broaching pgtd

7/31/2019 Broaching PGTD

1/21

SEMINAR ON

BROACHING


2/21

Broaching is the process of removing metalwith a tool which has teeth arranged in arow. Each tooth is successively higher than theprevious tooth and removes more material. In

broaching, one stroke or cycle of the machineproduces a finished part.

Broaching is used to produce both internal and

external features. Production rates are highand tolerances of +/- .01mm are possible.


3/21

Broaching Principles

Broaching is similar to

turning, milling, and othermetal cutting operations in

that each tooth removes a

small amount of material

The broaching tool has aseries of teeth so

arranged that they cut

metal when the broach is

given a linear movement

The broach cuts away the

material since its teeth are

progressively increasing

in height

Cutting action of a broaching tool


4/21

Broaching


5/21


6/21


7/21


8/21

Broaching Principles

Broaching is a machining process that pushes or pulls acutting tool (called a broach) over or through thesurface being machined.


9/21


10/21

Chip Formation Chip formation involves three basic requirements:

The cutting tool must be harder than the partmaterial

There must be interference between the tool and thepart as designated by the feed rate and cut per tooth

There must be a relative motion or cutting velocitybetween the tool and workpiece with sufficient

force to overcome the resistance of the partmaterial.


11/21

Tool Feed DirectionGullet

Workpiece

Depth of

cut per tooth

Tool


12/21

Chip Formation

As long as these three conditions exist, the portionof the material being machined that interferes withthe free passage of the tool will be displaced tocreate a chip.

Many combinations exist that may fulfill suchrequirements.

Variations in tool material and tool geometry, feedand depth of cut, cutting velocity, and part material

have an effect not only upon the formation of thechip, but also upon cutting force, cuttinghorsepower, cutting temperatures, tool wear andtool life, dimensional stability, and the quality of thenewly created surface.


13/21

The Mechanics of Chip Formation

Empirical metal-cutting studies reveal several importantcharacteristics of the chips formed during the broachingprocess:

The cutting process generates heat

The thickness of the chip is usually greater than the thickness ofthe layer from which it came

The hardness of the chip is usually much greater than thehardness of the parent material, and

The other three relative values are all affected by changes incutting conditions and in properties of the material to be

machined


14/21

The Mechanics of Chip Formation These observations also indicate that the

process of chip formation is one of deformationor plastic flow of the material, with the degree

of deformation dictating the type of chip thatwill be produced.


15/21

Plastic Deformation Originally, it was thought that chips formed in

metal cutting were created in much the same waythat wood chips are formed when split by an axe.

This may be partially true for brittle materialssuch as cast iron, but it does not hold true for themajority of metals. The process by which chips areformed with metal-cutting tools is called plasticdeformation.


16/21

Plastic Deformation

What actually happens in this shearing process isthat the metal immediately ahead of the cuttingedge of the tool is severely compressed resulting in

temperatures high enough to allow plastic flow. When the resisting stresses in a material exceed

their elastic limit, a permanent relative motionoccurs and further deformation is withstood.

This strengthening is called work or strainhardening, and is characteristic of all steels, butdemonstrated most dramatically in stainless steels.


17/21

How and Where Heat is Generated The force or energy that is put into the tool creates

movement in a group of metal atoms in the workpiece. Thisgroup is a finite number of atoms which are forced tochange their positions in relationship to each other.

As the atoms in the metal ahead of the tool are disturbed,the friction involved in their sliding over one another isthought to be responsible for 60% or more of the total heatgenerated.

This internal friction, and the heat it generates, can becompared to the friction and heat caused by bending apaper clip back and forth until it breaks.


18/21

How and Where Heat is Generated As the tool continues to push through the work piece, a chip

eventually slides up the cutting face of the tool. This slidingcreates an external friction which again releases heat. Thisexternal friction accounts for about 30% of the total heatgenerated.

The third area of heat generation is on the land or flank of thetool. This area accounts for about 10% of the heat generated.

This is assuming that the tools are sharp and made correctly asfar as clearance angles and face angles are concerned. As thetool wears, the above percentages will vary, especially whenthere is excess wear on the land, or if the clearance angle isinsufficient for the material or the part configuration. Thiscontact zone will actually increase as the part continues toclose in after the cut resulting in extremely high pressures onthe land area of the tool.


19/21

Advantages

Rough to finish in one pass

Production rates are high

Cutting time is quick

Rapid load and unload of parts

External and internal features

Any form that can be produced on a broaching tool canbe produced

Production tolerances are excellent

Surface finishes are better than milling

Operator skill is low


20/21

Disadvantages

Tooling cost can be high

In some cases--not suited for low production rates

Parts to be broached must be strong enough towithstand the forces of the process

Surface to be broached must be accessible


21/21

broaching pgtd

Documents