Every machinist has done it. The job is almost complete, the shift is nearly over, the surface finish is still in tolerance—whatever the rationale, they’ll decide a worn cutting tool can squeeze out “one more part,” only to have it fail.

The result might be nothing more than a broken tool and a scrap workpiece, but it could also mean thousands in lost material or damage to a CNC machine tool, costs that far outweigh that of a fresh drill bit.

Chatter, Deflection and Surface Finish: Early Warning Signs You’re About to Scrap

“Successful drilling is about process reliability, not just tool life,” says Paul Larson, product manager for drills, thread mills and grooving systems with Guhring Inc. “Shops should also recognize that the lowest tooling cost per hole doesn’t always produce the lowest cost per part.”

The reason is simple: As drills wear and cutting forces increase, hole quality deteriorates. Diameter, roundness, straightness, surface finish and burr formation may all be affected long before the drill actually fails.

“Replacing it based on a validated tool life standard will help maintain consistent hole quality and process reliability while eliminating the costly surprises associated with running a tool beyond its predictable performance window,” he says. “The goal isn’t to maximize the life of every drill; it’s to maximize productive spindle time and consistently produce good parts.”

Why a Scrap Part Is Costlier Than the Tool Change You Skipped

That logic applies to all cutting tools, but especially when the wear isn’t always obvious. Such is the case with taps—the product line Guhring’s Daniele Mintrone manages. As taps wear, the torque required to produce the thread gradually increases, he explains, and if nobody is paying attention, it’s easy to fall into the mindset of “it made the last part, so it’ll probably make the next one too.”

“That’s where the real cost of running one more comes in,” he says. “In addition to the broken tool, you now have a bad part, machine downtime, lost production and the time spent recovering from an avoidable failure.”

Mintrone points out that torque-limiting tap chucks help eliminate this angst, as do CNC machines that monitor and, in some cases, limit torque. Otherwise, he echoes what Larson and others here advise—plan tool changes, and don’t postpone them.

“These approaches help remove the guesswork and prevent tools from being pushed beyond their useful life. In most cases, replacing a tool a little early is far less expensive than dealing with the consequences of replacing it too late,” Mintrone says.

What Happens to Your Part When You Run a Worn Tool

Jake Rutherford, a research and development engineer with KYOCERA SGS Precision Tools Group, says worn or broken tools do more than make bad parts. They make a bad situation even worse. As a tool wears, the cutting edge begins to round, making it more difficult to shear the material properly. This increases force and heat at the cutting zone and impedes heat from flowing into the preferred location, the chip.

“Since the heat has to go somewhere, it transfers to the tool, causing the carbide to break down and wear faster. It can also have a detrimental effect on the workpiece through work-hardening, creation of an alpha layer in certain high-temperature alloys, and so on.”

Clay Platt agrees. The regional product manager with Kennametal says worn tools can negatively impact multiple elements of a machining operation. “The primary ones are poor surface quality and increased cutting forces leading to spindle damage, both of which have a direct influence on operating expense,” he says.

Worse, once these effects begin, additional negatives pile up. As Rutherford stated, metallurgical integrity of both the component and the tool may suffer. But so will chip control, creating an unnecessary safety hazard for lathe operators, while excessive cutting forces can lead to fixture and spindle damage.

How to Set Tool Change Intervals Without Relying on Instinct

Platt says tool change intervals can only be established by first defining acceptable tool life. “The pathway to achieving it is influenced by many variables, including the type of machining operation, the part’s quality requirements and, ultimately, the integrity of the cutting tool within a given run time.”

Rutherford concurs, recommending that shops prioritize quality when establishing “Acceptable Tool Life” guidelines. This starts by eliminating the disconnect between departments and their often disparate goals. For example, the procurement team is usually laser-focused on price and delivery, while manufacturing engineers care more about process integrity, with quality and repeatability being top of mind. “Both are very important roles that have direct influence on operational profitability,” Rutherford says.

What Is the Real Cost of Running One More Part?

Kyle Bisson, Sandvik Coromant’s turning product specialist, thinks the answer comes down to the amount of risk the shop is willing to accept. Sometimes a worn tool makes another good part (or even several dozen good parts) and the potential costs are avoided. That said, this “successful” result only reinforces the decision to continue running the tool.

“The problem is that once the tool reaches the end of its predictable life, running additional parts becomes a gamble,” Bisson adds. “The tool might still be cutting, but this doesn’t mean the process is stable, or that it can consistently produce the required part quality.”

This is why planned tool changes are so beneficial, he emphasizes. They’re short, controlled interruptions rather than unplanned changes that occur after the process is in trouble, which then require stopping the machine for troubleshooting, sorting the good parts from the bad, and other recovery work that far exceeds a simple tool replacement.

In line with Rutherford’s earlier comments, Bisson indicates that many manufacturers place too much emphasis on cutting tool spend and not enough on process security. “The cutting tool,” Bisson says, “is usually one of the smallest costs in that equation. But because of the tracking disconnect just described, the shop may fail to comprehend the total financial impact of a seemingly innocuous and all-too-common event: the machinist’s decision to run one more part.”