Another turning job for
James Hilton, this time a stovepipe chimney to be fitted to a 3D printed Polish Px48. This is another first for me as I've not attempted to turn a flanged chimney before. I've been reading up on the subject and the two most popular methods to shape the flange so that it sits on the boiler are by flycutting and filing and by squeezing. For simplicity I chose the latter method.
James sent me a drawing which I printed out to size, measured and made a rough dimensioned sketch from. The largest diameter, the flange, scaled at 6.3mm so I started with 7mm dia. brass turned down firstly to the flange dia. and then to 5.4mm, the size of the top lip;
The next job was to turn the middle section, paralell at first to the top dia., then with the headstock set at an angle (judged by eye) taper turned;
For this part of the job I used a pointed tool set at an angle.
The flange was shaped with a round file and the lip filed to shape, then the funnel drilled out to 3.5mm;
At this stage I realised that I should have turned it the other way round, with the flange outermost so it could be thinned down internally to aid forming. So I parted the chimney off and turned it around in the chuck, the jaws lightly gripping on the thin lip. However, this was enough for me to file out the bore to thin down the inner part of the flange;
The flange thinned down;
Now the flange can be squashed against a suitable piece of bar, 16mm in this case. The method usually described involves holding the chimney and bar in a vice. My vice is of the modelmakers lightweight bench type and not really up to the job, so I brought out my bench drill. I turned a small mandrell to sit in the chuck and hold the chimney upright, held some 16mm dia. brass in a V block, lined everything up by eye and pressed the flange over the bar to form the curved bottom;
And here's the finished chimney alongside the mandrel;
Not bad for a first effort.
All turning and drilling at 1600rpm, parting off at 365rpm. Cut dry.
Paul.
Paul, great job. Not really difficult, is it. Nice to see some scratchbuilding. And an old SL90 to do it on!
ReplyDeleteQuick hint. Before squeezing the flange, if you have a blowtorch, soften the chimney in the usual way...cherry red and leave to cool, then the sqeezing will go like butter and you can persuade that last bit down the side with a suitable smooth bar by hand.
Thanks Odds.
ReplyDeleteIt really wasn't as difficult as I imagined. I did think about annealing, but decided to try forming the chimney as is before applying heat. On a larger turning I imagine that I wouldn't get away without softening first.
Wonderful stuff.
ReplyDeleteHow light or heavy were the uts you made, when shaping the chimney?
Thanks Simon. (And apologies for the late reply, I've been away over Christmas).
ReplyDeleteI took quite light cuts, no more than .5mm on the diameter. I'm still getting used to using this machine, and anyway I'm not on a time limit! I was gentle with the file as well, taking a little off at a time and checking frequently. The file was a smooth 4" rats tail, used in preferance to a needle file simply because it has a wooden handle.