CNC machine tools frequently operate at greater feedrates and speeds than those used in conventional machining, so larger forces may be applied to the component and therefore to the clamping/holding devices. As the programmer, it is part of your task to decide what type of work-holding device is required for a particular operation. The following conditions must be met: 1When clamped, the component must not be able to move in any direction, either linearly or rotationally. It is essential that the tool cutting forces are exerted against a part or parts of the work-holding device that cannot be moved (Figure 5.1). This can be difficult to achieve when machining complex components of an irregular shape, such as castings, and you may need to use more than one holding device for all machining operations to be executed.2The component must not ‘chatter’ or deflect as it is being machined; this would produce a poor surface finish, dimensional inaccuracy and possibly tool damage. This may be overcome by using steadies for turning or fixture inserts when machining thin-walled components.3The component must be positively located and precisely positioned within the work-holding device. The part program datum point (that is, the point from which the co-ordinates of toolpath are programmed), is generally the same as the component datum point; so if the component is not positioned accurately the required component shape may not be achieved. A grid plate with a holding device attached can provide positive location and position in several directions.4In order to avoid collisions, the toolpaths must not coincide with any part of the work-holding device or clamps. When proving out the program, you may find you need to insert moves in the program to ensure this condition is met.5The work-holding device must provide adequate location and support to achieve the geometric tolerances specified on the component drawing.6All information relating to the work-holding device(s) used and the setting of datums must be documented for the machine setter/operator.
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