Although it's not one of the most common machining operations, broaching has been around the metal working industry for well over a century. The process–one of the quickest and most precise metal removal methods available–has come a long way since the first broach "drift" was hammered through cast iron well over 100 years ago. Curiously enough, the evolution of the broaching process has nearly brought it back full circle, to the first belt-driven flywheel machines developed early in the 20th century. Figure 1
The earliest incarnation of the broaching process utilized incrementally sized "drifts" that were hammered through the bores of parts one at a time. Numerous drifts of different sizes we required to finish a bore to the required size. The process, of course, was slow and extremely labor intensive. Figure 2
The first milestone in the evolution of the broaching process was the introduction of the first broaching machines, which were simple beasts with very few moving parts. The principle was simple: attach a belt to a flywheel, turn the flywheel, which turns a gear, which then transfers rotational energy to a screw or rack. The turning of the screw, or moving of the rack, would then move a slide, which would draw the cutting tool through the workpiece. The accuracy, repeatability, and precision of parts cut on this type of screw-driven broaching machine was excellent, but the process was slow.
The next step in the evolution of the broaching process saw hydraulic powered machines replacing the old screw-drive machines.
Hydraulically driven machines yielded faster cycle times, and the increased cutting forces that hydraulic power was able to achieve allowed many new operations to be performed by broaching that could not be done prior to the inception of the hydraulically powered machines. Broaching grew from strictly an internal machining process, creating such features as splines and keyways, to a machining process that was able to duplicate many milling operations in a fraction of the cycle time, and often with better part finishes and tolerances. Figure 3
The floodgate was opened. Operations never before done on a broach were now being processed more quickly and with better tolerances by using the new, hydraulically powered machines.
The broaching process continued along in much the same way for decades. While new technologies were adopted in electrical controls, hydraulic systems were developed, and new broaching processes were invented, the machine was still very much the same as ever; "a glorified log-splitter."
The inception of cell type manufacturing and tougher environmental preservation standards were the catalysts in the next evolution of broaching equipment. Traditionally, a machine possessing more than 36" of cutting stroke necessitated either a pit mount or an operator stand.
Machines mounted in pits were closer to groundwater. If any fluids leaked out of the machine, or were spilled during the filling or draining process, there was a danger of groundwater contamination. On the other hand, a floor-mounted machine, which required the operator to operate the machine from a platform, was unacceptable in the new manufacturing "cells," where a single operator was expected to run multiple machines–unless the operator happened to wear a cape and a blue spandex suit with an "S" on it, climbing up and down stairs all day was not an option. This was the rebirth of the "table-up" broaching machine, and I say "rebirth" because the technology was developed in the early 1950's but abandoned. In the case of Broaching Machine Specialties, an automotive manufacturer in Oregon was instructed to get their machines out of the pits because contamination was already evident. This requirement led to the development of our first "Cell-Mate" table-up machine in 1993. Since then BMS has shipped over 40 of these machines to manufacturers throughout the world. Figure 4
The table-up machines operated differently than the traditional pull-down version. Instead of the workpiece being held stationary above the floor and the tool moving down through it, the table-up design holds the cutting tool stationary and moves the part up over the stationary tool. This allows the part to be loaded at floor level and eliminates the need for a pit or operator stand; no groundwater problems, and no need for a flying operator. Cost cutting and tighter surface finish requirements drove the next evolution of the broaching process. Hydraulically powered machines utilize an electric motor to power the hydraulic system. This motor runs constantly even if the machine is in a static condition, such as between cycles or idling. The constant running of the motor uses valuable electricity and wastes a considerable amount of money in the course of a year. Broaching machine manufacturers answered the call to save energy by implementing the use of electromechanically driven machines. Figure 5
The hydraulic cylinder was replaced by a planetary roller-screw and a motor that runs only on demand, such as during the cutting or return stroke, replacing the constantly idling motor. Furthermore, the constant torque of a mechanically driven system yields better surface finishes than are achievable from hydraulically driven machines. This is due to the compressibility of hydraulic oil. High-speed photography of a hydraulically powered broaching machine filmed during the cutting stroke will show the slide and cutting tools pulsating or chattering through the cut. An electromechanical machine with its roller-screw drive has no compressibility and moves at a constant speed with a constant torque. It is the smooth, non-pulsating cutting action that yields a far better surface finish. In the case of BMS, an air brake manufacturer whose surface finish requirements were extremely rigorous led to the development of our first "Electro-Mate" screw-driven, electromechanically powered table-up broaching machine. Other advantages afforded by the electromechanical drive machines are:
• Because no hydraulic unit is required, the machine takes up 50 percent less floor space. This can be a major factor when designing a machining cell (see Figure 1 here).
• No pumps, valves, hydraulic leaks, or maintenance.
• No need to stock hundreds of gallons of hydraulic oil.
• No need to filter or dispose of spent hydraulic oil.
• Smooth cutting action yields better tool life and part finish.
• Because no excess heat is generated by a constantly idling motor, a cooler working environment results.
So here we are, having come nearly full circle. The newest technology in broaching, screw-driven broaching machines, is a new twist on an old technology. Use any clichÈ you see fit, whether that be "Old habits are hard to break" or "Keep it simple, stupid." But the one I prefer is "What goes around, comes around."