Multipurpose drills move more metal

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

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.

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.

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.

Other factors to keep in mind are workpiece material and the number of holes being drilled.

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

Job Shop Strategy

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.

“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.”

However, the machine and its setup matter.

“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.”

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.

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

“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.

Using Multipurpose Drills

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

1. During short production runs of non-repeating parts.
2. Drilling holes with small length-to-diameter ratios.
3. Producing large holes when dedicated solid tools are not cost-effective.
4. Making holes with relaxed diameter tolerances.
5. Preparing a hole by pre-machining it before another operation like boring or reaming.

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

Avoid Use in Wonky Setups

It’s also a good idea to avoid using these tools if there is poor runout on the machine spindle.

“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.

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.

Just as no machine shop is identical to another, no two shop’s tooling usage is exactly the same.

“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.

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.

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

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

“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.”

Raise Your Grades

The biggest effect that a grade has in the cutting process is on tool life.

“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.

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.

“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.”

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.

Wear Modalities

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

ISO H (hardened steels between 45 and 65 HRC)

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.

ISO K (cast iron)

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

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

ISO M (stainless steels)

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.

ISO N (non-ferrous metals, including aluminum, copper, and brass)

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.

ISO P (steel)

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.

ISO S (heat-resistant nickel- and cobalt-based superalloys and titanium-based materials)

These materials create BUE and often work-harden.

It should be noted that use of multipurpose drills usually is not recommended for ISO S materials.

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

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

“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.”

“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.

Then it’s time to examine your chips and tool.

Looking for Wear

Examining the centrepoint will show chisel wear.

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.

“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.

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.

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

“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.

Solving the BUE problem is a relatively straightforward process.

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.

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.

“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.”

Chipping also can be solved by increasing the surface feet per minute.

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

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 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.

“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.”

Coolant Deployment Strategy

It’s no secret that as holes get deeper, coolant delivery becomes more important.

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.

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.

“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.”

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.

“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.

Coolant choice typically is a byproduct of material use, too.

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.

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.

“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.”

Vetrecin’s rule of thumb: The more, the better.

Editor Joe Thompson can be reached at jthompson@canadianmetalworking.com.

Horn USA, www.hornusa.com

Iscar Tools, www.iscar.ca

Sandvik Coromant Canada, www.sandvik.coromant.com