Your shopping cart is empty.
Product Qty Amount

[email protected]
/ Categories: Archive, engine-structure

Casting back

heads-blocksThe trouble with advancing years, or so I was always told, was that you can always remember how it used to be. In the dim and distant past, it seemed to be much more fun (even though it probably wasn’t) and if the job couldn’t be precise then we always made it as accurate as we could somehow adapting the product to the limitations of the manufacturing process.

A typical case of this, I was reminded recently, was in the casting of cylinder heads and crankcases. In those days and I am only talking about the 1960s here, most of those with casting skills resided in the Pattern shop. A most unusual place and quite unlike everywhere else in the works, the Pattern shop smelt of pine wood forests and the floor had a liberal coating of sawdust rather like the local drinking establishment just around the corner. In place of the milling machine was a rather frightening bandsaw and the lathes seemed to lack a proper saddle and cross-slide so familiar in other parts of the works. You see, this place was used to dealing in wood and when sawn, machined and stuck together with glue, together with a liberal application of polyester filler for the fillet radii, (oh yes, this was technology at its best) a wooden facsimile of the external parts of the cylinder head would evolve. For the internal parts, life was a bit more difficult. Working from a 2D drawing and using a pattern maker’s rule, the patterns to produce the internal cores were produced which looked totally unrecognisable to anything on the drawing. You see, the pattern maker had to think in negatives and produce the patterns to make the cores, which would be placed in the finished mould to create the voids where the metal was to go. To the apprentice this was all rather too confusing! At the time we often doubted if the designer had ever thought about things like draft angles or split lines or runners where the molten metal would flow. The great skill of the pattern maker therefore was in deciding where all these were to go and, in effect, how the thing was to be made in the first place!

But how casting technology has changed in the modern world. Gone is, in effect what I would recognise as the pattern shop. Gone is the evocative smell of wood and polyester filler. Gone too is the pattern maker’s rule, with proportionately larger scales to account for shrinkage upon cooling - one side for aluminium and the other for cast iron.

Today, cylinder head designs are encapsulated in 3D virtual form, and core boxes will be machined directly using 5-axis machines. Indeed using rapid prototyping technology the cores themselves can now be made directly on a laser scanning machine. Sadly, gone also are the cylinder head drawings. Comprising sometimes of up to three or four full size A0 sheets, each one would be detailed with lots of little gems of knowledge, passing on titbits of information to the pattern maker. As a historical statement to the design philosophy, students of the art could pour over these for hours on end and learn much.

However, in its place we have castings that are not only accurate but thanks to modern casting software are of far better quality and free from porosity or blow holes. Only quality patterns can make good castings from which good components flow. As an engineer, I know which one I would sooner have.

Written by John Coxon.

Previous Article Sintered Aluminium Liners
Next Article The cylinder head gasket