Tech Talk
What is involved in the “Grinding of Broaches”?
Cutter bars, cylindrical and surface broaches are usually sharpened by grinding on the face of the teeth with a dish wheel. A small cup wheel is used for backing off the teeth to provide proper relief.
Special grinding machines are available for the sharpening of broaches of all kinds, with precision and speed. Some of these machines are designed especially for handling round, spline and cylindrical broaches. Another style is designed to take care of flat or surface broaches while a third is of a universal type for sharpening both cylindrical and surface broaches.
For sharpening square or rectangular broaches the machines are equipped with a swivel head slide which can also be turned in a horizontal plane. Thus broaches with teeth cut at an angle can be sharpened as easily as those with the teeth cut straight. With the broach held stationary, the grinding wheel is moved back and forth across the face of the tooth, taking light cuts.
Round broaches are sharpened by revolving on centers. Depending on the type of sharpening machine, the grinding wheel is lowered, or fed horizontally, into position adjacent to the tooth to be ground and the table then moved to bring the tooth into contact with the wheel.
The manufacturer of the broach should be consulted in regard to the correct top relief and amount of rake or undercut. These angles depend upon the material being cut. As a general rule, for cutting steel, the top relief is 1 ½ and the teeth are undercut 10 to 12. For cast iron and non-ferrous metals, the angles are usually less.
In the process of resharpening, it is quite important to grind the gullet between the teeth so that it is uniform, follows the original contour and blends into the back of the preceding tooth so as to permit the proper curling and disposal of the chips. Tearing of the surface being broached, creation of excessive heat, or unusual demands on the power required to pull the broach, may be caused by careless grinding of the gullet.
