Automatic Tool Changes

Any time spent by a metal cutting machine tool when no chips are generated is a waste. Therefore considerable effort is made by the machine tool designers to reduce or eliminate most of such idle times. There are a significant number of operations involving tools, which can be classified as idle times. They are tool engagement & disengagement, tool alter and tool setup. The automatic tool changer or ATC is one such component of CNC machine tools that reduces the tool change operation. These are particularly useful in machining applications where a number of tools are to be used for finishing the job.

For an ATC to operate, there should be some way of keeping a number of tools in close proximity to the tool change arm. Tool magazines are utilized for that purpose. Storage capacity of tool magazines vary between 12 and 60 in the most common machine tools. Higher capacities upto 200 may be found in some special machine tools. The common types of tool magazines used are turret for small capacity (about 12), drum or disc for medium capacity (12 to 30) and chain for big capacities.

A chain type tool magazine provides the necessary flexibility for storing large number of tools. A typical chain type magazine is shown in Figure 13. The tools are attached to the pockets, which are in turn attached to the chain that is moving on appropriate sprockets. The chain allows for a very large variety of arrangements.

Figure 13: A Typical Chain Type Tool Magazine

In the case of turret the tool changing is relatively simple, because of the turret indexing. However, in the case of other tool magazines, tool changing arm along single or dual grippers will do the necessary operation of removing the new tool from the magazine, and replacing that with the old tool from the spindle in a series of planned motions. The tool magazine is placed close to the spindle such that the actual tool transfers do not consume a lot of time. Typical tool change times vary from 2 to 10 seconds.