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Enhancing productivity in multi-part production

Enhancing productivity in multi-part production

Workholding cubes are essential for secure and accurate multi-part machining processes, but there are some critical considerations for their successful use. Phil Holyome of WDS looks at how they can be used to the best advantage.

Secure workholding is essential if machining is to be accurate. But that is far from the only concern, it also has to be accurately positioned, easy to set up and designed to not impede tool paths. Increasingly manufacturing industry is adopting multi-part production procedures, in which machining centres are used to produce several products at once.

The potential time and productivity gains of such practices are clear, with further gains if the machining is run unmanned. However, there are some special requirements that also need to be considered, such as the need to make sure all the component parts are positioned accurately and securely clamped in the machining area.

Typically, the parts are mounted on the sides of a workholding cube (perhaps more than one per side), which itself is mounted on the bed of the CNC machining centre. The CNC will have been programmed to run through an extensive sequence of machining operations to produce the finished products. It’s quite possible that the machine will be set late in the day shift and the machining will run unmanned through the night.

In effect each face of a workholding cube is the same as a machine bed or work holding plate, so the same principles of parts mounting apply. There are four main types of cube: standard stock cubes, dowel location cubes, edge location cubes and tenon location cubes. WDS cubes are made of cast iron, so are rigid and therefore stable, even in raised temperatures. Cubes made from fabricated steel or aluminium are less stable. The majority of cubes in use are not standard products but are tailored to match the exact needs of the particular application. Most users know what they require from a workholding cube, but need some support in realising an appropriate design. WDS is able to provide the design expertise to help develop such cubes, whether the need is simply for initial advice or for a full design and build service.

The first requirement is that the workpiece is located and clamped securely and does not move when being machined. However, the fixture must not mark or damage the workpiece in any way, and accuracy of positioning is important, with tolerances often measured in micrometres. Related to this is a need for simple loading/unloading - this should not require special skills and should not take too much time, particularly when there are many workpieces to mount.

The next requirement may take a bit of thought, in that there must be clear unhindered access for the tools. It may be necessary to consider up to 12 axes of movement: back and forth, left and right, up and down, clockwise and counter-clockwise in the X, Y, and Z planes. This will have to be considered for every part in a multi-part operation.

This can seem daunting, but while it is detailed work it is in fact based on some simple and consistent principles, so for experienced cube designers it is really only a matter of working through the process.

Workholding jigs and fixture are usually made up from standard components, such as clamps, support and location elements. These are mounted onto the cube so that they grip the workpiece but allow tool access.

In high volume work, the clamps may be powered and automatically deployed. In other cases, they will be manually set and care will have to be taken to ensure the correct gripping force, which may be considerable in metal cutting operations but less for assembly, spray painting etc. In all cases, it is good practice to spread the holding force over a large surface area rather than concentrating it onto one small part of the workpiece. Safety and failure events also need to be considered. Most modern machining centre have covered machining areas, so there is a high degree of protection of personnel, but thought must be given to guarding, failsafes and secondary effects.

A well-designed holding cube with effective fixtures will bring advantages in terms of cycle time, product quality, tool wear and operating life and ultimately pay for itself well within its expected design life.

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