In this article we will discuss about the techniques of internal and external broaching.
Internal Broaching:
Simplest of all the internal broaching operations is the broaching of internal holes. This operation is cheaper than drilling and reaming if a hole already exists, because of the accuracy and speed of operation. Broaching is preferred even for shaping thinner parts compared to press due to accuracy obtainable.
(i) Rotary-Cut Broaching:
In the case of round broaches the first tooth which encounters the hard scale layer gets worn out first; then the second tooth takes is its own load plus the share of first tool and wear out even faster. This problem is overcome in rotary cut broaches by arranging the teeth to take relatively deep but narrow cuts below the hard skin.
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In this design, the starting teeth are produced with narrow cutting points and the rise per tooth is more. The semi-finishing teeth are of the same diameter but are arranged in a staggered pattern, to remove the stock left by the first group. Finishing teeth are of conventional type. Such cutters are used for round as well and other shapes.
(ii) Double Cut Broaching:
In this type of broach, first teeth have deep chip breakers and second teeth are completely round and these remove the metal left by the grooves in the first teeth. This type of pair of teeth is provided with the rise per tooth only between the first tooth and successive pairs. These are also available for all types of shapes. Their length is obviously more than conventional broach but power requirement is less.
Other broaching processes include polygon broaching, key way broaching, spline broaching, turret broaching and rotary broaching.
External or Surface Broaching:
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This operation is performed on a surface broaching machine. However, with special fixtures, external broaching operation can also be performed on internal broaching machine. Suitable fixtures are required in case of external broaching to resist the side thrust.
In surface broach, the teeth are usually inclined at an angle across the face to produce the shear effect and thus cutting easily and smoothly.
Surface broaching is a fast operation to produce accurate smooth finish. Life of surface broaches is excellent. Workpiece must be strong because of high cutting pressure involved or else they must be properly backed up in fixtures. Surface broaching fixtures should be designed as ruggedly as possible.
Clamps should be heavy so that they will not bend or spring under the load of the cut. It is possible to surface broach the intricate shapes successfully. All indexing mechanisms should be designed such that no back lash can occur during broaching operation. Before broaching, it should be ensured that the work is firmly and fully seated in the fixture and is properly aligned.
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(i) Progressive Broaching:
Progressive broaches are used for wide surface and for cutting through hard skin. Fig. 17.6 shows a typical progressive broach in which two sets of roughing teeth are positioned in the form of an inverted-V. Each tooth of these is designed to take a cut to full depth but it covers only a small width. Finishing teeth are provided at the end.
(ii) Double Cut Broaching:
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It has similar concept as for double-cut internal broaching.
(iii) Straddle Broaching:
In this case two, similar, parallel, opposed surfaces are broached simultaneously.