Getting to grips with the problem

A major customer asked Production Equipment, builders of industrial hydraulic units, ‘How do we hold H section aluminium extrusions ranging in length from 2m up to 20m, where more than 80% of the original material is machined away, in three stages.’

After understanding what the three machining processes involved, Production Equipment’s answer was to put forward workholding proposals for each of the three manufacturing stages.

The first stage involved machining a flat face to form a datum face from which all subsequent operations are located and machined. It is estimated that to clamp two of the longest components using manual mechanical methods would take about 30min.

As the component is almost designed for clamping, the eventual first stage fixture incorporates 120 Romheld hydraulic swing clamps to hold it down. Clamping and unclamping is virtually instantaneous by means of hand operated control valves spaced at intervals down the length of the machine.

The flat surface created in Stage 1 is now used to locate and hold the material for the next two operations. The second stage involves machining intricate patterns along the length and profile of a pair of sections, so the material had to be held down by vacuum and clamped as well.

First, a vacuum labyrinth groove was machined in to the second fixture, so that a vacuum, can be used to initially hold the material flat. The problem was clamping on the flange wedge on either side of the section profile, as grip was limited to about 1.5mm to 2mm.

After the development of various ideas, a CAD drawing was produced demonstrating the swing clamps operating vertically, and horizontally as recommended by Production Equipment.

The advantage offered by horizontal operation is that, as there is quite a long stroke on the swing clamp arm, it is much easier to accommodate any discrepancies in the section. The swing clamps then secure the component against a datum edge.

It then became necessary to produce manifold blocks which accepted the 73 swing clamps with 6.0mm oil ways. As deep drilling 6.0mm for 20m is impossible, the manifolds were produced in modular form 14.48m in length. Although the fixtures are ‘tubeless’ their production still involved complex cross drilling to accommodate the valve gear to turn the clamps on and off. The design exercise became quite complicated.

There was the initial swing clamp mechanism to pull the section down, while the vacuum was applied. Then the swing clamps at 200mm centres were applied to secure the component along the edges before the complicated machining began. The workholding device is entirely leak free with no vulnerable leak points and presents a clean fixture without the swarf traps normally associated with pipework.

The third and final stage involves the contouring of the section, which entailed turning the component over and machining the original flat face. The final stage was perhaps the most difficult workholding solution. With around 80% of the original material removed, the delicate structure needed an innovative workholding solution for the final machining/contouring operation. This was solved using standard self-centering vices specially adapted by Production Equipment.

The Hilma Romheld long precision bed vice used has a centre fixed pedestal, with two hydraulically operated jaws either side, enabling two components to be gripped at once. On these it was at first proposed to fit special jaws and a special pedestal. Eventually, with a change of the datum face, it was decided to use hydraulic pistons to lift the section against datum nibs set on the vice jaws. This design developed into a double system.

Originally, the spacing between vices was set at 600mm, but this distance was reduced to 400mm, as by now the component was rather thin and flimsy. The operations carried out by the hydraulically operated vices are first to lightly grip the centre web of the section, then to raise the component against the vertical datum pads, then finally to apply a high force grip for workholding during the machining operation.

The result was a successful series of work holding applications supplied and installed ahead of schedule with the customer achieving highly satisfactory production rates and quality.