Yesterday was a difficult day. I was really pleased with my first valve and took a lot of time over it! Yesterday I ruined it. Ok, it was always going to be a test piece to sort out the machining stages, but I had hoped it would survive after so many chances to ruin it.

Where did I go wrong? Or, where were the problems discovered if I want to be positive. Well the reduction in length to finished size went well to 1/2 a thou (how accurate can we measure with the tools available to model engineers?) and facing the valve to finished depth also was achieved with the little fixture I made. So the process is now set for the production run. So how did it go wrong?

Looking at the plans I have found yet another error! and one that has **&^^%^&* me off. The write up in Edgar's article brought no comfort on his brief description on making the valves and his plan shows the wrong size BA thread! It reads 8BA and the diameter for cutting 8BA according to British fasteners is 0.086" and the plans shows only one diameter of valve stem, 3/32nd" or 0.09367" a difference of nearly 8 thousandths of an inch!

So when I decided to finish the valve by threading the end it was obviously much too large. With previous experience I know the plans are riddled with errors so I thought it must be perhaps 6BA instead. It was over to the books to find out what BA size came nearest 3/32nd". In the publications there are many data tables and drill sizes for tapping threads but little or no data for threading (well not in my books) however a quick search on the internet found the size of 7 BA nearest but the valve stem was 0.00445"  under 7BA. If we go to a thread that is under the stem diameter (and as stated on the plans) then a further 0.00775" need to be removed. This also does not account for a further reduction of between 5 and 10% recommended in some of the literature to stop pushing metal in front of the cut.

Perhaps I should have seen it and checked but I think that the plan should show the reduction in stem diameter, shouldn't it? Another wind up point is the length of the thread is not shown either on the plan or in the article! Measuring against the plan shows it near to 0.250" so that's the plan and I will try the next one with a 1/4" end reduction.

On the positive side I  made a suitable collet for holding the valve in the chuck for facing and bring it to finished size. After the messing about with the first valve I decided to make the final collet in brass for durability.