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Milling to tight tolerances at Loadpoint

Specialist saw manufacturer brings machining in-house and achieves ±5µm on straightness and flatness

Posted on 04 Jun 2019 and read 1496 times
Milling to tight tolerances at LoadpointMore than 40 years ago, Cricklade-based Loadpoint Micro-Machining Solutions developed what became the industry-standard saw for dicing semiconductor wafers.

Sawing avoids the current leakage in electronic components caused by the old method of scribing and snapping wafers.

The company recently brought all of its metal-cutting in-house, following the purchase of three new machine tools from High Wycombe-based Hurco Europe Ltd (www.hurco.co.uk); previously, larger castings that form the bases of Loadpoint’s products were sub-contracted out for milling.

That cost is now saved, and Loadpoint’s control over lead times and quality is much improved. Drawing tolerance is ±5µm for both straightness and flatness over the 750mm length of the largest base casting for a Macroace dicing saw.

This accuracy is being exceeded on a 20-tonne bridge-type Hurco DCX22 machining centre, which has a 2,200 x 1,700 x 750mm working volume.

Clive Bond, Loadpoint’s managing director, said: “We need to hold a high level of accuracy when manufacturing our machine components to underpin the precision our customers need when sawing their materials with resin- or metal-bonded diamond grinding blades that can be as little as 15µm wide.

“In addition to semiconductor wafer dicing, many applications these days involve cutting PZT, a piezo-electric ceramic material used for a multitude of applications — from parking sensors to ultrasound scanners.

“Glass for making optical filters, for example, and alumina for the manufacture of hybrid circuits are also frequently processed.

“Generally, our equipment has to saw material within a tolerance of ±3µm over a working area up to 12in in diameter.

“However, a recent application involved producing an ink-jet printer head from 200µm-thick PZT to a significantly higher level of precision.

“Over a 60mm length, 600µm-deep cuts had to be spaced out at 100µm intervals with a pitch-to-pitch accuracy of under 1µm.

“Tolerances of this order require that the structure of our machines is extremely precise.”

A 1m-deep concrete foundation was prepared to support the DCX22, and Hurco engineers spent considerable time and effort during the commissioning phase to ensure that the required machining accuracies could be attained, using a Taylor Hobson autocollimator to verify this.

Fine-tolerance techniques

Explaining the technique that allows tolerances within ±5µm to be held over a comparatively large distance entails unclamping the heavy castings and ‘simply restraining’ them in position on the table during the final operation, which involves taking only very light passes with a milling cutter.

Milling 2The process was successfully proved out at a Midlands sub-contractor (using a similar Hurco DCX machining centre) before Mr Bond went ahead with the machine purchase.

Not only do Loadpoint's FEA-optimised structures that support the three linear-axis motions and rotary-table movement have to be rigid and accurate (all CNC axes offer a 50nm resolution thanks to the use of Heidenhain encoders), so does the assembly carrying the 60,000rev/min air-bearing spindle (the run-out of this has to be less than 50nm TIR).

To ensure that this level of accuracy is achieved, a Hurco TM10i lathe replaced an old manual lathe as part of the project to re-equip Loadpoint’s machine shop.

It turns the stainless steel (or titanium) flanges that support and clamp the circular-saw blades.

To control the bore and complex flange profile to a tolerance approaching a single micron, they are sent to a sister Loadpoint company for cylindrical grinding, followed by precision balancing.

Another role of the new lathe is to turn a stainless-steel disc that forms the carrier for a vacuum chuck, which secures material during dicing.

After heat treatment, the component is held in a bespoke fixture on the third new Hurco machine on-site, a smaller VM20i three-axis machining centre; this mills recesses (over one face) for the adhesive that holds a high-precision ceramic insert in place.

Many of Loadpoint’s smaller castings and components are also produced on the VM20i, which replaced a manual-tool-change CNC mill.

Mr Bond said that because machinists at Cricklade were familiar with Heidenhain and Fanuc controls, there were initially some concerns about using a new CNC system — Hurco’s own WinMax.

However, he said that his operators were convinced of its suitability by a demonstration at a Hurco Open House event.

Indeed, Mr Bond says the software’s ease of use is such that menu-driven programming on the shopfloor using the touch-screens on the controls is carried out all of the time at Cricklade, to the exclusion of offline program preparation via CAD/CAM, even though Loadpoint machine components are created in CAD and are available as solid models.