I am considering making up patterns for some pre- grouping 6 wheel coach bodies, specifically as built for the SM&AR and subsequently the MSWJR by the MRCW Co. (although the panelling etc was quite common elsewhere).
I have in mind the modular system requiring patterns for a door , handed end pieces, and window panels to go between doors, from which resin castings can be produced. Coach sides can then be built up from mix 'n matched castings as appropriate.( I believe GRS are doing that with their Gresley's?).
Apart from a general request for any words of advice or caution, recommended casting company etc, I do have one specific enquiry- What shrinkage allowance should be made? I have been told that modern resins require almost no allowance whatsoever, but don't want to find an accumulated shrinkage from each component results in a measurable reduction in overall coach length against scale. Yes, I have to admit I do want it to be accurate.
Thank you in advance,
Jamie.
Hi Jamie,
I always allow one percent for shrinkage when preparing patterns for resin moulds.
This works for me but I do my own casting and always use the same brands of mould silicon/RTV rubber and resin.
Mike Williams will be able to give a definitive answer; some of his kits use the resin castings made from my moulds to produce the master patterns for commercial moulding of his kits (this produces a "double risk" of shrinkage since the final product is two mould-making/casting processes away from the initial patterns).
Regards,
John.
John,
Thank you for your reply. I have been pointed in the direction of a company called CMA Moldform so I'll see what they have to say about shrinkage for their materials, after the holiday period.
All the best
Jamie
Jamie,
CMA Moldform are used by all the major model railway people except Pete Waterman who also provides the service. They are not cheap. Shrinkage is generally stated by them to be 0%, but does vary + or - 1%. They have an excellent website which gives some guidance on pattern making.
CMA charge for the mould and then for each casting. The mould life is generally reckoned to be 50 castings and is then scrap. The price of both is largely dependent on size and the amount of resin required. For a van body the mould costs several hundred pounds each, but your small ones should be much less.
If you can possibly manage with a single-sided mould and the parts being flat on the back, somebody like Port Wynnstay will be 10-20% of the price that CMA charge. His castings may need more hand work to clean up though.
One query - on many carriages the ends were thicker than partitions and therefore the panel at the very end of the coach is wider than all the others. That means a special pattern and mould are needed for the left- and right- ones. Is the MSJAR like that? It was that variety which persuaded me to use etched brass instead.
Mike
Hello Mike, thank you very much for that info. Yes, the end panels would need to be handed specials. So an all third would need four patterns- two handed ends, a door, and a 'bridging' section with two windows. It seems to be a lot less effort when compared with making a whole side from plasticard or somesuch, which is my only other realistic alternative because I wouldn't know where to start with etching.
Actually, I think it could be an interesting process- until it goes wrong of course!
The MSWJR had two types of all third 6 wheelers, one a five compartment, the other six. Both shared identical dimensions (bar the extra length) so I have set myself an initial target of a model of each body type. If, and it's a big if, they work OK, two more 'bridging' patterns of increased dimensions would allow 3rd/ 1st. and 1st/ 1st. class combinations to cover most other coach types.
By the way, buffer housings were identical to your LNWR coach buffers , as of course is the roof furniture so I hope I can purchase those from you?
All the best
Jamie
PS Very much enjoying the building of your Composite coach, some nice touches in the kit design.
Thank you Jamie. Pleased to help if/where I can.
If anyone thinks that a shrinkage of 1% is rivet counting and to be ignored, it isn't. My wagon solebars are c80mm apart, so shrinkage of 1% could make them 0.8mm closer together, and my laser cut and CNC bent W irons wouldn't fit, requiring 0.4mm to be scraped off the inside of each solebar - a time-consuming job.
Of course, if you know it will shrink by 1% that's fine, but CMA state a tollerance of + or - 1%. Fortunately, the vast majority of castings I've had from them show no shrinkage at all and only a rogue batch of resin caused problems. I believe GRS suffered the same problems at the same time.
Mike
Mike
Would you be able to message me the contact details for Port Wynnstay please. I have a number of parts that I am looking for prices on, I have a big push to finish the mark 1s
Keith
Hi Keith.
http://www.portwynnstay.co.uk/
Phil Traxsom. Very helpful chap.
Regards,
Mike
If i have this right (and after much help from John) there should be two attached photos of the first castings back from CMA. I'm very pleased with them; no measurable shrinkage, no distortion, minimal very thin flash (photos show the worst and it really is inconsequentially thin). The full side shows the pieces slid together to represent an all third- they are not yet fettled and fitted.
Very nice Jamie. You did the right thing going to CMA. Not the cheapest, but the best. Did you pay for the moulds up front, or guarantee to take a fixed minimum number of resins?
I particularly like the edges which look to be straight and square, which makes butting them up and hiding the join much more likely.
A really nice job.
Mike
Mike,
Thank you very much.
Yes, I paid upfront for a bulk order- more than enough for all my future needs with this design of coach probably, plus a healthy allowance for mucking things up.
When making the patterns, I milled the blanks to get square edges before adding the cosmetic layers. That, coupled with the fact that the pieces are quite solid (approx. 1/4 in. thick above tumblehome), will hopefully make it easier to keep all straight and true as the sides get assembled. We'll see.
All the best
Jamie
looking most impressive ;D
Very nice
Again with thanks to John, a snap of sides for three coaches including bits for a brake third. The brake bits wouldn't be needed as patterns so were used as is. A central (ish) ducket will bridge the gaps.
Have you thought about making these available for sale? I know I would buy a set
simon
Simon,
Thank you. I hadn't, to be honest, they came about as a means to an end because I wanted to model some SM&AR/M&SWJR rakes. And the prototypes are pretty obscure to be frank.
However, I'm learning a lot from these and I do have an idea to offer a small run of horseboxes for which patterns are being made- either RTR or as kits.
Having said all that, I would be very happy to sort out availability etc of some coach bits if they really are of use to you. PM would be best.
All the best
Jamie