This may be a boring topic, but it’s good, or after all, boredom is by no means boring for many mechanics. People involved in the manufacture of various general-purpose components usually need to produce highly accurate holes in size and location to minimize wear and extend the life of the parts. The task of others is to make holes for bearings and pistons. These holes must be very round and straight, and also require excellent surrounding surface finish.

When fine boring is required, the workshop can use a variety of tools that can meet the demanding requirements. The latest products include standard fine boring tools, which can be made more effective by adopting best practices, and custom tools that can save time and cost compared to off-the-shelf similar products.

For those engaged in boring operations, one of the main challenges is to obtain consistent hole quality due to changes in semi-finishing processes.

"Hole location, straightness, and roundness issues are often byproducts of under-prepared holes," said Matt Tegelman, application and product manager at BIG KAISER Precision Tooling Inc., Hoffman Estates, Illinois.

The EWE fine boring tool head uses an embedded digital screen with Bluetooth technology, which is designed to allow the operator to easily monitor and configure the tool head when using a special mobile application. Image courtesy of BIG KAISER Precision Tooling

He believes that rough boring is the best way to prepare holes. He said that a double-hole roughing tool with two cutting edges can be used to ensure that the hole is round and suitable for finishing.

A fine boring step is required to meet strict tolerance requirements for roundness, straightness, position, and surface finish. For example, Tegelman stated that as long as the diameter tolerance is less than ±0.05 mm (±0.002"), fine boring is required.

The cutting edge of the finishing rod is moved by the micrometer mechanism to adjust the hole diameter. When using the boring head of BIG KAISER Precision Tooling, it is usually done with a micrometer screw, but other boring heads use a different mechanism. Regardless of the mechanism used, he said the goal is the same: adjust the cutting diameter in as small increments as possible to accurately control the hole diameter.

Although this sounds challenging, fine-tuning fine boring tools is easy. For example, using the Romicron fine boring system of Kennametal Inc. headquartered in Latrobe, Pennsylvania, you only need to turn the adjustment ring until you hear a click to perform radial adjustments of every 1 µm (0.00004"). The advanced global product Michael Hacker said the hole processing manager of Kennametal Shared Services GmbH in Fürth, Germany.

In addition to having a finer adjustment mechanism than rough boring tools, fine boring tools usually have only one cutting edge, while rough boring tools usually have two or three cutting edges.

"When you have two or three cutting edges, it is difficult to set them all to the exact same size," Tegelman said. "Therefore, with roughing tools, it is often difficult to obtain a surface finish that is comparable to the surface finish that can be obtained with a single-point tool."

However, with the correct parameters set, Hacker said that if both tools are equipped with the same insert, the rough boring tool can usually produce the same results as the fine boring tool. He said that usually the main difference between fine boring tools and rough boring tools is the relative ease of pre-adjusting the fine boring tools.

"Using rough boring tools instead of fine boring tools in the presetting room may require a lot of trial and time," he said.

Tegelman said that the reamer can also be comparable to fine boring tools in terms of accuracy and finishing quality. Reamers usually include a cylindrical shank and one or more carbide or industrial diamond cutting edges. Due to the allowable feed rate of the multi-blade reamer, the productivity can be very high when using a multi-blade reamer.

The Romicron fine boring system is designed for simple and precise manual adjustment. Image courtesy of Kennametal

But the downside is that compared with indexable inserts on boring cutters, the cost of wearing parts associated with reamers is higher.

"When (the reamer) reaches the end of its useful life," Tegelman said, "it is usually recycled or thrown away."

In addition, he said the tip adjustment allowed by the reamer is usually small and limited to compensating for wear. In contrast, he said that boring cutters usually have a larger adjustment range and can be used for a variety of different apertures.

In order to increase productivity and shorten cycle time, fine boring tools and reamers can be used together. Hacker pointed out that fine boring provides better positioning accuracy than reaming, but the process is relatively slow, and it is not better than reaming in terms of roundness, straightness and surface finish. Therefore, the shop can first use a fine boring tool to make a pilot hole—a depth equal to the hole diameter or less is usually sufficient—and then switch to a reamer with six or more teeth to quickly complete the hole.

When cycle time is not the main consideration and the workpiece is a precious workpiece that the manufacturer cannot lose due to boring errors, he recommends a two-step process, leaving twice the normal amount in the hole is boring. In the first step, the tool removes half of the blank and then removes it from the hole in order to insert a probe to measure the hole. This data is sent to the machine control system, which uses it to calculate the deviation from the required aperture. According to the calculated deviation, the process parameters are adjusted so that the boring will be more precise when the tool is reinserted into the hole in the second step to remove the other half of the blank.

Hacker said that this two-step fine boring process is particularly useful when the workpiece is unknown—for example, the exact hardness of the material affects the hole diameter of the tool.

When it comes to the quality of fine boring, Tegelman said, “The first problem I dealt with was inappropriate inventory balance”, which would lead to poor surface finish.

For most standard applications, he recommends that the diameter allowance be equal to or slightly larger than the blade nose radius of the tool. For holes with extremely tight tolerances, he said that the blank margin should be reduced, and the blade should have a sharp cutting geometry.

For adjustment purposes, the precision grinding blades in these precision boring tools are pushed radially by tapered screws. Image courtesy of Hollfelder-Gühring

Tegelman believes that in recent years, fine boring has been improved from the appearance of a boring head with a digital display to show adjustment increments. Today, he said that the EWE digital fine boring head of BIG KAISER Precision Tooling can communicate with the software, not only can display and record the offset adjustment information, but also use the last measured aperture value to determine the output diameter of the next cutting, and then display Where the diameter lies within the tolerance zone of the application.

The company is currently developing a closed-loop boring tool that uses back and forth communication between the tool and the online probing device in the machine to adjust itself. These tools have a boring head that uses a sensor instead of a fine-tuning screw mechanism, and a small motor that adjusts the size based on the sensor's measurement data. BIG KAISER Precision Tooling hopes to release products equipped with this technology later this year.

In addition to standard fine boring tools, the workshop can also choose customized fine boring tools, such as those provided by Hollfelder-Gühring GmbH of Nuremberg, Germany. (Guhring Inc. is located in Brookfield, Wisconsin.) One of the main differences between the company’s custom knives and competing products is that Hollfelder-Gühring does not use typical ISO blades, said Henry Hack, production and technology manager.

Typical inserts are held in place by screws.

"If the blade moves in the pocket holder, the customer will lose part accuracy," Hack said. "However, if your insert fits tightly to the pocket seat, there will be no movement."

Hollfelder-Gühring's fine boring tools have a prismatic pocket seat. He said that instead of using screws to hold the blade in place, these tools use a self-clamping system in which the cutting force pushes the blade into a prismatic seat and holds the blade firmly in place.

In addition, Hack said that the company's precision grinding blades can produce tighter diameter tolerances and better surface finishes compared to similar ISO products. He also touted the positive cutting geometry of the blade and the very sharp cutting edge, which can reduce the cutting force, thus making the blade suitable for fine boring of thin-walled parts and other precision parts.

He said that Hollfelder-Gühring's fine boring tools benefit from a system that allows users to make micrometer adjustments to the blades in their pocket holders to strictly control the hole diameter. He said that the combination of precision ground blades and high-precision adjustment systems allows the company to produce precision boring tools with two or three blades instead of a single cutting edge on traditional precision boring tools.

This is a great advantage for mass production applications in the automotive industry and other industries.

Generally, “if your part needs to use three different diameters and step lengths in one hole, it means you need three tools and three manufacturing steps, which takes a long time,” Hack said. "But we can use a customized tool made for all three diameters to complete the entire hole in one go, thereby reducing cycle time."

Although Hollfelder-Gühring's custom fine boring tools are "a little more expensive" than competitive standard products, he says the benefits outweigh the higher price tag.

"In the field of modern metal cutting, everyone is talking about the cost of each part," Hacker said. "We believe you will be able to reduce the cost of each part by using our tools."    

William Leventon is a contributing editor of "Cutting Tool Engineering" magazine. Contact him by phone 609-920-3335 or email to wleventon@gmail.com.

BIG KAISER Precision Tooling Inc. 888-866-5776 www.bigkaiser.com

Guhring Inc. 800-776-6170 www.guhring.com

Kennametal Corporation 800-446-7738 www.kennametal.com

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