Broaching is a machining process that uses a toothed tool, called a broach, to remove material. Linear broaching, which is the more common process, the broach is run linearly against a surface of the workpiece to effect the cut. Linear broaches are used in a broaching machine, which is also sometimes shortened to broach. In rotary broaching, the broach is rotated and pressed into the workpiece to cut an axis symmetric shape. Broaching is used when precision machining is required,and even though broaches can be expensive, broaching is usually favored over other processes when used for high-quantity production runs.

Broaches are shaped similar to a saw, except the teeth height increases over the length of the tool. Moreover, the broach contains three distinct sections: one for roughing, another for semi-finishing, and the final one for finishing. The profile of the machined surface is always the inverse of the profile of the broach. The rise per tooth (RPT), also known as the step or feed per tooth, determines the amount of material removed and the size of the chip. Because all of the features are built into the broach no complex motion or skilled labor is required to use it. The cut of a broach is effectively analogous to multiple passes of a shaper.

The internal broaching process begins by clamping the workpiece into a special holding fixture, called a workholder, which mounts in the broaching machine. The broaching machine elevator, which is the part of the machine that moves the broach above the workholder, then lowers the broach through the workpiece. Once through, the broaching machine's puller, essentially a hook, grabs the pilot of the broach. The elevator then releases the top of the pilot and the puller pulls the broach through the workpiece completely. The workpiece is then removed from the machine and the broach is raised back up to reengage with the elevator.

Because each broach is specialized to cut just one shape either the broach must be specially designed for the geometry of the workpiece or the workpiece must be designed around a standard broach geometry.

Broaching can be used for a high variety of materials including steel (42CrMo, 20CrMn, 31CrMoV9, 40Cr, 50Mn …) and casting.

Why choose broaching? “Just” for three principal reasons:

• • Time saving: comparing with gear shaping, broaching takes up to 40%-60% less, reducing lead times with better quality levels. Again, the type of processing also let us to machine more pieces at once in some cases;
• • Quality consistent: broaching needs no human intervention to make regulations on the machine before or during the process. Eliminate the human component means quality consistent lot to lot;
• • Money saving: less time and less waste lead to lower costs and that means lower prices as better quality means lower costs for the customers too, during the assembling processes;

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Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case hardened surface. It is predominantly used on steel. Typical applications include gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools, forging dies, extrusion dies, injectors and plastic-mold tools.

In gas nitriding the donor is a nitrogen rich gas, usually ammonia (NH3). When ammonia comes into contact with the heated work piece it disassociates into nitrogen and hydrogen. The nitrogen then diffuses onto the surface of the material creating a nitride layer. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required.

The advantages of gas nitriding over the other variants are:

• • Precise control of chemical potential of nitrogen in the nitriding atmosphere by controling gas flow rate of nitrogen and oxygen;
• • All round nitriding effect (can be a disadvantage in some cases, compared with plasma nitriding);
• • Large batch sizes possible - the limiting factor being furnace size and gas flow;
• • With modern computer control of the atmosphere the nitriding results can be closely controlled.

Gas nitriding can be used both on steel and cast iron. Ideally, the material for nitriding should be in the hardened and tempered condition, requiring nitriding take place at a lower temperature than the last tempering temperature.
For application in ring gears, after years of experience, the best material seams to be 42CrMo, quenched and tempered before machining and nitriding at the end.

Achievements:

• • High surface hardness;
• • Wear/abrasive resistance and anti-galling properties (the coumpond layer is extremely efficient in tribological application);
• • Improve fatigue life (compressive stress at surface);
• • Improve corrosion resistance (except for SS material);
• • Small deformations (low temperatures): reduce coupling problems; reduce noise.
• • No final grinding necessary: no removing part of the hardened layer creating inhomogeneity in the surface characteristics of the workpiece.

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