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This tool enables high metal removal rates (MRR) by separating long, stringy chips from deep cavities and dynamic milling paths by cutting the material into shorter, more manageable sections.

Hanita, a Widia brand, has released the VariMill Chip Splitter end mill for dynamic and trochoidal milling operations in steel, stainless steel, and heat-resistant superalloys.

In this case, he recommends the use of an NC spot drill, which produces an accurately located little hole that serves as the starting point for a secondary drilling operation. NC spot drills are stubby and offer good centering characteristics, he said.

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1. Process of locating the center of a workpiece to be mounted on centers. 2. Process of mounting the workpiece concentric to the machine spindle. See centers.

When it comes to irregular surfaces, curved ones are much more difficult for tool designers to deal with than inclined surfaces, said Gary McCarel, senior application engineer at Star SU. Designers can compensate for inclined surfaces by adjusting drill point angles to make sure the point engages with the part before the corner of the drill. This is important because the drill will deflect if the corner contacts the surface before the point is fully engaged. But this is harder to prevent when a surface is curved. Designers can’t always accurately predict the entry position of a drill in such cases, he explained.

“We come up against them often,” he said, “but we have been very successful in developing geometries to counter the situation.”

In addition to flat-bottom drills being a faster, more accurate option for creating pilot holes in many cases, it’s become easier to find the right drill for a specific application.

If a drill is large compared with the diameter of the round part, the drill will walk more than it would if the drill diameter were smaller.

He reports that users have found the SD245A design to be particularly beneficial when exiting holes with irregular surfaces.

Flexible-sided device that secures a tool or workpiece. Similar in function to a chuck, but can accommodate only a narrow size range. Typically provides greater gripping force and precision than a chuck. See chuck.

“We have a great deal of success with it in diesel engines,” McCarel said. “It cuts very straight and very quick, and it’s strong enough that it doesn’t always require spotting on an irregular surface.”

Strip or block of precision-ground stock used to elevate a workpiece, while keeping it parallel to the worktable, to prevent cutter/table contact.

William Leventon is a contributing editor to Cutting Tool Engineering magazine. Contact him by phone at 609-920-3335 or via email at wleventon@gmail.com.

On the downside, using a spot drill or an endmill to prepare the way for drilling of irregular surfaces adds both a tool and a tool change to the process. One result is additional cycle time that may be problematic in some manufacturing situations, Dunneback said.

In almost all situations involving irregular surfaces, McCarel recommends reducing the drill feed rate on entry to 40% to 50% of the normal feed rate until the margins are fully engaged, which normally occurs at a depth of up to twice the drill diameter. But during this “soft start,” drill speed is not reduced.

Star SU doesn’t sell off-the-shelf tools for regular or irregular surfaces. Instead, the company uses CAD software to design custom tools for specific applications. To start with, the company requests part prints from customers “so we have a theoretical idea of what we’re up against,” Dunneback said.

Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

Hull said walking is less of a problem if the drill diameter is small compared with the diameter of the round part being drilled. This is especially true, he said, when the drill bit is stubby and has good centering characteristics like a split-point drill. Most high-performance drills have a split point, which features two additional edges ground into the chisel edge, he said.

Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.

He reports that flat-bottom drills are becoming a more popular choice for paving the way for deeper drilling on irregular surfaces. As for why, he points out that endmills used for this type of work are offered only in certain fractional sizes, such as 3.175 mm (0.125") and 6.35 mm (0.25").

Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.

However, Hull said a spot drill works only in situations in which the hole to be drilled is right down the center of a round part. For off-center holes in round parts, he said one option is to use an endmill to produce a round, flat starting point for a subsequent drill. The flat starting surface helps keep the forces on the drill vertical rather than lateral.

The 5-flute chip splitter is suitable for steels and the 7-flute chip splitter is suitable for stainless steel and heat-resistant superalloys.

“People are usually breaking drills coming out of an irregular exit,” Lenz said. “But we put this drill in there, and it solves the problem.”

“Four-facet drill points center a little better than all the other drill points we have seen,” said Manfred Lenz, product manager for holemaking and tooling systems at Seco Tools LLC in Troy, Michigan.

“I think because of all the potential sizes a customer may need, there weren’t a lot of off-the-shelf flat-bottom drill options” in the past, Hull said. “But in the last five years, we have started to see companies come out with standard offerings you can pick out of a catalog on a more regular basis.”

Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.

Beginning a drilling process with an endmill can serve another useful purpose. Scale that builds up on a cast-iron surface during the casting process is harder than the inner material and difficult to penetrate, McCarel said. An endmill, though, can break through that scale and also create a flat area suitable for drilling on a curved or an inclined surface.

Any controlled equipment that allows an operator to program its movement by entering a series of coded numbers and symbols. See CNC, computer numerical control; DNC, direct numerical control.

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As for the machine, “this means the gibs and ways are all nice and tight, so there is not going to be any movement while you are drilling,” he said. This can be true of new machines, as well as older machines that have been maintained well.

These tools are available in 5- and 7-flute configurations from ½ to 1 in. standard dimensions in both WP15E and WS15PE grades. The end mill can cut up to 4.5xD.

To minimize drill deflection, Hull suggests dropping the feed rate by 50% to 75% compared with the program feed rate when entering and exiting an irregular surface. For the same reason, he recommends that the entire drilling system — including the fixturing, toolholder and machine — be as rigid as possible. Instead of an ER32 collet that gives, for example, a hydraulic or shrink chuck would be a better choice of toolholder in these cases.

Drilling holes in parts becomes a bit complicated when part surfaces are irregular in some way — curved, inclined, uneven, etc. But armed with a little knowledge about the subject and the right equipment, shops can complete the task efficiently and to the satisfaction of customers.

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Included angle at the point of a twist drill or similar tool; for general-purpose tools, the point angle is typically 118°.

McCarel said if a tool deflects when entering an irregular surface so that it is not parallel to the axis of rotation, the tool will break.

Process of generating a sufficient number of positioning commands for the servomotors driving the machine tool so the path of the tool closely approximates the ideal path. See CNC, computer numerical control; NC, numerical control.

Featuring four facets, the new Feedmax MS drill point is suitable for aerospace applications in which centering is important. Image courtesy of Seco Tools

An endmill can be used to spot an irregular surface and prepare it for a subsequent drilling operation (left). Once the endmill has created a pilot hole, the drill has a flat surface to enter (right). Image courtesy of Star SU

“You have to run at the rpm recommended for the application to have the angular momentum required to shear the material,” he explained.

A variation of the flat-bottom drill designed to be helpful on irregular surfaces is Seco Tools’ Feedmax SD245A. Lenz describes this solid-carbide tool as “partially a flat-bottom drill” because the drill point is flat except for a very small 140-degree tip in the center. The SD245A also features four land margins — cylindrical shapes ground near the cutting edge for guiding purposes — as opposed to just two on standard drills. Two land margins aren’t enough to guide a drill on an irregular surface, “so it’s going to walk, and it’s going to break,” he said.

When entering an irregular surface like one that is curved or has a good deal of variance due to scale, a drill can “walk” because it is subjected to lateral forces, as well as the normal axial forces, said Brandon Hull, vice president of product management and business development at Guhring Inc., a tool manufacturer in Brookfield, Wisconsin. This can cause breakage, particularly if the drill is made of carbide, he said. Even in the best-case scenario, he said the result will be some deviation in hole position and diameter.

Irregular surfaces are a fairly common feature of parts for a variety of industries, including oil and gas, aerospace and automotive, said Jamie Dunneback, North American round tool sales manager at Star SU LLC in Farmington Hills, Michigan.

Endmills “don’t give you the exact diameter you need for in-between drilling sizes,” Hull said. As a result, “you often have to do a circular interpolation to get the correct diameter to properly pilot a drill. So a lot of people are moving to flat-bottom drills that can give them the specific diameter they need for a hole.”

This end mill is designed with non-overlapping chip splitters along the flute cutting edges. The result is more efficient MRR, while maintaining the ability to make finishing passes. The unequally spaced design feature allows each sequential cutting edge to remove any remaining material left by the chip splitter of the previous flute. The tool’s design reduces breakage and makes it versatile enough for both roughing and finishing passes.

Another option that can eliminate the need for a spot drill for irregular surfaces is a flat-bottom drill. It can create flat-bottom holes because it has no point angle, so it is actually flat at the top. Hull said this geometry can drill accurate holes in curved or angled surfaces without a spot.

In these cases, however, the drill needs to be short because side deflection will increase as drill length increases. To drill deeper holes on irregular surfaces, Hull said shops can start the process by drilling a pilot hole with a flat-bottom drill, which will leave a round, flat surface for a standard drill to finish the job.