Steels wear on the tool’s margins, but this wear typically is slow with dialed-in cutting conditions. The machining of steel is relatively easy, but it becomes more difficult depending on its hardness and the carbon content.

“What Sandvik does is design a [multipurpose tool] with a geometry that is productive and secure in different materials. Then we design the grade that is safe and secure through all the material groups as well,” said McEachern.

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Operators should constantly try to promote a thick-to-thin chip milling strategy to help reduce the amount of shock forces on the tool. Photo courtesy of Iscar Cutting Tools.

“That's the good thing about drilling: Chipping and BUE are both remedied by increasing the cutting speed,” said Vetrecin.

“I’m not concerned about the pressure anymore with the large-diameter drills. It's all about flow. It allows you to keep a cool temperature, and there is plenty of lubricity and flow to help evacuate chips out of the flutes,” said McEachern.

Operators and programmers need to work together to ensure consistent depth of cut, engagement, and chip thickness. Howard also noted that the depth of cut can be larger with certain ceramic geometries when compared to CBN inserts.

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“Switching tools for different materials has gotten a lot easier thanks to exchangeable-head systems,” said Dave Vetrecin, holemaking product manager for Iscar Tools, Oakville, Ont. “If you have one tool body and you're drilling stainless one day, you can use a head for that material, and then later, if you're cutting steel or cast iron, you can have a different head for that. These interchangeable heads are very versatile and also have the ability to hold quite accurate tolerances.”

Heat-resistant superalloys (HRSA) include a number of high-alloyed iron, nickel, and cobalt-based materials. They are very similar to the ISO M materials, but are much more difficult to machine. With such a wide spread of materials under the generic heading of HRSA, the machining behaviour can vary greatly even within the same alloy group.

“The chips when you first start will have a nice, smooth surface,” said Vetrecin. “If you develop a built-up edge, the surface of the chip will be rougher and have lines on it. You can see exactly where there's a piece of material stuck on the cutting edge because of a scratch on the chip.”

“Toolpaths are critical,” said Howard. “If you are nice to the cutter, the cutter and inserts will be nice to you. It’s important to treat them gently.”

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When it comes to the exchangeable-head drills, you're not just indexing a single insert, you change out one big piece of carbide – at the drill’s head, which is a more expensive changeover.

However, these materials generally tend to be sticky, create built-up edges, work-harden, and generate heat, which is often given back to the tool. While some HRSA are very abrasive because of their material composition, almost all place particular demands on the cutting tools. This is why many shops are turning to ceramics for milling of HRSA.

And while drilling is a seemingly simple and common metal cutting process, its success or failure is based on many quality measurements, including concentricity, wall and floor surface finish, straightness, and hole size tolerance.

Cast iron causes wear on the transition from the margins to the cutting edge because its silicon carbide (SiC) content makes it highly abrasive.

Second, inspect the cutting edge after each test cut as you increase the speed. You should see the BUE move toward the centre of the cutting edge until it becomes minute or even non-existent. That’s the time to stop increasing the speed.

“SiAlON (silicon, aluminum, oxygen, nitrogen) is a mixture of silicon nitride and aluminum oxide,” he said. “It has the best chemical stability and resists notch wear. There are a number of variations under this category. Whiskered ceramic provides improved toughness and bulk strength compared to the traditional ceramic, and fibres are included.”

Other types of multipurpose drills, including Horn USA’s Supermini HP, can perform multiple machining functions, such as drilling, boring, face turning, and skimming. Horn USA

In busy job shops, machines typically are used for multiple materials, and changeovers happen regularly. Because downtime is a pure productivity sink, efforts should be made to reduce this non-productive time.

However, the speed should be balanced between creating enough heat in the cutting zone to plasticize the chip but not so high to as unbalance the ceramic, added MacNeil. To produce a proper chip thickness, the feed should be selected that is high enough to not work-harden the material but not so high that it causes edge frittering. To get the most out of the tool, air is recommended to clear chips away and prevent them from being recut.

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“Most materials can use synthetic water-based coolants with success,” said Tonne. “I also have seen some cases where neat oil improved performance in nickel-based alloys because the shearing action was improved.”

The most common wear that affects a drill’s cutting edges is BUE. When this wear occurs, material sticks to the cutting edge before eventually popping off and taking with it part of the tool’s edge and coating.

The type of wear that affects drills often depends on the material being processed. Some examples of the material/wear relationship are:

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Iscar’s LOGIQ3CHAM assembled drills carry exchangeable carbide heads with three effective cutting edges. Three polished flute surfaces help ensure easy chip evacuation, and the variable flute angle design results can sustain high axial forces. Iscar Tools

To produce a proper chip thickness, the feed should be selected that is high enough to not work-harden the material but not so high that it causes edge frittering. Photo courtesy of Sandvik Coromant.

“If indexable or exchangeable-head drills are suited to the hole size, I would not use solid-carbide multipurpose drills when I see applications in which the setup might be very weak,” said McEachern.

“This speed is a requirement for the tool,” explained Brian MacNeil, milling products and application specialist for Sandvik Coromant, Mississauga, Ont. “The minimum speed to generate enough heat for ceramic usually starts at 550 m/minute or 1,700 SFM. Depending on the material and its condition, you could achieve 975m/min. or 3,200 SFM, whereas carbide in this area typical averages 36 m/min. or 120 SFM.”

And measure your mixture right with a refractometer. It’s the best method to get an accurate idea of what’s going on with your cutting fluid.

“Once initial tool life and productivity baselines are achieved, experimentation with increased depth of cut and SFM should be completed to maximize productivity,” added Corneil.

“With exchangeable-head drills and indexable insert drills, the coolant flow is really critical,” he said. “This is because it's more important to have good flow on larger-diameter drills than it is to have high pressure. So, 1,000-PSI output may be no good to you if you're not getting enough litres per minute.”

“The transition to ceramic cutting tools is typically a learning experience for the machinist, as what is happening in the shear zone is completely different than what we expect with carbide tooling,” said Jeremy Corneil, milling product manager for Iscar Tools Canada, Oakville, Ont.

Ceramics, unless whiskered, tend to be extremely brittle. This makes it important to use the best milling practices.

The importance of the individual quality data points also depends on the parameters of the hole being drilled. The type of hole (through, blind, chamfered, stepped, entry, exit, and cross), hole diameter (micro, small and medium, and large), and hole depth all matter.

For drilling on a lathe, McEachern’s first impulse is to use indexable or exchangeable-head drills for multipurpose drilling because they have a steel body that flexes a little bit. If you use a solid-carbide drill on a lathe, you need to have a solid setup with good alignment.

If you are cutting stainless steels or heat-resistant superalloys, synthetic and semi-synthetic coolants should be shelved to make use of an emulsion water-soluble oil with a 10 to 15 per cent coolant concentration.

“Less experienced machinists might interpret that as a chip in the cutting edge, when it's in fact just the coating being ripped off when the buildup pops off. This usually means that the tool is moving too slowly, which can be remedied by increasing the cutting speed,” said McEachern.

There are three things to examine when looking for wear, and according to Iscar’s Vetrecin, it’s best to do this at the testing or prove-out stage, before real parts are being produced.

First look at the chips that are being produced because they can tell you a lot about your process right off the bat.

When the grade is combined with tool design features such as the chisel point, cutting edges, flutes, helix angle, and coating, you get a tool that works well across numerous materials.

“It’s crucial to understand the setup to recommend a drill,” said McEachern. “When we're working in machining centres, the drill is rotating in the spindle through to a centre line. These machines are pretty well aligned with the spindle perpendicular to the workpiece. However, when you're on a lathe, the drill is stationary and mounted in a turret. They can get out of alignment over time from a lot of different factors.”

Iscar’s SUMOCHAM drills feature exchangeable heads, cylindrical shanks, and internal coolant holes. They can be fitted with four standard drilling head types for drilling on four different material groups while keeping the same steel body. Iscar Tools

According to Edwin Tonne, training and technical specialist, Horn USA, Franklin, Tenn., many reasons exist for using a multipurpose drill. These include:

This group of steels requires a strong chisel point to battle the higher pressure or axial thrust at its centre. These steels generate a lot of heat during cutting and are very abrasive to the cutting edge. Tools for hardened steel need to be strong and chemically stable at high heats. This heat also can lead to plastic deformation.

“With ceramic indexable cutting tools, the insert pockets should be inspected and replaced if there is any damage,” said Corneil. “Due to the high RPM required to run these tools properly, it is also recommended that the insert screws be tightened with a torque wrench to proper specs.”

“Tool selection, especially in job shops, all starts with the material and batch size. If our customer has a large enough batch size, then I always recommend a dedicated geometry and grade for high productivity. However, if they only have five parts to make, and there's only one hole per part, I recommend a more economical drill and maybe a drill that serves multiple purposes,” said McEachern.

“You have to be dead-on perpendicular to the workpiece. If there's any angularity in the setup whatsoever, a solid-carbide drill can snap. That’s why I very seldom ever recommend a solid-carbide drill for a lathe setup,” he said.

Compared to other tooling such as carbide, fewer options are available when it comes to ceramics, particularly geometry.

With so many variables come many drilling options, and solid-carbide, indexable, and exchangeable-head tools all have their place.

Corneil added that ceramic inserts typically suffer from depth of cut notch wear, built-up edge, and chipping. In most cases he recommends changing inserts based on an excessive burr, a large increase in machine power consumption, or wear that is 0.04 in. wide.

Image

A look at the cutting edges, either with the naked eye or, even better, under a microscope, will tell you a lot about the operation. This works best during the testing phase as the cutting parameters are dialed in. Iscar Tools

“Speed is not an enemy of ceramic inserts,” said Howard. “If ceramic is run too slow, a result will be chipping of the insert cutting edge. Push the feed rate as hard as possible; this will give you better tool life.”

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Coolant provides lubricity at the cutting edge and margin, helps break the chip, flushes chips out of the hole, and controls the cutting zone’s temperature to eliminate thermal shock.

He added that ceramic tools can be produced as indexable inserts for larger features and as a solid round tool for smaller features.

“Often there are problems that are not spotted until it's too late because these machines are running at a high rate of speed, chips are flying, and doors are closed and coolant is flying,” said McEachern. “You can’t see much, but you can still listen. Sometimes even leaning on a machine or putting your hand on the door of the machine to feel for vibration helps diagnose a problem.”

“It is important for machinists to be able to evaluate and understand the wear factors and understand how to react to them,” said McEachern.

Ceramic inserts need to run at very high cutting speed with reinforced geometries, and in order for it to work, it needs to generate an extreme amount of heat to plasticize the material and then the tool will displace the heat. Ceramics can have a negative effect on the surface integrity and topography so are not used for machining close to the finished component shape; rather, they are recommended for roughing operations.

Stainless steels are quite heat-resistant and, therefore, produce wear on the cutting edge of the drill. Notching and built-up edge (BUE) commonly occur on the cutting edge.

“It’s also important to make sure the SFM is high enough to plasticize the material,” said Corneil. “If the material ahead of the shear zone is not softened from the heat of cut, the ceramic inserts will simply chip and break. Due to the brittle nature of the ceramic insert a moderate feed rate should be tested first. Typically, a good starting feed rate is between 0.0025 in./z to 0.004 in./z depending on the geometry.”

As previously mentioned, programming toolpaths specifically for ceramic inserts is necessary to ensure long tool life. The ceramic inserts should stay in contact with the part as much as possible. Every time the inserts come off the part, the insert’s tool life is decreased.

When it comes to milling HRSA, the experts agree that developing a suitable cutting strategy is important. Avoiding interruptions and keeping the tool in the cut is ideal. Climb milling is preferred to conventional milling. Soft entry or arching into the cut is also highly recommended.

“New HRSA materials are constantly being developed,” said Steve Howard, engineering and marketing manager for NTK Cutting Tools, Wixom, Mich. “These new materials will withstand the demanding environments in which certain components must function, and they can have higher hardness characteristics, even exceeding 45 Rockwell C.”

If you start seeing crater wear, which occurs on the rake face of an insert when using an indexable drill, for example, it is caused by excessive speed and too much heat.

“Wear patterns for ceramic usually resemble edge frittering, top slice, and notch wear,” said MacNeil. “When you get it right, you can achieve nice, even flank wear just like carbide.”

Sandvik Coromant’s CoroDrill 860 -GM multimaterial drill promises precision and accuracy at higher productivity rates. Sandvik Coromant Canada

Speed is not an enemy of ceramic inserts. If ceramic is run too slow, a result will be chipping of the insert cutting edge. Photo courtesy of NTK Technologies.

Ceramic inserts need to run at very high cutting speed with reinforced geometries, and in order for it to work, it needs to generate an extreme amount of heat to plasticize the material. Photo courtesy of Sandvik Coromant.

When Sandvik Coromant designs a multipurpose tool, it starts with a geometry that is productive and secure in different materials. Then it pairs with a grade that is safe and secure through all the material groups as well. Sandvik Coromant Canada

Beyond geometry, a shop also must decide what ceramic grade is best suited for its applications. According to MacNeil, generally speaking, two types of ceramic cutting tools are available: SiAlON and whiskered.

“Shops use these tools to save tool positions and decrease cycle time caused by tool changes,” said Tonne.

An award-winning writer and graduate of the Sheridan College journalism program, he has published articles worldwide in a variety of industries, including manufacturing, pharmaceutical, medical, infrastructure, and entertainment.

He added that at all times operators should try to promote a thick-to-thin chip milling strategy to help reduce the amount of shock forces on the tool. Also, coolant should never be used as the inserts will suffer from catastrophic thermal shock.

For example, in an operation that is drilling to a depth of 7xD, the chips come out of the first part of the hole easily and quickly. Then as the drill moves toward the bottom of the hole, a different shape in the chip flute helps ensure that the chips will still evacuate well. Also, the coolant flow needs to provide enough lift to get the chips up the flutes and out of the hole.

Lindsay Luminoso, sr. editor/digital editor, contributes to both Canadian Metalworking and Canadian Fabricating & Welding. She worked as an associate editor/web editor, at Canadian Metalworking from 2014-2016 and was most recently an associate editor at Design Engineering.

Because machinability of HRSA tends to be poor, the experts agree that opting for a ceramic cutting tool is the best choice.

His general rule is to have 1 litre of coolant per minute per 25 mm of diameter of the drill. So if you have a 25-mm-dia. drill, you should have 25 litres per minute of coolant flow.

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“You can tell by a chip’s colour, shape, and how they come up out of the hole if the tool is working well,” said Vetrecin.

First, increase cutting speed while keeping the feed rate (feed per rev.) the same. If you change more than one parameter at a time, you won't know what solved the problem.

Image

Corneil echoed this by adding that negative round button inserts are one of the most popular insert geometries with the trend moving toward solid ceramic end mills.

The most common cause of tool failure and poor surface finishing is tool wear. However, flank wear on a ceramic insert is a normal result. Wear is on the top of the insert is a result of a SFM or speed that is too high.

“The flute is one of the most important features,” said McEachern. “You need a flute that enables good chip evacuation so there's no jamming. Often we even have multiple helix angles in the flute for evacuating chips at different sections of the hole.”

According to McEachern, it’s important to have high-pressure coolant when cutting with solid-carbide drills, and smaller diameters typically need higher pressure than larger drills. For larger indexable and exchangeable-head drills, it is the volume of coolant flow that becomes important.

Beyond the cutting tools, quality toolholding with a low axial or radial runout is recommended to ensure long, consistent wear, according to MacNeil. When it comes to solid round ceramic tools, a quality hydraulic or heat-shrink toolholder is recommended to reduce the tool’s tendency to pull out in difficult materials.

These tools also typically have symmetrical cutting edges and can drill holes to much closer tolerances than an indexable drill.

Solid-carbide multipurpose drills typically have symmetrical cutting edges, so they create very accurate positional tolerancing. Even exchangeable-head drills can have a strong chisel point at the centre to give good centring capabilities. Indexable drills have multiple cutting corners and, therefore, create good edge economy and cost efficiency, but struggle with tight tolerances.

Drilling often is a late-stage machining process, deployed only after many other operations have had their turn removing material from -- and adding value to -- the part.

Positive geometry, with an optimized edge-rounding, will also prevent chip adherence at the point where the edge exits the cut. However, MacNeil noted that tool geometry with a negative land can provide a stronger edge line.

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MacNeil pointed out that higher feeds and depths of cut require a reduced cutting speed. However, these boundaries will change depending on the component material hardness and grain size.

If chipping occurs, the chips will show a smeared, scratchy surface. You essentially can see one scrape for each corresponding chip on the cutting edge.

“If your chips are not glowing orange or white, ceramic milling tools simply won’t work,” said Corneil. He added that making the choice to experiment and implement ceramic milling tools on HRSA applications is an exciting and efficient transition. Due to the increased wear resistance of the ceramic substrate, machine rigidity, fixturing stability, gauge length, and proper parameter selection are of the utmost importance.

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This group of materials tends to create wear on the cutting edges because of material adhesion. However, aluminum often contains silicon, which makes it abrasive in nature.

“I always give a lot of credit to the operators when it comes to diagnosing a problem because they know more about their specific process than we do,” said Vetrecin. “We know how to fix these problems easily once they are identified, but every process is different in terms of fixturing, clamping, rigidity, and coolant use and pressure. There are so many variables. A skilled operator can tell you exactly what does work and what doesn't.”

Choosing ceramics for HRSA helps reduce the time in cut, which can mean significant cost and time savings, which are important because machine times can be expensive in the component segment. Ceramics provide higher metal removal rates and can achieve up to 20 to 30 times the speed of conventional carbide in HRSA.

“Positive geometry inserts typically provide the best results,” said Howard. “These styles generate less tool pressure compared to negative-style inserts.”

A look at the cutting edges, either with the naked eye or, even better, under a microscope, will tell you a lot about the operation. Wear is going to happen. But as long as it’s in the right area of the tool, and it is predictable, drilling operations can be successful.

“Smaller shops that are doing small-batch runs have a lot of variety in their shops,” explained Randy McEachern, product specialist, Sandvik Coromant Canada, Mississauga, Ont. “This lends itself well to using multipurpose drills.”

“What we really want is even flank wear along both cutting edges, because if we are getting a nice, even wear pattern there, then we know that we're going to get some good life out of the tool. It means that we are very close to having the correct cutting data in terms of speed and feeds to allow this drill to produce many holes with good quality,” said McEachern.