies, the last proprietor sold them separately. Rhymney Models also offer them as castings.

Many thanks for the leads. I hadn't realised that Rumney Models did anything other than items much too modern for my projects! I'll contact Justin for information.
Regards,

Andrew Nummelin

andrewnummelin
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Little & Large - The Socially-Distanced Challenge

Postby andrewnummelin » Fri Apr 17, 2020 12:23 pm

LITTLE
It became clear as I was doing the model drawing that the original had highly detailed and dimensioned boxes but the underframe was probably best regarded as the equivalent of a loco weight diagram - a good impression but not necessarily correct in all aspects. So the changes & assumptions I made were:
- the brake block and lever were slightly changed to make them mechanically more logical (on the original drawing the top of the brake block would pull away from the wheel when the brake lever is depressed)
- solebars assumed to be 1mm thick and "buffers" 2mm and extending 4mm beyond the headstock.
- W irons to be 10 thou thick, bearing carrier 8 thou.
- floor 0.5mm thick (dimension on drawing is illegible) and does not actually extend beyond the boxes. (Longitudinal planks like the VoNR broad gauge coal box wagons)
Capture.JPG

Also on the drawing is a standard P4 wheel and pin-point axel. This makes it clear that my model wagon is going to need a shorter, thinner axel and wheels with narrow tyres.
So the next stage is to start cutting material for the underframe.

LARGE
Axelboxes are on order - I'm hoping that the gap between the ending of pandemic restrictions and the following show will allow enough time for delivery and installation. A study of the Rumney Models sprung bogies has given me an idea of how to make ones for my wagon.
Regards,

Andrew Nummelin

andrewnummelin
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Little & Large 2 - The Socially-Distanced Challenge

Postby andrewnummelin » Sun Apr 26, 2020 7:03 pm

Not much progress to report as the weather has meant the garden took priority! Still some things have been done.
little and large 002.jpg


Little

Solebars and one headstock have been fitted to the floor. The second headstock can probably only be fitted once the underframe is running OK as it will block access to the spring wire. Next job is to cut out the W irons from 10thou: at the same time I'll probably cut out the box retaining lugs and the brake lever.


Large

As information was collected about the prototype bogie design I was able to determine how I would attempt their construction built around channel and L sections. I did consider making all of these from sheet but decided to be sensible and buy commercial sections where available. The order has gone in and while I wait for it to arrive I made some progress by cutting out flat bits that will be needed.

Firstly, the top plate was cut from 5thou sheet: I thought this would be easy but I found the sheet to be rather delicate and almost all my stock had small creases or indentations. I did not envisage making the underframe in the same manner as the prototype as I guessed the model would be rigid enough with just solebars, heasdstocks and cross members to support the bogies. However, I noticed that there were some features like the storage pockets and brakes that would need support from various underframe components so the underside was marked out to indicate where the underframe component sections would have been on the prototype. I then had to cut the openings to give access to the storage pockets: I did not find it easy to make a very neat job of this, sawing and filing such thin sheet is not easy and I have created some distortions that I trust will be levelled out when the underframe is added. From the drawing it appears that the top of the wagon was made from 11 plates the full width of the wagon, so the top surface was scribed to show the joins: I made no attempt to indicate the rivets as these were finished flush and I guess with a coat of paint and some dirt they should not be visible on the model.

The top plate is clearly a job that would be much better achieved by etching: the cut-outs would be easy and one could also accurately form the small edge indentations for the stanchions. A question to be considered would be whether or not to half etch the underside to facilitate fixing of the underframe - probably sensible. Tab and slot construction would not be appropriate to avoid any things showing on the top surface.

The vertical plates for the solebars were cut over length as the ends will need to be trimmed to fit exactly into the headstock sections (easy to design if one were etching). Fortunately there are no visible rivets in this section, they are all in the L sections or other items fitted to these items.

The little brackets for stowing the stanchions alongside the solebar were cut from a strip of 10thou with the ends rounded by eye. I've made more than needed... These would be easier to make as etchings, but I would not aim for a sharp bend so just a minimum mark to show the location for bending. For an etched kit I would look to have some sort of registration mark to show exactly where they should be fixed, a half etch on the back of the solebar perhaps. One might also consider forming them as a U to make assembly even easier.

The "skirts" for the bogie support were rivetted and cut out from 0.17mm sheet. Fixing these to the solebars in exactly the right place may not be simple!
Regards,

Andrew Nummelin

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Little & Large 3 - The Socially-Distanced Challenge

Postby andrewnummelin » Sun May 10, 2020 12:12 pm

Two steps forward and seven steps back

Large

The profiles and other bits ordered from Eileen's arrived quickly but nothing has been done with them yet fortunately!

Little

A start was made with the wheel sets: to check that everything else would work, I decided to use standard wheels first before embarking on trying to make seven spoke ones. Firstly I made a mandrel to hold a standard wagon wheel and to turn down the face of the wheel to make it thinner.
little and large 003.JPG

This worked exactly as intended on an old OO wheel but it was a failure with P4 wheels with moulded split spokes. First attempt resulted in the spokes being distorted so I assumed I was being a bit heavy handed with cutting down the face, being much more gentle with the second and third attempts resulted in the rim coming loose from the centre. Perhaps this should have been expected.
So I resorted to use of the mill and thinning the wheels making very fine cuts with a 1.5mm end mill - a tedious job but it worked.
little and large 004.JPG

Then on to the axels: to fit the narrow I estimated that an axel length of around 24mm would be needed so I made one roughly and trial fitted the wheels. Unfortunately the coning of the axel ends extended into the wheels which were then wobbling on the axel. So the next idea was to mount the wheels on tube and fit a 1mm axel through this. Rather than basing the axel on my drawing, I decided it would be best to put the basic underframe together and ensure the axel actually fitted.
The underframe was cut out and tried between the solebars, I was a little concerned that I would not get the bends in exactly the right place and might find that the W irons would be a bit too far apart. The opposite happened and they were actually a little too close together. This is clearly something that would require care in designing something for photo etching but I'm sure there's enough information available to be pretty confident in getting the bends to be correctly spaced. Anyway, I cut the underframe in half and glued it to the wagon floor, taking care to ensure that it was set up square (two long rods viewed against the rulings on a cutting mat).
The bearing carriers were next - very fiddly to cut out. I put a lug at the top for the spring wire rather than use the approach of Bill Beford (or of High Level) that probably requires half etching to hold the wire.
So now I could make 1mm steel axels to fit exactly.
Fitting the wheels to the tube was not as easy as I expected: despite them not being apparently loose, it was easy to mount them not square to the axel. I ended up by using a drop of Loctite 603 with a small amount of epoxy at the rear of the wheel for reinforcement and holding against a back-to-back gauge to keep the wheels square. This was not completely successful, one wheel has a slight wobble. So when the final ones are made a different approach will be needed. I'm thinking towards a wheel centre that is significantly longer than the wheel thickness and making a jig to hold the wheel rim square to the axel during assembly and glue setting.
A quick check with a wheel set, without axel, showed that there would be the needed clearance to the W irons.
little and large 005.JPG

Now disaster struck! The lug on the bearing carrier fouled the wheel rim.
little and large 006.JPG

First thought was that I had not filed them down enough, then thinking the concept might be in error I went back to the drawing and looked very closely. Even allowing for the wheels being measured as slightly larger diameter than I had drawn, it was clear that the lug should not foul the wheel tread (there should have been about 0.2mm minimum clearance). So, had I made the bearing carriers too the wrong size? Difficult using the way I marked them out? The answer was indeed that they were too small. So I set about measuring everything and it soon became clear that all the metal bits were too small despite them matching the drawing.
So the printed drawing had to be wrong. So instead of printing onto a label just the bits I wanted to cut out, I printed the whole of the drawing including the ruler round the edge. I'm used to printers not working exactly to scale so I normally first print a drawing on plain paper, measure a known length and then adjust the scale in the printer dialog to give the correct output. It's a long time since I last did this, but with various printers an error of 0-2% is what I remember: I'd checked this latest drawing and found exact alignment with a steel ruler so I'd assumed that no adjustment was needed. How careless I had been: reprinting the full drawing with ruler showed that what should have been 150mm was actually 140mm and I'd only spotted alignment of a major division!
little and large 007.JPG

Had I measured anything else it would have been obvious that something was wrong.
However the story does not end there, with my current printer one can only have a whole % so I could print at 107% but not 107.2%. I suspected for for small components this difference would not matter being less than the precision to which I can work so I printed the drawing at 107% and measured various bits carefully. There were still some small errors (different in x&y directions) so I wondered if the basic CAD program I was using might not have been giving a perfect output. I therefore opened it in a different program but found an identical output. Hence the errors are in the printer driver or the printer itself. So the next step will be to scrap what I have done, and produce a special version of the drawing stretched differently in the x & y directions but printed at 1:1, adjusted until the output is correct. I'll probably also add dimensions around individual components to facilitate final checks.
So not, measure twice and cut once, but measure several times and cut twice! (I hope not more than twice.)
Regards,

Andrew Nummelin

Philip Hall
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Re: Little & Large - The Socially-Distanced Challenge

Postby Philip Hall » Sun May 10, 2020 3:08 pm

Andrew, I read about your thinning the wheels with interest, as I have just thinned down two Gibson driving wheels to 1.85mm so as to fit a wheelset within the rear splashers on a Hornby LSWR 700 class 0-6-0. The splashers/cab sides are diecast and a real pain to try and excavate to gain more clearance, so I went for the rear wheelset instead.

I made up a true faceplate from a lump of brass to secure the tyre to with double sided tape, having removed the moulded centre, after all I was only taking gentle cuts, and I wanted to be able to remove the tyre without distortion. It took a fair time even taking a fraction off at a time, and I still resorted to a fine file to thin it to the required amount. For the second tyre I simply filed a little off the face of the tyre, a few strokes at at time, revolving the wheel after each segment and checking with the vernier, finally finishing by mounting the tyre on the double sided tape again and giving it a final polish.

This second method was actually much quicker and also easier to control. There was not a lot left of the face of the tyre after all this, but I just got away with it. I checked the centres for concentricity before gluing the tyres back on with Devcon 5 minute epoxy.

It eventually took about 2-3 hours to complete the two wheels, so I suppose it might have been quicker to attack the splashers! Fortunately the front splashers have more clearance...

Philip

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Re: Little & Large - The Socially-Distanced Challenge

Postby andrewnummelin » Mon May 11, 2020 9:54 am

Philip Hall wrote:...
I made up a true faceplate from a lump of brass to secure the tyre to with double sided tape, having removed the moulded centre, after all I was only taking gentle cuts, and I wanted to be able to remove the tyre without distortion. It took a fair time even taking a fraction off at a time, and I still resorted to a fine file to thin it to the required amount. For the second tyre I simply filed a little off the face of the tyre, a few strokes at at time, revolving the wheel after each segment and checking with the vernier, finally finishing by mounting the tyre on the double sided tape again and giving it a final polish.
...

Philip

Philip,
Thanks for this - I'll try it next time (assuming I don't make too many more stupid mistakes).
Regards,

Andrew Nummelin

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Little & Large 4 - The Socially-Distanced Challenge

Postby andrewnummelin » Wed Jun 03, 2020 12:21 pm

Progress! And a request for help.
Things have moved slowly as, to relieve the tedium of fiddly bits, I've also built a Slater's van kit that has been in the cupboard for over 10 years: and there was the garden and good weather!

Little

So the CAD drawing was resized differently in the x & y directions and reprinted giving a drawing that was now correct and all the components were cut for the second time. Assembly now went much better but there were still a few problems. Firstly, the bearing carriers did not always remain aligned so small guides were made from 5 thou plastic: if I ever make a rake of wagons with etched components this is something plan for in the design. Because of the tight clearances I felt that every last little bit of vertical movement of the bearing carriers would be useful so tiny plastic wedges were glued in to force the spring wire to the bottom of the (0.4mm) holes in the chassis. The underframe would now run on a bit of track but the tiny bit of remaining wheel wobble meant that it did not run freely: the clearances as so small that the wheel rims sometimes touched the suspension.
little and large 008.JPG

Examination of the wheels suggested that there were two points of weakness arising from them having been thinned down so much: between the tyre and the centre, and more significantly between the axel and the centre. I've decided not to attempt to correct the existing wheels but to start again: I'll go for a new thicker centre to make alignment with a 1mm axel somewhat easier: I'll also consider using Ultrascale tyres as these can be supplied in a smaller diameter than the standard commercial products for wagons and the extra clearance could be useful. (The drawing of the prototype scales at wheels of 11.8mm diameter for the model.) Having a lathe I should be able to make my own tyres; but I'm not confident that I have the skills needed to make a number of exactly the same diameter.

The real challenge will be the spokes - has anyone any bright ideas of how to make a wheel with 7 split spokes? Is 3D printing a realistic option, and if so, in what material?

Around 5-6 hours was spent on making the couplings... for making more I think the design of some bending jigs would be essential! Attaching the pin to fix the shackle took several attempts - the fine wire broke too easily during manipulation and I doubt if it will survive use on a layout. If one ever has a rake of wagons with this type of coupling on an exhibition layout, the temptation to revert to toy train couplings would be exceptionally strong!
little and large 009.JPG

I've not fitted axelboxes, springs the brake or detailing on the solebar as these bits would be too vulnerable before the running gear is fixed permanetly to the wagon and that awaits the 7 spoke wheels. While I sit in the armchair and contemplate how to make them, I'll make a start on the coal boxes. Would it be madness to consider making them openable?

Large

The problem with the printer had also resulted in bits cut for this one being too small so the skirts were made again. The bogie side frames have been cut from 0.25mm sheet and "riveted". 1mm L profiles were cut and soldered to the frames to form a U shape as guides and supports for the bolsters. The question was then how to join the frames while still leaving enough flexibility between them: the prototype arrangement of two 3"*6" channels between the tops of the frames with 5"*2½" T shaped bracing between the channels and frames would, I'm sure, not be flexible in a model. The first trial is with two 0.9*0.2mm nickel silver strips at the level of the axels and appears to have given an OK performance in a simple push along test - bogie will happily ride over a 0.5mm obstacle for one wheel with just a little bit of weight on it. A pleasant surprise!
little and large 010.JPG

However it is clear that I have made a silly mistake in fixing the frames at the distance apart of a normal etched W iron: the frames had a 3" L angle re-inforcement around the inside edge and there's no room for this. So I'll need to make longer frame spaces an put some packing between the bearings and the frames. Time to study the GA drawing again carefully. It will also be the time to think how to mount the brake gear - I think it will be VERY fiddly.
Regards,

Andrew Nummelin

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Little & Large 5 - The Socially-Distanced Challenge

Postby andrewnummelin » Thu Jun 25, 2020 10:29 am

This should have been posted on 11th June but for some reason, probably my carelessness, it did not appear.


Little

The first coal box has been made - quite fiddly. The basic box was made from 2thou brass shim, being the nearest in thickness to the scale size of the prototype's 3/16". I wondered if this would be strong enough but I'm glad I had it available. It is easy to cut with a scalpel but it does bend very easily and needs the strapping to reinforce the structure. I think the flexibility has been an advantage as, with all the handling, the final box has sides that are far from very flat and I suspect this is what the prototype would have looked like. The strapping on the prototype was of various thicknesses from 1/4" to 11/16", so I settled on just two thicknesses of nickel silver (5 and 10thou): with so many little bits I guessed that the lower conductivity of nickel silver would be an advantage over brass during assembly. I started with the bottom strap using high temperature solder (288) and doing the rest with ordinary electrical solder or solder paste. Three different soldering irons came into use! The strapping was not always easy to do: putting rivets into cut strips caused distortion and cutting, especially finishing, sheet with rivets was not easy to do without damaging the rivets. The corner plates were particularly difficult, bending the rivetted plate without squashing the rivets or having bad distortions was not very successful (despite a well scored bend line): perhaps 3 thou brass would be easier than 5 thou nickel silver for the second box. I was able to make a pin and "chain" for the floor retaining strap, so making an openable bottom should have been possible; but I decided not to do so as I do not envisage ever having a layout where such a feature could be used.

If I ever decide to make a rake of wagons like this I would not contemplate making the boxes in the way I did the first one. I would only consider etching - economically it would not be sensible to use several different thicknesses of sheet and I have some ideas as to how a pretty good representation could be achieved with only one thickness. I'll do a design later and ask for opinions.

Large

Two different approaches for simulating the reinforcement round the bogie frames: a piece of 30 thou plastic, that should also act as a spacer for the bearing, and metal strip made from 1mm brass angle with most of the angle filed off to allow it to be curved. The latter is a more satisfying approach but the edge may not be as neat as the plastic version. I'll have to wait until the model is finally painted to see which is visually the best (assuming both turn out to be mechanically OK).
Regards,

Andrew Nummelin

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Little & Large 6 - The Socially-Distanced Challenge

Postby andrewnummelin » Thu Jun 25, 2020 10:36 am

Only just realised that part 5 had not been posted when it should have been and this one should have appeared yesterday.

Little & Large


Today is a bad day! I'm missing some bolts and will have to go out shopping, now that it's allowed. It's not just the missing bolts as some new tools have arrived so as of tomorrow I'll have no more excuses for not getting on with the machining of the bits I'll need to make some real progress - but the weather is forecast to be so good that I might restrict cutting activities to the roses... A new variation on armchair modelling.

Still I thought I ought to show how far I have got.

little and large 011.JPG


Two boxes made for the little wagon - I chickened out of making the bottoms hinged but I did manage a pin and "chain" to secure the catch. At the moment the wagon leaves the track with the slightest provocation - perhaps the underframe should have been carved from a solid block of metal as I'd like to be able to run it with the boxes empty.

The bogie is waiting for me to work out how to machine the suspension parts: the bolts are currently loose. The odd underframe arrangement of the large wagon is so that there is enough to make it adequately rigid, provide a base for the bogie pivot pins, make boxes for the storage compartments and to leave room to fit buffer springing and orientation control. I think it will be better to fit the buffers before the bogies to allow as much room as possible for working.
Regards,

Andrew Nummelin

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Guy Rixon
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Re: Little & Large - The Socially-Distanced Challenge

Postby Guy Rixon » Thu Jun 25, 2020 3:45 pm

A while ago you asked about print special wheel-centres. It's certainly possible and might also be feasible if the right material can be selected.

First off, I suspect that the resins commonly used in desktop printers that have become popular are not quite what is needed. My experience of these parts is that the resin is a bit prone to fracturing. If you needed to skim it concentric I think it might shatter.

Similarly, Shapeways "Smooth Fine Detail Plastic" is not really strong enough.

I know that aerospace-grade polyurethane resin printed by DLS would do the job. This resin is fairly rigid --- more so than than a moulded wheel-centre --- but not too fragile. I've trimmed parts in this with hand tools and it doesn't seem to explode on contact. I know a company with a DLS printer that will run small jobs for hobbyists, but their pricing assumes that you fill the print bed, which is a bit larger than a desktop printer; one set of wheels would cost as much as a bucket of wagons, which is to say too much.

Shapeways have a nylon material called PA12 (PA for Polyamide). It is "engineered to produce final parts and functional prototypes with fine detail and dimensional accuracy" which sounds like our kind of game. Its natural finish is grainy and utterly tragic in finescale terms (a friend bought a coach printed in this and it's unusable), but SW also sell it dyed black and smoothed which sounds better. The smoothing is some fancy chemical process rather than tumbling and is claimed to "[alter] the geometry by no more than 0.4% dimensional change". That sounds like they remove 0.4% smaller in the smoothing, so the outer radius of a wheel might shrink by 0.05mm and the axle bore might slacken by 0.008mm. I think this might answer for wheel centres to be mounted in steel tyres using Loctite.

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Re: Little & Large - The Socially-Distanced Challenge

Postby andrewnummelin » Fri Jun 26, 2020 3:02 pm

Guy,
Many thanks for the very interesting and useful information. PA12 does indeed appear to be a good candidate material for this sort of application but it may be touch and go for minimum thickness - I’ll do a drawing and submit it to see how it is rated.
I’ve been thinking through how to make the wheels, particularly the order in which to do things so as not to damage fragile bits. I think it will soon be time to experiment.

Guy Rixon wrote:A while ago you asked about print special wheel-centres. It's certainly possible and might also be feasible if the right material can be selected.

First off, I suspect that the resins commonly used in desktop printers that have become popular are not quite what is needed. My experience of these parts is that the resin is a bit prone to fracturing. If you needed to skim it concentric I think it might shatter.

Similarly, Shapeways "Smooth Fine Detail Plastic" is not really strong enough.

I know that aerospace-grade polyurethane resin printed by DLS would do the job. This resin is fairly rigid --- more so than than a moulded wheel-centre --- but not too fragile. I've trimmed parts in this with hand tools and it doesn't seem to explode on contact. I know a company with a DLS printer that will run small jobs for hobbyists, but their pricing assumes that you fill the print bed, which is a bit larger than a desktop printer; one set of wheels would cost as much as a bucket of wagons, which is to say too much.

Shapeways have a nylon material called PA12 (PA for Polyamide). It is "engineered to produce final parts and functional prototypes with fine detail and dimensional accuracy" which sounds like our kind of game. Its natural finish is grainy and utterly tragic in finescale terms (a friend bought a coach printed in this and it's unusable), but SW also sell it dyed black and smoothed which sounds better. The smoothing is some fancy chemical process rather than tumbling and is claimed to "[alter] the geometry by no more than 0.4% dimensional change". That sounds like they remove 0.4% smaller in the smoothing, so the outer radius of a wheel might shrink by 0.05mm and the axle bore might slacken by 0.008mm. I think this might answer for wheel centres to be mounted in steel tyres using Loctite.
Regards,

Andrew Nummelin

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grovenor-2685
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Re: Little & Large - The Socially-Distanced Challenge

Postby grovenor-2685 » Fri Jun 26, 2020 7:55 pm

Perhaps this would be a good place to start, David seems to have the process sussed out.
https://www.scalefour.org/forum/viewtopic.php?f=20&t=5419
Regards
Keith
Grovenor Sidings

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Mike Garwood
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Re: Little & Large - The Socially-Distanced Challenge

Postby Mike Garwood » Fri Jun 26, 2020 11:54 pm

What ever became of this ? Anyone know?

Mike


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