In this, “A” shows the casting as supplied and which needs to be surfaced on the
two outer faces to make them at 90° to one another. With the short face mounted vertically,
as at “B”, much more has to be removed compared to the longer face being mounted
vertically, “C”. Having first machined the casting as at “C” it can then be mounted
as shown at “B” and machined equally over the larger surface.
Fortunately, the pattern maker will have chosen in most cases to make the taper on
the shortest sides, see Sk. 4A, so as to reduce the depth of the pattern in the mould
and minimising the amount of metal that will have to be removed at the machining
stage if made as per Sk. 4B. Another benefit of this is that it reduces the amount
of iron used thereby reducing the cost of the casting as a result.
With castings that conform to the above one of the shorter faces is very likely to
be the one to machine first, that is certainly the case with three out of the four
castings illustrated. I should add here that the angles shown in the sketches are,
for clarity, more than those normally present on the casting which is usually between
one and two degrees per face.
Machining Cast Iron
As the details of the process are mainly for the benefit of the workshop owner new
to metalworking I will cover the most important points.
1. The surface of iron castings can frequently posses hard spots that will rapidly
blunt high speed steel tooling.
2. Because of the hard spots, if possible use carbide tipped tools for initially
machining the casting's surfaces.
3. If the surface starts to take on a glazed appearance this is because the depth
of cut is not sufficient to get below the hard skin.
4. This can either be because the castings surface has a shallow hollow or that it
has been mounted on the machine with the surface being machined slopping down very
5. In this case, return to the start, deepen the cut, and recommence to machine the
6. If you do not have a tipped tool, reserve an old HSS cutter for the task, keeping
your sharp cutters for more appropriate tasks.
7. If you have to use a HSS cutter, grind a chamfer, about 1 to 2mm, around the edge
of the casting so that the cutter does not have to break through the hard surface.
8. Whilst less important, the chamfer is also worthwhile if using a tipped tool.