Turning a Swedish spark arrester chimney in OO9 - part 1.

A long time ago Charlie asked me if I could turn up a chimney. Not just any old chimney, but a Swedish spark arresting type. These have a peculier bulbous lower section (the actual spark arrester) with a stovepipe top, a nice challenge which I simply couldn't refuse.

I decided that it would need to be made in two sections, the spark arrester and seperate stovepipe, which would slot together once finished. As the lower section would be the hard part I made that first. Actually that's not quite true, the first thing to be made was a flycutter to cut the base of the spark arrester to suit the boiler. I re-scaled the drawing that Charlie provided and found the boiler to be 12mm diameter.

To make the flycutter first a length of 6mm dia. silver steel was cross drilled and reamed 4mm, then the end drilled and tapped M4 for the clamping screw. I then filed the end of a length of 4mm dia. silver steel into a cutter, which could then be sawn off to 11.5mm long. This was then heated to cherry red and quenched in water to harden it with a normal DIY store blowtorch. I didn't bother with tempering, its only going to be used on brass. Here's the cutter;

To set the tool to cut a 6mm radius (to suit a 12mm dia. smokebox) I set the cutter 3mm from the shank of the holder, which as its made from 6mm dia. material will position the cutting edge 6mm from the centre line of the holder (3mm shank radius + 3mm from shank to cutting edge).

For the lower section I chose to use 10mm dia. brass. The first job was to cut the bottom radius so the rod was clamped in a toolholder on the saddle at 90 degrees to the bed. I did this by leaving the toolholder loose, fitting the face/drive plate to the spindle and offering up the bar to the plate then clamping everything up tight;

To set the rod at the correct height I measured from the saddle to the top of the rod. The centre height from the saddle is 12mm, add half of the rod's diameter (5mm) and that measurement should be 17mm. To get that measurement I made a shim from 1mm thick aluminium to sit between the toolholder and the saddle. With the flycutter in the collet chuck the base can now be cut;

Action shot(!);

I set the machine at 1100rpm and took .3mm cuts. Easy work.
Once the base profile was cut I chopped the end of the bar off at 35mm(ish) long to hold in a 3 jaw chuck. Then the bar could be turned down, the smaller diameter is to suit the base flange;

With my compound slide fitted and set to 45 degrees the lower taper was cut along with the smaller flange diameter;

I had to grind up a special left hand turning tool for that operation.
Using the parting-off tool the uppermost flange was turned along with a recess;

The recess is there to make it easier to file the bulbous shape that makes this type of chimney so interesting;

Done with a needle file and finished with a rubbing stick.
The lower flange, where the chimney is attached to the smokebox, is as far as I can tell oval in shape. This needed to be filed but to make a start I milled two flats on the flange;

Note the 2mm dia. hole, this is for the upper section to slot into.
Once I had milled the flats I could start on the hardest part of the entire job, namely filing the lower flange to shape. I'm always slightly nervous when taking a file to a chimney as the shape can be  quite awkward, needing a good eye and steady hand. I'm very aware that one slip of the file, or a lack of concentration, and the job could be ruined. But so far I've not made any mistakes, I work steadily with a decent smooth file and check often with a magnifying glass. Finishing is again done with a rubbing stick.
The base was first filed oval, then the sides were shaped to suit the smokebox. I haven't got any really good photos to work from, just some black and white shots in a Bradford Barton book (European Narrow Gauge Steam volume 2). Once I was happy with the shape I parted the base off, ready for the next operation.
Here it is after parting-off, showing the underside;

And from the front;

Clicking on the photos, which I recommend you do, brings up a larger image. More soon.

Paul.