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Broaching fires up competition.

Broaching continues to be one of the more effective metal removal processes being used today, especially in quality- and production-conscious applications, such as for automotive powertrain components, transmission gears, and shafts.

It can produce splines, gear teeth, cam surfaces, special radius lobe forms, flats, keyways, and many other special shapes and forms both internally and externally in alloy steel, mild steel, malleable and cast iron, and various nonferrous metal components.

Broaching is a manufacturing process in which a single round or flat tool has many individual cutting surfaces arranged in such a way as to successively rough-cut, semi-finish-cut, and finish-cut a workpiece as the broaching tool is pulled or pushed around or through the workpiece. Finished internal or external involute splines and special forms are thus produced in either helical or spur configuration in one pass.

The cutting action for a single point on the broach may be likened to that of a shaper or planer. In fact today, broaching is making its greatest inroads competitively against shaping and hobbing.

The reason for these advances is that broaching as a process provides high production rates with low operator input requirements, repeatable high accuracy, low cost per piece, and high finish quality.

Broaching has provided solutions to some of the stickier production problems in the automotive industry. For manufacturing a hub synchronizer--formerly machined as two parts--attaching the hub to the shaft prior to pot broaching makes it possible to guide with the broach tooling on a closely ground bearing diameter, which is an integral part of the shaft. The splines and slots are then broached on the hub concentric to this diameter and in perfect alignment. Previously, broaching the parts alone and mounting them on the shaft often didn't produce a good match up of hub and bearing.

In this instance, two separate parts and two separate machines and tools are involved. One part required 27 involute splines (30 index), 11 diametral pitch, 30 deg pressure angle, 2.7272 pitch diameter, and 1.150" length of cut in 1053 mild steel. The second part involved 24 involute splines on a 1.020" length of cut in 1153 steel.

For cutting a blind spline, an eight-station automatic load and unload machine is used. Because the largest outside diameter of the piece is greater than that of the spline, the broach has to stop dead after cutting the spline and retrace its steps. The part involved 74 involute splines, 1.00 module, 45 deg pressure angle, 2.9134 pitch diameter, and a 0.366" length of cut in 1022 steel.

Producing the precision broaching tools themselves is a time- and manufacturing-intensive operation. This requires carefully monitored green machining, heat treating, grinding, and EDM wirecutting--all under controlled environmental conditions. The broaches, which may reach 9 feet in length and weigh more than 2000 lb, can have as many as 20,000 cutting teeth on a full-form broach assembly. The investment in grinding time alone can run to three weeks of two shifts to grind involute gear teeth on a broach.

Heat-treating is essential to providing control of hardness, wear life, and quality for such tools as round broaches, pot broaches, internal spline and running gear broaches, broaching lead bars, and related wear components.

Heat treating is also necessary to stress-relieve the broaches and can be performed as many as five times during the broach manufacturing process at National Broach to ensure broach tool stability. At National Broach & Machine Co's facility, computers control exacting furnace temperatures to monitor broach tool quality. All broaches are heat treated in special vertical furnaces to eliminate internal stresses and sagging.

Internal broach tools up to 20" in diameter and 150" in length are manufactured and heat treated by the company. Oil bath quenching produces the fiery show seen here.

Recent advances

New developments in broaching technology are extending its uses. Introduction of National Broaches' Red Ring workpiece transfer vertical broaching machine, for example, answers one of the familiar objections to broaching operations: i.e. the need for the operator to work on a platform or for the manufacturer to dig a pit to accommodate the long broaching tool. Since no pit is required, line change is easy, and the lower working position facilitates operation and maintenance.

Pot broaching is a favorite of the automotive industry--especially for front-wheel drive transmission gears--because of the high-production rates of up to 500 or more pieces per hour that are attainable. The process is rapidly becoming a viable alternative to gear hobbing in producing external spur gears, involute splines, and other special and interrupted tooth configurations because of the speed and tooth-finish that is possible.

Pot broaches are used to produce external forms on parts that are passed through the pot, which holds ring-type and/or stick-type cutting tools. A round blank is loaded on top of a hydraulic cylinder and is pushed through the mounted pot, producing external shapes in the form of the spline, involute gear teeth, slots, cam surfaces, and now sprocket teeth.

The addition of a series of individual precision, high-speed steel annular rings or wafers behind the sticks and holder bushings in the main pot shave the entire form, i.e. the involute flanks, the full-filet, root-radius tangent to these flanks, and the root diameter of the parts.

Finish pot broaching of sprockets represents a significant capital equipment saving over the present accepted process, says National Broach. It can take up to ten shaping machines to duplicate the output of one pot broach machine. Up until recently the process wasn't effective in producing the sprockets, but wire EDM and precision grinders have made it possible to produce the intricate shapes that are needed for the application.

Also National Broach is manufacturing an internal, helical-toothed, cutting shell with spiral tooth gullets to be used on a pot broach machine, which will then produce a part with external helical broach teeth through a very short length of cut.

A totally new design innovation in the pot broaching machine itself is aimed at opening it up to still more applications. The new design would make quick change pot broaching a reality. The benefit would be that pot changeover time, which can run from three to four hours, could be accomplished in ten to fifteen minutes. Several different parts could be broached in small runs on the same machine with each part requiring a different set of broach tools.

EDM leads to improvements

The list of viable broaching processes is getting longer because of improvements in the manufacturing of broach tools. Precision CNC grinders and wire EDM machines enable National Broach to provide customers with tools that meet or exceed industry standards.

It is possible to manufacture single-tooth cutting tools such as rings and blind spline dies, which cut cleaner and to closer accuracies. Broaching machines are also holding greater tolerances due to precision laser metrology checking systems that detect variations down to one millionth of an inch.

Ring gears have been successfully produced by Red Ring broaches with side-shaving shells developed by National Broach for many years. In this process, called full-form finishing, a two-piece broach comprising a roughing section and a finish broaching tool are able to complete an internal gear to finish size in one broaching operation.

The full-form broaching method has been favored by the automotive industry for making internal running gears over the nibbling-type broaches. A full-form shell, for example, can have 1000 to 2000 cutting edges to finish the same form as a nibbling-type with 20,000 cutting edges. In addition, the full-form broaching operation produces a superior finish, often as low as 7 Mu, vs nibbling-type broaches, which usually produce finishes in the order of 50-80 Mu. Accuracy and precision are also said to be enhanced. Currently, National Broach is manufacturing these shell assemblies with spiral tooth gullets instead of conventional round gullets. (Gullets are the troughs between teeth in which chips are formed and pocketed to be carried through the workpiece.) This design enables the helical-toothed broaches to cut smoother and gain much added broach life, improved part quality, and finish.

National Broach is a manufacturer of broaches and broaching machines, gear finishing equipment, including shaving, honing, grinding, and roll finishing and roll forming machines, cutting and forming tools and precision lead bars, master gears, and expanding and inspection arbors. The company's affiliation with Nachi-Fujikoshi Corp has broadened its lines of broaching and gear finishing machines and cutting tools.

For more information on National Broach & Machine, circle 229.
COPYRIGHT 1992 Nelson Publishing
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Lorincz, James A.
Publication:Tooling & Production
Article Type:Cover Story
Date:Oct 1, 1992
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