Boring Big: Drilling Performance

Horizontal boring mills are behemoth in size and flexibility.

If machine tools were dinosaurs, horizontal boring mills would be brontosauruses. Even the smallest of these brutes have axis travels best measured in meters, and spindle horsepower sufficient to drive a small car. But what is a boring mill? Isn’t it just a gigantic horizontal machining center?

Courtesy of PAMA

A Speedram machine drilling mounting holes on a large housing.

Boring mills do share many of the same characteristics as HMCs. They have X, Y and Z axes, are frequently equipped with rotary tables for multiple-sided machining and generally have large-capacity tool magazines. But the defining characteristic of a boring mill is its bar spindle, or W-axis.

Similar to a quill, the bar spindle is an extension of the Z-axis. With conventional machining centers, the ability to reach deep into a workpiece is limited by the size of the spindle housing and the Z-axis travel. Manufacturers can sometimes get around this by using an extended-reach toolholder, but this can cause chatter, tool wear and loss of accuracy.

On a boring mill, however, once the Z-axis, or ram, is in position, the W-axis can extend to machine features that would otherwise be inaccessible—without the need for long-reach toolholders.

Also, a bar spindle is inherently more rigid and accurate, according to Bob Conners, vice president of sales and marketing for United Precision Services Inc., Cincinnati. “The W-axis allows for better access into tight areas, greater rigidity as well as overall parallelism of the spindle to the machine axes,” he said.

Parallelism is key. Conners noted that the fixed spindles on traditional machining centers are relatively short. Of course, builders strive to perfectly align those spindles, but depending on a number of factors, including less than perfect installation, improper machine maintenance and wear, slight misalignment may occur. “But in a boring mill, a spindle might be 10 ‘ long,” Conners said. “Compared to a traditional machining center, the spindle on an HBM has wider support and a shorter lever arm. This makes it easier for the builder to dial-in and maintain proper alignment.”

He should know. United Precision represents five brands of machine tools, and all of them are big. Union, United Precision’s boring mill line, claims to be the oldest machine tool builder in Germany, being in business more than 150 years.

Courtesy of Union

A twin-table Union machine performing end work on a pump housing.

Like most boring mill builders, Union offers several iterations of its equipment, each suited to a particular type of work. A table-type boring mill uses a conventional compound-saddle design similar to most HMCs, but with the addition of the W-axis. These machines are for smaller workpieces, but take that with a grain of salt: You can still machine a block of steel the size of a Smart car on one of these machines.

“In general, the limit for a table-type design is a 100″ cube weighing 30 tons,” Conners said. “But you still need to consider how much of the load is going to be hanging out beyond the support of the guide ways.”

For anything bigger than a 100″ workpiece, a T-style, or planer, machine should be considered. In this design, there is no saddle—the X-axis is separate from the rest of the machine and slides on its own set of guide ways, making heavy loads less of a consideration. The column, which contains the spindle as well as the quill, rides on a perpendicular set of guide ways, providing Y- and Z-axis movements.

It’s like a car wash. You drive your car onto a track (the X-axis) while the spray head moves up and down the rocker panels (the Y-axis) and across the hood (the Z-axis). And this car wash is even equipped with a deep-clean W-axis, for getting inside those tough-to-reach wheel wells.

Planer-style boring mills, because they do not rely on a saddle design, have higher weight capacities and X-axis travels than their smaller-table brethren. Think large equipment frames and housings or components for earthmoving machines, parts up to 20 ‘ long and weighing as much as 60 tons.

Courtesy of PAMA

Finish machining a machine casting on a PAMA traveling-column machine.

Looking to machine something bigger, say the size of a railroad trestle? For mammoth parts, you’ll need a floor-type boring mill, or traveling-column machine. With virtually unlimited X-axis travels and weight capacities limited only by the foundation on which the machine sits, there’s not much these big guys can’t handle—aircraft frames, rock crushing equipment, military products and even other machine tools.

As its name implies, the business part of a floor-type machine—the traveling column—moves the entire length of a fixed bed, which is typically sunk into the shop floor. And because the bed is at floor level, you don’t need to climb a ladder to check a cutter or measure a just-milled counterbore. You can even drive a forklift on to the bed to load and unload workpieces.

Monster Builders

There are a number of horizontal boring mill builders. One is the Italian company PAMA s.p.a., which has been supplying horizontal boring mills for more than 80 years. According to Sergio Scotti, general manager for PAMA Inc., Elgin, Ill., the company tends to cater to larger corporations such as GE, Caterpillar and other companies in the energy and mining industries, but sells machines to smaller companies, including machine shops.

Courtesy of United Precision

Contouring a steel casting with an articulated head.

PAMA offers two horizontal boring mill versions: the floor-type Speedram and the table-type Speedmat. Each machine comes in a number of configurations, but all have cast iron construction throughout and offer spindles rated from 70 to 250 hp. PAMA can also equip its machines with pallet changers, rotary tables with capacities up to 600 tons and swappable multiaxis, programmable indexing heads.

While Grandpa probably never had them on his boring mills, programmable indexing heads give the ability to rotate the cutter in multiple directions. The benefit is obvious—an indexing head lets a user machine up to five sides of a workpiece without repositioning. They also make it possible to drill angled holes or reach inside a workpiece to mill internal features.

A more-costly full-contouring version permits milling of complex 3-D surfaces, such as those seen in die and mold work. Better yet, many builders offer an automatic head-changing option, which permits faster changes between different jobs. Said Conners of United Precision, “There are a number of options, including number of axes, attachment methods, motor type and through-spindle coolant. You might spend anywhere from $30,000 all the way up to $400,000 on a spindle head.”

Another builder offering this technology is Soraluce. A member of the Spanish Danobat group of machine tool builders, Soraluce was founded in 1962. Steve Richards, sales director of Soraluce America Inc., Rockford, Ill., said: “An articulating head makes a really big impact in reducing setups, due to the ability to rotate the head to whatever angle is needed. This also reduces the expense of multiple fixtures. And with today’s CAD/CAM systems, it’s much easier to program up to seven axes than it was in the old days, when programs were written by hand.