31/1/17 - Pansy

This evening's work was to finish the barrel clamp jig and cut the hole for the dome.

A rather elaborate style holding jig for a 5" barrel was made to be able to get the hole square to the barrel and on centre with a rigid setup. A test piece of copper was used to try out a 40mm holesaw and see what sort of finish was possible. The diameter even with eccentricity on the holesaw was perfect for the dome flange.

The jig was clamped to the mill table and centre punch mark was lined up with the spindle. I ran out of Z travel on the table to fit the holesaw so it had to be done in two operations, drill the pilot hole and then fit the holesaw in. (The 4 inch head raising piece for a Bridgeport mill would be an advantage!)

The mill was set to low speed and the quill was driven off the handwheel with the quill lock half on to prevent any grabbing. Once the holesaw had cut a groove it didn't complain or chatter and rather uneventfully finished out a nice hole.

Clamp jig was dismantled to try the dome, which was just a little tight. A quick file to remove a burr made all the difference and it fitted perfectly square without any forcing one way or another, aligning off the machined hole as expected. The scriber traced the barrel inner profile  onto the dome to file that to shape.

This is a key thing to be aware of as the protrusion of the dome into the barrel sets the maximum height of water in the boiler. Once the steam entry into bottom of the dome is blocked by the water, a column of water goes right up the dome and into the main steam pipe.

A couple of boiler shapes I've seen,  notably the NSW 12/13 class could pick up water quite readily at half a glass if using the conventional steam collecting setup of dome and regulator. The stepped height between top of outer firebox and the barrel and the positioning of top gauge glass fitting on top of the firebox means the water level can very well be half a glass and begin priming! A long gauge glass in that case can be deceptive as to how much water can be carried before priming.

Some locos with tapered barrels have a cheat of a parallel barrel under the cladding, this too can be one to watch for. Naturally the steam should be coming from the highest point in the boiler, so a dry pipe should be run to the top of the firebox to eliminate this.

I have well digressed, it must be late!

Happy steaming!

Nigel