Car gauge component re-engineered
A 15 per cent reduction in the cost of producing a family of five brass components is being achieved by subcontractor, Qualiturn, using one of the 18 CNC mill-turning centres at its Hertford facility. The saving results from altering the design slightly and producing the parts in one uninterrupted machining cycle on a sliding-headstock lathe.
The component goes into round, dial-type gauges used in modern kit car instrument panels and found in vintage cars worldwide. Previously, the customer machined them in-house in five separate operations from brass extrusion, which had to be purchased in consignments of up to two tonnes – enough for a full year’s production – to secure a favourable price. Even so, it was expensive.
Qualiturn reassessed the design, which dated back to the 1930s, and concluded that it could be made from less costly 20 mm round brass bar on a Star SV-32 sliding-head lathe. A 32mm capacity machine rather than a 20 mm bar model was chosen, as the SV-32 has a turret with enough driven tool capability to complete the extensive prismatic operations in cycle.
To achieve the manufacturing change successfully, an alteration had to be made to one feature of the component. A radius around part of the outside was changed to a series of four flats that are two to three time faster to mill on a sliding-head lathe than a continuous arc. Function is not compromised, as that area of the component sits on the inside diameter of the gauge cylinder and it does not matter whether the feature is faceted or smooth.
Batches of 400-off are regularly produced for each of the five variants and the customer confirmed that, compared with the previous in-house production cost, Qualiturn’s charges are 15 per cent lower. Furthermore, lead-time has been halved to two weeks.
Additionally, there is no need to buy and stock expensive extrusion; or to machine larger batches than necessary to achieve an economic price, with consequent on-costs associated with work-in-progress and large stocks of parts.
Components vary from 30 to 55 mm in length and are machined in one hit on the Star SV-32 in cycle times ranging from 165 to 215 seconds. Operations on the front end are centre, drill, counterbore, followed by rough and finish turning of the front OD and turning of an abrupt start leading into a 1/8” BSP external thread.
Still in the main spindle, the irregular polygonal profile is milled, a cross hole is drilled to meet the bore, a Bourdon slot is milled in the side which encompasses the cross hole, and a form tool comes in to drill and countersink three larger cross holes in a different orthogonal plane, followed by cross tapping.
The component is gripped in the counter spindle and parted off. Using driven endworking tools, two off-centre 1.6 mm diameter / 9BA holes are drilled and tapped in the reverse end while machining starts on the front end of the next component. The amount of simultaneous machining in the main and counter spindles in this application is of necessity small, otherwise the savings would have been even greater.
Part of Qualiturn’s service to the customer included using up the remaining stock of extrusion by turning it on the SV-32, for which a separate program had to be written. The machine was run at around 1,500 rpm to suit the material, rather than at 5,000 rpm which is the spindle speed used to machine round brass bar.
