A method of work holding and location that has gained full acceptance for computer numerically controlled milling and drilling setups is the grid plate.
The grid plate is attached to the machine table, often permanently, and since the part programmer can identify the exact position of any hole and will know the dimensions of any locating dowels or blocks used in the work-holding arrangement, he or she can establish datums when writing the program and instruct the machine setter accordingly.
The setting of a grid plate does not involve the use of dial indicators, edge finders, etc., and therefore is not demanding on manual skills on the shop floor. Once set, it provides for quick, simple, and accurate location of the workpiece.
It is often possible to load more than one component at each setting and known pitches. By using the “zero shift” facility (see Chapter 6), the machining program can be repeated in a new position with a resulting saving in machine downtime.
Apart from clamping directly to the grid plate, components can be held in fixtures, vises, or a set of fasteners and accessories, which themselves are accurately located and clamped in position. Complex shapes can be accommodated by using specially shaped locators, while fixtures can be provided with finding devices so that they may be accurately located and clamped in a known position.
Production engineers aim to minimize downtime, that is, the time when the machine is not fulfilling its principal function of cutting metal. A significant source of downtime is work loading and unloading. The use of preloaded pallets considerably improves the situation.
A pallet is merely a table, which, like the grid plate, is provided with a series of holes or slots to facilitate location and clamping of the component.
The most straightforward arrangement will involve the use of just two pallets. A workpiece is located and clamped on the first pallet in a position predetermined by the part programmer, and the pallet is then moved into the machining position. As machining is taking place, the second pallet is loaded. When machining of the first component is complete, the stretchers are interchanged and, as the second component is being machined, the first pallet is unloaded and reloaded with another element.