Wednesday, 18 July 2012

The Perfect Joint (1)

Sentinel 7109 works at 275psi, a higher pressure than most steam locomotives, waggons and traction engines. Many steam industry suppliers have off-the-shelf items for up to around 180 psi but, for Sentinels, more specialised items need to be sourced, in particular jointing material.

For the steam fittings to the boiler and piping to the engines, I've chosen a robust 1.5mm jointing sheet made by Klinger and sourced from Seddon & Black (now Craftmaster Steam Supplies). Identified as PSM/AS, it is a tanged stainless steel sheet sandwich with exfoliated graphite either side (instead of bread!). It is certainly up to the task but it presents a challenge of its own.
Safety valve flange adaptor (1)
The tanged stainless steel sheet makes the material difficult to cut. Tin-snips are fine for outer edges where the curvature is slight; however, inside edges of smaller radius cannot be done this way. Small holes can be cut with hole punches so long as there is a solid backing of just the right hardness for the punch to cut through to. The best combination I've found is to use a chunk of heavy paving stone and a length of old oak from an ancient desk.

In the photo above, I used hole punches for the stud holes, tin-snips around the outside and a chisel for the centre hole - which is clearly not too tidy!

Safety valve flange adaptor (2)
Wandering off-subject for a moment, this is what it looked like before it was removed from the boiler. EN3B studs have been used to replace the originals.
Before removal from the boiler!
The standard type of Sentinel safety valve manifold, similar the one pictured below, would be fastened to the four-hole flange.
Sentinel Safety Valve manifold
I had to buy half a square metre of the PSM/AS sheeting which was more than I wanted just for the boiler fittings' joints. As it had seemed pricey to use large copper washers for the steam pipe joints, I pondered whether there was a way of making the washers using PSM/AS. This would of course mean cracking the problem of cutting holes with diameters of around an inch or two and only a quarter inch total material width.

There are gasket punch sets available but, as they come at a price and don't claim to be able to cut the tanged sheet sandwich materials, I'm looking to find a better alternative.

Hole Cutters
A hole cutter looks possible but, without care, could easily tear the material surface away from the inner tanged sheet.

I've an idea how to do this effectively but some R&D is needed to perfect the technique and the outcome of that will have to wait for the next article!

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