by Kip Hanson
When drilling thousands of holes in a workpiece, choosing the right drill is critical to success
Holemaking is familiar territory to most machinists. But imagine having to convert a metal plate larger than a dining room table into a hunk of Swiss cheese. That’s the challenge faced by those working in the heat exchanger industry, who machine parts that are more hole than metal and whose greatest fear is breaking a drill on the 999th hole, possibly scrapping a part that may have cost more than the company car.
Peter Mah understands these challenges; as the planning supervisor at heat exchanger manufacturer Titanium Fabrication (TiFab) Corp.’s St.Laurent, QC, facility, he suggests facing them head on. “As a rule, you want to start off with very aggressive feeds and speeds and then back off to find the stable zone. Of course, you need a rigid setup to do this, and you especially need the right drill.”
As their name implies, heat exchangers transfer heat between two fluids. One familiar example is your refrigerator, which keeps food cool by blowing air over condenser coils, thus transferring its heat to the refrigerant within. The same process is applied in a variety of industrial settings, from petrochemical plants to steam turbines. Many of these rely on shell and tube style heat exchangers to remove heat from waste gas or to keep machinery cool.
Looking like a bundle of straws, shell and tube exchangers are constructed of hundreds or even thousands of tubes contained within a series of metal plates. Each of these plates requires an equal number of holes to be drilled, one for each tube. Because high temperatures and corrosive fluids are the norm for industrial heat exchangers, tough metals such as titanium and duplex steel are often used, although a large number are made of carbon steel, especially those in refineries and other heavy industries.
Most of the holes drilled at TiFab range from 16 mm (0.625 in.) to 38 mm (1.500 in.) in diameter, through material a couple diameters thick. That doesn’t sound too bad except for the fact that a typical plate contains several thousand such holes, and that the recommended cutting parameters for the materials TiFab machines are often wrong. “We discovered that some cutting tool suppliers test their drills using billet material or bar stock,” says Mah. “But the grain structure of forged and plate materials is much different. We broke a lot of drills before we found the right feeds and speeds.”
Keep it consistent
Once dialed in, consistency is the key to making high volumes of holes, Mah says. And much to the chagrin of his tooling providers, he notes that multiple brands of drills are needed when machining a wide array of materials. “We tried to stick with a single supplier for everything but it didn’t work out that way. Carbon steel cuts differently than Inconel, and Inconel isn’t even close to tantalum or zirconium. We ended up with several tooling partners, each with solutions that work best for certain applications.”
One of these is Guhring Corp. in Kitchener, ON. TiFab uses the company’s RT 800 WP series insert drills, a spiral fluted, interchangeable carbide insert drilling system with a self-centering 140° point. Brandon Hull, director of product management at Guhring’s corporate headquarters in Brookfield, WI, says the RT 800 covers a broad range of materials, hole depths and diameters, and should be used with a hydraulic or whistle notch toolholder. “We actually have a newer version of that series–the HT 800 WP–which has a stronger clamping mechanism on the tip and might provide better results.”
For those unfamiliar with interchangeable, a.k.a. replaceable tip drills, Hull says they offer a best of both alternative to solid carbide or indexable drills. Replaceable tips generally run faster and provide better hole quality than indexable drills. They cost a bit more (although far less than carbide) but that’s easily offset by their greater productivity. Also, depending on the diameter, you can re-sharpen them up to three or four times, provided you don’t push the drill past the point of no return.
Randy McEachern, product and application specialist for Sandvik Coromant Inc., Mississauga, ON, says that replaceable tips are a good solution for heat exchanger companies, or indeed for anyone interested in high performance drilling. “We’ve been working closely with this industry and have just released a new series in North America–the CoroDrill 870–that was designed with these customers in mind. Some of our customers are seeing penetration rates of 1,800 mm/min (70 ipm) in low carbon steels.”
As with any machining application, McEachern recommends as rigid a setup as possible. When drilling with an exchangeable tip drill (ETD) such as this, 70 bar (1000 psi) or greater through-spindle coolant should be used, with a clean, well filtered cutting fluid mixed to a concentration of 8 to 9 per cent.
He notes that some heat exchanger manufacturers stack multiple plates on top of one another for drilling. In this case, traditional indexable insert drills are wholly unsuitable due to the slug that’s formed when the drill breaks through the workpiece. This slug can catch or bind between plates, damaging the tool, the workpiece, and possibly the machine.
Putting on the squeeze
Someone who can attest to this is Tony Deschner, machine shop manager at Exchanger Industries Ltd., Calgary, AB. He’s been making exchanger plates for more than 30 years. He’s drilled tens of millions of holes and is well aware of all that can go wrong. When stacking plates, he uses a baffle drilling system from Quickmill (see sidebar on page 34) to draw them together, often drilling stacks up to 300 mm (6 in.) thick. “It works sort of like a drywall anchor, and keeps the plates from moving around while drilling,” he says.
Deschner agrees that high pressure coolant is a must, especially when drilling through multiple sheets, but adds that you shouldn’t skimp on the volume of cutting fluid either. “The most important part of any drilling operation is to get the shavings out of the hole. We typically run around 60 L/min (15 gpm) through the tool. There were a lot of drill failures before we started doing that.”
Interestingly enough, Deschner does not send drill tips out for re-sharpening, saying that the results are too hit and miss. “It depends on the material, the hole size and depth, but we typically get between 46 to 56 linear meters (1800 to 2200 in.) on a new drill tip. A reconditioned one might do that, but it also might fail after one meter (40 in.) and create a lot of rework in the process. It’s just not worth sweating over a $90 tool.” SMT