Effective methods of evaluating tools in use can pay big dividends
Machine shops use a variety of techniques to track the condition of their cutting tools, ranging from simple to sophisticated. No matter what monitoring method is used, it can be crucial in preventing catastrophic tool failure. At its best, monitoring also significantly boosts tool life and slashes tooling costs.
Among the simpler methods of monitoring tools is entering information into a tool-life table provided by the machine control. In this table, for example, operators can note that a particular tool usually lasts for a certain number of parts or for so much time.
“You can type in that normally you get 20 pieces out of a tool, or that it can be used for 30 minutes,” said Jeff Rizzie, director of digital machining for the Americas at Sandvik Coromant, Fair Lawn, N.J. “That is the dominant [form] of tool monitoring. You’re not checking anything on the tool, just using some historic data.”
Other monitoring methods ascertain the condition of the tool. The most common way is simply by looking at it—taking it out of the spindle or opening the machine doors and evaluating it visually, said Michael Dieken, a product manager at Ingersoll Cutting Tools USA, Rockford, Illinois. A visual inspection could be triggered by the appearance of parts—if operators notice burrs or lower finish quality, for example—or how the tool is reacting to the workpiece, according to Dieken.
“We usually encourage customers to either eyeball [the tool], which most people do, or use some type of visual device like a microscope to examine the edge,” he said, adding that wear in the range of 0.010 to 0.015" (0.254 to 0.381 mm) is usually an indication that it’s time to index the insert or change the tool out.
In addition to microscopes, USB plug-in imaging devices can be helpful by magnifying cutting-edge images 100× or more so users can see wear, noted Thomas Raun, chief technical officer at Iscar USA, Arlington, Texas,
Also useful are laser measurement devices that operators can point at a cutting edge to measure wear. Laser measurement is a much more accurate way to determine whether a wear benchmark has been reached than simply eyeballing. It’s also a growing trend, according to Ingersoll’s Dieken.
“I think you will see a lot of laser measuring equipment at larger manufacturing companies whose business mentality is to hire lower-skilled workers and count on equipment to be smart,” he said.
Instead of guessing when they need to be concerned about tool wear, some shops keep an eye on the horsepower consumption of their machines. Dieken pointed out that a rise in horsepower during a cutting process is a reliable signal that the tool is worn and should be checked.
Best Monitoring Practices
To better understand the relationship between horsepower consumption and the wear of a particular tool, shops should perform some experiments before using the tool in production, according to Steve George, senior manager of product engineering at Kennametal Inc., Latrobe, Pa. In these experiments, George recommends tracking the load on the tool as well as the wear during a trial cutting process. As horsepower increases, shops should periodically examine the tool and note the amount of wear on a chart, which will be a valuable aid in estimating tool life when production begins.
Iscar’s Raun also advises shops to do a little preproduction work to help them gauge the life of a tool. He recommends using the tool to make a benchmark test cut using the maximum values for the cutting parameters (feed rate, depth and width of cut, etc.) the tool will encounter in production and noting the spindle horsepower.
“Let’s say the test cut uses 30 percent of the spindle power,” he said. “Then set a wear value of 10 percent. As soon as you hit 33 percent on the spindle load, you can say that you’ve reached the end of tool life.”
The 10 percent wear value “is a pretty safe benchmark to use across the board,” he noted, though he added that he has seen shops bump it up to 15 or 20 percent, depending on what they were doing with a particular tool.
Another preproduction step recommended by George is to think about the cutting process that will be employed to produce the desired part quality and dimensions and what your primary tooling concern should be during that process.
“Consider an end mill with a corner radius,” he said. “Is it more likely that you’ll have to change the tool based on wear in a certain area than breakage? That is normally the case in that situation. So plan your monitoring around something like how the radius of that tool changes.”
For heavy roughing, on the other hand, the primary concern will probably be tool breakage or insert failure. In that case, George said, shops should be focused mainly on horsepower changes in order to avoid the costly consequences of breakage or failure, which can include scrapping a part as well as machine and toolholder damage.
Another factor that should be considered in the planning stages of a job is how workpiece material affects the tool monitoring strategy. For example, George noted that cast iron is generally very abrasive, so shops cutting that material are usually more concerned about tool wear than breakage. Tool wear should also be a concern when cutting titanium or a high-temperature alloy, he added, but issues like built-up edge can cause spikes in forces that might cause tool breakage before the wear limit is reached.
Training Pays Off
In shops where tool monitoring procedures are rudimentary and/or almost nonexistent, the result can be “extreme underutilization” of the tools they buy, Raun said. “I can’t tell you how many times people are [replacing] an indexable insert that has cutting edges on it that haven’t been used.”
When Sandvik Coromant’s Rizzie and his colleagues analyze customers’ used inserts, they sometimes find that they still have as much as 50-60 percent unused life. This is before they teach these customers about tool wear, using charts showing different tool wear mechanisms.
“We show them what [things like] flank wear, chemical deposition and plastic deformation look like,” Rizzie said. “Then we have the operators or engineers monitor it. Every so many parts they run, they look at the insert and make a determination.” Trainees usually start monitoring with the naked eye, then switch to a visual aid like a loop magnifying glass.
The result is normally a significant increase in tool life and usage at the customer’s facility, Rizzie reported. In one case, he said, a customer’s monthly insert purchases dropped from 6,000 to 2,000 within six months of the training.
Automating the Process
To get more accurate tool monitoring than normally possible with humans playing the lead role, some shops turn to automated systems. Rizzie estimates that a good automated monitoring system can increase tool life by 25-30 percent compared to a more operator-centered process.
Automated systems from Marposs Corp., Auburn Hills, Mich., generally work with digital and/or analog sensors in a machine tool to obtain information about the condition and performance of tools. The systems evaluate sensor data using special software strategies, noted Jorge Pena, product manager for the company’s Artis division. Artis tool- and process-monitoring systems are designed to capture tool condition information throughout the machining process.
Manufacturers select an appropriate Marposs monitoring system based on tool type, machine design, workpiece characteristics and the cutting process. Systems offered by the company can monitor the power of the motors driving the cutting tools, as well as tool vibration and strain. Evaluating a mix of different sensor signals produces optimal results, Pena said.
Like Marposs systems, CoroPlus Process Control from Sandvik Coromant pulls data from the machine in order to monitor tool condition. The system also adds a sensor of its own to the machine, normally an accelerometer, which can provide vibration information when attached to the spindle of a milling machine, for example.
Connected directly to the machine PLC, the system uses edge analytics to look for patterns and anomalies. “When a tool breaks, you see some kind of spike in the sensor data,” Rizzie said. “It also monitors vibration and forces on the tool [to determine] when it is worn out and needs to be changed.”
When a set limit is reached, the system can automatically signal that a new tool is needed. It can also shut the machine down and trigger an alarm so the operator knows something needs to be done.
Even if a shop’s personnel have been trained to monitor tool wear, Rizzie maintains that CoroPlus Process Control is worth the investment. Besides providing more tool-cost savings than what’s possible even with well-trained operators, he said the system can detect and react almost instantaneously to tool breakage, shutting a machine down within five milliseconds of the event to reduce or prevent damage to the part, toolholder and machine tool.
Monitoring Plus Adaptive Control
Another option for shops interested in automated tool monitoring is TMAC 3.0 from Caron Engineering Inc., Wells, Maine. TMAC (short for Tool Monitoring Adaptive Control) monitors “everything about the tool” over time and saves the data, noted Rob Caron, the company’s president.
Parameters checked by TMAC include power, vibration and strain. “Especially on Swiss machines, the tool can be so tiny that power doesn’t really reflect” tool condition, Caron explained. Drawing the necessary conclusions from vibration data is also challenging for tools that small. As a result, TMAC uses a sensor in the toolholder to monitor strain, which is deformation resulting from the force on the cutting tool. This can be measured “down to a tiny amount,” he said.
Operators can view real-time data on a screen. Among other things, TMAC provides a tool-degradation percentage, a digital representation of tool condition as the tool degrades throughout its life. When a user decides that a tool isn’t cutting the way it should anymore, the values of all relevant parameters at the time are recorded so the user has those exact values for future reference. When tool condition reaches a point where action is required, TMAC can notify an operator that a tool change is needed or trigger an automatic change to a redundant tool if there is a spare in the tool magazine.
As its name suggests, TMAC also includes an adaptive control feature, which is meant to reduce cycle time when the material being machined is not consistent or is difficult to cut. In these cases, users can set a desired horsepower level for the cutting process and the system will automatically override the feed rate of the tool through the material to maintain that power level.
According to Caron, adaptive control is particularly useful in casting and forging applications, “where material is changing all the time and it is pretty much impossible to programmatically modify feed rates because you don’t know where the thick and the thin spots are. With adaptive control you just program the horsepower for that particular tool and as it hits thinner and thicker material, the system will automatically override the feed rate.”
A Good Investment?
How can shops decide whether automated tool monitoring systems like TMAC are a good investment? Kennametal’s George believes the answer can be found in the overall level of automation in a shop and amount of what he calls “operator content” the shop is willing to have in its processes. “If you are investing in other automation, I would be [leaning] toward the automated system,” he said.
Also, robot-loaded manufacturing systems need automated tool monitoring to allow unattended, or lights-out, operation, which is increasingly popular, Caron noted.
To those who opt for an automated monitoring system, Caron advises them not to rush it right into production. “Before you start applying limits and trying to optimize tooling, just let the system show you what cutting data looks like,” he said, “and get used to looking at data in a graphical format versus watching and listening to the outcome of the cut.”
Caron also thinks shops that install an automated system should make sure that someone on the premises is knowledgeable about it. “None of these systems just goes in and works completely unattended without any effort right out of the box,” he said. “So have at least one person, an expert or champion, who is going to make sure the system is applied correctly.”