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Cutting Chamfers The cutting part of a tap is the teeth of the chamfer on the leading end of the tap. The rest of the thread length is the cylindrical guiding part, which is slightly back-tapered for clearance. A decision on the best type of chamfer form has to be carefully made as both the tap life and quality of thread are greatly affected. Generally, the form and length of chamfer depend on the type of hole to be tapped. Though holes do not normally cause difficulties whereas the tapping of blind holes can create certain problems associated with chip evacuation.                              Blind Hole                          Through Hole The length of the chamfer is determined by careful consideration of the following factors: The number of chamfer cutting teeth cannot be kept too low so as to avoid overloading, premature dulling and oversized or rough threads. A chamfer lead that is too long, however, increases the torque and the danger of breakage. Commonly used chamfers are taper, plug and bottoming Eight to ten cutting teeth per land are produced by a taper chamfer. A plug chamfer produces three to five cutting teeth per land and a bottoming chamfer one to two cutting teeth per land. The recommended radial relief behind the cutting edge of the chamfer portion is .004" to .005" relief per 1/16 of land width. Regrinding Taps The cutting edges of a tap become dull during cutting at a rate dependent on cutting conditions and the material being cut. With increased wear the cutting torque increases and breakage will occur if sharpening is not carried out in time. Increased wear of the cutting edge is also indicated by poor chip formation. In addition, the surface finish of the thread flanks is poor and burr formation is perceptible. Special attention should be paid to dull cutting edges of small diameter taps because the weak tap cross-section cannot endure the higher cutting torque and breakage may occur. For good thread cutting the following principle is valid: “Sharpen frequently.”