CNC Machining by Murray Productions

020 8573 4354

www.murray-productions.co.uk

Murray Productions Limited

Silverdale Road

Hayes

Middlesex

UB3 3BN

Telephone: 020 8573 7836/4354

Customer Service and Sales ext 101

Facsimile: 020 8573 0152

Email sales@murray-productions.co.uk

Machining describes the action of removing material from the initial billet (a block of material from which the component is to be made), or a plate, bar or other form, by moving the material against a static cutting edge, by moving cutting edges against the static material. or both. Turning, using a lathe, involves rotating the billet and bringing a static cutter against the moving surface. Since the billet is rotating, the resulting cut will be at a constant radius about the axis of the billet, so turning is used to produce aerospace turned parts such as shafts, tapers and cylinders. Using a suitably shaped cutter and passing it down the length of the rotating billet can create a screw thread by a process called thread rolling.

Aerospace milling

If the billet is clamped firmly and the cutter is rotated, the process is known as milling, and can produce both flat and curved surfaces. Using shaped cutters allows more complex cuts to be produced, and in some cases a cutter will be specifically produced for a certain task.

A shaping machine uses a cutter that moves in a straight line across a fixed billet, a little like a wood plane. A variation on this technique is broaching, which has a long tool (the broach) carrying a large number of shaped cutting edges, each of which takes off a little more material than the previous edge. The final cut will conform to the profile of the final cutting edge of the broach. Broaches may be several metres long. The process is used for cutting the complete root attachment slots in turbine discs, to receive the individual turbine blades.

Other machining methods include drilling, grinding and spinning.

Aerospace precision engineers find themselves machining in specialist alloys and all manner of advanced materials. For helicopter components for example, Murray Productions offer machining complete manufacture.

Helicopter engines and transmission systems

Many helicopters today have a number of engines to supply motive power to the rotors and transmission systems. In fact, all but the smallest helicopters usually have two engines, and some larger ones have three. The need for multiple helicopter engines is obvious; helicopter lift is wholly dependent upon rotor speed, which in turn depends upon the power provided by the engines. In the event of engine failure it is still necessary to have power available to drive the rotor, therefore multiple engines are needed so that the remaining engine(s) can satisfy this requirement. Although it is possible to land a single-engined helicopter following engine failure, using a technique called auto-rotation, this mode of unpowered flight takes time to establish. If the helicopter is flying at around 500 ft or less then it is unlikely that safe auto-rotation recovery can be carried out. Engines are usually sized so that the aircraft can fly for a period of time with one engine failed, except in the most extreme flight conditions: when the helicopter is flying heavily loaded or `hot and high'. EH 101 Merlins are fitted with a variant of the General Electric T700-GE-401 turbo-shaft engines in the naval version while civil and military versions are powered by the General Electric CT7-6, a variant of the T700 developed specifically for the EH 101 Merlin.

A more recently developed engine available for this class of helicopter is the Rolls/Turbomeca RTM 322 which is designed to operate at 2,100 shp (shaft horse power) and weighs around 530 lb. This engine is of a suitable size to power up-rated versions of the EH 101 Merlin. It is being produced with a 50/50 work share by RollsRoyce and Turbomeca.

The majority of new helicopters use gas turbines rather than internal combustion engines, for a variety of reasons. Most engines are electronically controlled using computers and over recent years control has become digital in nature, using Digital Engine Control Units (DECUs). These units are usually configured with two lanes or channels of control, though, for a single-engined helicopter, a dual channel and a hydromechanical stand-by channel may be provided. Typical control laws which would be embodied are:

• Acceleration control.

• NH control.

• Surge prevention.

• Fuel flow max/min.

• Torque limiter.

• Torque/free turbine droop governor.

A system comprising more than one engine/DECU may also incorporate features whereby one will be accelerated to maximum power if one of the other engines fails or the thrust drops below a predetermined level. Such a system is likely to apply power more quickly than the pilot when operating in a critical flight mode such as the hover.

Helicopter gearbox components.

The transmission systems and hence, the helicopter gearbox components, needed for three-engined helicopters are complex. Each of the three engines drives though a series of reduction gears to third-stage collector gears. The collector gears drive the rotors at 210 rpm through sun and planet gears. The tail rotor shafts are driven off the collector gears at 3,312 rpm. The accessory gearboxes are also driven off the collector gears, however when the rotor is stationary it is possible to drive the accessory gearbox by the APU or from No. 1 engine by pilot selection. The accessory gearboxes drive two of the three hydraulic pumps and the two AC generators. The third hydraulic pump is driven directly off the main gearbox. The main gearbox lubrication system comprises two independent lubrication circuits, each with its own oil pump filter and cooler... a complex assembly of engine components and gearbox components.

Accessory gearboxes are located on the front of the main gearboxes and the main rotor drives rise vertically from the main helicopter gearboxes. Due to the obvious importance of helicopter transmission systems, a considerable degree of monitoring is in-built to detect failures at an incipient stage. Typical parameters which are monitored are oil pressures and temperatures, bearing temperatures, wear, and in some cases accelerations. The role of the health and usage monitoring system hums on board helicopters is assuming paramount importance.

Spark erosion and edm machining.

Spark erosion is unusual for its lack of physical abrasion, it is a process of erosion. The workpiece and tool are placed in the working position in such a way that they do not touch each other. The voltage applied between the electrode and workpiece and the discharge current have a time sequence. Starting from the left, the voltage builds up an electric field throughout the space between the electrodes. They are separated by a gap which is filled with an insulating fluid. The cutting process therefore takes place in a tank. The workpiece and tool are connected to a D.C. source via a cable. There is a switch in one lead. When this is closed, an electrical potential is applied between the workpiece and tool. At first no current flows because the dielectric between the workpiece and tool is an insulator. However, if the gap is reduced then a spark jumps across it when it reaches a certain very small size. In this process, which is also known as a discharge, current is converted into heat. The surface of the material is very strongly heated in the area of the discharge channel. If the flow of current is interrupted the discharge channel collapses very quickly. Consequently the molten metal on the surface of the material evaporates explosively and takes liquid material with it down to a certain depth. A small crater is formed. If one discharge is followed by another, new craters are formed next to the previous ones and the workpiece surface is constantly eroded.

Wire Erosion removes material using electrical discharge erosion action, but with a wire electrode traveling longitudinally through the workpiece. The relative moving passage between the wire electrode and workpiece is controlled by a CNC system according to a preprogrammed CNC program to cut the workpiece into desired shapes. Wire Erosion is widely used to cut punches and dies, shaped pockets and many other applications

Aerospace Presswork

Presswork too is offered on a start to finish basis, with press tools design and press tools manufacture or refurbishment.

www.murray-productions.co.uk