Making Perfect Holes

While drilling is a commonplace operation in many shops, it is still important to control the machining environment precisely, minimize runout, and use new technology where applicable.

Without a stable, well-maintained machine tool, the quality of a holemaking operation, both in terms of hole quality and placement, will be negatively affected. Specifically, the machine’s spindle must be maintained properly by replacing the bearings when necessary to eliminate vibration. Care must be taken to ensure that runout is not being transferred from the machine’s spindle to the toolholder and then to the tool.

“Runout in the drill itself can often be traced back to the toolholder,” explained BIG Kaiser Precision Tooling Applications Engineer Michael Bojanowski. “If you are not controlling how straight the drill is being held, you can’t expect a straight hole to be produced.”

If runout is uncontrolled, it doesn’t matter how precise the drill is. Spinning a drill with high runout prevents it from cutting in the way it was designed to. Not only does this cause cylindricity and straightness problems, but tool life will be shortened and breakage will occur more often.

“By reducing the amount of runout by half, you can often increase tool life by three to four times,” said Bojanowski. “You will also reduce downtime and the time needed for tool changes.”

Fewer tool changeovers means lower consumable costs and a better overall manufacturing strategy.

Drill Walking

Another issue can arise when drilling into a difficult-to-machine material. At times the cutting forces created when drilling into these troublesome materials are enough to push the tip of the drill away from the material. This causes the drill to “walk” along the surface.

“Drill walking can happen if the tool has an incorrect tip geometry for the material being cut,” said Bojanowski.

If the drill’s point is not correct, the forces that are pushing back on the drill when it meets the workpiece are enough to misalign the drill. Then the drill body will follow, and the hole will not be straight.

“Tool selection, and specifically point geometry, is important,” said Bojanowski.

Some drills are designed to be material-specific. For example, if you want to drill a hole in aluminum, there are drills with point geometries specifically for that application. Every part of the drill from the flutes to the point has been designed to give the best performance in aluminum cutting in terms of  straightness and cylindricity, but also for tool life and the necessary speeds and feeds.

However, working with many different materials can get expensive if drills must be stocked for all of them. This has led to the creation of high-performance drills that are more versatile and can be used for different materials without the need for a tool change.

“These versatile drills are slightly different,” said Bojanowski. “For example, at BIG Kaiser we have designed a four-facet, split-point geometry drill. The unique geometry reduces the load on the drill and therefore minimizes the cutting forces that the drill experiences. The more you reduce those forces, the less stress will be on the drill, and the less likely it is that the drill will walk.”

Another way to eliminate walking is by first spot-drilling the hole. Spot drilling entails drilling a small hole or the creating an indentation in the surface of a workpiece to serve as a centering guide.

However, some newer drills have a self-centering point geometry that allows them to work without a spot drill by being able to center themselves.

The Importance of Toolholding

Toolholding is an important variable in controlling runout. Also, the smaller the hole you are making, the more important toolholding becomes.

“The drill is only as good as the toolholder that you put it in. A new drill in an old or out-of-spec toolholder is a waste,” said Bojanowski. “Not only that, but any runout caused here will become amplified at the tip of the tool.”

Recent trends in toolholding have centered on perfecting collet chuck design.

When a tool is placed in a collet, the collet collapses evenly around the circumference of the tool and centers the tool as it does so. This even gripping force at the tool’s circumference is important in reducing runout.

“This design has been used for many years, and now we are beginning to see refinements,” said Bojanowski. “It is still important for collet chuck users to be able to use different-sized tools and switch them out quickly and easily. It is important to control runout and increase the clamping force on the tool, and this can be done easily with today’s collet chucks.”

Stable Workholding

Another contributing factor to accurate holemaking is workpiece holding and clamping. When metal is being cut, vibrations are introduced. If rigid clamping is not applied, that vibration will transfer through the material and move the workpiece.

“If the workpiece is moving, even at a microscopic level, it can cause premature wear or even failure of the tool,” said Bojanowski.

It’s very important to look at how you hold the piece and to make sure it is stable.

Once the drill is fully engaged in the cut, proper chip evacuation becomes a primary goal. If chips get caught in the hole they may be recut. If the chips get packed in the flutes of the drill, there is nowhere for new material to go, and the tool will break.

Chips that don’t evacuate the hole can get in the way of the drill and bind between the drill and the workpiece. This will lead to poor hole quality and ID surface finish.

“This can also negatively affect cylindricity,” said Bojanowski. “If your tool is dragging a chip, it can easily cause an out-of-round hole to be produced. Making sure that the chips get out of the hole is very important. That’s why drills will have flute geometry based on specific materials.”

Using coated tools is also a good idea. Coatings increase lubricity and reduce friction so the chips can travel up the drill faster.

“You should have a very robust coolant delivery system that ideally transmits coolant through the tool. This allows you to not just cool the work zone, but also increase lubricity at the tool point where it is needed the most, and aid in chip removal,” Bojanowski said.

For more information, visit www.bigkaiser.com.