To Study about Broaching Machines

Introduction:


            Broaching is a m/c operating in which a tool, having a series of cutting teeth called broach is either pulled or pushed by the broaching m/c past the surface of a work.

The term broaching may have derived from an ancient roman word braces which meant an object having projecting teeth the operation itself dates only to the 1850’s when broaching tools then called “drifts” were hammered in black smith shops through the work or pushed through the work or pushed through with an arbor press.

Broaches:


A broach is a multiple edges cutting tool that has successively higher cutting edges along the length of the tool.

Types of Broaches :

It can be classified in various ways :

1.  According to type of operations.

a. Internal

b. External

2.  According to method of operation

a. Push type

b. Pull type

3.  According to type of construction:

a. solid type

b. built-up inserted tooth type

c. progressive cut type

d. rotor cut type

e. double jump or overlapping tooth type

4.  According to function:

a. Surface type

b. Keyway type

c. Round type

d. round hole type

e. splint type

f. spiral burnishing type

Broaching of inside surfaces is called internal or hole broaching and of outside surface, external or surface broaching.

 

Different types of broaching machine:


1. Horizontal machine:

 1.  Single or duplex ram made for ram operation.

(a) hydraulically

(b) Mechanically by screw and cut

(c) Electromechanically with rack and pinion similar to a planning m/c.

2.  Drum or turret m/c.

3.  Special surface broaching and rifling m/c.

Vertical machine: (Mostly adapted for hydraulic operation.)

1.   For internal broaching 

(a)  pull up

(b)  pull down

(c)  push down

2.  For surface broaching

Broaching methods:


Internal or hole broaching:

In this the work normally remains stationary and the broach is either pulled or pushed through the same to produce a hole of desired shape and size.

External or surface broaching:

In this either the work or the broach is moved past the other to produce a groove or surface of desired shape and size on the external surface of the work.

Pull broaching:

Mostly adopted for the internal broaching, in this the work remains stationary and the broach is pulled through the same to produce to hole of desired shape and size.

Push broaching:

Adopted mostly for the internal broaching of relatively lighter jobs. The work remains stationary and the broach is pushed through the same. However it can be used for external broaching also.

Continuous broaching:

It is a method suitable and largely adopted for the broaching of indentical components on large scale in this method the broach remains stationary while the work move continuously past the same along a horizontal or circular path.

Calculation of tooth load on a broach :


            Calculation of the tooth load on a broach. This calculations also helps in judging the capability of the m/c for pulling or pushing the broach this load is calculated as follows.

1.   For the circular holes:

F=  π.d.T.t.P

d = Diameter of the m/c ed hole

T= total no. of teeth  cutting at a time.

t= Rise per tooth

P= force required to cut square of 1 m.m. of the metal at a given         rise 1 tooth.

2.  For square holes:

F= 4l . T.t.P

l.= machined length of one side of the d\square hole.

The main items to be decide in designing a broach are its length,no.of teeth and total rise. The following are the recommended rises per tooth for different operations.

Round broach for internal broaching                          0.05 m.m.

Splnined broach for internal broaching                       0.0625 m.m.

External broach for surface broaching                        0.075 m.m.

Broach for keyway broaching                                     0.0875 m.m.

Total rise = (n) rise per tooth

Where n = No. of teeth in the broach

Effective length L = n π pitch

n =       depth of cut    +  Ts + Tf

            cut per tooth

                             =   Total rise         +  Ts + Tf

                                 Rise per tooth

            Where   n= total no. of teeth in the broach

                        Ts = No. of  teeth preceding cutting

Tf = No. of finishing teeth  (usually 3 to 6 in no.)

Pitch P = 1.25  (square root of l) to 1.75 (square root of l)  is the length to be broached m.m.

            Machining time= effective length of broach in m.m.

                                         Cutting speed in m/min.

                                   Effective length in m.m.

                                         1000 π m/min.

Advantages and limitation of broching:


Broaching has been adopted for mass production work because of the following outstanding features and advantages.

  • Rate of production  is very high with properly  more pieces can be turned out per hour by broaching than by any other means
  • Little skill is require to perform  a broaching operations in most cases the merely loads & unloads the work.
  • High accuracy and high class of  surfaces  finish is possible A tolerance  of  +0.0075 m.m. and a surface  finish of about 0.8 microns (1 micron = 0.001 m.m) can be easily  obtained in broaching.
  • Both roughing and finishing cuts are completed in one pass of the tool.
  •  The process can be used for either internal or external surface finishing.
  • Any from that can be reproduced on a broaching can be m/ced.
  • Cutting fluid may be radily applied where it is most effective because a broach tends to draw the fluid in to the cut.

Certain reasons however limit the application of the broaching process


  • high tool cost a broach usually does only one job and is expensive to make and sharper.
  • very large work can’t be broached.
  • The surfaces to be broached can’t have an obstruction.
  • Broaching can’t be used for the removal of a large amount of stock.
  • Parts to be broached must be capable of being rigidly supported and must be able to withstand the forces that set up during cutting.

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