Rotary Broaching Passes the Test

     The world of precision machining and production of small components continues to move away from classic lathes, screw machines and manual operations. Tools for these techniques may stand the test of time or be replaced by new and improved ones. Many tools continue to be reinvented with coatings, strengthened materials and improved designs. These tools are great for both the classic screw machine and the latest CNC machining center. Rotary broaching tools are one such tool passing the test, expanding their roots in the automatic machining sector, while also being adapted for production on CNC swiss type machines.

     Rotary broaching is a fast, accurate method of producing internal and external forms on the end of a work piece. The entire operation takes seconds, and is capable of producing accurate forms within 0.0005 inches. The most common shapes are hexagons and squares in blind holes, but rotary broaching is also capable of producing various splines, serrations, six lobe shapes and other polygon forms. The practical depth for rotary broaching is up to one and a half times the smallest diameter of the form being broached. This type of broaching can create forms with a major diameter up to 2 inches in aluminum, 1.5 inches in brass and 1inch in steel. Micro broaching forms smaller than 0.050 inch is also possible. Broaches for hexagons and squares are available in a variety of inch and metric sizes.

     One advantage of this operation is that it almost always costs less than accomplishing the same result with a secondary operation on another machine. In some cases, rotary broaching may be performed at the same time as other operations to increase speed without sacrificing accuracy. The increase in profit during production has made rotary broaching increasingly popular in the medical, aerospace, automotive and plumbing industries. Due to the large assortment of rotary broaching tools available for immediate delivery, even small machine shops can afford to switch broaches and create a different polygon form with minimal or no setup.

0700 Series Swiss Type Adjustment Free Rotary Broaching Tool Holder by Slater Tools Inc.

     The latest advancements in manufacturing have affected both rotary broaching tool holders and rotary broaches. Tool holders are now available in compact adjustment free models. Adjustable broach holders have multiple set screw adjustments which can be used to help compensate for any misalignment between the spindle and cutting tools. Swiss type adjustment free tool holders require no adjustment. They are ready to be installed and have grease fittings which are accessible for easy maintenance. This helps swiss type machine operators by reducing the size of tools in the tooling zone, and helps CNC machine operators who do not need to adjust the tooling.

     A second advancement in rotary broaching tools involves the broaches themselves. Due to the increased use of stainless steel, titanium and other difficult to machine materials, such as in bone screws for example, multiple materials are used to manufacture broaches. Standard broaches are typically made from M2 high speed steel, but are also available in PM4 (powdered metal tool steel) and cobalt metals like T15. Although a harder material does not always improve tool life, some operators have been able to increase tool life by upgrading the broach material. If broach wear becomes a problem, coatings such as TiN, TiCN and TiAlN are all available for improving performance.

     Part preparation for rotary broaching is simple. Broach manufacturers can provide suggested cutting speeds, feeds, and dimensions for pilot holes or major diameters. The edge should be broken at 45 degrees to a diameter greater than the largest diameter of the form before broaching. This can be removed later if desired. Enough room should be left in the bottom of a blind hole for chips to accumulate, and a recess at the bottom of the hole allows chips to break clean. Chips can be removed with a boring tool or a flat bottom drill.

     Fluids play a minor role as the amount of heat generated is minimal, any type of water or oil based fluid is acceptable. When broaching a blind hole, fluids should be applied on the broach tip and not into the pilot hole. Trapped fluid may be unable to escape, but can be eliminated by ordering a vent hole in the broach.