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.
About Mike Freeman
Hello, my name is Mike Freeman and I am a retired Chief Fire Officer from the United Kingdom with a keen interest in model engineering, silver caddy spoons and sea fishing. I live in the pretty fishing port of Brixham, South Devon, in the United Kingdom.
I am a sufferer of degenerative osteoarthritis, which impacts on the amount of time I can spend in the workshop, and is the reason why you will see seats and a stool in some of the photo's. I have only recently added the above sentence after a discussion with one of my Doctor's from the excellent pain clinic, based in Torbay. This does affect concentration one of the reasons why I double then re-double my measurements and set ups.
Before completing nearly 30 years of public service I and my wife Sandy, owned and ran, a small restaurant in Okehampton Devon. Prior to marriage I worked for my father in his various businesses, in the early days these were garages, which he bought in a run down state, then built them up before moving on, to start again.
I took every opportunity in those early days, to work in his workshop's
learning 'on the job' rather than as an apprentice. This, I suspect, is the reason why my building various model's in the early days, turned into model engineering, when funds allowed the required equipment to be bought.
My workshop comprises one half of a detached double garage. It has a
stud wall separating the two halves and unusually perhaps, its own
shower/toilet/washbasin compartment! (the true reason for buying the
bungalow - don't tell the wife!!!)
It is fully insulated with a ceiling and fluorescent lighting supplementing the one window. Several double electrical sockets are dotted around in
relevant positions.
Equipment consists of 2 lathes (1 Myford ML 10 and 1 Chinese variable
speed motor with etched glass DRO's), 1 milling machine (RF25 far East) floor standing pillar drill, Proxxon bandsaw, Warco bandsaw, 6" wire brush and polishing mop motor, 6" coarse and fine grinder, Proxxon mini drill, various benches and an engineers vice. I have just acquired a third lathe, a Myford ML7 which I am at present evaluating so I can decide which of the Myford's I will keep.
I have only just bought the Chinese lathe and had it fitted with DRO's prior to delivery. And what a bonus they are! So good in fact that I decided to buy another set for the Milling machine. Whilst my engineering experience can only be described as limited, I find the use of DRO's has affected my accuracy levels which have improved tremendously (although that's not saying much!) and would recommend their fitting to any model engineer.
Prior to retirement I built for my son's 17th birthday a Locost car. This
was a tremendous project and a great feeling when it passed its test. The book it was based on suggests it can be built for £250. Ours was nothing special in the sense of all new parts but still cost about £900 to build!
