Pat­tern mak­ing

The Shed - - Scale-model Steam Engines -

Win’s work­shop shelves are packed with wooden pat­terns, from the tini­est fit­tings to the huge base for his Penn­syl­va­nia A5S ten­der. Pat­tern mak­ing is the start­ing point for build­ing metal mod­els and the suc­cess of the fin­ished en­gine de­pends on its ac­cu­racy. Cus­tom­wood is used in pref­er­ence to wood as it is more sta­ble.

“You can get sets of cast­ings sent out from the UK but it’s a hor­ren­dous price,” says Win, who in­stead gets hold of the draw­ings and works it out from there. “It’s like mak­ing a huge 3D jig­saw,” he says. It’s not a sim­ple mat­ter of repli­cat­ing the fi­nal form, as both shrink­age and the neg­a­tive spa­ces have to be taken into ac­count. “The metal shrinks when it so­lid­i­fies so the cast­ing ends up a bit smaller. The shrink­age is dif­fer­ent for each type of metal so you’ve got to think it out and do the maths.” To make the moulds for cast­ing, the Cus­tom­wood pat­terns and cores are set in resin sand, with a gat­ing sys­tem built in to chan­nel the molten metal.

“It can be a bit of a brain­twister, es­pe­cially get­ting the neg­a­tive and pos­i­tive shapes in the pat­terns. It’s a painstak­ing process, es­pe­cially mak­ing cylin­ders with com­plex cor­ing. The pat­tern doesn’t look any­thing like the cylin­der, as it has out­side core prints on it,” says Win, who has built about 14 boil­ers in his time, the big­gest for his Bur­rell trac­tion en­gine. “I make all the moulds and cores and fit them to­gether so [that] I don’t hold up the blokes at the foundry,” he says. Once they have been cast, the metal cast­ings are ma­chined and filed to fit. On the bright side, if it all goes wrong, at least you can turn them back to liq­uid again, he says.

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