Practical Engineering

The Foden safety valve casting, although looks like the drawing, once finished, dimensionally this isn't the case. The distance between the stack centres is dimensioned as 1 1/8" on the drawing, but the actual distance to be correct and central on the casting needed to be 1 3/16". This had a knock on effect. Firstly the bolt holes had to be adjusted to enable a 4BA nut to fit at the bottom of the stacks. The stacks were set 1/8" and 1/32" off true dimensions. Secondly the cover plate was machined with minimum clean up to accommodate these adjustments . Even with these adjustments a 4BA one size smaller hexagon nut will have to be used.
Photo below shows the additional marking out to get everything to fit within the casting.

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