SECTION 5.6

Sharpening a round or spline broach requires following a slightly different procedure. The diameter of the grinding wheel is normally the same as the root diameter (gullet diameter) of the broach tool. This is due to the cutting action between the grinding wheel and broach tool. Since the grinding wheel is now in contact with another cylindrical shaped object and is following an existing face angle, the process becomes slightly more complicated. Now the grinding wheel sharpens with the entering and exiting sides of the wheel, which should cause a cross-hatching pattern on the face of the cutting tooth. This pattern can help determine if the center of the grinding wheel is central to the broach tool; the pattern should be symmetrical.

Figure 5-3.
Formula For Setting Sharpening Head

The formula for calculating the amount of tilt to place on the grinding head is (see Figure 5-3):

Face Angle x (Wheel Diameter / Root Diameter) = Head Angle

If the wheel diameter is the same as the root diameter, the grinding head angle would be the same as the broach face angle. This is not always practical, however, and the formula should then be used. For example, if the required face angle is 15 degrees, the root diameter is 0.850", and the closest wheel diameter in stock is 2", the head angle would then be set at 35 degrees (see example).

Example : [15° x (2.00" / 0.850") = 35°]

Since an internal broach is a tapered tool, the root diameter at the finishing end of the broach tool is larger than that of the first cutting tooth. The standard practice is to start with a particular set-up (head angle and diameter of grinding wheel). The first tooth to be sharpened is normally the last tooth of the broach. As the gullet decreases with every tooth you progress forward, the size of the grinding wheel also decreases from wear and constant wheel dressing. The number of spaces per interval to move forward and the amount of the wheel to be trimmed comes from experience.

Once the sharpener reaches the front end of the broach tool, a new wheel must be mounted on the head and the process repeated, this time with a polishing wheel. The polishing operation should be given extreme attention to insure the gullet radii are perfectly blended. If the face angle was ground too deeply during the face sharpening operation, the polishing operation will not properly clean up the gullet. The polishing wheel should also barely touch the face, removing a minimal amount of stock.

Throughout the polishing and sharpening operations, the broach should rotate in the same direction as the wheel. This allows greater cutting action, as opposed to a rolling action. The speed of rotation depends on the root diameter of the broach...the smaller the diameter, the faster the speed