Cutting Tool Engineering
May 2012 / Volume 64 / Issue 5

Big and bigger

By Susan Woods, Contributing Editor

BIG Kaiser 317 Roughing Action.tif
Courtesy of BIG Kaiser Precision Tooling

BIG Kaiser’s 317 large-diameter boring system roughing a 10.5 "-dia. hole. The system can bore holes up to 46.46 " in diameter.

For practically any large-diameter boring job, there is a system that can get the job done.

Boring is a common hole finishing operation, but boring large-diameter holes is a bit of a niche market.

“Depending on the industry, you may only find a handful of shops in some areas capable of doing this type of work,” said Ron West, global product manager for Kennametal Inc., Latrobe, Pa. “They have the larger equipment necessary to do this type of work on a regular basis.”

Many boring tool manufacturers consider an 8 "-dia. hole large, a diameter that is widely bored. However, the definition of a large hole is a moving target. “A large boring tool is 40 " and above,” said Karl Hochuli, president of Wohlhaupter Corp., Centerville, Ohio. “Typically, the industry goes up to 40 " as a stock standard item, but stock standard for us is up to 128 ".”

Industries that require large-diameter boring include energy, marine, mining and construction. “Toward the smaller end (of large), 20 " and under, we do a lot with oil and gas,” said Matt Tegelman, applications manager for BIG Kaiser Precision Tooling Inc., Hoffman Estates, Ill., noting that parts include valves, blowout preventers and frac pumps.

Rough, Finish and In Between

Before a large hole is bored, a shop typically performs circular interpolation with a milling tool to remove a big portion of the stock, leaving just enough for the rough and finish boring tools.

Rough boring tools are usually twin cutters. “With rough boring, depending on the application, we can take up to a ½ " depth of cut or greater,” said Pat Nehls, product manager for Walter USA LLC, Waukesha, Wis. “If the requirements of the hole are not very precise, for instance, a hole with a fairly open tolerance such as ±0.005 ", then we can consider a tool for roughing and finishing in one operation.”

Hochuli pointed out that, for larger diameter holes, metal-removal capacities are limited by machine torque capabilities. “For example, a 40 "-dia. boring tool attempting to remove ½ " of stock per side would require more than 3,000 newton-meters of torque, which is not possible with today’s machine tools,” he said. “For that diameter, you could remove 0.062 " to 0.125 ". I would start at 0.062 " per side and work my way up until the machine and tool tell me to stop.”

Finish boring tools are single-point tools. When finishing, 0.020 " to 0.030 " of stock is usually removed in a large hole.

There are alternatives to rough and finish boring tools. “You can employ combination roughing and take out twice the amount of stock in one pass by using two roughing inserts set on different axial planes,” Hochuli said.

With a twin-cutter rough boring tool, both inserts are the same height and contact the part at the same time. A combination rough boring tool has one insert leading the other insert by some length, such as 0.012 " to 0.015 ". “You stagger the cut, and the higher insert touches the part first and might take out 0.375 " to 0.400 " of stock on one side of the part,” he continued. “The lower insert takes out the last portion of the cut, and you could take out that same amount of stock on the opposite side of the part. So you are basically taking out more stock in one pass.”

Courtesy of Wohlhaupter

A combination rough-and-finish setup was used to bore this cast iron stator housing with a 16.12 "-dia. × 19.69 "-deep bore. Even with severe interruptions in the boring operation, a P7 bore quality was achieved.

The same principle can be applied to a roughing and finishing boring operation. The roughing insert would contact the part first, and then the finishing insert would be set to the exact diameter to achieve the finish. “If you have a roughing insert on one side that sits 0.012 " to 0.020 " higher axially than the finishing insert, you can achieve good finishes and take out a lot more stock in one pass. We call that combination semifinishing and finishing,” Hochuli said.

However, a true fine-finishing operation requires a single-point tool. Finish is typically dictated by the nose radius. For instance, with an 0.008 " nose radius on a finishing insert, if the operator feeds the tool into the hole beyond 0.008 ", or beyond the nose radius, the finish starts to deteriorate. “If you keep your finish somewhere below that 0.008 " nose-radius feed rate, then your finish will be good,” Hochuli said. “The heavier the feed, the worse the finish.”

Typically, as the diameter increases, the tolerance loosens. “You don’t see too many holes bored over 10 " where you are trying to hold less than a thousandth or two,” BIG Kaiser’s Tegelman added.

Building Big

To reach these large sizes, boring tool manufacturers typically use modular systems with slides and bridges. Customers can change diameters quickly and efficiently, avoiding the need for specials.

“On our finish boring tools, for example, there are four basic components: the flange, extension slide, finish boring head and counterweight,” Tegelman said. “To cover the next range of diameters requires changing only the extension slide.”

To lighten the load, some components are made of aluminum. “Weight is critical,” Hochuli said. “The heavier the tool, the harder it is on the machine spindle. If you have too heavy of a tool, the front of it will be at one height, or plane, but at another one as the tool extends through the hole, so you are going to have runout. This is called the ‘moment of angle.’ ”

BIG Kaiser 318 Roughing 45 inches.tif
Courtesy of BIG Kaiser Precision Tooling

BIG Kaiser’s 318 large-diameter boring tool set to rough bore a 45 "-dia. hole. The 318 system includes lightweight aluminum components.

KM Bridgetool2.tif
Courtesy of Kennametal

Kennametal’s ModBORE boring bridge tooling system is for hole diameters from 25 " to 86 ".

The automatic toolchanger must also be powerful enough to take the large boring tool in and out of the spindle. With an aluminum construction, the need for manually loading large, heavy boring tools into the machine or with an overhead crane can be eliminated.

Walter USA’s Nehls noted that a large boring tool that goes back into the machine tool magazine might cover up the neighboring tool pockets, reducing the number of available tools. “We have an adapter to rotate our tool 90°, making it stand more vertical so we can open up those tool pockets,” he said.

Depending on the size, machine tools run large boring tools at around 2,000 rpm or slower, and, therefore, balance is not an issue. The twin-cutter rough boring tool is somewhat naturally balanced because there is a cutter on each side. The finish boring tools have a counterweight on one side to offset the insert and holder on the other side. Finishing tools 8 " and larger typically have a counterweight.

Machine Matters

Large-diameter boring is typically done on horizontal boring mills and horizontal and vertical machining centers with spindle sizes of CAT 50, HSK 100 or larger.

“If it is, say, 10 ", 12 " or 14 ", you can use a conventional milling machine,” said Chris Merlin, portfolio manager, general engineering for Kennametal’s Strategic Marketing Group. “It depends more on part size. If you can fit the part in the machine, you can machine the hole. But you see a lot of special equipment for really large parts. The machine has to be able to handle the part.”

Walter B3220_tiger.tif
Courtesy of Walter USA

Walter USA’s MAXI bridge-type rough boring tool has an NCT modular connection. The steel tool has a diameter range of 5.9” to 25”.

Because large boring tools are run at low spindle speeds, vibration is minimal. However, the higher the length-to-diameter ratio, the more vibration comes into play. “Typically, you want to be at less than 5:1 to be most effective,” Hochuli said.

Conventional machines might have trouble reaching into deep holes because of limited Z-axis travel. Toolholders with extensions are available, but they can cause problems with vibration, tool wear and accuracy. But on a horizontal boring mill, a quill can extend the spindle to reach deeper bores. The Z-axis (quill) and the W-axis (table) move toward each other.

“Even with the use of aluminum components that reduce tool weight, large- diameter boring tools are still too heavy to reach depths greater than 20 " from the spindle,” Tegelman said. “The quill spindle machines provide a much more stable deep-hole boring platform for large diameters.”

Machine tool supplier Methods Machine Tools Inc., Sudbury, Mass., offers Feeler horizontal boring mills, which have long quill travels. “With Z-axis travels up to 67 " and additional quill travels in the Z-axis plane to 35 ", holes can be extremely deep,” said Paul Hurtig, Feeler product manager for Methods. The company also offers a variety of quill sleeve extensions, which can be mounted to the spindle face to enhance quill support when cutting with the quill mostly or fully extended.

Mitsubishi Heavy Industries America Inc., Machine Tool Div., Addison, Ill., offers two new floor-type MAF horizontal boring mills with a 5.9 "- or 7 "-dia. boring spindle. “The quill-type spindle is not used,” said General Manager Koji Tanizaki. “The boring is done by the ram (Z-axis/headstock) and the boring spindle (W-axis). The 16.5 "-square ram extends and retracts in two stages while supporting the boring spindle, enabling boring up to 88.5 " deep within a narrow workspace—49 " ram length in Z-axis plus 39 " boring spindle length in W-axis.”

Depending on the hole diameter, typical toolholders are CAT 50, CAT 60, HSK 100 and HSK 125. For very large diameters, typically more than 40 ", the boring tool bolts directly into the spindle.

Kennametal’s ModBORE tools mount to a standard shell mill adapter or clamp directly to the spindle nose. “A shell mill adapter is a common tool,” Merlin said. “The ModBORE uses the same kind of adapter, typically a 2 " or 2½ " one, that a milling cutter mounts on.”

Cool Down

Coolant is recommended where possible to help enhance tool life and chip evacuation when large-hole boring. However, chip evacuation is not much of a problem because the hole is large and the boring tool only consumes a small portion of the hole.

FEELER FBM-Series Boring Mill-PR-image.psd
Courtesy of Methods Machine Tools

Feeler FBM-Series horizontal boring mills come with a 4.3 " or 5 " quill and a W-axis with 27.6 " of travel.

“Whatever you do, you want it to be consistent,” Wohlhaupter’s Hochuli said. “If you are running dry, as you can easily do with cast iron, you want to run it dry the whole time and use an air blaster to remove chips.” Another option is a mist to keep the cutting edge cool combined with air to blow chips forward.

Coolant can be an issue on horizontal boring mills because they are open, eliminating the possibility of using flood coolant. Through-tool coolant can be applied, but it requires guarding around the machine. “Through-coolant needs to be well controlled,” Methods’ Hurtig said. “Typically, the high-pressure coolant is not turned on just after a tool change, like in a machining center, but when the tool is just in front of the hole location or just after the boring has begun.”

The success of a large-hole boring operation ultimately comes down to the machine itself. “Understanding machine force capabilities is key,” Hochuli said. “Everything is dictated by the quality and setup of the machine. I know what our tools are capable of, but I can only run to the limits of the machine.” CTE

About the Author: Susan Woods is a contributing editor for CTE. Contact her at (224) 225-6120 or


BIG Kaiser Precision Tooling Inc.
(888) 866-5776

Kennametal Inc.
(800) 446-7738

Methods Machine Tools Inc.
(877) MMT-4CNC

Mitsubishi Heavy Industries America Inc.
(630) 693-4700

Walter USA LLC
(800) 945-5554

Wohlhaupter Corp.
(937) 885-1878

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