Dull Tool Replacement
The importance of improving the task of dull tool replacement is related to the frequency at which cutting tools become dull.
With large lots and aggressive machining conditions, it is likely that cutting tools will often become dull during a production run. This is true even with smaller lots. Also, cutting tools will eventually get dull when they are used from job to job. Given this criteria, most companies replace dull tools on a regular basis and should be interested in improving the related tasks.
CNC machine tools use a variety of cutting tools, and the specific tasks required to replace dull tools varies based on tool type. Even so, I think of cutting tools in this respect as falling into two categories: inserted tools and integral shank tools.
Inserted tools utilize carbide (or similar) inserts, and only the cutting edges (the inserts) are made from carbide. The rest of the cutting tool is usually made from steel. Only the inserts must be replaced or indexed during dull tool replacement. Changing or indexing inserts will not have much affect on the overall configuration of the cutting tool.
Integral shank tools use the same cutting tool material for cutting edges and shanks. Replacing the cutting part of a dull tool requires replacing the entire tool, which can affect its overall length. If the cutting tool’s length changes, some kind of measurement and offset entry will also be required during dull tool replacement. In a similar fashion, the diameter of an integral shank end mill will change if it is replaced with a sharpened end mill (not a new one). Again, this will require measurement and offset entry.
Though we’ve divided all cutting tools into just two types, there are also many cutting tool components used to adapt them to the CNC machine tool. The amount of time and effort required for dull tool replacement is closely related to how much disassembly is required to remove the cutting part of the tool.
My first suggestion is to question how much dull tool replacement can be done offline. That is, can your workers replace dull tools while the machine is in operation? Some machines have unique accessories (such as
tool life management systems) to help in this regard, and they allow cutting tools to be safely removed and replaced during the CNC cycle.
Tasks your workers do offline will not affect production run time. So the more they can do while the machine is running, the less impact it will have on throughput. However, with the current trend of lean manufacturing, it’s likely that your CNC operators are replacing dull tools—and they may not have time to do much while the machine is running.
Regardless of how much is done offline, it is still desirable to improve the related tasks of dull tool replacement—especially if operators are performing these tasks. If they are doing so offline, improving the related tasks will provide more time to do other things. More urgently, if they are replacing dull tools while the machine is waiting, improving related tasks will, in turn, shorten production run time.
If the cutting tool must be removed from the machine prior to dull tool replacement (as is necessary with most machining center tools), look for ways to streamline the task. Are the dull tools removed through the machine’s spindle, or are they removed using the machine’s magazine? In most situations, and if multiple tools must be removed, it is faster and easier to use the machine’s magazine.
When the cutting part of the tool is removed and replaced, ensure that the required hand tools are readily available and easily identifiable. Many of the related hand tools (especially Allen wrenches) look similar. Use some kind of color-coding system to make it simple to identify which hand tool is used for each cutting tool component.
The tasks of measurement and offset entry can be eliminated if the cutting part of the tool is qualified. Carbide inserts, for example, can be replaced without changing the length or diameter of the tool (much). For integral shank tools, such as twist drills, it may be possible to make some kind of end stop that is placed inside the collet to which the shank end of the drill can be bumped during replacement. This will make it possible (with new drills) to keep the cutting edge of the new tool in exactly the same location as the replaced tool, which eliminates the need for a measurement. If you doubt this technique, think of how a tap holder works. When you replace a tap, the tip of the new tap is in the exact same place as the replaced tap.
Even inserted tools will change slightly during dull tool replacement. A few thousandths of an inch may be all it takes to cause a critical surface to stray from its tolerance band. The tighter the tolerances machined by the cutting tool, the more necessary trial machining will be. When you know trial machining is necessary, make it as simple as possible for operators to do so. Past Tech Talk columns have addressed methods of programming the trial machining operation using the block delete function.