About face milling

Large-diameter milling cutters can use a toolpath with a large width of cut. KYOCERA Precision Tools

Three certainties exist that are unavoidable for every machinist: Death, taxes, and insert wear.

And, of the three, unfortunately only insert wear is somewhat controllable. In fact, it should be planned for and monitored, with adjustments being made when necessary because uncontrolled insert wear shortens tool life, creates unpredictability, and, in the worst scenario, creates unplanned downtime.

In a single-point turning operation, indexing inserts isn't that big of a deal. But what happens if you are performing a face milling operation with a cutter that holds a dozen inserts?

Face milling is by definition an intermittent cutting process. It creates a lot of impacts.

First off, let's define the operation. Face milling is one of the most common milling operations found in metal manufacturing shops today. In this operation, the cutter is placed perpendicular to the workpiece. And, though many entering angles can be used in this operation, the two most common are 45 degrees and 90 degrees.

Face mills typically are used for milling a flat surface, or face, on a workpiece. Large-diameter milling cutters can use a toolpath with a large width of cut. Depending on the manufacturer and even somewhat the preference of the machinist/shop, this width of cut can vary.

"In face milling, the optimal combination of chip formation conditions and loading of the mill's teeth is observed when the cutting width is 70 to 80 per cent of the mill's diameter," said Andrei Petrilin, technical manager for ISCAR Tools, Oakville, Ont.

According to Petrilin, this relationship of width of cut to tool diameter remains the same in both the roughing and finishing stages.

"Rough milling focuses on high metal removal rates [MRR] while finish milling assures precise accuracy of the milled surface. Finish milling features significantly smaller depths of cut and higher tool precision characteristics when compared to rough milling. Yet, the recommended relation between the width of cut and a milling diameter remains the same," he said.

Todd Rucker, technical center engineering manager, Indexable Tools Division, KYOCERA Precision Tools, Hendersonville N.C., also advises using a large engagement.

Effective chip evacuation is a key factor for high performance and prolonged tool life in face milling. ISCAR Tools

"I generally recommend, and this is any face milling cutter, that you should engage at least 50 per cent of the cutter diameter. If you’re at least half engaged to the diameter of the cutter, then you're hitting the strong part of the insert's edge," he said.

So, in other words, if you're engaged with at least half the diameter of the tool, then every insert that's coming into the cut is hitting on an insert's strongest part.

"Instead of coming in at 90 degrees when you're hitting the material and hitting an insert where it's strongest, you cause excessive wear to your insert, and it can even damage the insert. That's why we generally recommend at least 50 per cent engagement," said Rucker.

One thing to look out for when you are making multiple passes with your mill is the creation of pickup lines. As the mill transverses the part, a tiny "ridge" is created as the material is deformed.

"This is one of the biggest downfalls in face milling when you have to make multiple passes," said Rucker. "There's always some kind of a pickup line created between passes. To minimize this, you have to look at how your specific cutters are designed. Ones that are more tightly toleranced are the best."

Using a single wiper insert in the group of inserts also can help improve the surface finish that is left behind the milling pass. According to Rucker, only a single wiper insert is used, no matter how many other inserts are in the cutter.

Don't forget to monitor your spindle load. Like any other milling operation, it can be an indicator that something isn't correct.

Selecting the type, shape, coating, and grade of an insert depends on multiple factors. These choices work together and are dependent on each other to achieve their goals (typically, but not always meaning a high MMR or fine surface finish).

"An indexable insert is not a separate element but an integral part of the entire system," said Petrilin. "In rough milling, for example, the target is high MRR by use of face mills, which enables greater depths of cut under hard cutting conditions [high mechanical and thermal loads]. When using indexable milling cutters, the maximum depth of cut is a function of insert dimensions, which, in turn, determines the required insert size. To achieve the utmost economical use of the indexable insert, it requires the effective use of the cutting edge. If the cutting depth reaches at least 70 per cent of the insert cutting length, a highly efficient outcome is assured."

Carbide inserts can be single or double-sided, which, naturally, affects the number of indexable cutting edges that can be used. During roughing, the insert sustains a high load because of the large DOC being used. When combined with a maximum feed per tooth, this load increases. Inserts being used for roughing should be robust.

Visual inspection remains a common method for checking wear. ISCAR Tools

Inserts with a 45-degree entering angle have a chip thinning effect that enables increased productivity partly because it reduces vibrations, especially in setups with long overhangs.

Inserts with a 90-degree entering angle are used when a 90-degree form needs to be made. They also can be used successfully in milling thin-walled components.

"An insert is probably less than 2 per cent of the total cost of manufacturing," said Rucker. "It's all about how many parts you can make. It's all about metal removal measured by inches removed per minute. Worry more about your metal removal rates than how many times you are indexing your inserts."

The math works. The more material you remove per minute, the more parts you make, and the more money your shop can make in a day.

A good milling insert needs a tough grade, but other factors matter too. For face milling operations, in particular, choose an insert with a PVD coating. Although, this rule can be broken for specific materials. When milling cast iron, for example, a CVD-coated insert can be used because cast iron is not usually as difficult to machine as other metals.

"Milling operations are a lot easier when you choose the grade correctly," said Rucker. "We have a grade for steel, and we have another grade specifically for stainless. Some of the steels and stainless crossover, especially if it's a 300 series stainless. But when you get to a pH stainless, you’re going to want a different, tougher grade."

High-temperature alloys also have dedicated grades. You need to pay attention to the manufacturer's recommendation.

Choosing a cutter means choosing how many inserts will be active in the cut. Fewer inserts in the cutter means there will be less tool pressure, but it also means less metal removal.

Some tooling manufacturers will designate their cutters as coarse, fine, and extra-fine. Your particular operation likely will lend itself to one of these three cutters. Coarse is the lowest number of inserts, fine increases the number of inserts, and extra fine increases even more, typically in a two-insert jump.

"A finer pitch cutter likely has more inserts," said Rucker. "If you put two more inserts on that cutter, you also have to put more flutes on that cutter. That can affect chip evacuation."

Milling operations are a lot easier when you choose the cutter, grade, insert, and coating correctly.

Pitch and flute number affect tool pressure. It could mean the difference between needing to run to project on a 40-taper machine or a 50-taper machine.

Cutting speeds and feeds are set depending on the material being machined, the insert with all of its variables, and the overall operational stability. Milling at the wrong speeds and feeds leads to poor cutting performance, diminished tool life, and even tool failure.

During the pre-processing or testing phase, conditions are created to help determine the insert's lifespan and discover how many parts or part surfaces can be milled before the insert needs to be indexed. All inserts in the cutter should be indexed at this time.

Most inserts have numbered corners to help the machinist keep track of whether the insert has been indexed or not. Starting with edge No. 1 and proceeding through each numbered edge until the entire insert has been used makes it easier to keep track of the number of indexes that have been performed.

"For each cutting [new] operation, the machinist should rely on the recommendations of the tool manufacturer as a basis for tool life estimating, and then ‘adjust’ tool life by checking for wear by visually inspecting the cutting edges of the inserts," said Petrilin. "Under shop floor conditions, the indicators of insert replacement or indexing are based on the visual inspection of inserts, increased power consumption, changes in surface quality of machined surfaces, burr formation, vibrations, and even a change in the sound that is being made."

Tool life is affected by many factors, including the speeds and feeds being used, DOC, the physical and chemical properties of the material, and the machine tool setup. Wonky workholding can be as detrimental to insert wear as running the tool using the wrong cutting data.

Machinists constantly look for longer life from their inserts, but predictability is just as important, if not more important, in many operations.

"Predictability of insert tool life is an important factor for successful cutting. It ensures stable machining results, enables effective process planning, and greatly contributes to reducing production costs," said Petrilin.

"The biggest killer of milling inserts is notching," said Rucker. "This happens a lot when the depth-of-cut line of the insert keeps getting hit in the same spot and beat up. The thing with notching is it really is a visible notch that may look too bad, but eventually you will need to flip the insert because if you let that notching progress, the insert's corner will break."

The next wear pattern that is common in face milling is crater wear.

Face milling when the width of cut is 70 to 80 per cent of the mill diameter provides the optimal combination for chip formation and mill loading. ISCAR Tools

Crater wear happens as chips keep washing across the insert over and over again until a crater appears on the insert's top face.

Both notching and crater wear can lead to a catastrophic failure of an insert, and that will stop the machining process unnecessarily, a.k.a. unplanned downtime.

Providing proper chip evacuation is a key factor for successful face milling. It can be accomplished in a couple of different ways, depending on if you are cutting wet or dry.

Use Geometry. According to Petrilin, the design of an insert's rake face contributes to both chip formation and chip flow. Chip-splitting and chip-chopping shapes on an insert's cutting edge divide wide chips into small segments that make chip evacuation easier.

Use Coolant. Coolant is needed to reduce the thermal load on a tool's cutting edge. However, that's not all it does.

"Another important function of coolant is lubrication, which enhances material removal and improves chip flow," said Petrilin.

Coolant use strategies and coolant choice depend on the workpiece material and tool choice, including its material makeup.

"Coolant strategy is important," said Petrilin. "For example, high-pressure cooling (HPC) creates thin, manageable chips. Minimum-quantity lubrication (MQL) in near-dry milling creates a machined workpiece and chips that are nearly dry, making part cleaning and chip disposal much easier and quicker."

Use Air. In true dry cutting, chip evacuation can sometimes be aided by an air blast. Choosing to use coolant depends on many factors, including the material being cut and a shop's preference. Even expert opinions vary.

When coolant is used, a hot insert is cooled down very quickly, which thermally shocks it. According to Rucker, this thermal shock increases wear and reduces tool life.

"Coolant and milling generally don't mix," said Rucker. "We do not recommend running coolant in any milling operation except when milling high-temp alloys. The heat generated during cutting does not go out of the work zone in the chip in titanium-based or nickel-based components."

If this generated heat stays in the workpiece, it will work-harden these ISO S metals.

"In these instances, we definitely recommend using coolant," said Rucker.

All three of these choices are used to keep the toolpath clear and the tool cutting. If chips keep getting trapped between the mill and the machined surface, it may be time to look at the process as a whole. Trapped chips reduce surface quality, cause vibrations, and decrease overall milling performance.

According to Petrilin, the cutting forces generated during machining boil down to the resistance of the machined material to the cutting tool.

These cutting forces are divided into three variants: tangential, radial, and axial. The tangential cutting force determines power consumption, and the two other two affect the mill's load along its axis and radially to its axis.

The interrelation between these forces mainly depends on the mill's cutting geometry and the cutting edge's angle. Decreasing the cutting edge's angle, for instance, leads to a higher axial cutting force and lower radial cutting force. Increasing the cutting edge's angle reduces the axial force but causes higher radial force.

Petrilin said that the cutting tool's edge angle is a design characteristic of face mills that directly relates to its application. For example, he said, if a face mill is intended for machining open plane surfaces, a mill design with a 45-degree angle is preferrable. However, when milling faces are bound by a shoulder, 90-degree mills are required.

The type of milling operation matters too. Rough face milling using high-feed milling should be performed by mills with a cutting edge angle 10 to 17 degrees, according to Petrilin.

By design, face mills are paired with inserts that support the load produced by these forces, whether they are radial or axial, to help ensure that machinists get the desired performance and tool life.

Editor Joe Thompson can be reached at [email protected]

ISCAR Tools, www.iscar.ca

KYOCERA Precision Tools, www.kyoceraprecisiontools.com