Turning a Swedish spark arrester chimney in OO9 - part 2.

To finish the lower section I had to turn up a mandrel, which was a light press fit in the bore, to hold the base for further turning;

I first faced the top to length, then had to cut a groove into the top face. A form tool was ground and the job done with the lathe running at its lowest speed (155rpm, made possible by the slow speed motor mount that I've fitted);

The groove should probably be deeper, but as this will be used as a casting pattern I'm leaving it shallow. To make sure that the tool had adequate clearence and would do the job I practiced first on an offcut of scrap brass. This also meant that I could set the tool to the correct diameter before using it on the actual workpiece.

On the prototype the spark arrester is fabricated from several parts bolted together. There's a top plate which is seperate from the main body, so to represent the 8 bolts holding it down I drilled a series of .35mm diameter holes around the top rim. To do this I set the Unimat up as a drill press and used my dividing head to space out the holes. The drill proved to small to hold in my drill chuck so I held it in an Eclipse pin chuck which in turn was held in my collet chuck. With the machine running at 2600rpm each hole was drilled 2.5mm deep, here my depth gauge proved its worth;

Each hole had a length of .33mm brass wire soldered into it. The end of the wire was first rounded off with a file, then it could be inserted into a hole and cut down overlength. Whilst still in the hole it was filed until there was just .4mm sticking out, then the wire could be removed, turned around so that the rounded end was on the outside, covered in Carr's 179 degree solder paste and replaced into its hole. Any excess paste was then wiped away and the wire soldered in with my RSU, used at its maximum setting. A time consuming job, but hopefully the results are worth the effort. I now have even greater respect for pattern makers!

The upper stovepipe section was just plain turning, although my compound slide saw use again to turn the taper. It was made to be a light press fit into the spark arrester.

Once I'd parted it off the chimney could be held on the lower, smaller, diameter and a shallow hole drilled into the top. Both parts could then be pressed together.
Here's the thing finished;

I just hope that Charlie's pleased with it!

Paul.