Chamfers and Countersinks That Halt Burr Formation
Chamfers and countersinks help control burr formation, improve thread starts, and protect hole quality.
Quick take: Chamfers and countersinks are small features that have outsized impact on burr control, thread starts, and downstream hole quality. This page is strongest when it is paired with tapping and threading references rather than treated as a stand-alone edge-break step.
Related references: Tapping Deep Holes: Guide to Chip Evacuation and Tool Life, Pipe Tapping: Guide to Taper Pipe Threads, and Comprehensive Threading Calculators.
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. 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.
A chamfer or countersink may also be specified on a part drawing to help a bolt properly align or start. And adding a chamfer or countersink to a hole that will be tapped will always aid when starting the tap.
For reasons such as these, the vast majority of threaded holes require some type of chamfer or countersink. As a result, this is one of the most common machining operations. And it’s one that can usually be improved.
All images courtesy of Emuge.
Threaded-hole chamfers normally have included angles of 120° or 90°, with 90° being most common. The OD and depth of the chamfer sometimes are specified on the blueprint. However, it is frequently left to the discretion of the machinist or programmer to determine the depth or OD of the chamfered hole.
While a DIN standard does not exist, there are a few well-worn machinist rules of thumb. 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. This will eliminate the burr and provide enough depth to act as a starter for a mating bolt.
Machining Methods
Creating a chamfer or countersink can be accomplished in various ways, some of which are more efficient than others. 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.
Review the print ads from this magazine to continue
This quick advertiser review unlocks the rest of the article and keeps the full-screen reader focused on the ads instead of the page chrome.
