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Why Not Rotary Broaching?

by Matthew Jaster, Associate Editor

Broaching has always been a numbers game. The ability to manufacture a large amount of automotive components, for example, was an important part of the technique's appeal in the early days. Today, broaching machines (horizontal and vertical) boasting advanced controls and CNC technology have attempted to take broaching into mainstream manufacturing and have done so with some success. This, however, has been primarily accomplished in linear broaching where a broach is run linearly against a surface of the workpiece. Rotary broaching is a different technique that offers its own advantages and disadvantages.

The two necessary components to achieve the rotary broaching process include a rotary broach tool holder and a rotary broach, according to the Polygon Solutions website. The principle that makes this process work is the 1 angle of the cutting tool to the centerline of the workpiece. This causes the rotary broach to shear into the workpiece, with a chisel or scalloping effect, as the broach is fed into the part to the depth required. The rotary broach tool holder has a live spindle that holds the broaching tool, allowing the spindle to spin freely within the tool holder. In a lathe, the spindle is driven by contact with the rotating workpiece.

Rotary broaching produces polygon and gear-shaped forms, usually holes, in machined parts. A rotary broach is fastened into a broach holder spindle, and 'wobbles' as it is fed into the hole at the desired speed and feed. The result is a hole with the same profile as the broach. "This method is commonly used in the gear industry to create involute splines in small lots of parts where grinding is nearly impossible, and EDM is too costly," says Peter Bagwell, product engineer at Polygon Solutions. "Rotary broaching is ideal for aluminum, brass, mild steel and some plastics. Broaching tougher materials like stainless steel, titanium, Inconel and other materials is possible; however, tool life is substantially reduced. Polygon Solutions continues to research materials and broach construction methods to help machine tougher materials."

Rotary broach tool holder

A Polygon Solutions GT Series Adjustment Free Rotary Broach Tool Holder (all photos courtesy of Polygon Solutions).

Application examples of rotary broaching include small orthopedic fasteners (bone screws) in the medical industry, shafts with splines or serrations in the automotive, bicycle, motorcycle and aftermarket industries and complex connectors that need to be machined in the aerospace and electrical industries. "Polygon Solutions redesigned the traditional adjustment free rotary broach holder and alignment rod patented in 1976 by Rinaldo Mucci and Paolo Bremi. Polygon's GT Series includes a pressure relief hole and sealed bearings for grease free maintenance."

For customers concerned with hydraulic pressure building up inside of blind holes, the pressure relief hole allows fluid and air to escape without sacrificing the integrity of the broach or the holder. "This also allows the operator to use cutting oil or coolant freely. The sealed bearings also help the operator, eliminating the need to monitor the broach holder and refill the leaking grease chambers. The environmentally clean design also eliminates messy grease guns and surrounding contamination," Bagwell adds.

Changing the Perception
Bagwell says that because the broach is held on a one degree angle, many machinists assume they have to set up the tools at one degree, or that the tools cut oversize or undersize. These rumors contribute to the belief that broaching is a 'black' art. "Continuing to patiently educate customers has been the most effective way of countering this perception," Bagwell adds.

All of Polygon's product data is maintained on the website including a rotary broaching forum and blog to reach out to potential customers, solicit their comments and display the company's manufacturing capabilities. "Social media plays a small role with the exception of YouTube," Bagwell says. "Directing customers to watch YouTube videos gives them insight into how other people are doing rotary broaching, what machines rotary broach tools can be used on and what tools are available in the marketplace. There's also quite a few websites where amateurs are making their own rotary broaching tools. At first, this may not seem like a great marketing tool, but it does 'spread the word' about rotary broaching and helps future customers see the value of our easy to use tooling."

Rotary broach tool holder

Custom rotary broaches are made by Polygon Solutions Inc. to fit most standard rotary broach holders and may also be used as punches or shaping tools.

All of these marketing initiatives have played a significant role in educating the manufacturing community on rotary broaching. Much like many machining operations, it really comes down to what it can and can't do for potential customers. "The most common advantage to rotary broaching over every other method is that on-machine broaching eliminates secondary operations," Bagwell says.

Rotary Broaching Advantages/Disadvantages
EDM vs. rotary broaching: While the EDM process includes costly electrodes or labor intensive setup, rotary broaching offers a lower cost per part and a one-time setup.

Conventional broaching vs. rotary broaching: The conventional method requires a costly broaching machine expense, extensive design and engineering and blind holes cannot be broached with a single broach. Rotary broaching offers a low cost rotary broach holder expense, it's easier to design and it can broach blind holes easily.

Grinding vs. rotary broaching: Grinding wheels can't fit into holes while very accurate forms can be rotary broached.

Watts Drill vs. rotary broaching: The Watts Drill leaves round corners in the hole while rotary broaching can produce forms with sharp corners.

Some of the disadvantages of rotary broaching includes a limit to hole sizes up to about 1"diameter and as small as .050" (small rotary broaches are further weakened by the back taper required in rotary broach designs), a limited hole depth, limited to materials with good machinability and limited to forms with small tooth height. "A chamfer is required at the front of the hole," Bagwell says. "Sometimes if the part does not include a chamfer, material is added to the process, chamfered and then removed after broaching."

Rotary Broaching Market
Bagwell believes the rotary broaching industry has ups and downs that seem to follow metal manufacturing trends. "In the end, it comes down to the time and financial savings that the process can offer potential customers now and in the future," Bagwell says. "While we believe additive and other future machining technologies will have a toll on the need for cutting tools like rotary broaches, the relocation of existing advanced lathes and mills to new customers and the need for more batches of smaller customized parts will keep the rotary broaching industry vibrant for years to come."

For more information:
Polygon Solutions Inc.
16770 Link Court, Suite 106
Fort Myers, FL 33912
Phone: (239) 628-4800
www.polygonsolutions.com