![]() The NC spot drill is used as a lead tool prior to drilling to create the 90° included angle. Today, many machine shops apply an NC spot drill with a 90° point angle to accomplish spot drilling, countersinking and chamfering, which saves tool-change time and reduces tooling costs. Formerly, the customary way of machining a chamfer on a hole to be threaded was to apply a 120° or 90° countersinking tool after the initial drilling operation. This will eliminate the burr and provide enough depth to act as a starter for a mating bolt.Ĭreating a chamfer or countersink can be accomplished in various ways, some of which are more efficient than others. The most common practice is to apply a chamfer diameter that is 0.010" to 0.015" (0.254mm to 0.381mm) larger than the thread’s major diameter. While a DIN standard does not exist, there are a few well-worn machinist rules of thumb. However, it is frequently left to the discretion of the machinist or programmer to determine the depth or OD of the chamfered hole. The OD and depth of the chamfer sometimes are specified on the blueprint. ![]() Threaded-hole chamfers normally have included angles of 120° or 90°, with 90° being most common. And it’s one that can usually be improved. As a result, this is one of the most common machining operations. ![]() And adding a chamfer or countersink to a hole that will be tapped will always aid when starting the tap.įor reasons such as these, the vast majority of threaded holes require some type of chamfer or countersink. ![]() In addition, when the mating part is placed on a burr, the burr can be forced down, effectively deforming the internal thread and increasing the potential for cross threading.Ī chamfer or countersink may also be specified on a part drawing to help a bolt properly align or start. The purpose of chamfering or countersinking an internal threaded hole is typically to avoid creating a raised burr that can prevent a mating part from properly seating with another flat surface. ![]()
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