BHI Practical Exam - Second Year

Over the last few weeks Rosie and I have been working on the BHI practical exam, in which you are given a technical drawing for a multi-part item made from different materials and finishes with tolerances of 0.05mm, and required to make it - this year it was a cock and plate. You're given three weeks, but this is to allow you time to make mistakes. I was definitely glad for the excessive time last year, but this year having made a more similar, but much more complex, practical piece as part of the degree, I found this exam to be largely straight-forward. See after the break for a picture diary!

The piece consisted of four parts; a plate which would be jewelled, the cock, with a countersink for the screw and two steady pins, an arbor which was somewhere between a balance staff and a barrel arbor (more of a 'proof of ability' thing rather than any sort of real horological component), and a shouldered screw.

I decided to start work on the arbor first as I expected this to be the part that would be the most complex. I began with the square section, as doing this when the rest of the part was down to size could put unnecessary strain on the thin parts and risk breaking it if I wasn't careful! I began by turning the appropriate length of bar down to just larger than the diagonal dimension required for the square. from there I turned down a very small length to have a diameter the same as the distance across flats. This would help me to know when to stop filing without having to constantly measure! I used a drill bit as a size guide which can be seen in the picture below.

Once the square was the correct size I turned down the other parts, measuring as I went. The lengths of the parts under the cock are not stated and thus need to be derived from other measurements, and then chosen so as to make everything look in proportion.

The diameter for the pivot wasn't given either, so it required trial-and-error, reducing the diameter until the supplied jewel fitted well.

Once I was happy with all the dimensions I needed to harden then temper it, as is standard practice for arbors. before doing this I spent some time polishing, as it is much easier to remove material from soft steel! I would give it a final polish at a later stage.

Below you can see the arbor after tempering to blue. I heated it slightly more than is desirable, as can be seen from the pale blue colour, which should be more of a royal blue.

Then came the final polishing stage! This was probably the most time consuming stage as hardened steel takes a long time to do anything to. Below you can see a picture from half way through. After doing a final polish I realised there were still marks for rougher grades of paper. That meant I had to start all over again!

 
The finished part! Behind you can see technical drawings I made, using the provided dimensions. It's really useful to see a 1:1 scale drawing of the part you're making, and these can be stuck to material to act as a guide as well, which is why I printed so many!

Next I chose to make a tool for the screw head countersink. I came up with a tool (though I'm sure I wasn't the first!) to do a similar job for my first year mini clock washers, and so used the same design with different dimensions.

The first step is to drill a hole down the centre to allow cut metal somewhere to go!

The bar is turned down to the diameter of the hole it will cut. I then cut a small channel around the diameter a distance from the tool tip; this distance is the depth I want the hole to be, and acts as a guide while cutting.

I then use a slotting file or piercing saw to cut an X shape down the bar, which creates the four cutting teeth. Once the cuts are a reasonable depth (at least 5mm) I file down the back faces for relief. is is important in this stage to ensure that all cutting faces are at the same height, otherwise not all of them will cut!

I then harden and temper the tool to give it the strength to cut, and finally use an arcansas stone to make the cutting faces very sharp and flat. We'll see the tool in use later!

My next job was to start on the cock. As I mentioned before we made a much more complex one as a project for the degree and as such I already had a plan of attack. I stuck a piece of brass to my wax chuck and turned it down to about 1mm thicker than the cock height. I then removed the brass, turned it around, and, re-measuring the size, turned it down to be the correct height. Once I was happy with this I drilled a hole through the centre, and began to bore out the underside. This was a very slow process, taking over 60 0.1mm cuts!

Cutting it so slowly requires a lot of patience but means the outcome requires a lot less finishing.

I then needed to cut a hole for the screw. I used a technical drawing print out (as mentioned earlier) to find where the screw needed to be, and used a centre punch to make a mark in the correct place. This allowed me to use the lathe tailstock to position the cock in the correct place on my brass chuck.

I started the hole with a centre drill to ensure acuracy, then used a regular twist drill to brill all the way through the brass. When I face off my wax chuck, I always drill a small hole in the centre, and this was how I could tell I had gone all the way through the brass!

I then used the tool I made earlier to cut the countersink. I made sure to use oil and very little pressure to allow it to make as clean a cut as possible, as it would be a real challenge to clean up the hole later. Same as boreing the underside, taking the time to be careful at this stage would save a lot of time later!

A nice, clean countersink.

Next I needed to make the brass into the wedge-shape required. I began by making a hardened steel filing butting of the same diamter of the small end, and attaching it to the underside. I then used another technical drawing printout (this is why I print so many!) super-glued to the top to act as a guide. Once I was close to the printed line I used a vernier guage to ensure I had reached the correct dimensions.

Once I was happy with the shape, I used acetone to remove all the glue, and put a chamfer on the top edge. I used a pair of compasses to mark guide lines the appropriate size, then removed the material with a needle file, being careful to ensure I kept it a 45° angle as required. I then did a quick polish, just to make it look nice!

Next was the steady pins. I drilled the holes for these using the same method for the screw hole, but didn't drill all the way through. I then made pins to be friction-fitted into the holes. I deliberately made them too long to give room to then trim them to the desired length. I used jewellers burrs to give the tips an even radiused finish. I also made the plate, which was some very simple drilling and filing, to allow me to test fit everything and ensure all the holes were in the right place. If they weren't I would have a real problem!

Unfortunately I forgot to take pictures of the process of making both the plate and screw, but neither was particularly interesting!

The test fit! Putting everything together allowed me to see that I had made the arbor about 0.1mm too long, so I corrected this before proceeding. Once I was happy everything fitted well, I Just had to polish it!

When polishing brass I use 40 micron 'micro-finishing paper', then 9 micron, and finally Autosol polishing compound. It seems like very few steps, but gives very good results indeed! It allows me to get a very good finish in very little time.

Once I was happy with the finish, I cleaned all the parts, and assembled them for the final time! If I had more time, I could have definitely done some things better, but overall in the context of the exam I was happy with the outcome and hopefully I'll get me a good mark!