The Deep and Short of It
Drilling is one of the most common methods of metal removal. But as with any manufacturing process, there are a number of factors that can affect quality. When it comes to making the perfect hole, considerations range from hole depth and correct workpiece setup to coolant qualities and chip evacuation methods.
Normally, the first thing to consider when planning a drilling job is the print tolerances, says Karen Tucker, product manager of drilling at Carboloy Inc. (Warren, MI), a metal cutting tools vendor. What are the dimensions of the hole? What are the surface finish requirements? "There are a variety of types of drills that you would select based on those requirements," Tucker says.
One key determinant in drill selection is whether the job calls for short-hole or deep-hole drilling, notes Tony Yakamavich, senior product specialist at Sandvik Coromant (Fair Lawn, NJ), a materials technology company. Industry definitions vary, but Sandvik defines a short-hole job as one in which the depth of the hole is less than or equal to five Arial the diameter of the hole, says Yakamavich. Anything deeper is considered to be deep-hole drilling, he says.
Short-hole drills provide the best penetration rates, but they do not supply a qualified surface finish or good hole tolerance, says Yakamavich. "They're basically just to put the hole in. Finish and tolerance would have to come later with another type of tool."
Deep-hole tools, by contrast, "are qualified to produce excellent surface finish and excellent hole tolerance," Yakamavich says. Because of those features, in fact, many of Sandvik's customers rely on deep-hole drilling equipment for short hole drilling, as a way to obtain superior surface finish and hole tolerance, Yakamavich notes. In many cases, this approach can save money in the end, he says, because it eliminates the need for subsequent hole finishing operations.
Setup and workholding
The stability of the setup and the workholding also plays a significant role in hole quality. "As far as your setup goes, it's not only the clamping of your material, but also the total runout of the drill in your holder," says Tucker. "Correct setup ensures better quality."
In many instances, Tucker says, shop personnel assume that a bad hole must be the result of a bad tool. But often, the problem is instead caused by an incorrect setup. The importance of correct setup in achieving high quality drilling results is often overlooked, she says.
Yakamavich agrees. "No matter how rigid or accurate the tool is, the setup plays a very important part," he says. "Setup and choosing the correct drill go hand-in-hand."
Setup and workholding affect quality in many ways, Yakamavich continues. If the workholding device is not rigid, for example, it can not only negatively affect the hole tolerance and surface finish of the component, but it can also shorten tool life, he points out.
Workpiece alignment is equally important. "If we see that we have experienced centerline deviation, we know that one or two things could be out of alignment," Yakamavich observes. "One could be the spindle of the machine," he says. But most of the time, it is the work that is being held that is out of alignment, he notes. And the resulting eccentricity leads to quality problems.
Other factors that play a big role in hole quality are coolant and chip evacuation methods, says Tucker. In addition to preventing overheating of the work area, liquid coolants are also needed to flush the chips out of the hole. "Sometimes it's the concentration of the coolant mixture itself that makes a difference, as well as the lubricity," she explains. Tucker suggests that if an application seems to be absolutely perfect--the parameters have been checked, the setup has been checked, the material is without problems--and things still don't work, then checking the coolant and improving the concentration could make a difference. "That's something we generally don't think about when we drill."
If the requirements for the hole have a surface finish tolerance or limit, the way in which chips are transferred away from that hole can affect quality. In short-hole drilling, the coolant is often fed through the center of the drill forward, says Yakamavich, and the chips are evacuated externally through troughs or flutes in the drilling tool. The result is that the chips have a tendency to hit the wall of the hole, and can cause surface finish problems, he says.
But in deep-hole drilling, this is typically not an issue, because the chips are extracted internally, says Yakamavich. The Sandvik product line, for example, features single tube and double tube versions for internal evacuation. With these tools, the coolant is forced in around the drill and brings the chips back through the center of the drill to be exhausted at the spindle or through the machine.
While external chip evacuation can negatively affect hole surface finish, however, that is not always an issue. "If you're doing a finishing operation with a real tight surface finish requirement, then it's a consideration," says Tucker. But it depends on the job requirements. "If you're going to do another operation after drilling, such as reaming or boring," she points out, "then it doesn't really make a difference."