Most Common Profile for Countersinking

Though various tool profiles exist for countersinking, the most common profile type is with a pilot. This design features positive stops which allow for a true coining, controlling both material roll-over and crowding. This ensures a consistent profile each time the tool is used.

The disadvantage of this tool is that the stops are set for a given material thickness. However, with the interchangeable inserts for form-down countersinks, you can easily change coin sizes and reduce tooling costs. You no longer need to trial and error on pre punches and struggle with an inconsistent profile.

Pre-Punching a Pilot Hole

In addition to positive stops, there is another parameter to control the quality of the countersink. It is the process of pre-punching a pilot hole prior to coining. This pre-punched hole provides an opening for the pilot stop to enter the material, control material flow, and ultimately result in a burr free pre-determined bottom hole size. A rule of thumb for determining a pre-punch hole size is: Pre-Punch Hole Size = B – [(B – C) x .75], where “B” is the largest opening at the top of the countersink and “C” the desired opening at the bottom.

What controls the location of the screw? 

A screw placed in a countersunk hole is located by the angle. Holding a close tolerance on the “C” dimension is unnecessary and can be risky. The reason that location is often tied to the “C” dimension is that physically checking the hole location is easer on the “C” dimension than trying to find the theoretical center of the countersink angle.

Another misconception associated with this special is that the “C” dimension must be held to a close tolerance so that there will be as much material to support the screw as possible. The truth of the matter is that in many cases the thickness of the material at the “C” dimension is so thin that holding a close “C” dimension adds an insignificant amount of support. 

A common problem is not allowing enough material in which to hold the “C” dimension, (see example 2). It is not uncommon for a customer to complain that they are having trouble holding the “C” dimension. Upon examining the height of the “C” dimension we often find it to be only a few thousandths. It should never have a height of less than 0.010 (0.25mm). With some of the steel coming in on the low end of the thickness tolerance, problems with the “C” dimension are not uncommon. 

For more information on creating countersinks in punching machines visit wilsontool.com/countersinks or contact a Tooling Technician. 

February 10, 2020