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Viewpoints Blog: Rotary Broaching: Getting More from Your Shop Floor

Jeff Tryles







By Jeff Tryles
Director of Sales
Slater Tools Inc.

A fast and efficient machining method, rotary broaching is used for making squares, hexes, splines, serrations, keyways, Torx-type and a range of many other internal and external polygon shapes on a workpiece. If your shop uses machines such as CNC lathes and mills, Swiss-type machines and machining centers, then you already have the capability to use this efficient technique and benefit from it as an alternative to outsourcing or having to use slower and more costly methods of producing a shape in a part.

Small and large machine shops alike, with a variety of customers in varying industries, benefit by having a rotary broach toolholder on hand. When new applications arise, simply switch broaches to run a different form with minimal effort. Rotary broaching has seen increasing demand in recent years from industries such as medical, aerospace, automotive and fasteners where special shapes are machined into various materials. For example, bone screws used in orthopedic implant operations, and aerospace port connectors, are produced quickly and in one pass.

One significant advantage of rotary broaching is that it can be performed on any CNC lathe, mill, machining center or other turning machine. In most cases, rotary broaching can be performed at the same time as other turning operations, increasing efficiency and profitability during production, without sacrificing accuracy. Unlike conventional broaching, where a series of stepped polygon forms are pushed through a hole until the desired size and form is achieved, the rotary broach cuts the full form, one corner at a time.

Additionally, broaching into a blind hole is possible with rotary broaching. Using a toolholder with a built-in one-degree offset (relative to the workpiece center line), a controlled wobble action is created as the broach sequentially cuts from tooth to tooth upon engagement with the workpiece. The resulting benefits are reduced cutting forces, increased tool life, tight tolerances (0.0005"), fast cycle times (1500 rpm and 0.006 ipr on average) and the ability to eliminate secondary operations on another machine.

Because of the free rotating spindle on the rotary broach toolholder, live tooling is not needed to rotary broach. On a mill, the rotary broach toolholder rotates in the machine spindle while the broach and the part are stationary. On a lathe, the toolholder body remains stationary while the free-spinning broach rotates in sync with the part.

The best applications for rotary broaching are small forms (0.028" up to 2.00" diameter) that need to be machined at shallow depths. A general rule of thumb on recommended depth is not to exceed 1.5 times the minor diameter of the form. Materials such as plastics, aluminum, brass, alloys, and stainless steel are ideal for rotary broaching. Difficult materials, like titanium, can also be successfully broached with enhanced broaches in a variety of coatings (TIN, TICN and TIALN) and substrate materials (M-2 HSS and PM-4).

Simple part preparation is the key to successful rotary broaching. First, for internal operations, the part must be pre-drilled and chamfered prior to broaching. It is recommended to ‘open the pre-drill’ larger than the minor diameter of the form to allow for easier cutting, reduced pressure and longer tool life. Additionally, the depth of the pre-drilled hole must extend past the form to leave room for chips to accumulate. When machining a through hole, or if a recess can be added at the bottom of the form, the chips will break off cleanly. If an undercut is not acceptable and the chips must be removed, they can simply be machined off after broaching with a drill or boring tool.

The second requirement for success is to machine a 45° lead chamfer slightly larger than the major diameter of the form. The chamfer helps to guide the broach, ease the cutting forces, and extend tool life.

External operations are similar with respect to part preparation. The outside diameter of the part must be pre-turned below the major diameter of the broach, and a 45° chamfer machined onto the leading end. If the form is the full length of the part, the chips will simply break off. However, if you are not broaching past the end of your part, a back chamfer or undercut is required to break the chips.

With minimal set up time and part preparation, many shops can successfully reduce costs and increase efficiency with rotary broaching. ME


This article was first published as a digital exclusive feature for the December 2013 edition of Manufacturing Engineering magazine.  Click here for PDF

Published Date : 11/22/2013

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