Implementing A Scrap Reduction Program

Companies vary as to how they handle scrap. Some plan on it as part of the normal manufacturing process.

Columns From: 5/19/2002 Modern Machine Shop,

Companies vary as to how they handle scrap. Some plan on it as part of the normal manufacturing process. Others don't throw anything away. Regardless of how a company handles scrap, everyone can agree that scrap is a bad thing. But companies vary as to what they'll actually do to reduce scrap. Companies should be willing to do more to minimize scrap if raw material is costly; there are numerous prior machining operations; cycle time is long; the machining process is complicated; and/or many repeated jobs are run.

Assuming that raw material is acceptable and that you have a stable machining process (two scrap causes that may be beyond the control of people in the CNC environment), most of what causes scrap workpieces can be traced to human error. Excessive scrap-causing mistakes should be a signal that your CNC people need more training. Well-trained people will, of course, scrap fewer workpieces than poorly trained people.

Scrap caused during setup may be difficult to avoid. With complicated workpieces, and especially if there are surface relationship tolerances (parallelism, flatness, concentricity) that cannot be measured while the workpiece is clamped in the workholding device, an incorrectly machined workpiece may be impossible to rework once it is removed from the machine.

But don't be too quick to give up on eliminating scrap during setup. Using some ingenuity, you may be able to help setup people make the first workpiece a good one.

One common technique is trial machining. When the setup people recognize a tight tolerance, they adjust an offset in such a way that the cutting tool will leave additional stock on the critical surface. After machining, they'll measure the surface and adjust the offset accordingly and rerun the tool. The second time the tool cuts, it will be within its tolerance band. This is a very common technique, but we have a few questions.

First, why would you force the setup people to recognize critical tolerances? If they miss one, the first workpiece will likely be scrapped. Why not specify all surfaces that require trial machining right on the setup sheet? Better yet, why not program the trial machining operations? If setup people can recognize the need for trial machining, surely a programmer can, too. The commands required to trial machine can be included right in the CNC program under the influence of the block delete code (/).

Do you have a spindle probe? Machining center operators who have spindle probes on their machines can actually automate the entire trial machining process by programming all commands related to trial machining. First, the offset will be adjusted in the program. Then the tool will machine the workpiece. Next, the probe will measure the surface just machined, and the program will adjust the offset accordingly. And finally, the tool will be rerun.

Scrap caused during a production run is commonly the result of operator error. Incorrect calculation and entry of offset change during sizing and tool replacement are the most troublesome scrap-causing problems. Again, repeated mistakes should be a signal that your operators need more training. Or maybe you should consider lowering the skill level required to make offset changes. Are operators given target values for all dimensions that must be sized? Do you specify dimensions with plus/minus specifications? Are all dimensions for similar workpieces specified in the same manner? Can you incorporate a parametric program that allows operators to enter a measured value and let the control calculate offset change amount and make the change? (E-mail me to get the Tech Talk column that shows how).

Scrap can also be caused by programming mistakes. Your setup people should be able to find gross motion mistakes during the program's first dry run. However, very small motion mistakes don't show up during a toolpath display or dry run. Unfortunately, there may not be much you can do to eliminate scrap caused by these tiny mistakes. In similar fashion, no current toolpath display (that I'm aware of) can spot mistakes with cutting conditions. If you have critical work surface finishes to be machined, cutting conditions must be appropriate. This may be next to impossible to confirm for a new program until a workpiece is machined.

Once a program is verified, and especially after it has been used for several production runs, it should never be the cause of future scrap. Many companies consider these programs as proven or qualified programs, never changing them again once they're deemed qualified.

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