Irregular drilling demystified

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.

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.

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

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.

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.

Without a spot, a flat-bottom drill can create accurate holes in curved surfaces. Image courtesy of Guhring

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.

“Once that starts,” he said, “the deeper you go, the more pronounced it becomes.”

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.

Split-point drill bits can have four facets.

“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.

Start With Spotting

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.

“This happens quickly,” Hull said, “and the drill can slip off to the side.”

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.

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.

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.

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.

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

So instead, shops may want to consider a triple-margin drill, which has three margins per flute and a 30-degree helix.

“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.”