The Knee Mill: Still Standing

The design of the knee mill goes back to 1936, when Rudolph Bannow’s company produced a knee-and-column vertical mill with a rotating turret and sliding-ram head. The mill became known by the company’s name—Bridgeport—which, like Kleenex, became the generic signifier for mills of similar design.

Eighty years is a good run for a single design. But now we often hear that the venerable knee mill’s time must be closing. Newer, multiple-axis CNC machines that can do much of what the knee mill can—and more—will fill its niche.

But that hasn’t happened—at least not yet. The knee mill remains popular with many manufacturers. Recently, one builder of knee mills has even seen a noticeable uptick in sales.

Why does the knee mill still stand tall?

Machinist Bruce Lloyd sets up an operation on one of the two Clausing manual knee mills at Munn Manufacturing. Photo courtesy of Munn Manufacturing.

Simplicity and Speed

Munn Manufacturing, Freeport, Mich., uses 12 CNC mills and 11 CNC lathes for complex tooling and production runs for aerospace, heavy truck, automotive and aftermarket work. Started in 1963, the company has sections for production machining, tooling and pipe bending. About 2 years ago, the company purchased two Clausing manual knee mills “for use on jigs, fixtures and the like,” said owner Steve Buehler.

“Sometimes it’s just easier to put a part into a knee mill, pop in a couple of holes or tap something, make a little cut in it and you’re done.” Buehler contrasted that with the longer work of programming those tasks on a CNC mill. “Sometimes the knee mills are just easier for us. That’s why we keep them here.”

Kevin Mungovan, president of Clausing Industrial Inc., Kalamazoo, Mich., thinks cost is an important reason why the knee mill remains popular. “Why pay more to drill a hole than you need to? You could put a 2-axis control on a knee mill, take the part off the $100,000-plus machining center and put it on a $30,000 knee mill. Let it do what it does well—drill the bolt pattern. Let that other machine do what it does best—make chips,” he said. “We’re seeing more applications where the knee mill provides support for a CNC machining center. Or it is used for a second operation.”

That’s in line with how Munn Manufacturing uses its knee mills, according to Buehler. “Suppose a run on a CNC machine is from 30 minutes to an hour,” he said. “While it’s going on, our machinist can make other parts on a knee mill—or on a manual lathe, if that’s what is needed—instead of trying to program the tasks into a CNC mill or lathe.

It all began with the Bridgeport, back in 1936. This is the newest iteration of the Bridgeport Series 1, made by Hardinge. Image courtesy of Hardinge.

“It’s absolutely easier,” Buehler added. “You don’t have to write code or set up tools on the CNC mill. With the knee mill’s collet, you can just put in a drill or tap or endmill. We get the throughput we’re looking for.”

This fits with what George Young, sales manager for knee-mill builder Kent Industrial USA Inc., Tustin, Calif., sees in the industry. He said manual knee mills “are still popular in most toolrooms, most machinists are familiar with them,” finding them much faster and easier than a “full-blown” CNC machining center for setting up and machining simple parts.

Kent has provided many multiple-machine installations for large companies, such as Plantronics, Gulfstream Aerospace, Honeywell, Schneider Electric and Magna, in addition to selling knee mills to many small- and medium-size job shops.

Kent makes a CNC version of the knee mill, as do others. The Kent version features “an easy-to-use, conversational control that’s ideal for small shops learning or getting into CNC machining at a more affordable starting price,” Young said. “They are also handy for full CNC shops when doing a second operation, prototyping or quick, small-lot-quantity part runs.”

Another benefit for small shops: “Many of our models can also run on single-phase power, so they can be put into a ‘garage’ shop for someone who is just starting out,” Young said.

Wide Open Spaces

Young pointed to the traditional advantages of the knee mill’s design to explain its popularity. “The tiltable head, movable ram and turret make the machine versatile and allow for a flexible work envelope,” he noted. “Its ‘open’ nature means an operator has easy access for quicker setups.”

Dickinson Manufacturing Solutions, Madison, Wis., takes full advantage of that open design. The company, which works with materials from plastic to aluminum, steel and titanium, employs six machinists on a single shift who use eight Haas CNC machines and a Clausing manual knee mill, purchased in 2011.

“We do everything from work for engineering firms selling specialized equipment to the oil and gas industry,” including prototypes and small-lot first runs, said founder Keith Dickinson. “We do work for the biotech industry, repair work for local companies like DuPont and low-tech work, such as all of the tap handles for startup Wisconsin Brewing Co.”

Between a manual knee mill and a multiple-axis CNC mill is a thriving niche for a knee mill set up for CNC programming. This is the Kent 4VKF CNC knee mill. Image courtesy of Kent USA.