Precision mould and die making gets boost from probing

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Renishaw plc

Ramada Acos

Complex mould tools require good fixturing, probing

“Probing is the only way to achieve better than 10 μm repeatability which cannot be done by craftsmen with hand-tools afterwards,” explains Tunks. Initially the metal billet is set on the machining centre using the spindle-mounted probe, then first machining operations are completed, but this is not the end of the story. The billet then has to be removed, rotated and re-set to be perfectly positioned relative to the first machined features, ready for spark

“The fixturing is very good but the reality is that you have to cope with thermal errors and clamping forces as well,” Tunks explains. A 3R fixturing system is used, which re-positions fixture plates to within 5 μm, but the other factors mean that a rapid probing operation, taking a matter of seconds, ensures machining will be within tolerance.

The XYZ machine also has a table-mounted TS27R tool setting probe fitted, which sets tool lengths for all the ball cutters used, at multiple diameters, down to the smallest at Ø 0.4 mm. All sizes must be checked for length, which is done on the machine rather than using a pre-setter, taking into account thermal effects such as spindle growth. The range of cutting tools is limited to about 60 different cutters, with machining programs designed to use this library of tools. Luckily, the machining of mould tools doesn’t require processes like drilling and tapping holes, making tooling selection more straightforward.

Bob Tunks has had a number of machines fitted with probing - as he says. “Even if I had bought a less capable machine I would have had it fitted with probing, all the machines I’ve had in the past have had it.”

High-speed machining centre is versatile, rigid

The latest machining centre is a 12,000 min^-1, 43 m/min rapid traverse XYZ 1060 HS vertical machining centre. “Much of the mould work we do here is complex 3D machining involving a lot of small cuts using radiused or ballnose cutters. However, because I prefer to replace ‘old’ technology rather than simply add to my plant list, I need a machine that can be ‘all things to all men’. I need to rip pockets out of steel bolsters as quickly as I can using carbide-tipped cutters; I need to machine copper and aluminium for EDM electrodes and prototype ‘soft’ tooling; and I need to rough out cores and cavities, and then to finish machine mould tools when they come back in their hardened state. On top of this, I have to be sure that these demands will not affect the accuracy of the machine or, worse, cause a breakdown.”

“So it’s not just about spindle speed, it’s about the overall rigidity of the machine, the quality of its build, and the way in which it accelerates and decelerates The 1060 HS is also equipped with a Siemens 840D ShopMill control that looks ahead to convert a multitude of straight lines into a smooth curve. This machine can drive a 6 mm diameter cutter at 2500 mm/min with a 0.1 mm step-over and produce two halves of a mould tool that fit together perfectly on a three-dimensional split line – that is five times faster than the last machine!”

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