by Marlon Blandon
There’s more than one way to drill, thread and chamfer that hole
The majority of threaded holes will require some type of chamfer, and this operation can often be improved upon. Threaded hole chamfers are normally included angles of 120° or 90°, with 90° being the most commonly applied. The outer diameter of the chamfer or depth can be specified on the blueprint, however this is frequently left to the discretion of the machinist or programmer to determine the depth or final outside diameter of the chamfer hole. While a DIN standard does not currently exist, there are a few well-worn machinist rules-of-thumb applications. The most common practice is to apply a chamfer diameter, which is 0.010 to 0.015 in. larger than the major diameter on internal threads. This will eliminate the burr and provide enough depth to act as a starter for a mating bolt.
Applying a chamfer or countersink can be accomplished in various ways, some of which are more efficient than others. The customary way of applying a chamfer on a threaded hole was the use of a 120° or 90° countersinking tool as a secondary operation after the initial drilling operation. Today, many machine shops will use a 90° NC spot drill to accomplish the spot drilling, countersinking and chamfering, to save tool change time and reduce tooling cost. The NC spot drill is used as a lead tool prior to the drilling operation to create the 90° included angle. While this may be convenient, it is not recommended for holes that are drilled with newer, high penetration rate carbide drills. High penetration rate carbide drills are manufactured with a 140° point angle. Using a 90° spot drill prior to a 140° carbide drill will lead to chipped corners on the drill and premature failure. Therefore, the tooling sequence needs to be changed if a drill for a carbide tapped hole is to be employed.
Proper tool sequence for producing a chamfered threaded hole with a carbide drill: (1) Drill (2) Chamfer (3) Tapping
Following this sequence, a 120° or 90° countersink tool or a chamfer mill is the best choice for applying the threaded hole chamfer, and tool life of the carbide tapped hole drill will be increased dramatically. Care should also be taken when aligning to countersink or chamfer mill the tapped hole to ensure part quality and function. A chamfer mill can be used to cut a circular chamfer with a circular interpolation, but may not be the best design for a straight plunging routine. A traditional HSS countersink tool is designed for plunging applications, but they are not designed for a chamfer milling path.
Applying the chamfer
The most efficient way to apply a chamfer to a hole to be threaded is by using a combination drill and countersink step drill. By applying a 90° countersink step angle to the drill, the hole and chamfer are applied in one operation. Combining two machining operations into one reduces tool change time, reduces the machining time and also reduces tool inventory levels.
Another benefit of a step drill is that the location of the chamfer is concentric with the hole when a step drill is used to produce the chamfer. If a step drlll is selected as the preferred method of hole preparation, the operator should be wary of using modified standard drills where the pilot minor diameter is ground onto an existing single diameter drill. The web thickness of the minor diameter will most likely be too thick and chip flow will be impeded. The minor diameter is commonly applied without a margin for clearance, which will also lessen the performance of the drill by creating friction on the outside diameter.
Proper step drills, especially carbide drills, should be ground from a carbide rod so that the web and margin is designed for the application. Step drills are offered from stock. Some manufacturers offer quick shipment programs that allow the user to vary the tapped hole depth in blind hole applications. SMT
Marlon Blandon is thread mills product manager for Emuge Corp.