Precision Boring

In-process gauging and real-time diameter feedback for bore creation

September 1, 2009

In many boring applications it is up to the operator to make fine adjustments to the boring bar in order to create the correct bore.

Now with technology advances, in-process gauging and real-time diameter feedback can automatically control bore size during machining. These closed-loop systems have a bore gauge coupled with a microadjustable boring bar. This self-regulating process was designed to automatically control bore sizes without operator intervention.

This new technology comes from machine tool supplier Makino, which also created its own boring bar for this system.

“Traditional boring bars have only one insert, which makes them uneven,” explained Makino Process Development Engineer Stan Weidmer. “These boring bars have to be manually adjusted, have a tendency to chatter, and can deflect under load. In some applications these work very well, but in long L-to-D applications or in hard materials, this is not the best system.”

The use of a roughing boring bar followed by a finishing boring bar means that two machining passes must be performed to create each bore. Not only is this time-consuming, but the chance for error can occur as tools are moved in and out of the centerline of the bore.

Makino has developed what it is calling a coolant-adjustable boring bar (CABB) which uses through-spindle coolant to adjust the diameter of the boring bar. By controlling the volume of coolant that passes through the bar, users can control the diameter of the bar itself.

In addition, this CABB has two inserts on each side.

This new technology can replace a standard boring bar setup or a helical interpolation tool. While each type has its pros and cons, for a true closed-loop system, Weidmer suggests the coolant-adjustable boring bar.

System Feedback

Three choices of tooling are a standard boring bar setup (left), coolant adjustable boring bar (middle), or a helical interpolation tool (right).

“Unfortunately, when it comes to precision boring, the diameter that you set is not always the diameter that you get because of the forces involved in the cut,” said Weidmer. “That is why getting feedback in real time and using that feedback to make adjustments in the process are important.”

Feedback options that are available for this system include an air gauge, touch probing, or a linear variable displacement transducer (LVDT) gauging system.

“By collecting this data and feeding it back into the system, we are letting the machine do the thinking and make the necessary adjustments,” said Weidmer. “It creates correct first-time bores and eliminates the need for rework.”

This replaces the traditional process of bore creation, which generally comprises a rough pass, a finish pass, and finally an inspection process. If the diameter is incorrect, the operator must manually adjust the boring bar and repeat the process until the correct diameter is achieved. Also, as the insert wears during use, and therefore cutting conditions change slightly, it becomes increasingly difficult to hold the optimal diameter.

For more information, visit www.makino.com.