Work on the Boiler continues with the finishing and installing of the Economiser (pre-heats the incoming water to the boiler using waste heat from just before the flue) and the Superheater (adds energy to the steam on the way to the engine, again using waste heat from the flue gasses.
Milling and Drilling the Econo/Superheater Headers
As noted in the last post, I decided to mill the recesses in the two halves of the headers, as there is a lot of metal to shift, and with the “ripping” milling cutters this was by far quicker. (some pics)….
Tapping Machine 😦
As there were such a lot of M8 holes to tap (about 48 per boiler) I thought I would “invest” in a tapping head for the mill, and went for a cheap Chinese one – and you certainly get what you pay for… I was worried when it arrived as it appeared to not have a load-sensitive clutch, so decided to try it gently on a test piece… the pictures show how that worked out…
I can imagine it would have been OK with tapping thru holes, but even being as careful and slow as I could, it was useless on a blind hole -sheering the tap instantly. I was just very happy that I snapped the tap in a test piece, and did not scrap a header that I had spent some time (and money) on making… So it’s back to standard/hand mode… and we have a full set…
Cutting the O-ring Grooves
The final job on the headers is to cut the 4.5mm wide groves for the o-rings that seal the joint between the two halves. I was a little nervous about cutting these in the lathe, as you need to produce a narrow tool with additional clearance to avoid the outer edge of the groove as it is cut.
So, again I decided to mill these, using a small slot drill and the rotary table. This did work, but was slow and tool wear was a problem – I might revert to lathe cutting these for the drum ends. The setup for milling the grooves was to fit new clamps in the recess before removing the clamps on the rotary table that were holding the header while recess milling (see pic). This meant we knew that the part was still centred on the rotary table, ensuring the recess and o-ring groove were concentric.
With all this complete we just needed to fit the tubes with the same expanding process as used on the main drums.
You will recall I drew the parts for the inner casing of the boiler in ViaCAD and then had them plasma cut directly from the drawing; this meant assembly was quite simple. I used M5 rivnuts and screws to make the assembly, spotting through from the plasma cut holes to locate the rivnuts. This went well. I also purchased an American design of tool for fitting the rivnuts, this is based on a pair of wedges which produce the compression to expand the rivnuts. The more normal “pop-rivet” style of tool requires bigger hands and forearm muscles that I currently own!
The last job is to pressure test the assembly, as the boiler is rated to 250psi, the hydraulic test (with the assembly full of water to avoid any large bangs!) is 375psi. Once we located the one tube we failed to expand (light shower ensues) the assembly proved pressure-tight – which is more than could be said for the plugs and pump, which took several hours of work to fettle to a point where they would stand the pressure. (In the process I became a complete fan of Dowty Seals, which rendered the plugs in the headers pressure-tight with little more than finger pressure inserting the plugs. Here is a video of one of the completed assemblies under pressure.
Not a bad months work in total…