Encoders take control of nanogrinding process

The Tetraform C Nanogrinder breaks new ground in ultra-precision grinding technology with the ability to generate optical quality finishes without recourse to lapping or polishing.

Breaking new ground in ultra-precision grinding technology with the ability to generate optical quality finishes without recourse to lapping or polishing, the Tetraform C Nanogrinder is able to produce 2D and 3D forms using creep-feed techniques and relatively high material removal rates.

The Tetraform design is based on a triangular, pin-jointed 'tetrahedral' space frame structure and is primarily aimed at the highly accurate machining of silicon, glass, ceramic, tool steel and similar materials.

However, the process is already under the microscope of the automotive industry where its potential has been seen to produce key transmission components which need to be of 'optical' quality.

Jointly developed by Loadpoint of Swindon and Cranfield University, the Tetraform Nanogrinder is based on an original design concept by the UK's National Physical Laboratory.

However, with the Loadpoint development to achieve positional capability and precise control of the process, the multi-axis TNC 426 contouring control and linear encoders supplied by Heidenhain GB of Burgess Hill in West Sussex were critical.

The first machine to be built by Loadpoint has already achieved exceptional levels of static and dynamic stiffness and combined with an advanced aerostatic spindle bearing and hydrostatic slideways, leadscrews and thrust bearings; the machine is able to demonstrate exceptional high levels of precision.

Meanwhile, for a highly precise grinding operation it is also able to maintain relatively high material removal rates.

Equipped with cubic boron nitride (CBN) or diamond cup wheel, the machine is able to achieve Ra finishes in the 1 to 20nm range with the added processing advantage of creating very low subsurface damage to the material.

With the addition of electrolytic in-process wheel dressing (ELID), surface finish can be improved even further and ensures the surface structure and damage are not compromised as the grinding feed rate is increased.

The basic configuration of the Tetraform design uses three tubular struts of equal length.

These are secured with ball-ended joints to form a horizontal base while three further struts, joined in a similar manner, support a connecting unit which houses the grinding spindle and z-axis vertical oil hydrostatic slideway.

The horizontal x and y axes use similar hydrostatic slideways and are attached to the base struts giving a maximum stroke of 220 and120 mm, respectively.

Motion to each slideway is created through hydrostatic oil based leadscrews and the vertical aerostatic grinding wheel spindle which is able to rotate at speeds up to 6000rev/min.

The Heidenhain TNC 426 system is fully integrated into the structure of the machine and is able to control five axes in addition to the spindle.

It uses a 2Gbyte integrated hard disk that provides virtually unlimited memory for part programs while the powerful microprocessor hardware enables high speed block processing of 4 ms to be achieved, even on long programs.

Programming of the machine can be carried out very easily, either on the machine, using the Heidenhain conversational system with graphics support, or off-line, using a CAM system.

To ensure high levels of positional feedback, the Nanogrinder is fitted with Heidenhain LIP 300 high-resolution exposed linear encoders on each axis.

These high precision Zerodur glass ceramic scales have accuracy grades better than +0.5um and ensure high repeatability and feed back to monitor positioning of any axis movement.

Based in Swindon, Loadpoint has over 30 years' experience in the design and build of some 890 high precision dicing, scribing and grinding machines.

More than 85% have been exported to the USA, Pacific Rim and Scandinavia.

It also produces high performance air bearing spindles at one of its two Dorset factories.

The selection of Heidenhain for control and glass scale encoders was never a point of debate as Loadpoint's director John Sweet explained: "We first specified Heidenhain's reflective metal scales back in 1966 for one of our early dicing machines and since then have established a close working relationship.

We use the company's linear encoders on our MicroAce dicing machine while the NanoAce model also features a TNC control as well as glass scales.

The TNC 426 system specified for the Tetraform gives us the level of control we require and of critical importance it provides a known and proven stable platform on which we can run this high accuracy machine".

The first Nanogrinder is currently undergoing trials at Cranfield University in order to create a more cost-effective process for damage-free or ductile machining of brittle materials for use in the optical, communications, computer peripherals and semiconductor industries.

Already, the Tetraform has produced a one nanometre Ra optical quality surface finish on glass using a diamond wheel cutting at a depth of 4um with a feed rate of 6mm/min.

Meanwhile, tests on M50 bearing steel using a CBN cup wheel have resulted in surface finishes of 3.7nm Ra.

One natural area of application for the machine will undoubtedly be the dicing of silicon wafers into chips for the semiconductor industry as the demand for thinner sections continues to grow for use in the latest generation of smart cards and smart tags.

However, the exceptional dynamics of the Tetraform will have major implications on other processes such as the automotive industry.

Already, this volume-hungry sector is talking of targets of 10nm Ra surface finish, but it needs to be economically produced on hardened steel by direct machining, without the need for polishing.

These parameters are already well within the capability of the Tetraform C, while the rapidly developing medical sector is seen as another growth area.

As well as reducing or eliminating the need for lapping or polishing, the Tetraform is also quicker and a much cleaner process than traditional optical quality finishing techniques.

It can be used to grind 2D and 3D forms as well as performing less complex axis moves for dicing and scribing operations on silicon wafers.

(This was Engineeringtalk's Top Story on 24 October 2002).

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