Essex-based Braintree Precision Components (BPC) is a division of linear-motion system specialist Hepco Slide Systems, which is based in Devon and produces over 300,000 inner guide component products and bearing systems a year. These are currently sold in more than 60 countries world-wide.
The company is no stranger to hard turning, as it uses this process on fixed-head lathes to produce larger bearing types. However, it was keen to transfer this proven technology to the finishing of smaller inner ring components with bores in the diameter range 6-20mm. Moreover, these parts have a critical 35deg angular face on specific outer races. Following in-depth hard-turning trials at Bushey-based Citizen Machinery UK Ltd (
www.citizenmachinery.co.uk) in 2013, BPC installed a Miyano GN-3200 with integrated work handling. BPC is no stranger to Citizen Machinery UK, having installed nine Citizen CNC sliding-head turn-mill centres since 1990.
The Miyano GN-3200 subsequently installed was soon creating overall savings of up to 50%. General manager David Ford said: “These were achieved not only from shorter cycle times compared with grinding, but also improved and more-consistent surface finish, an improvement in bore geometry and the elimination of taper due to grinding-wheel breakdown. In addition, overall size control was more effective than could be achieved with grinding.”
The Miyano GN-3200 is compact (it takes just over 1m2 of floor area and is just 700mm wide); and thanks to the symmetrical design of its frame and bed, it is not only thermally stable but also highly rigid and with a high level of vibration damping — characteristics that allow hard turning to be fully exploited.
The machine has an integrated high-speed gantry-type loading system working from a pallet that can hold up to 35 bearing rings. Furthermore, this loader can overlap most of its programmed tasks within the overall cutting cycle, so it has a minimum effect on productivity (its rack-and-pinion drives give speeds of 130m/min left and right and 154m/min in the vertical axis). The spindle has a single 40mm collet chuck and is rated at 1.5kW 8,000rev/min. Travel is 18mm in X and 200mm in Z; the rapid-traverse rate for both axes is 16m/min. A linear platen carries the tooling for both roughing and finishing operations (on a second GN-3200 installed recently, the platen also carries a Renishaw probe).
The first machine, which was installed to finish-bore the bearing rings, originally had to cover four basic sizes of component; this has now been extended to 18 in order to cover special product applications in both stainless and non-stainless steels. Batch sizes can range from 100 to 500, with cycle times that allow up to 1,200 parts to be made a day. Standardised tool kits, which can include grippers, collets and cutting tools according to component size, see change-overs take just over 1hr — including a first-off check. Re-setting is often carried out as many as five times during the 18hr covered by the day and night shift.
Mr Ford says: “Initially, tooling development was an important issue. When we started, inserts lasted for around 40 components; this has now risen to 1,000 parts per insert edge. Furthermore, because we rough- and finish-bore, we downgrade used finishing inserts to the roughing position to minimise insert usage.”
The second auto-loaded GN-3200 was installed in June this year for the rough and finish hard-turning of a 35deg taper that provides the interface between the track roller and the linear guide track. Moreover, a concentricity of better than 2µm has to be maintained. “Because of the critical nature of the angle, we incorporated a Renishaw probe on the tool platen of this machine in order to check the component prior to turning. Even with the additional probing cycle, we are still achieving the same overall cycle time previously required for grinding.”
The Braintree facility,which opened in 1989 and employs 70 people, is housed in three units (3,300m2 in total) — two for production and the third for assembly and stores. Investment is planned to further improve productivity and double Hepco’s turnover and profitability over the next decade.