General Optics Taunton, now part of the GSI Group, has been manufacturing beryllium and aluminium beryllium composite products for more than a decade, producing high precision components for various industry sectors such as defence and aerospace, semiconductor, metrology, medical and nuclear. Machining takes place in a dedicated 3400 sq. ft. sealed negative pressure environment within the 8500 sq. ft. single-source Lisieux Way, Taunton facility, which in its entirety represents an investment of some £3.5 million.

The downside is that while beryllium can be machined, it fractures easily and sharp corners contribute to crack propagation. Tapping can be particularly difficult because of the high surface stresses involved and the material’s lack of ductility. So every aspect of the machining process is carefully monitored and controlled – which is where Heidenhain (GB) Limited comes into the picture.

Although the VMC had been performing well on general machining work, General Optics Taunton’s exacting requirements for the machining of beryllium proved a step too far. New ballscrews were fitted to the machine in an attempt to eliminate the problem of inaccuracies resulting from thermal growth but this was unsuccessful. However, as the machine was fitted with a Heidenhain 426M CNC control, the company then approached Gary Blanch of Bournemouth-based Engineering Equipment Centre Ltd, an authorised Heidenhain distributor and retrofitter since 1986, who suggested fitting linear incremental encoders – an option that the machine tool manufacturer had previously dismissed as impractical.

When it came to actually fitting the Heidenhain LS 486C encoders to the VMC, Gary Blanch was required to comply with the strict provisions of the European Integrated Pollution Prevention and Control Regulations (as airborne beryllium particles are toxic) by donning full body protective clothing with a filtering system supplying air to his breathing hood. He was also limited to a maximum of 2.5 hours exposure per visit and, when exiting the machine shop, required to take a full body shower while still wearing the protective clothing to remove any traces of beryllium dust.

After the Heidenhain encoders had been fitted to the VMC, an equivalent machining test confirmed that the 3 micron inaccuracy previously experienced had reduced to zero in the X axis and 0.5 micron in the Y axis.

“Without these scales we would have been dead in the water as far as machining tight tolerance, high value components is concerned” says Jeremy Dugdale, Production Engineer with General Optics Taunton. “Beryllium is extremely expensive to buy and, typically, most of the material is machined away, so mistakes become progressively more costly.”

From this it can be argued that the typical jobbing sub-contractor has neither the means nor the expertise to machine beryllium successfully and safely. Although milling and turning operations at General Optics Taunton are undertaken on relatively commonplace machining centres and CNC mills and lathes, clearly it is the acquired knowledge in terms of the methods employed – component design, tooling and fixturing – that is the differentiating factor.

“The key machining requirements are a sharp, keen edge on the cutting tool, a well-maintained machine tool and adherence to the correct manufacturing processes,” says Jeremy Dugdale. “This is the only way to avoid surface stresses and the resultant cracking that could lead to component failure.”