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Designed to fill a currently un-served niche in the marketplace, Curtis Machine Tools has high hopes for its new Vector production grinding machine. More so now that the company has found a high performance encoder solution in the form of an MHG linear encoder from Newall Measurement Systems. Fitted with the MHG, the Vector machine is already taking the market by storm since its launch at the turn of the year.
Colchester-based Curtis Machine Tools is part of the Douglas Curtis Group of companies. The majority of its 33-year history has been spent building special purpose machines based around grinding processes – applications such as grinding diesel injection components or diamond drilling bits.
“We identified a niche for a fast cycling, accurate, auto-fed grinding machine with a small footprint, pitched somewhere between the super precision high end CNC grinders and the very low cost machines built in the Far East,” explains group chairman, Rick Curtis, who has been heavily involved in the machine's design.
The key drivers for the Vector machine are precision, reliability and price. Many automotive Tier 1 and Tier 2 suppliers opt to have their CNC grinding machines rebuilt towards the end of their life cycle. While Curtis Machine Tools knew it could build a machine capable of chuck, between-centres, face and concentric grinding that could compete on price with rebuilt CNC grinders, it also recognised the need not to compromise on accuracy, performance and reliability. To achieve this goal Curtis had to source an encoder solution for the Vector that could match-up to the high performance machine features that the company had in mind.
“In the past we've used traditional measuring systems such as glass scales,” says Mr Curtis. “However, glass is prone to contamination, usually by way of oil or coolant droplets suspended in the air that build up over a number of years, affecting the reflectivity and eventually causing damage. Glass scales also require careful handling by maintenance staff. As a result of these factors we decided to source an alternative, which took the form of the MHG linear encoder from Newall.”
Newall incremental encoders such as the MHG range provide sine-cosine or quadrature square wave feedback signals. An electromagnetic field is generated by inducing a 10kHz sinusoidal current through a single drive coil within the reader head. This field interacts with the nickel chrome elements contained in the scale. A set of four pick-up coils detect variations in the induced field that are then combined and processed by the electronics to generate a signal that varies as the head moves along the scale. Depending on the position of the reader head as it passes over each element, the phase shift of this pick-up signal relative to the drive signal will vary between 0 and 360°. A high-speed digital-signal processor converts the analogue signal to an industry standard differential quadrature signal. Because Newall MHG encoders are fully enclosed, all risk from contamination is eliminated.
“The prime reasons for choosing the MHG were its method of measurement and the fact that it is intrinsically more robust than optical alternatives,” explains Mr Curtis. “It also integrates well with the Siemens control system. Overall we are very pleased – we simply take the MHG out of its box, and it works – which is exactly what we wanted.”
Using the MHG, the Vector grinding machine offers a resolution of 0.0001mm with micron accuracy. Launched in January, three machines have been built – two of which are already sold. Curtis anticipates sales in the region of 25-50 machines a year.
Pitched as a high-accuracy grinding machine for normal production use, the Curtis Vector can accommodate components up to 30-40mm in diameter by 120mm in length, although typically most parts will be smaller. It has five different interchangeable workheads that can be positioned for either straight or angle approach grinding. The machine features a unique layout in which the grinding wheel guard is fixed as an integral part of the machine's structure. There is also an innovative loading system (optional), in the form of a three or four-axis Cartesian robot. Typical customer applications include fuel injection parts, automotive systems such as brakes and steering components, turbochargers, steel and tungsten carbide tool blanks, and ceramic components.
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