TREPANNING
 BY P J M SOUTHWORTH

I HAD the problem of making a brass shim 0.025 inch thick having an inside diameter of 4¼ inches with a radial thickness of just under ½ inch, in other words a ring. When finished it was to be a spacer in a steam turbine running at 14,000 rpm so it had to be right. The inside diameter fitted on to a spigot and the outside had to be within 0.005 inch, also there needed to be no distortion in the ring when finished.

There are a number of ways of cutting a ring out of sheet brass, I guess the common one is to drill a series of holes on the inside and file it to shape, the outside being rough cut using a hacksaw and filing it up. The other way of cutting out the middle would be to trepan it out using a single cutter in the milling machine, but with such a large diameter I could see all sorts of problems, neither would it be all that accurate. The solution I came up with was to mill both the inside and outside diameters using a rotary table. The photograph shows the set up on a rotary table to enable the outside to be machined after the middle had been removed.

The centre on the brass is pop marked and the inside and outside lines scribed, after which a hole can then be drilled to help centre it on the rotary table. The outside is then rough sawn close but not on the scribed line, say

0.050 – 0.060 inch away. The blank is then centred, packing placed underneath, in this case 0.125 thick, to get the metal above the table, then clamped using bolts that enable the chuck to pass over them. Moving a clamp to let the chuck pass could upset the setting.

With a slot drill, which is better than an end mill, of no more that 3/16 inch diameter, and away from the inner scribed line, a hole is cut through the brass and the table can then be turned to cut out the middle. Once this has been done the hole is measured and a cut put on. At this stage don’t be too greedy and go straight to size on either inside or outside cuts, it is better to take small cuts. If the metal is on the thin side there is a danger with a large cut for the spiral of the cutter to pull the metal up and distort it.

Once the inside has been machined to size the clamps can be changed from the outside to the inside one at a time so as not to move the ring on the table and alter the setting. The outside can then be machined to size. Both on the inside and outside removing 0.001inch was very easy so the finished job was accurate, neither was there any distortion.

The photograph shows the set up on the rotary table with the centre cut out, which is lying at the side, the clamps changed over and ready to machine the outside.