Moreover, as the Brother machine is equipped with a twin pallet changer – Greenway Pepper’s first – parts are presented more quickly to the spindle so floor-to-floor times are reduced further. It is not surprising that Greg Pepper, joint owner and managing director, reports such dramatic improvements in productivity.
The Brother TC-32BN QT has a top spindle speed of 16,000 rpm and 1.5 g acceleration to rapids of 70 m/min in X,Y and Z. These figures are much higher than the 8,000 rpm and 30 m/min on the subcontractor’s other VMCs recently purchased from another supplier. Mr Pepper said, “It was the relative slowness of our other machines and a sudden upturn in medical business that prompted us to buy the Brother machine.
“I had seen the TC-32BN at MACH 2008 and visited the Whitehouse technical centre in Kenilworth a year later, where I was impressed with the demonstrations. It was like having the output of two machines in the footprint of one. “However, the opportunity to invest did not materialise until work for an AIDS blood sampler and a DNA testing machine arrived in quick succession, pushing our company turnover past £1 million.”
Trials were carried out by a number of potential machine suppliers, including Whitehouse. The TC-32BN QT was at least a third quicker than the other shortlisted VMCs and 2.6 times faster than a twin-pallet model offered by a recent supplier to the Newcastle-under-Lyme factory. Part of the productivity advantage on the Brother is down to high speeds and feeds while the machine is in-cut, and part is due to short idle times.
Promoting the latter is acceleration of the BT30 spindle from 0 to 16,000 rpm in 0.46 sec, enabling a chip-to-chip time of 2.1 seconds. While a tool is being exchanged from the 40-station magazine, the workpiece can be repositioned ready for the next feature to be machined, or else the twin pallet table can start its rotation. By the time the part is in position, the spindle is hovering over it at 16,000 rpm, ready to cut immediately with the correct tool.
Not a moment is wasted. Installed in March 2010, the TC-32BN QT is operated flat-out by Greenway Pepper all of the time. High-pressure coolant at 90 bar through the spindle and tool dispels the large amounts of swarf generated. Coolant of a relatively thick consistency prevents tap breakage, which is important as a large amount of synchronous tapping is carried out at 8,000 rpm. This feature, a Brother first, involves true linear interpolation in Z with spindle rotation, resulting in higher speed, better control over thread quality and depth, and longer tool life than with conventional rigid tapping. Workpiece clamping is configured differently on the two pallets, allowing Greenway Pepper a high degree of versatility. On one pallet there is a MicroLoc baseplate with clamping elements for workholding.
The other pallet supports a Nikken trunnion-and-tailstock indexer that rotates a MicroLoc plate through 360 degrees to provide a 4th CNC axis. Looking in a little more detail at a couple of the applications put on the Brother so far, one involves machining solid aluminium billet to produce a well plate for storing blood samples. The component has a 12 x 8 array of tapered cones that have a drawing tolerance of half a degree and a fine internal surface finish. Positional tolerance of the individual wells is within 20 microns. Flatness on the back of the component must be within 25 microns, which can only be achieved using a relatively small, 4 mm diameter milling cutter, as a larger tool would introduce stresses.
The maximum 70 m/min cutting feed rate and 16,000 rpm spindle speed of the Brother are ideal for removing material quickly from both faces without distorting the plate. Total cycle time is 40 minutes, which Mr Pepper describes as “incredible”, considering the amount of aluminium milled away by such small cutters. The cycle time quoted includes the exacting routine of drilling 0.8 mm diameter blind holes in the top face of the component, drilling similar holes in the side to meet the first holes. This creates a vacuum-less hole for thermometer insertion. Another medical component, referred to earlier, whose cycle time has dropped from 70 to 26 minutes, is the side plate for a new instrument that will allow on-the-spot DNA profiling by medical staff and police forces.
Due for global launch in July 2010, the US-designed equipment has an aluminium casing comprising a family of parts, prototypes for which Greenway Pepper produced at the end of 2009 in batches of 50-off on its slower machining centres. The subcontractor is looking forward to reaping the benefits of the 63 per cent saving in production time on the Brother machine once volume manufacture starts. To extract more benefit from investment in the twin-pallet machine, Greenway Pepper has moved from single- to double-shift operation, 7.00 am to 10.00 pm. The approximately 250 mm by 100 mm side plate for the DNA equipment is machined from 10 mm thick, 6082 T6 aluminium plate.
As before, it is essential to keep machining stresses low, so an 8 mm diameter cutter reduces the thickness to 7 mm, rather than a much larger cutter that would normally be used. High-speed drilling and tapping are again important, as the component incorporates many M2, M3 and M4 holes, all of which need to be tapped. Mr Pepper continued, “Securing the DNA machine contract took three months and underlines our evolution from a jobbing shop into a value-added machining facility, offering production of small to large batches and a fully integrated design, development and assembly function.
“The back-up provided by Whitehouse was very good in transferring this and other work to the Brother machine. It included writing early programs and comprehensive operator training both at Kenilworth and on our shop floor.”
