I appreciate this is an old topic however, I am in the process of creating the 5" gauge Britannia in CAD before cutting material. I have an old set of drawings, copyright Spink Newbold and Chesterfield redrawn C.J. Gilbert 6.84. Are these the current drawings and the same as the one's available from Blackgates ?
I have come across several discrepancies the latest of which is a 1/32 difference between the planes of the crosshead (15/32) and the axes of the piston / valve (7/16)
If you have come across this please can you tell me how you rectified it, or has this been corrected on later drawings ?
If you are putting all those drawings onto CAD that is most interesting.
I have tried to trace the history of the design, and would be pleased to hear comment from anyone who knows more. The design is always attributed to Perrier and blue prints of those drawings show hand written numbers and lettering. We could not find a date on the few sheets looked at and I have never found information regarding where or when they were published. After searching I could not find the build in Model Engineer. Norman Spinks name appears with copyright on the next sets of drawings and he was a supplier of castings around 1976. I can only assume he obtained the design from Perrier and published it as his own. REDRAWN C.J.GILBERT 6.84 also appears on the drawing set available today from Blackgates who do acknowledge 'Designed by J Perrier' at the head of their drawings list.
Does anyone know anymore about the Perrier part of the story? I guess a question to Blackgates is the obvious start.
You are saying that the crosshead misaligns with the piston centre by 1/32"? Could this be a misidentification or error regarding whether there is or is not a gasket between the cylinder block and frames?
Off the subject, and just for your information, are you aware that Hewson has done his own, more prototypical drawing set?
The 7/16 dimension comes from sheet 8 as does the 1/32 dimension for the 'Hallite' gasket material and the 1.578" lateral offset of the cylindewr from the 1/8 backplate.
The 15/32 dimension is derived from the 3/16" - 3/16" - 3/8 +.005" - 3/16" stack shown in the crosshead cross section on sheet 4
If I am correct the mid plane of the crosshead slide is 3/16", the outboard leg of the crosshead fork is 3/16" and the mid plane of the crosshead link arm is 3/32" thus totaling 15/32" Consequently irrespective of gasket and casting / material thickness the valve and the piston cannot align.
The alignment of the expansion rod and the con rod could be put down to gasket thickness / compression but the distances are not the same, the expansion rod is 0.0131" out and the con rod is 0.0111" out.
I've bought one sheet of the Hewson design for comparison. There are very few dimensions so I presume it is a kit.
I am sorry I am not much use in helping you find the 'error'! You might have detected something in CAD which earlier builders probably made an adjustment for as they fitted parts? Agreed the crosshead has to be exactly on the cylinder bore; builders might have adjusted the brackets?
The Hewson drawing set is fully dimensioned. It is not a kit. There are a few people building one at the moment and I would guess it is going to replace the Perrier/Spinks as the 'chosen one' for the very small number of future new-builders.
I think this may be tied up with the need to provide packing behind the slidebar brackets. Somewhere along the line, the dimensions for the casting were wrong which led to a 1/8" packing plate behind them. I don't remember specifically adjusting for this error but I generally change things on the fly to suit and seldom document my changes.
Norm, Jim in Australia has an earlier set of drawings and a conversation with him might give you some of the earlier background.