BROACHING MACHINES

Broaching is a machining operation in which a tool, having a series of cutting teeth, called broach, is either pulled or pushed by the broaching machine past the surface of a work piece.

In doing so, each tooth of the tool takes a small cut through the metal surface. The surface to be cut may be external or internal. When the operation is performed on internal surface it is called Internal or hole Broaching and in case of external surface External or surface Broaching.

Most of the cutting is done by the first and intermediate teeth, where as the last few teeth finish the surface to the required size.

Types of Broaches:

There is large variety of broaches and they are classified.

1. According to the method of operation:

Push, Pull or Stationary.

2. According to the kind of operations they perform:

Internal and external.

3. According to their construction:

Solid, built up, rotor cut, inserted tooth, over lapping tooth, progressive etc.

4. According their use:

Single purpose or Combination.

5. According to the functions:

Key way, spline, burnishing, roughing, sizing, serration, rifling, surface, spiral etc.

Push broaches are shorter in length than the pull broaches, of the same cross-section in order to ensure adequate stiffness to resist bending. Push broacher is used where a shorter length is to be broached and less material is to be removed. Where a considerable amount of metal is to be removed and a longer surface is to be broached a pull type broach, which carries more number of teeth and is longer, and hence removes more material is preferred.

Internal broaches are generally made of solid construction, but where chances of wear are more and high accuracy is desired a shell type construction is always preferred, which consists of several replaceable shells mounted on a bar. They are known as built-up broaches.

External or surface broaches are generally of built-up type having replaceable sections or teeth. The broaches used to produce single surface such as a round hole, are known as single purpose. Against this, many broaches, called combination broaches are designed to take two types of cuts simultaneously and produce two different surfaces or perform two different operations such as sizing and burnishing a hole or sizing a hole and cutting splines in it. Both the operations are done in a single pass of the tool. A burnishing broach is used for producing a highly finished and glazed surface. It is the tool (broach) which moves, while the work is stationary, but in certain cases the broach remains stationary, where as work pieces are moved past it as in continuous broaching machine. A broach made in single piece is known as solid broach.

The internal broaches which are normally of solid type are commonly used for enlarging and sizing an existing hole and/or providing specific shapes to the existing holes.

These holes in the components exist due to earlier operations on them, such as drilling, casting, forging, punching etc. Rotor cut broaches are used for heavy stock removal in castings and forgings.

Tool materials and Heat treatment:

For light work broaches made of high carbon steel are used. High speed steel is the most commonly used material for the manufacture of broaches and they give satisfactory performance in mass-production, and heavy duty work. They give fine surface finish and have long life. Broaches having their teeth tipped with sintered carbides used for hard materials and abrasive materials. Their use is mostly confined to mass production work in surface broaching.

Proper heat treatment and subsequent grinding are two very important aspects in manufacturing a broach. Long broaches are heated in vertical type of electric furnace, so that there is a uniform distribution of heat throughout the entire length of the tool and the distortion is minimum. This is followed by cooling in air under pressure. The broach is hung vertically during air cooling also in order to avoid war page. Short broaches are heat treated in horizontal furnaces. Specially designed grinders are used to grind and finish the teeth of the broach.

Broach Construction:

The front pilot enters the hole to keep proper alignment. The cutting teeth follows the front pilot, gradually increase in size. The first set of cutting teeth, called roughing teeth, does most of the cutting. They are fallowed by semi-finished teeth, which remove comparatively less stock. The variation in their sizes will be smaller than the roughing teeth. They bring the size of the hole to the required size. The finishing teeth which follow after semi finish teeth do not practically remove any stock but they smooth finish the hole. When the first finishing teeth are worn out, those behind them start doing the sizing operation. The rear pilot supports the broach and keeps it aligned after the cut is over.

PRINCIPLE OF BROACHING:

The operation of broaching involves the use of a multi tooth cutter, called broach. The teeth of the broach are so designed that the height of the cutting edge of the following cutting tooth is slightly more, equal to the feed per tooth, than that of the preceding tooth. Thus when the broach is fed in a straight line, either, over an external surface or through an internal surface, the metal is cut in several successive layers by successive teeth of the broach. The thickness of each layer is same and is known as feed per tooth. The sum of thickness of all the layers taken together is called the depth of cut.

During the operation either the broaching fed past the stationary work piece or the work piece past a stationary broach, the former practice being more common. The surface produced carries an inverse profile to that of the broach teeth. A specific point regarding broaching is that out of all the basic machining processes, it is the only process in which the feed is in built in the tool (broach). This feed is equal to the chip the thickness.

A push type broach is fed past the stationary work on a horizontal broaching machine, to machine an external surface on the work piece. A pull type broach is fed in to the hallow work piece on a vertical pull-down type machine, to machine an internal surface of the work piece. In this case also, the work piece will remain stationary. Both the operations are performed in a single linear stroke of the broach. After end of the stroke in both the above operations, the broach is retraced to the original starting position, the finished part is replaced by a new work piece and the operation repeated as usual.

TYPES OF BROACHING MACHINES:

There are a number of different designs of broaching machines available in different sizes and capacities. A few of them like arbor press are manually operated and the rest all are operated by power. Manually operated machines are used normally where only a few pieces are required to be broached and the components are small in size. Where broaching is done on mass scale, a power driven machine is used.

The common types of broaching machines can be classified as follows:

1. According to the power employed---Manually operated or Power driven.

2. According to the direction of broach movement in cutting---Horizontal or Vertical

3. According to the method of cutting---Pull, Push or Continuous.

4. According to the condition of movement of the tool relative to the work---Moving or

Stationary broach.

5. According to the type of drive---Mechanical or Hydraulic drive.

6. According to number of pull heads---Single or Multiple pull head.

Broaching Press:

A small number of jobs can be easily broached on a manually operated arbor press. This is simplest and lightest of all the presses used in broaching work and is manually operated.

Modern power presses, used for broaching on mass scale, usually carry a hydraulic drive. Push type broaches are commonly used on these machines. Both internal as well as external broaching can be done, but internal broaching is more commonly performed on these machines.

These machines are made in various different sizes, ranging in capacity from 250kg to 35 tons pressure. These machines are generally available in vertical type. The work piece is placed on the machine table and the vertical ram of the machine, carrying the broach, pushes the broach past the work. The main advantage of using presses for broaching is that they are relatively less expensive as compared to other broaching machines.

Another advantage with these presses is that, they can easily be switched over to perform other operations like bending, swarf cutting or trimming and drawing etc, when broaching is not being done. Thus their idle time is fully utilized. It is for these reasons that presses are mostly preferred over other regular types of broaching machines, where broaching is not a regular requirement.

Horizontal Pull Type Broaching Machine:

All the modern horizontal pull type broaching machines carry a hydraulic drive for reasons of getting the required power and efficient drive. A pressure gauge always fitted which readily indicates the pull being applied on the tool. These machines are used both for internal as well as external broaching. Those used for hole or internal broaching carry a bed quite similar to that of a lathe and the broach moves like the tail stock on the bed ways. The other class which is used for external or surface broaching, carries the guide ways on a vertical surface, normal to the bed, along which the broach moves.

Fixtures are invariably used on these machines. In addition to this, the cutting pressure or to say the pull, exerted on the broach further helps in clamping the work in position. The broach is pulled by a horizontal ram, which is driven by a hydraulic piston and cylinder mechanism incorporated in the body of the machine.

The mouth or front part of the ram carries a hole to receive the shank of the broach puller. The shank of the broach is passed through initial opening of the job and connected to the broach puller or pulling head.

The rear end of the broach is usually held in a supporting slide, which travels along with the broach during the operation, just like a travelling steady on a lathe.

These machines are manufactured in both fully automatic and semi-automatic types. In both the type’s automatic stops and limit switches are provided to control the length of stroke of the ram.

Vertical Pull Type Broaching Machine:

Vertical broaching machines, using the method of pull-broaching, are of two types:

1. Pull - Down Type.

2. Pull - Up Type.

Both these types are used for internal or hole broaching, an additional advantage with these machines is that more than one broache can be mounted and made to operate simultaneously.

A single ram sometimes carries as many as four broaches. Single ram machines cut in one stroke only and the return stroke is idle, which is 2 to 3 times faster than the cutting stroke.

The Pull-Down type machine carries an elevator at the top from which the broaches are suspended in an upside position, the tail being gripped in the elevator. The work piece is mounted over the table and broach lowered to pass its front pilot through the work.

This pilot is gripped by the pulling head attached to the top of the ram, which is enclosed in the bed. The ram pulls the broach down through the work piece to produce the desired hole.

After the cut is over the work piece is removed and the broach pushed up by the ram, so that the elevator again grips the rear end of the broach to take it back to the starting position. The work piece is removed and a fresh work piece loaded to start the next operation.

In pull-up type machine the ram is provided at the top, which carries the pulling heads at its bottom. The elevators are provided inside the bed to hold the broach in vertical position.

The work piece is clamped to the underside of the table and the elevators raised to pass the front pilots of the broaches through the work piece and the table where they are gripped by the pulling head.

The ram, then, starts its upward or cutting stroke and pulls-up broach through the work piece.

After the end of the stroke the work piece falls down and is removed and the ram lowered.

The rear ends of the broaches are gain gripped by the elevators and the formers is brought down to the starting position.

Duplex-Head Broaching Machines:

these machines carry two rams instead of one. They are made in both horizontal type as well as vertical type.

These machines give very fast rate of production as they provide a sort of continuous cutting in the sense that when one broach is providing the cutting action the second one is returning to the starting position and viceversa.

These machines use push method of broaching and are commonly used for external or surface broaching.

Something happens in case of loading and unloading of the jobs that while one fixture is being unloaded and reloaded with fresh job the other is holding the other job in operation and vice-versa.

Continuous Broaching Machines:

these machines are manufactured in both horizontal as well as vertical type.

Horizontal machine mainly differs from other types in that the broach remains stationary while the work pieces move continuously past it to perform the cutting.

The horizontal machine consists of two sprockets, one on each side of the machine, on which continuously travels an endless chain. A series of fixture are mounted on this chain to travel along with it.

The broaches are rigidly held on the machine in horizontal position over the chain. Work pieces are loaded on the fixtures on one side of the machine and unloaded on the other side.

The speed at which the parts can be broached depends upon how fast the loading of work pieces is done. The vertical type carries a number of platens on a continuous chain.

Broach holders are mounted on these platens to carry the broach sections. The work is clamped on the horizontal table of the machine and the broach sections moved past the work by moving the chain.

Another useful type of continuous broaching machine is the Rotary table Horizontal Continuous Broaching machine. In this machine a rotary type of table continuously rotates about axis. The fixtures are mounted on this table. The broach is held rigidly on the broach holder above the table.

The body of the broach also carries a curvature similar to that of the table. Work pieces are loaded in the fixtures. As the table rotates at a slow speed, the work pieces are fed past the stationary broach and the finished pieces unloaded, followed by loading the fixtures with fresh work pieces.

Thus the cycle of loading, broaching, unloading and reloading continues without any break. These machines are very useful in mass production of identical broached components.

Machine Size:

The size of the broaching machine is the length of stroke of ram in mm and the pressure applied on the broach in kgs or tons.

The other main specifications of the machine are:

1. Range of speeds and feeds.

2. Type of drive.

3. Power rating of electrical motors.