Sorry for the long silence, we have been working hard to relocate to Cumbria and convert the garage into a workshop – with that approximately complete it allows me to get started on the boiler for Befur. So this post concerns itself with this topic.
A description of the Boiler
As we have mentioned elsewhere (see design choices – engine, boiler and machinery) I had opted for a water-tube boiler of a Yarrow-type. This arrangement involves a set of “drums” which contain the water and steam, linked by a large number of tubes with the heat source installed inside this assembly. These are arranged in a triangular shape like a ridge tent, with a large drum at the “ridge pole position” which will hold the collected steam (and hopefully some water!) this is the “steam drum”. The two smaller drums form the base of the triangle (known as the “mud drums”), and then there are 206 12mm cunifer “water tubes” linking the steam drum to the mud drum. The steam is created in these tubes and rises to the steam drum. The whole assembly is then encased in a sheet metal “casing” lined with ceramic insulating material (no asbestos these days) 🙂 .
After some further thought I opted for a boiler design from John King of the SBA. Following some discussion a group of us have decided to build a set of three boilers to this design, and are operating as a work-sharing co-operative, so I am currently making the drums (basically drilling some 1,236 12.1mm holes in the nine mud and steam drums). These are made from 4″ and 6″ diameter steel tube – basically lengths of the kind of stuff you would find in an oil refinery!
Importantly, this boiler can be built without any welding (as I am not a certified welder). The drums are closed with ~20mm steel end-plates attached with threaded stays passing down the drum and large dome-nuts. The mud-drums have a single 20mm stay (bolt) holding the ends on, and the steam drum has four 16mm stays – pretty substantial!
Interestingly, the tubes are attached to the drums by expansion – the tube is fitted into the hole in the drums then an “expander” is inserted into the tube and squeezes the tube out into the hole until it is tight. Then the expander is removed and the squeeze holds the tube in place – no glue, sealant, soldering or welding… and this is then able to withstand the test pressure of 500psi….(testing has actually shown that this method of fixing tubes provides a joint with the strength necessary to withstand about ten times this load!)
Drilling the drums
This is a simple, if very repetitive/dull process. The holes are arranged radially around the drum, with the holes normal to the surface. Actually it was this work that prompted me to buy the larger mill to be sure I could fit and drill the drums in “one go”.
The set-up was borrowed from John Richardson (another SBA member) who has built one of these boilers. The drum is clamped to the milling table, using one of the table slots to ensure it is square and level. I then fitted an end-stop, and two side supports to the table to allow the drum to be rotated for each row of holes and reset into the correct position.
Positioning the holes – DRO & Digital Angle Gauge
The position of the holes to be drilled is set out using a DRO (“Digital Read Out”) and a digital angle gauge. The DRO is one I made to an open source design published by Shumatech. This is a powerful tool, it uses “glass scales” attached to the table that are able to measure the position of the milling table to within 5 microns (about a tenth of the thickness of a human hair or as long as a typical bacteria!). It then allows you (amongst other things) to define a variety patterns of bolt hole grids or PCD arrangements.
The process is then a sort of semi-automatic (manual-matic) CNC. You load the pattern you want, set the DRO running and the DRO tells you how far away from the current position the next hole is to be drilled. Then you wind the handles until the “distance” is zero; so you are then at the position to drill the next hole – this dramatically reduces the chance of a mental-arithmetic error screwing the job!
The distance between the rows of holes could be measured and marked out on the circumference of the tube, but I opted for a more direct approach. I clamped a small tool-maker’s clamp to the end of the drum and then placed a magnetic digital angle gauge on this. I zeroed it before starting, and could then rotate the drum at the end of each row setting it to the correct angle as indicated on the gauge.
This approach (DRO and Angle Gauge) has an additional benefit in that you avoid accumulative errors. As the position of the hole and the angle of the tube are both measured from the “origin”. So even if one hole or row is slightly misplaced, the next one will not “inherit” this error.
So the first mud drum was finished today! (ps – the ends of the drums had previously been turned true and to length on a friend’s Colchester, as they were marginally too long for my Harrison L5).
Given the number of holes we need to drill we would either need to buy several drills or have a method for sharpening them – I opted for the later.
It is interesting to note that if you sharpen a drill so that the point is not exactly central and even the drill will cut oversized holes. As the even-sizing of the holes for the tubes is important (as we need to ensure an equal amount of “squeeze” on each tube as we expand it) the business of well sharpened drills is critical.
I made a Quorn tool cutter-grinder some years ago and this allows you to sharpen drills using a method known as “4-facet sharpening“. This is not the way drills are normally manufactured, but has several advantages 1) the drills will start without a centre-pop, 2) you get drills that cut to size when resharpened.
I had a practice run on a few of my normal jobber drills and centre drills before starting work – I have been meaning to do this for a long time, so was happy to finally sharpen a number of my more “used” drills – here are some pictures….
New Workshop Layout
To close out this update, here are some pictures of the new workshop in a semi-complete state. It is fitted with a 4Kw Transwave 3-phase converter as North West Power’s estimate of £28,400 to bring the 3-phase 50ft from the substation seemed unreasonable!!!!