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A special-purpose hybrid turning/grinding centre supplied by Heidenhain is playing a key role in the production of special square-section polymer seals being used in extreme aerospace applications.

Based on the reground and reconditioned bed of a Hardinge HLV manual centre lathe, the machine was completely re-engineered by Chris Stinchcombe, managing director of Machine Tool Controls (MTC) of Pontypool.

As well as panel build, the project involved the installation of a cross slide and a headstock packer to raise the centre height by 65mm and to carry the Acu-Rite linear encoder and two tool posts.

The machine will help produce the critical closure areas of aerospace seals of diameters from 5mm to 350mm and between 2mm and 10mm in section.

A carbide knife blade in the front tool post is used to clean cut the moulded edge of the seals at 45deg and a grinding spindle in the rear tool post position is used to grind the inside diameter (when the seal runs on a shaft) or to produce a step on the outside diameter (if the seal is to be located in a housing).

Key for the machine operator to maintain the position of the slides and tolerances is the use of an Acu-Rite 200S LCD panel-mounted digital readout (DRO) supplied by Heidenhain, a provider of angular, linear and rotary encoders, DROs and CNC systems.

The 200S provides a clear display for the operator to maintain the + 0.025mm tolerance on the 5mm diameter of the smallest seal and + 0.1mm on the largest.

Thanks to the capability of the DRO, the operator can monitor the rotational speed of the grinding wheel and spindle motor as well as benefit from a full onboard help menu display and interlock guarding for the chuck.

Depending on the sizes of the seal, the machine will use one of four different sizes of vacuum chuck.

The chuck is located on the headstock with aluminium inserts tailored to suit the appropriate size of the component in order to locate it against the backing plate.

The seal, which has already been ground and pre-sized on a mandrel, will be pushed into the chuck and held by vacuum ready for processing.

Cutting and grinding cycles are very short, taking between 10 and 20 seconds according to the size of the component, and batch sizes vary from low call-off specials to around 5,000.

The MTC-supplied package also included a pair of SENC 150 five-micron-resolution precision glass linear encoder scales for the machine’s X and Z axes.

Each scale incorporates roller bearings to reduce any backlash effect when positioning and to extend scale life.

An integrated scanning sensor within the scale was also important to resist contamination and work in conjunction with the potentiometer within the machine’s hand wheels.

Control of the vacuum chuck is via dual foot pedals, one to operate the chuck and the other to control the extraction unit that removes any dust from the guarded machining zone.

The carbide knife blade is held in a Multiflex tool post that enables the manual adjustment of the angle of approach to the seal, while the Fimec 12,000rev/min electric grinding spindle that is in the rear tool post also has rotary position adjustment.

A special carbide open grit wheel was initially used but was subsequently changed to a diamond impregnated wheel to provide more consistent operation.

Stinchcombe services a range of industries, including aerospace, heavy engineering, steelworks and carpet manufacturers, as well as model makers requiring a mill or centre lathe retrofitted with a DRO.

He said: ‘The Acu-Rite 200S DRO has proven very popular in the retrofit market for turning, milling and grinding operations, providing a very clear and functional display of two or three axes of movement on its 5.7in [14.5cm] LCD display.

‘The large 3D tactile keypad makes data input easy for the operator and the ability to hold 16 tool offsets/diameters, preset and zero reset as well as near-zero warning,’ added Stinchcombe.

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