Building the Stuart boiler feed pump part 3

After several days looking at the drawings it became clear that there was no easy way to mark out the various ports and steam passages in the cylinder or the steam chest. The more I studied the drawing the more confused I became. I have to say that the draftsman was clearly a sadist as there is no convenient datum to work from and only the minimum number of dimensions given and to add to the fun all in fractions.

The much scribbled on drawing shown here is the final version, I had several other earlier attempts to work out a starting point. This was by far the most challenging part of building the model.

Once a starting point had been established it was then time to machine the two castings to size and bore the cylinder. I marked out all the steam passages using a height gauge and a small pair of draftsman's dividers. Trying to do it using the mill DRO was just two complicated with lots of opportunities for cock up.

There are a lot of holes to drill all of which require a high level of accuracy. I decided to use D bits as these are more likely to produce holes that are to size and not wander off. There are are two very deep holes for the main valve and the shuttle piston.

These were drilled first and then all the other passages drilled in turn. The most challenging were the steam inlet port which has to meet another drilling to connect to the shuttle valve chamber.

When all the passages in the steam chest were complete attention turned to cylinder, this was a little easier as there are fewer holes.

The whole process of marking out and drilling took place over a few days. It was a question of measure 3 or 4 times and then cut. there is a lot of investment in time, one false move and the component can be scrap.

The picture above shows the finished steam chest together with the D bits used to form the passages.

The next job was to attach the steam chest to the cylinder while making sure that all the holes lined up. I accomplished this by making a couple of register pins which located in the larger steam ports, it was a relief when everything lined up and the holes for the studs could be drilled.

The studs are within a few thousands of some of the steam ports so great care is needed in marking out and drilling.

The access holes needed to drill the internal steam passages were plugged with 16th dia brass using a retaining compound. having accomplished this frankly nerve racking process I needed a break and tidied the workshop.