Turning the unpredictable into predictable

Understanding insert wear patterns can help shops keep the turning process predictable and their machines running smoothly. Phuchit/iStock/Getty Images Plus

Inserts only last for so long, but there are few things shops can do to understand the life cycle of their tools and get the most out of their inserts.

“Everything starts with understanding tool management and having a clear test plan to establish given wear parameters and end of life to make the process predictable,” said Jan Andersson, director of global product management, indexable inserts, YG-1 Canada, Burlington, Ont. “Most shops don’t start with this. They put in an insert, run the machine, and it’s not until they start getting into trouble that they index it. When they are asked about the tool life, they have an estimate, but upon investigation, they realize that the tool life is unpredictable, which can cost them a lot of money.”

Having a clear understanding of an insert’s behavior and life cycle for a given application will help shops develop an indexing schedule that will help standardize production, keep wear to a minimum, and troubleshoot when unpredictable wear starts occurring.

“There's a lot of productivity left on the table without fine tuning the process,” said Steve Paulding, president, CERATIZIT of Canada Tooling Solutions, Cambridge, Ont. “Many shops are using inserts that are not designed specifically for the application, or are using the wrong corner radius perhaps, but that's what they have in the cabinet. It’s hard for customers to zero in on having the perfect insert. Utilizing the cutting data on the labels and seeking advice from the manufacturers will help achieve the best insert tool life.”

Inserts are not meant to last for a significant amount of time. Understanding wear patterns can help shops keep the process predictable and their machines running smoothly. Wear can be classified into two categories: predictable and unpredictable.

Predictable wear is the wear pattern that happens in every single application, and it can almost be predetermined if a shop has tested its tools and developed a tool life management strategy. Flank, notch, and crater wear are primary predicable wear patterns found towards the end of a tool’s lifecycle.

It takes a bit of knowledge and understanding to identify wear. But a simple Google search can provide some key visual indicators. Most insert manufacturers offer training classes to teach tool wear patterns. Many resources are available to help make the process more predictable, and shops should take advantage of this expertise to help use tools more efficiently.

With primary wear, it’s important to know when exactly the insert needs to be changed. There is a common factor to follow to ensure predictability of the process.

“Once you start seeing flank wear exceed 0.004 to 0.006 in., the workpiece may show signs that it's time to change the insert,” said Paulding. “It’s also important to note that a substrate that has smaller grain size or lower cobalt content will offer higher wear resistance, which will extend tool life in the right materials.”

There are a multitude of grades available, whether for roughing or finishing applications, so it’s important to choose the right one for the application. However, insert manufacturers are a good resource because they can ensure that the insert has the best characteristics and provide guidelines for proper wear patterns. Many insert catalogues also will provide troubleshooting information for most predictable wear issues.

Utilizing the cutting data on the labels and seeking advice from the manufacturers will help shops achieve the best insert tool life. CERATIZIT

“Metal cutting and turning is a thermal process, requiring enough temperature in the cutting zone to get near plastization of the material,” said Andersson. “Equally important to that is thermal evacuation. The chip area is defined as feed rate times depth of cut in relationship to the thermal conductivity of material being machined. The more mass you have, the more efficiently you can get the temperature away from the cutting zone. That is the key to control the secondary chemical wear and one way to do this is to simply increase the feed rate.”

There has been a lot of conversation around speeding up the process, but speed is one of the last parameters that a shop should adjust when trying to manage tool life and maximize productivity. Shops should find the speed that's needed to get to the right temperature in the cutting zone.

Predictable wear is, by nature, expected as the tool reaches the end of its life. Knowing in advance when to expect that wear will help shops manage processes better. Unpredictable wear, on the other hand, can spell big issues for insert tool life.

The goal of any shop should be to drive unpredictable wear to the predictable. Unpredictable wear includes fracturing and chipping, which can be classified as catastrophic failure.

“The No. 1 telltale sign of an unpredictable process is inconsistency in tool life,” said Andersson. “For applications where a shop understands the expected number of parts per life cycle, having an insert that doesn’t meet that standard is an indication that the insert is experiencing unpredictable wear. From a visual standpoint, chipping on the edge line and insert breakage point suggests serious issues that require troubleshooting. The answer to solve these issues is different based on how they start.”

The most common reason why shops start seeing chipping is because the grade-geometry combination is incorrect for the given application.

“There's the ISO scale of inserts where the lower the number is, the harder, more wear resistant it is,” said Paulding. “The higher number would be the tougher insert. Typically, if you're chipping an insert before you get that expected tool life, you should go to a tougher or a higher number on the ISO scale. If you're wearing out, then you should drop down to a lower number on the scale.”

Geometry also will give the toughness behavior for a given application. In turning, for example, using a wiper is an efficient way to stop chipping because of the strong edge-line microgeometry.

“Chipping can also be caused by running too slow,” said Andersson. “Because of the thermal process, if it’s running too slow, it won’t generate enough temperature, which will make material a lot harder to machine than it should be. And that usually overloads the microgeometry, which causes chipping.”

Whether it is overloading the microgeometry because of overfeeding or running over interruptions, or the cutting data is incorrect, it is important to understand why the insert is chipping to figure out the best way to solve the problem.

When a shop understands the expected number of parts per life cycle, having an insert that doesn’t meet that standard is an indication that the insert is experiencing unpredictable wear. Ladislav Kubeš/iStock/Getty Images Plus

Another common reason chipping occurs is when shops use one insert for both steel and stainless.

“Steel is a continuous chip flow,” said Andersson, “Whereas stainless is a laminar chip flow. And what this means is in stainless, you put a lot of compression into that chip, and eventually you have so much compression in that chip that it snaps off. That puts an enormous amount of stress in the microgeometry, causes chipping. That's why the geometries are so widely different for stainless than steel, and it’s important to use an appropriate insert.”

Unpredictable wear can first show up in surface finish quality. Also, if an insert is chipped, the operator may hear a different sound coming from the machine.

“For turning, if a shop is using the right insert, it should be relatively smooth sailing for 15 minutes at least,” said Paulding. “Most shops want or expect more and will run the insert right to the end of life, but that’s not a good approach. You want to index or change the insert before wear starts affecting the part. The insert is really an inexpensive piece of the manufacturing process but can have significant ramifications of part production.”

A general rule of thumb is that shops can expect 15 to 20 minutes of engagement time in most materials.

“If you run slower and get a longer tool life, you're probably not being as efficient as you can and your cost actually goes up more than it should,” said Andersson. “If you run faster, then you stop the machine too often to index, leading to more downtime. So make sure the process is predictable and scheduled indexing is set around that 20 minute mark. Don't run it down until the end of life.”

In testing, it’s important to look at various stages of the process, at part 10, 20, 50, and so on, to see how the insert behaves over the course of its life cycle. Going from start to finish without part observation won’t help shops fine tune the process to get the most out of the insert and limit wear.

“With issues, rarely is it the insert that's changed,” said Andersson. “Most of the time it is other parameters, for example, a different batch of material that is slightly harder than the last one. That can have a big impact. Coolant changes from the test batch can change how the insert wear develops. So all these factors need to be taken into account.”

Today’s insert manufacturers make it easy to choose the right insert and turning parameters, including speed, feed, depth of cut, and more. But these are just basic minimums and maximums.

“Fine tuning the process is so important to help shops get the maximum efficiency out of the machine, make the insert last longer, and save money,” said Paulding. “For shops that are using an insert and getting fairly decent tool life out of it, try increasing the feed rate override by 10 per cent, which can shed about six seconds off every minute in the process. And if that works well with no unpredictable wear occurring, go up another 10 per cent until you reach the maximum feed rate for the given application. This will help to maximize efficiency and get the most tool life out of the insert.”

Beyond the insert, there are wear components that can affect insert life. Make sure the toolholder is in good working order and that shims are replaced at proper intervals. For turning, it is recommended that the shim is replaced for every 10 pack of inserts. Without monitoring the overall process parameters, not only can inserts wear, but parts may not be produced to specification. Understanding the process and fine tuning the parameters may take some time upfront, but it will help shops ensure they are getting the most out of their turning insert life cycle and keep machines running well.

Associate Editor Lindsay Luminoso can be reached at [email protected].

CERATIZIT, www.ceratizit.com

YG-1, www.yg1.ca