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Home > AstroBroach Machines > Principles of Operation
Types of Broaching Operations: Broaching machines may be either vertical or horizontal, and broaches may be either pushed or pulled over (external broaching) or through the work (internal broaching). The following descriptions cover four major broaching operations.
Surface Broaching: This is external broaching usually done on a vertical machine with a broach that's either pushed or pulled through the part. On horizontal machines, a pulled broach is customary. Surface broaches are usually made up of broach sections attached to a holder and bolted to a ram. This built-up arrangement allows the broaching of multiple surfaces and intricate shapes.
Pull Down Broaching: This is internal broaching done on vertical machines with broaches that are pulled down. This method keeps the tool in tension, preventing buckling, so it is particularly suited to work requiring long slender broaches. Other advantages are easy workpiece holding, simplified chip disposal, and the ease with which the broach teeth pull coolant into the cut.
Push Broaching: This is internal broaching done on vertical machines where the tool is pushed either up or down. The broaches are short, to avoid buckling. This lack of length limits the amount of stock they can remove. This method is suited to light work and low production runs. Push broaches, however, are less expensive than Pull broaches.
Pot Broaching: This is external broaching done on vertical machines using a tool that may consist of a stack of concentric ring-shaped broaches held in a pot-type holder. The cutting edges are on the inside perimeters of the rings. The broaching is done by either pushing the work through a stationary holder, or by pushing a holder over stationary work.
Broach Description: Broaches may be used for either internal or external work. An internal broach may change the shape and size of an existing hole and is commonly constructed in one piece. An external broach removes stock from the outside of the work and is usually made up of assembled sections.
A broach has three main cutting areas. The teeth in the first section usually have deep chip-holding spaces and there is a rather steep rise from one tooth to the next. These teeth rough away the material.
The central section of the broach gradually establishes the workpiece profile at a progressively lighter feed, tooth by tooth. The last several teeth provide finishing and final sizing. (Burnishing teeth are sometimes used. They follow the finishing teeth, and have no cutting edges.)
Tooth configuration varies depending on the material being broached, and whether rough or finished cutting is being done. The length of a broach is determined by the amount of stock it will have to remove from the workpiece, the length of the cut, and by the allowable chip thickness for the material to be processed. The length of the cut principally determines the tooth spacing and stock removal determines the number of teeth required.
Chips: Broaching chips are much like those from any other single point tool. However, unlike milling, they are retained between the broach teeth during the cut. The chips remain here until the broach clears the work, at which time they fall or are brushed away. Good broach design depends upon tooth geometry that produces proper chip curling action, tightly curled chips and easy chip flow.
Coolants: The main functions of a broaching coolant are to:
Fortified petroleum coolants are used most often, but some of the new heavy duty water soluble coolants have excellent ability to dissipate heat, are cleaner to work with, and reduce fire hazards while, in some cases, allowing broach speed to be increased.
- Carry off the heat generated by the cutting action and the passing of the tool and chips over the work surface
- Lubricate the tooth surfaces and chips to facilitate chip flow
- Flush the chips from between the teeth
Broach Sharpening: A properly ground broach is almost razor-sharp. All internal and most external broaches are re-ground only on the front face, where just enough metal is removed to restore the broach to its original sharpness.
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