How to maximize material removal rates in turning

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Material removal rates (MRR) are important in any machining process. This is especially true in high-volume production environments. But even job shops can focus on ensuring they get the best MRR possible.

“The amount of material you can remove and how fast you can remove it will directly impact cycle times, how long it takes to complete a particular job, and ultimately how much profit can be realized on a particular job,” said Michael Cope, product technical specialist at Hurco Companies, Indianapolis.

Most machine shops strive to increase their bottom lines in any way possible, and one way to do this is through increasing MRR, which can help drive down cost per part. This concept is especially true for milling applications, but it begs the question: Is it possible to significantly increase MRR in turning?

“We find that people talk about material removal rates more in milling than in turning,” said Steve Schermann, applications manager at Milltronics, Indianapolis. “For the most part, there is more software and different techniques available in milling. But, turning is catching up in terms of software and new cycles.”

As industry demands shorter cycle times, a number of factors can help shops increase MRR in their turning applications.

The follow are just a few.

“Any lathe or machine that you're using will have some limitations regarding how fast and how far it can travel,” said Tyler Economan, technical sales manager for Index Corp., Noblesville, Ind. “The machine design needs to be able to handle the increased speed rates and have a control and motors that can accelerate and decelerate at the appropriate rates to minimize non-productive times. This allows for maximum material removal rates without sacrificing surface finish and part accuracy.”

There are many different lathe designs and styles available on the market. Machine shops looking to invest in equipment specifically designed for high MRR should consider a few design features.

“On a turning machine, the design can either have ball linear guides, roller linear guides, or box ways,” said Siroos Askari, product sales manager for DMG MORI Canada, Mississauga, Ont. “Box ways are suited better for heavy-duty cutting or high material removal. It has a larger contact area and an oil film between the saddles, which provides a dampening effect. It more easily absorbs the cutting forces, which can reduce vibration. Vibration causes poor surface finish and reduced tool life.”

Cope agreed that the traditional ball-type linear ways may not be as effective for heavy cutting and higher MRR and said roller linear ways are a good option, in addition to other design features like increased spindle speeds, casting materials, and overall design.

Milltronics ML Series combo lathes offer both manual and CNC operations for tool rooms and job shops. They can be configured with many options and other features such as bore sizes and bed lengths – including live tooling and C-axis (on most models). Milltronics

“Certainly, the maximum spindle and live tooling speeds and the overall machine rigidity all affect the amount of material that can be removed and how quickly,” said Cope. “Couple these features with the addition of faster live tooling RPMs, a Y-axis or subspindle, and you have a really strong recipe for getting these jobs off the machine very quickly, with good surface finishes and with the ability to hold very tight tolerances. All of this will go a long way in reducing the cost of each part.”

The bed design also is a consideration. With aggressive cutting strategies and high MRRs, machine stability and rigidity are essential.

Askari noted that the bed design should have a wider leg or enough width with a low centre of gravity. The lower centre of gravity makes the machine more stable and prevents vibrations.

“Cast iron beds can absorb cutting more efficiently,” he said. “Also, the casting of the bed should go all the way under the spindle housing. This is especially important with bigger spindles; the bed must extend where the spindle housing is to better support the spindle bearings from high cutting forces resulting from any heavy-duty cutting.”

Another consideration is the saddle. The turret is attached to the saddle with a coupling. According to Askari, this coupling should be as large as possible. A large coupling diameter often provides better clamping between the turret, which means that it can take heavier forces.

The torque on a machine plays an essential role in increased MRR. On large lathes that produce large parts, a gearbox is capable of delivering more torque than a standard cartridge-type lathe.

When looking at increasing MRR, it’s important to include tooling. Having the right lathe with the necessary capabilities is essential, but adding a poorly designed tool, even on the best of machines, is not going to yield great results.

Economan pointed out that machine tool builders frequently collaborate with cutting tool manufacturers, many of whom are willing to test different manufacturing techniques and strategies. Cutting tool manufacturers often are able to give recommendations about the geometry of the insert, the material the insert is made of, and the coating and chip breaking options. Companies can help ensure they’re getting the most out of their machine technology by taking advantage of the expertise of both machine tool builders and cutting tool manufacturers during their process development.

“Most of all, they will be able to accurately determine how hard you can push the tool in any particular cut,” said Economan.

However, as a rule of thumb, the experts said that it is important to work with the shortest tools possible. Outer diameter (OD) turning often is a more stable process than inner diameter (ID) turning by nature of having to use a boring bar in the ID.

The Hurco TM8MYi is a slant-bed lathe with live tooling capabilities and a Y-axis that makes it possible to machine a pocket with straight walls and flat bottoms or perform off-center machining operations with just the click of a box. It also features a BMT55 turret. This machine was introduced at IMTS 2024. Hurco

“In ID applications, use the largest-diameter boring bar possible to add robustness to the tool and reduce any vibration that it may incur,” said Askari. “As for inserts, choose the thickest insert with a large nose radius, which are more rigid and less prone to breakage.”

If the tool life is not good or the insert is constantly breaking in the middle of the cut, operators must stop and change it out, basically wiping out any gains related to increased MRR.

Increasing MRR is commonly associated with milling because the toolpaths selected can have a huge impact. This is not necessarily the case in turning. In order to increase MRR, operators need to rely on speed and heavy cutting.

“The CNC control software has to be able to accommodate the faster feed rates and the increased acceleration and deceleration speeds needed for increased MRR,” said Schermann. “The machine itself has to be rigid enough to support the increased cuts, heavier cuts, or faster cut depth of that process.”

Depending on the material condition and geometry, it’s important to find the optimum speed to achieve that maximum MRR. At the same time, it’s important to ensure that the chips break.

“If you cannot break the chips, these chips entangle around the tool, and either the tool or insert will break,” said Askari. “Any strategy should target maximum metal removal rate, but at the same time, ensuring it will break the chips to make sure that you have enough tool life.”

Beyond this, machine builders are finding ways to improve the machine and its controls, allowing for the development of new strategies to take shape.

For example, bidirectional turning is a high-efficiency strategy where the tool cuts in all directions as it moves towards the chuck and then retracts all the way back and starts over again, repeating the pattern.

“With bidirectional cutting, the tool is always moving back and forth on the part and cutting in all directions,” said Schermann. “It will reduce the cycle time and allow the tool to do more profiling and features with the same tool because it is able to come from a different side of the part than a traditional cutting strategy does.”

Another strategy is getting as many tools in the cut simultaneously as possible.

The DMG MORI NLX series was designed for production of large parts. It offers turning and milling properties and includes a rigid design, maximum drive power, and state-of-the-art equipment options. DMG MORI

Economan explained that certain machines include a main and counter spindle and feature the ability to get up to four tools in cut at the same time.

“Theoretically, this strategy is removing material four times faster than just one tool at a time,” he said.

On a multispindle machine, there can be many tools in cut simultaneously. This will depend on the geometry and complexity of the workpiece, but these types of machines can offer up to eight main spindles and the capacity to include two tools in cut per spindle.

“This approach divides the work amongst the spindles and drastically reduces cycle time,” said Economan. “Maximizing the number of tools in cut simultaneously is a straightforward philosophy that works.”

While there are several factors to consider when it comes to increasing MRR, there also are many ways to reduce the overall cost per part.

“The addition of features like a bar feeder, live tooling, Y-axis, and possibly a subspindle are all good ways to help get the job done faster, with fewer operations, fewer setups, and overall, less operator intervention,” said Cope. “The one-and-done mentality goes a long way in turning, and if we can couple these features with higher speeds and faster material removal rates, then we have an efficient strategy overall.”

According to Schermann, live tooling offers significant benefits when it comes to performing certain operations that are usually done in a secondary operation. For example, while turning on a lathe is common, live tooling allows the machine to mill features.

“When you can do everything in one operation, this increases the overall accuracy of the part,” said Schermann. “Dimensions are easier to hold. It also saves time not having an operator and machine sit idle waiting for the part for a secondary operation. The more operations you can do in one machine is for sure going to increase your production.”

Manufacturers are increasingly focused on reducing downtime or idle time, which is determined by how quickly the machine can move from cut to cut.

“Rapid rates, feed rates between cuts, how quickly the tool carrier can index from insert to insert, can all add up,” said Economan. “This is especially true if the process requires a lot of tool changes. Anything that can be done to reduce that time will have a significant impact on cycle time.”

In the area of kinematics, Index's patented SingleSlide system provides two degrees of motion in a single plane to allow faster responsiveness without increasing vibration. Index

Turning is a fairly straightforward process. However, some features, tools, and strategies can help improve MRR, leading to an overall reduction of cost per part.

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

DMG MORI Canada, en.dmgmori.com

Hurco, www.hurco.com

Index Traub, www.index-group.com

Milltronics, milltronics.com