Drilling Nonflat Surfaces

Concave, convex, and uneven surfaces all pose unique challenges in holemaking applications

October 1, 2010

When drilling nonflat surfaces, proper drilling techniques, chip evacuation, and coolant usage all affect the success or failure of the job.

As the drill approaches an angle, reduce the feed rate until the central insert is engaged into the workpiece

Manufacturers worldwide are always seeking higher productivity. Only by optimizing production can costs be reduced and profits maximized.

In holemaking operations, drilling techniques, chip evacuation, setups, and coolant usage all affect the success or failure of the job.

“In any drilling operation chip evacuation is key,” explained Sandvik Coromant Canada Product and Application Specialist Randy McEachern.

Chip packing is eliminated and a quality hole created only when the chips travel up the flutes of the drill and out of the hole. However, when an indexable drill drills into a material with an uneven surface, the tool behaves differently than a solid carbide drill, so considerations are different for the two types of drills.

Depending on the severity of the uneven surface, for example a 10-degree versus a 30-degree angle of incline in a concave surface, the drill will begin to wander off center, causing the hole to become oversized and out of position.

“If the drill is a solid carbide drill, there is a risk of breaking the drill because of the vibrations and wandering,” said McEachern.

With a solid carbide drill on a concave surface, the radius of the surface needs to be large enough to ensure the corners of the outside diameter of the drill do not hit the material before the centerpoint of the drill.

“You need to have the point engaged first to make sure the edges do not break off before the hole is even started,” said McEachern.

Drilling in the center of a convex surface usually allows the center of a solid carbide drill to enter the cut first, especially if drilling into the high point of the material.

“Indexable drills, such as Sandvik’s new CoroDrill® 880, have a step technology on the central insert that engages first. This step is what stabilizes the drill and keeps it from wandering,” said McEachern.

Entry and Exit

The drill will acts like an end mill until the two inserts are in the full diameter of the cut. Feed can then be increase to 100 per cent.

A soft entry – accomplished by a reduction in the feed rate -- helps reduce the tendency of the drill to wander and allow the drill to stay on-center. In extreme cases, a center drill can be used to create a pilot hole or an end mill can create a flat surface first. Most of today’s drills, however, don’t need any kind of a center drill or pilot hole prior to drilling on a flat surface.

When the drill exits a part there are similar necessities for feed rate reduction, but they are not as critical because the full diameter of the hole already has been created. However, a soft exit from the material can ensure that hole quality is kept within tolerances, and that vibrations are minimized, prolonging tool life.

Before drilling can even begin, however, for best performances steps need to be taken.

“If the tool is an indexable drill, I always recommend that it is held in a proper drilling holder and not simply loaded into an end mill holder as some people will do,” said McEachern.

This recommendation holds true for a milling machine moving vertically into a part and also for machines equipped with a universal head in which the head has been rotated.

“When drilling on a lathe, the tools will quite often be placed in a lathe bushing for a boring bar adapter. In these cases, it’s very important to have good alignment of the turret, both in terms of perpendicularity as well as angularity,” said McEachern.

Most drilling work done in shops today, however, occurs when the drill is rotating.

“For a rotating application with a solid carbide drill, I always recommend a hydromechanical chuck, such as the CoroGrip® or a hydraulic chuck, like the HydroGrip® is used because it is individually balanced and provides a smooth performance,” said McEachern. “These chucks provide an even clamping force, which means they are clamped symmetrically around the shank of the tool. When the tool is held correctly, tool life is prolonged.”

Chip Evacuation

Correct tool design, flute shape, and the ability to pump coolant through the tool can all help evacuate chips quickly and easily.

As holes get deeper, chips can start to pack into the flutes of the drill. Because they have farther to travel, they become more difficult to evacuate. Deeper holes also produce more chips. The correct tool design, flute shape, and ability to pump coolant through the tool can all help evacuate chips quickly and easily.

The function of the coolant is to aid the tool in the chip evacuation process while also providing cooling properties and lubricity to the cutting edge.

“Because the most important part of the drilling operation is the chip evacuation, through-spindle coolant is recommended,” explained Iscar Canada Rotating Tools Product Manager David Vetrecin. “Also, the higher the pressure, the better chip evacuation occurs.”

At the beginning stages of the drilling operation, small chips are created until the full diameter is achieved. Following that, full-sized chips are created through the rest of the cut. In the hole, however, there is nowhere for the chip to go except back up and out of the hole.

“If there is a problem in the hole, chip packing can occur, and in most, if not all, cases, this will cause tool breakage and can even cause a part to be scrapped,” said Vetrecin. “Chip formation is the biggest enemy in a drilling operation. If chip formation is good, and coolant pressure is adequate, then your drilling operation is likely to be a success.”

The hardness of the material also must be taken into consideration when drilling holes, and cutting feeds and speeds will be affected by the hardness of the material.

“As HRC goes up, speeds and feeds must come down,” said Vetrecin.

Common Drilling Problems

Built-up edges and Chipping. According to Vetrecin, although it’s difficult to tell the difference between built-up edges and chipping because built-up edges can cause chipping, the correction is fortunately the same for both problems: increase the cutting speed. This causes more heat to be put into the chip, because the tool coating is designed to resist heat.

Flank Land Wear. Flank wear is the most desirable type of wear problem to encounter in any cutting situation, not just drilling. This situation can be corrected simply by reducing cutting speed.

Chisel Wear. If wear is apparent on the chisel of the tool, a reduction in the feed rate can remedy the problem.

Flute Wear. Much like flank wear, if wear is seen on the flutes of the drill, speed should be reduced.

Breakage. Catastrophic failure of a drill is usually caused by chip evacuation problems such as chip packing in the flutes. Not only will a broken tool cause downtime, it also can cause the part to be scrapped. Coolant should be checked in these instances to make sure it is still flowing.

Oversized Hole. Drilling in a lathe (when the tool is not rotating) and drilling in a mill are two completely different operations. On a milling machine, oversized holes are most often caused by tool runout. Using a better-quality toolholding system can correct this. If the oversized hole has been created in a turning environment, misalignment between the turret and the center of the lathe usually is to blame. A realignment of the turret may be necessary in this situation.

Monitoring the process, especially the spindle load, is also important in a drilling operation.

“Fluctuation in the spindle load can indicate a problem, especially if it is increasing steadily,” said Vetrecin.

After Drilling

MultiTAP can produce threads within both 2B and 3B classes of fit, eliminating the guesswork of calculating H-limits.

Just as the selection of the correct drill, speed, and feed rates is important, it is also important to select the right tapping tool if a tapping operation is performed following hole creation.

“In the hole preparation for tapping, the correct drill size is essential as it established the minor diameter of the thread,” explained Emuge Corp. Technical Director Alan Shepherd.

Material type and material hardness also determine tapping speed and tap life.

“This is one of the first questions we ask when recommending a suitable tap,” said Shepherd.

Chip control in tapping, much like in drilling, is extremely important in determining the life of the tool. Chip control also affects tapping speed and thread quality.

Speeds and feeds in tapping can quickly become complicated because taps can be used on both simple manual tapping machines and sophisticated CNC machines with rigid (synchronous) tapping cycles. Whether the tapping operation is manual or CNC, the lubrication quality of the coolant is essential to ensure good tap performance and therefore good internal thread quality.

“We can tap with minimum quantity lubrication (MQL), which requires taps specifically designed for this, and the use of modern machine sand tapholders,” said Shepherd.

This is just one example of how taps are evolving to improve threading operations.

For more information, visit www.coromant.sandvik.com, www.emuge.com, and www.iscar.ca.