This article takes an in depth look at Broaching.
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Broaching is a machining process where a sharp, hardened, toothed tool removes material from a workpiece in a consistent, continuous, and accurate way. The process of broaching uses a tool with raised teeth of differing sizes that precisely cut away a specified amount of material with every pass over the workpiece.
In broaching, the multi-cutting tool moves while the workpiece remains static, or the workpiece moves while the tool is static. In some cases, there may be relative rotation between the tool and workpiece. The broaching process can be internal through the center of the workpiece or external across the surface of the workpiece.
The designs for exterior or surface broaching include slab, slot, contour, pot, and straddle, while internal broaching includes keyway, concentricity, and cut recut. The broaching process varies in accordance with the portion of the workpiece that will be broached or cut.
Broaching is a common machining process used to selectively remove material from a workpiece to achieve a certain dynamic or shape. The difference between broaching and other machining applications is the tools used to deform and transform the workpiece.
The purpose of the broach is to push against the workpiece to leave a set of holes or shapes. What makes broaching unique is the types of cuts it makes; they are odd and cannot be found in other machining methods.
A broaching machine is the simplest of all forms of machining tools and consists of a broaching tool, holding fixture, drive, and support frame. The two types of broaching machines are horizontal and vertical; they can have one or more rams depending on the application. With dual or multiple rams, the rams work opposite each other, with one on the cutting stroke and the other on the return stroke.
A broaching machine either pulls or pushes the broach through or past a held workpiece. In other designs, the workpiece is held in place as the broach passes over it. The power and force for a broaching machine comes from hydraulically supplied energy.
Horizontal broaching machines use the pull type method for broaching the workpiece and are used for interior and exterior broaching. A horizontal machine has a bed, a broach pilot, and a drive mechanism.
The main broaches on a horizontal machine are keyways, splines, slots, round holes, and interior shapes. Though horizontal machines take up floor space, they are perfect for long broaches and heavy workpieces.
A vertical broaching machine pushes or pulls the broaching tool, with push broaching being the most common type. The design and configuration of vertical machines makes it possible for them to perform multiple operations since it is easy to pass a workpiece from one machine to another.
The three types of vertical machines are push down, pull up, and pull down. The pull up design is the most popular. Vertical broaching machines can be powered by hydraulics or an electro-mechanical drive; hydraulic is the most popular and cost effective.
A surface broaching machine removes access material from the surface of a workpiece. The cutting tool passes over the workpiece to produce flat or contoured surfaces. The unique feature of a surface broaching machine is that the cutting tool is attached to a ram that is forced along past the workpiece. The ram can move horizontally or vertically depending on the design of the machine. It is possible for a surface broaching machine to have two rams; this is described as duplex broaching.
In continuous broaching, the workpiece is constantly moving while the broach is held stationary. The movement can be straight, horizontal, or circular. With horizontal continuous broaching, workpieces are loaded on a fixture that is mounted on a chain. Continuous broaching is used for cutting similar parts at the same time.
Rotary broaching is unlike the other forms of broaching and is known as wobble broaching. It is a precision method for producing polygonal shapes internally or externally. In rotary broaching, the cutting tool is placed at a 1° angle to the centerline of the workpiece, which causes the broach to cut with a chiseling or scalloping effect.
Rotary broaching can be completed with a lathe or mill using a horizontal or vertical spindle machine where the spindle is allowed to spin freely. The shapes that can be created using rotary broaching include hexagons, squares, serrations, keyways, involute, spline, spur gear, numbers, and letters.
The rotary broaching tool holder allows the broaching process to work and cuts one portion of the form at a time. The broach is fed into the part and cuts the full form in one pass. The broach bit is designed with a back taper to prevent it from thrusting into the workpiece while feeding the broach into the pilot hole.
Turn broaching machines are used for circular, linear, and spiral cutting where surfaces need a high finish. The tool has multiple inserts to remove material by rotating the crankshaft between centers. The components of a turn broaching machine are capable of finishing or roughing. The type of machine determines the tool.
Roughing inserts and their segments depend on the requirements of the stock removal rate. The finishing segments have adjustable cartridges, which are ideal for producing tight tolerances.
Computer Numerical Control (CNC) machines can be used for the broaching process due to recent technological developments. Parts can be broached using the rotary or linear processes in a single step. Broaching companies have designed tools, tool bodies, and holders that can be included in the CNC process.
CNC broaching has indexable insert broaching tools and tool holders. The special tool holders make it possible to have several specialized inserts. CNC broaches include rotary or wobble, punch, keyway, splines, polygons, and serrations to name a few.
The process of broaching produces parts with exceptional surface finishes and dimensional accuracy. A broaching tool has rough, semi-finish, and finish cutting teeth capable of finishing the machining of a rough surface in one stroke.
In the case of interior broaching, the tool may be pulled or pushed through the workpiece, or the workpiece may move across the tool. Internal broaching necessitates a starting hole for insertion of the broach.
A broach is a single point cutting tool with many points that cut like a flat ended sharp tool such as a chisel. In some cases, the teeth of the broach may be set diagonally to complete shearing cuts that produce a sliding or skewing deformation.
A broach is a cutting tool with several rows of teeth with each tooth slightly larger than the previous one. Simple or complex cuts can easily be produced quickly and accurately using a broach cutting tool. During the process, each cut of the broach is shallow along the length of the workpiece.
Of the various types of broach cutting, linear broaching is the most common. The tool moves linearly against the workpiece surface, making its cuts as it moves to the end of the workpiece The diagram is an example of linear broaching.
In pull broaching, the workpiece is held in place as the broach is pulled through. The type of broaches for this process are long and require a special head. The tension created during pulling builds up to make the broach more effective. The process is mainly used for interior or internal broaching but can be used for surface broaching. Pull broaches have a great many teeth and provide a longer cut.
Pull broaching can be completed up, down, or horizontally through or across the workpiece. Automobile cylinder blocks and heads are faced flat using the pull broaching method.
A push broaching tool is shorter to be able to endure the compressive forces applied as it goes through the workpiece. It has fewer teeth, which can be bent or broken due to the compressive forces. Push broaching produces shorter cuts to produce smaller parts.
Surface broaching is a common use for push broaching tools. When they are used for that purpose, they are referred to as surface broaching tools.
Pot broaching is used with cylindrical workpieces and has a pot shaped tool holder. In pot broaching, the tool remains stationary as the workpiece is pushed or pulled through it. Pot broaching produces a cut inverse that of interior broaching and is used with other machining or independently. The tool for pot broaching is extremely complex and requires a special fixture; this makes pot broaching a seldomly used process.
Spline broaching, also known as round broaching, creates round, plain, or irregular splines and spiral tooth forms. It can be used to create shafts, gears, and meshing mechanical components. Spline broaching is the most common form of broaching and uses a broach to form a series of ridges or teeth around the circumference of a pre-cut hole. The process of spline broaching is an efficient method for creating serrated, straight, and involute splines.
In the automotive industry, spline broaching is used to produce transmission and driveshaft parts. The grooves created on a spline allow the efficient transfer of torque by locking together separate components.
Blind broaching refers to the broaching process where the tool does not pass completely through the part, meaning the tool machines into a blind cavity. The process of blind broaching is used on parts that are designed to not have a pass through. In the case of keyways, they can be designed to be a blind broaching when the tool stops in the middle of the bore.
With internal broaching, material is removed from the interior or internal surface of the workpiece; this is known as hole broaching. Prior to beginning the internal broaching process, a starter hole is drilled into the workpiece; this is referred to as blank prepping. Internal broaching is used to enlarge, widen, and resize a hole.
The internal broaching tool has teeth that get successively larger to provide an exceptional finish on the workpiece. Internal broaching is completed using automated machines such as a CNC machine, which is capable of producing holes quickly and efficiently in the workpiece. The internal broaching method can be completed using either the push or pull process. The categories of internal broaches are round or spline.
External broaching removes material from the exterior of the workpiece and is referred to as surface broaching. Like the internal broaching tool, the teeth of an external broaching tool progressively increase. External broaching uses a guided ram with a broach holding fixture. The force of the cutting is transferred to the ram along the length of the broach.
External broaches are flat surfaces and form surfaces. With flat surface broaching, the tool passes over the workpiece to produce a desired surface. The form surface broaching process is designed to produce different contours and shapes, which include serrated, angular spline, gear forms, fir tree, compressor slots, and keyways.
The key to broaching is the design of the broaching tool. A properly designed tool can lessen the possibility of drift, deflection, and breakage as well as provide efficiency during the cutting process. Broach tool design is a specialized type of manufacturing engineering that involves analysis, planning, designing, manufacturing, and use of broach tools.
An essential part of a broach is the rise per tooth that indicates the amount of material each tooth removes on every pass. The teeth are broken into roughing, semi-finishing, and finishing sections and can perform internal/interior shaping or external, surface, or exterior shaping.
Slab broaching is the simplest form of broaching and is used to cut flat surfaces. The process of slab broaching square ends can provide a surface for future broaching.
Slot broaching cuts slots of various depths and widths in the workpiece. In high production operations, slot broaching is faster and more economical than milling.
Nibbling or egress broaches have narrow roughing teeth positioned at an angle to the centerline with full width teeth capable of making semi-finishing cuts.
Contour broaches cut concave, convex, cam-shaped, contoured, and irregular surfaces at exceptional tolerances.
Dovetail or pine tree broaches create forms in compressor wheels to hold the blades in a turbine disc. This form of broaching requires several passes due to the nature of the workpiece and the complexity of the forms.
Pot broaches cut external forms such as spur gears, splines, slots, and other tooth forms. The design of pot broaching allows the deformation to occur in a single pass. Pot broaching has replaced hobbing in the manufacture of external gears.
Straddle broaching uses two slab broaches that cut identical parallel surfaces on opposite sides of the workpiece in one pass. The process maintains a precise dimensional relationship between the two sides.
The two types of internal broaching are rotary and vertical; they are designed to increase the size of a hole in the workpiece. All internal broaching requires the creation of a starter hole referred to as blank prepping. Vertical broaching can be push or pull. Rotary broaching is used when it is necessary to create an asymmetrical hole in a single pass.
Internal broaches are located on their shank and held in place by a set screw clamp with a whistle notch.
Shell broaches are used for semi-finishing, roughing, and finishing. Any section of a shell broach can be replaced or sharpened. When used for finishing, the shell teeth have greater accuracy than long broach tools.
Round hole broaching is the simplest of the internal broaches. A round hole broach has a series of teeth that encircle the tool with teeth designed to cut the outer edge. With soft ductile metals, round hole broaches require a chipbreaker.
The keyway broach is one of the more common forms of internal broaches. A keyway broach cuts a slot in the bore of the workpiece and is supported by a fixture, referred to as a horn, which locates where the keyway is to be cut.
An internal gear broach nibbles at the internal surface of the workpiece to generate a gear profile.
A gun barrel broach is very long and capable of moving the full length of the barrel. It has very fine teeth since barrel grooves are very minute at a few thousandths of an inch deep. The broach machine pulls the broach through while rotating it to produce the spiral rifling pattern in the barrel.
With spline broaching, the pitch diameter has to be concentric. A concentric broach is a full form finishing tool that has round and spline teeth to shave the diameter and spline form.
The process of cutting a spline in a thin walled part is difficult since the walls of the part expand as the broach passes but then spring back. A cut and recut broach has a breathing area behind it that prevents the walls from springing back. The front of the tool cuts the spline and is followed by a second cutting tool that recuts the cut section.
Square broaches are used to enlarge, shape, and finish irregular shapes. Their purpose is to change a round hole into a square using a rotary broach holder that angles the broach such that only one corner is being cut at a time. Square holes are for holding square shafts and can be blind or through cut holes.
A broaching tool is designed to remove metal from a workpiece using a multi-pointed cutting tool that has several teeth arranged in rows with each row of teeth being higher than the previous row. The three sections of a broaching tool are roughing, semi-finishing, and finishing.
The pull end connects the broach to the pulling head on the broaching machine.
The shank is the part from the bridge to the root diameter that is held and pulled inside the machine. The length of the tool from the pull end to the root diameter is known as the shank length.
As the name implies, the pilot centers the broach in the hole that will be broached.
These are the teeth at the front of the broach that make first contact with the work piece.
After the roughing teeth come the semi-finishing teeth, which are smaller and take less of a bite out of the workpiece.
Finishing teeth are the broaching teeth and are all the same size. They finish the workpiece, taking it to the desired size and shape of the design for the product.
The rear pilot balances the broach and keeps it aligned.
The follower end has a function that is similar to the rear pilot—supporting the elements of the tool.
The push type broaching tool is shorter than the pull type and experiences compressive forces as it pushes through. Since the teeth of a push broaching tool are easily broken or bent, they are reduced in size and smaller than the teeth of a pull through broaching tool.
The parts of a push through broaching tool are the same as those for a pull through type.
External or surface broaching removes material from the surface of the workpiece. As with the pull through and push through broaches, the teeth for surface broaching increase progressively.
The land is at the bottom of the teeth and supports the cutting edge.
As the broach cuts the workpiece, the chips created by the cutting move through the rake.
The pitch is the distance between the cutting edges of any two teeth.
The clearance angle is the angle of the land in regards to the horizontal axis, which is designed to prevent friction between the teeth and the workpiece. It ensures that only the cutting edge of the teeth are in contact with the workpiece.
The depth is the height of each tooth.
The gullet radius is the radial space between two teeth through which the chips flow after being curled.
Chipbreakers are notches used to eliminate chip packing and assist in chip removal. They are ground into the roughing and semi-finishing teeth parallel to the tool axis. Chipbreakers are staggered such that a set of chipbreakers is followed by cutting teeth. They are a vital part of round broaching tools keeping the tools from forming ring shaped chips.
During the broaching of slots, the tool gets enclosed by the slot during cutting and carries the chips the full length of the tool. The broach teeth rub the sides of the slot and wear. A single relief angle on the sides of the teeth reduces wear and leaves a small portion near the cutting edge.
Though broaching is an ancient form of machining, it has become a valuable part of many industrial operations. The latest nuts, bolts, gears, and other components for aerospace and the automobile industry depend on broaching to produce precision reliable parts with exceptional tolerances.
The primary use of broaching is in the production of high volume parts that need accurate, repetitive, and complex cutting. A wide variety of metals and materials can be used for the broaching process. Preferred metals have a hardness on the Rockwell C hardness rating scale between 26 and 28.
Every type of broaching is different and designed to fulfill the needs of a specific application. There are areas where broaching is the best tool for the job, such as cutting parallel and multiple surfaces, fast cutting, automated cutting, exceptionally large production runs, and tough hard materials.
Several types of metal shaping operations can be replaced by the implementation of broaching to save time and use fewer tools. Any type of metal alloy can be broached, but it works best on soft metals like aluminum, copper alloys, brass, certain forms of plastic, wood, and midrange types of steel. Exceptionally hard materials, such as titanium, can be broached, but the broaching tool quickly dulls due to the hardness of the metal.
Below are examples of components that are produced for automobile transmissions using broaching.
A transmission gear for locating clutch plates:
An example of a transmission part produced using push broaching with the splines and teeth cut in one pass.
The part below is an example of pot broaching of a drive mechanism that has to be carefully controlled to ensure precise tolerances.
An example Internal broaching of transmission gears in different sizes and involute splines.
Though broaching is highly adaptable and capable of performing multiple machining functions, there are factors that should be examined before deciding on the process.
The first factor to consider is the surface of the workpiece, which has to be parallel to the direction at which the broaching tool moves. Any obstructions, faults, or additional elements on the surface make it impossible to use broaching since it is a unidirectional, continuous process.
When there are multiple contours, curves, or angles on two or more planes, it is impossible for broaching to form them in one pass except in the case of helical gear teeth.
The material to be broached must be able to endure the force and stress produced by the broaching process. Parts with thin walls that are fragile or those with delicate cross sections will not be able to endure broaching.
The best materials for the broaching process are softer, ductile, and pliable. There are some high tensile strength steels as well as other hard metals that can be broached. The primary factors determining if a certain metal can be processed using broaching is the equipment, tools, and method to be used.
The most common material to be broached is steel; it is used in a variety of broaching applications. The grade of steel determines its applicability.
Below is a short list of common materials used in broaching. There are several varieties of plastics and copper alloys as well as other materials that are shaped using the process. Broaching tools can be engineered and designed to fit the needs of any type of material required to meet the needs of an application.
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