Boring: Never dull start to finish

It may be a boring topic, but fine, or finish, boring is anything but dull to many machinists. Those involved in making a variety of common components often need to produce holes that are highly accurate in size and location to minimize wear and lengthen part life. Others are tasked with machining holes for bearings and pistons that must be very round and straight and also can require an excellent surrounding surface finish.

When finish boring is necessary, shops can turn to a number of tools capable of meeting demanding requirements. The latest offerings include standard finish boring tools, which can be made even more effective by employing best practices, and custom tools billed as timesavers and cost-savers compared with their off-the-shelf counterparts.

For those engaged in boring operations, due to variations in semifinishing processes, one of the main challenges is getting consistent hole quality.

“Issues with hole location, straightness and roundness are often byproducts of bores that were not sufficiently prepared,” said Matt Tegelman, applications and product manager at BIG KAISER Precision Tooling Inc. in Hoffman Estates, Illinois.

Featuring an embedded digital screen with Bluetooth technology, the EWE fine boring head is designed to make it easy for operators to monitor and configure the head when used with a special mobile app. Image courtesy of BIG KAISER Precision Tooling

He believes that rough boring is the best way to prepare a hole. A twin bore roughing tool with two cutting edges can be used to ensure that a hole is round and properly located for finishing, he said.

Finish Boring Basics

A finish boring step is needed to meet tight tolerance requirements for roundness, straightness, position and surface finish. For example, Tegelman said fine boring is necessary whenever diameter tolerances are tighter than ±0.05 mm (±0.002″).

The cutting edge of a finish bar is moved by a micrometric mechanism to adjust the bore diameter. When using boring heads from BIG KAISER Precision Tooling, this usually is done with a micrometer screw, but other boring heads employ different mechanisms. Whatever the mechanism, he said the goal is the same: to adjust the cutting diameter in the finest possible increments to accurately control bore size.

Although it may sound challenging, making very slight adjustments to fine boring tools can be easy. Using the Romicron fine boring system from Latrobe, Pennsylvania-based Kennametal Inc., for example, each 1 µm (0.00004″) radial adjustment can be made simply by turning the adjusting ring until a click is heard, said Michael Hacker, senior global product manager for hole finishing at Kennametal Shared Services GmbH in Fürth, Germany.

Besides having finer adjustment mechanisms than rough boring tools, fine boring tools normally have just one cutting edge while rough boring tools typically have two or three.

“When you have two or three cutting edges, it is very hard to set them all exactly to the same size,” Tegelman said. “So with a roughing tool, it is typically very hard to get a surface finish comparable to what you can get with a single-point tool.”

Set to the right parameters, however, Hacker said a rough boring tool often can produce the same results as a fine boring tool if both tools are equipped with identical inserts. Frequently, he said, the key difference between a fine and rough boring tool is the relative ease of presetting a fine boring tool.

“It might take a lot of trials and time in the presetting room to use a rough boring tool instead of a fine boring tool,” he said.

Tegelman said reamers also can equal fine boring tools in both accuracy and finish quality. A reamer typically includes a cylindrical shank and one or more carbide or industrial diamond cutting edges. Productivity can be high when using multiflute reamers because of the feed rates allowed by multiple cutting edges.

The Romicron fine boring system is designed to allow easy, precise manual adjustments. Image courtesy of Kennametal

On the downside, though, consumable costs associated with a reamer are high compared with indexable inserts on boring tools.

“When (a reamer) has reached the end of its life,” Tegelman said, “it is generally recycled or tossed.”

Moreover, he said allowable cutting-edge adjustments for reamers are usually small and limited to compensation for wear. By contrast, he said boring tools typically have a much greater adjustment range and can be used for several different bore sizes.

Best Boring Practices