Horn Broaching - Productivity, Precision and Competitiveness for Carbon Fibre Specialist
Live tool station powered broaching units utilizing the Horn broaching system of holders and inserts have allowed Banbury-based composite systems specialist CTG Ltd to reduce the time needed to produce a series of splines on the inner circumference of stainless steel trunnion-style shaft connectors/end fittings from six minutes to one minute. The broaching technology was integrated by Horn Cutting Tools Ltd, and is allowing CTG to meet increasing demand for 85 variants of the components; these are an interference fit on the ends of filament wound carbon fibre hollow shafts used in a multitude of aerospace, and automotive applications.
CTG production manager Martin Pacey. “The splines are produced to very tight tolerance as we require an extremely secure fit without recourse to the use of adhesives. We actually defined the basic spline pattern some years ago but we were reliant on using pull-broaching to produce it. This had various disadvantages attached as the tooling was fiercely expensive, each diameter needed a dedicated tool and we were reliant on a supplier whose future was uncertain. Pull broaching was also incapable of satisfying an emerging requirement for broaching splines in a blind hole.”
Horn’s initial contact with CTG came about due to the blind hole requirement and a successful solution was duly delivered. Like most successful applications of broaching on a lathe to date it relied on movement of the entire Z-axis saddle and turret assembly to move the tool back and forth, while the rotary index was taken care of by the lathe’s C-axis. Though successful in terms of fulfilling the machining requirement, the large masses being moved around meant it was much slower than pull broaching, taking around 6 minutes per component compared to one minute for the pull broach operation. However the flexibility that was afforded via use of the Horn broaching system offset any disadvantage in time. A further point to take into consideration, the component now came off the machine complete with orientation of the spline being controlled relative to a datum position on the component if and when required.
Mr Pacey continued. “We could tolerate the longer cycle for the small number of blind hole connections required but rising overall demand for products using the connectors dictated the need for a much faster method of machining through-hole splines. Pull broaching was not a viable long term solution and methods other than broaching were considered. However when Horn discussed the turret mounted broaching attachment it was an obviously ‘right’ solution provided that it could be made to work reliably.”
The attachment comprises a mechanism that is driven by the live tool facility on a lathe turret. This is enclosed in an oil bath, to minimise component wear; it converts the rotary drive into reciprocal motion that propels the broaching tool backwards and forwards. Each mechanism is custom built to provide a particular stroke length –CTG has thus far used variants offering 20 mm and 30mm mm stroke length.
Because Z-axis assembly on the lathe is stationary the major moving parts are relatively small and light. Generation of the splines relies on tiny incremental movement of the X-axis and periodic index of the C-axis. The broaching tool is based on the Horn Type 117 insert with a precision ground spline form, housed in standard and special Horn tool holders.
Mr Pacey concluded. “During commissioning trials the unit has demonstrated capability to maintain the same level of accuracy as a pull-broaching tool and also match its speed. Moreover, whereas the pull broach required a dedicated set of quite costly tooling for each size of connector, the Horn system can accommodate the majority of sizes with three insert variants. In addition the actual cost of the broaching unit and tooling is very competitive especially if you factor in the improved flexibility, relatively easy set-up and reduced wear and tear on the Z-axis of the lathe. It is proving to be a very satisfactory solution.”