May 2011 / Volume 63 / Issue 5|
By Susan Woods, Contributing Editor
Courtesy of BIG Kaiser Adjustable finish boring heads allow for a range of diameters to be machined with one tool.
inish boring can take some time but achieves tight hole tolerance and fine surface finish requirements. And if a fine finish and tight tolerance are not required, rough boring might suffice.
“The main reason you would want to perform finish boring is based on tolerance,” said Jack Burley, vice president of sales and engineering, BIG Kaiser Precision Tooling Inc., Hoffman Estates, Ill. “If the tolerance is tighter than ±0.005 " we usually recommend a finish boring operation. And when you use a finishing tool, you are going to not just achieve the tolerance but also a very fine surface finish.”
Finish boring imparts a surface finish from 0.4µm Ra to 3.2µm Ra compared to 3.2µm Ra and up for rough boring and from 6.3µm Ra to 12.5µm Ra for drilling, according to Donato Pigno, manager, product management, Komet of America Inc., Schaumburg, Ill.
There are numerous reasons part designers require a tight tolerance and fine surface finish on holes. Holes that have bearings, seals or shafts running through them are often fine-bored. Some of the more common parts include connecting rods for large diesel engines and cylinder blocks in car engines.
While other types of finish boring tools exist, the focus of this article is on adjustable finish boring heads used on vertical and horizontal machining centers. The biggest advantage of adjustable boring heads is they cover a range of diameters. It would be almost impossible for a shop to stock individual boring tools for all possible hole sizes. Having such a selection would also be cost-prohibitive, even though an individual tool costs less than an adjustable one. Adjustable boring heads for small diameters start around $1,000; larger-diameter boring heads start around $3,000.
Courtesy of BIG Kaiser
“From a practical standpoint, we like to know not just the size the customer needs but the production requirements for that boring tool going forward,” BIG Kaiser’s Burley said. “When we know they are looking at low production, we suggest systems that offer the highest degree of flexibility for other diameters and depths.”
Another way finish boring tools are flexible is insert independence. “Finish boring tools, especially when based on ISO or ANSI turning inserts, are independent from the manufacturer of the head,” said Thilo Mueller, senior product manager, drilling for Kennametal Inc., Latrobe, Pa. “With boring heads, you can take it off the shelf and put in any standard turning insert you have, so you can always use whatever is the latest in turning inserts.”Small, Medium, Large
Boring head styles vary based on hole diameter. For holes under 1", a boring head is too big to fit inside. Instead, the head remains outside the hole and a small boring bar with an insert that fits in the hole is attached to the head. When the boring head is adjusted for diameter, the bar and insert move in and out eccentrically from centerline.
In the case of intermediate-size boring heads, say, holes from 1" to 8", the head with the insert holder attached fits inside the hole. The main part of the tool is going to be on centerline so it is not moving off-center. Just the holder moves in and out for different diameters.
With large-diameter boring heads for holes from 8" to 40", the whole head moves eccentrically. The head, holder and insert are on one side and a counterweight is on the other side. “With large diameters, you are going to have a bridge and extension slide that the boring head sits on,” said Brent Godfrey, industry and applications specialist for Sandvik Coromant Co., Fair Lawn, N.J. “If you want to add an inch to the diameter, you push that slide out about ½" and the counterweight also moves out ½".”Fine Adjustment
To adjust them for the required hole diameter, boring heads feature a graduated micrometer screw, or dial screw. As the dial is advanced or retracted with a hex wrench or by hand, it moves the insert holder along with it. Typically, the boring heads adjust to within 0.0005" on diameter, which is accurate enough for most applications. Some can adjust to 0.0001" on diameter with vernier markings, a secondary scale. Other heads are available that can be adjusted in very small increments—0.00008", for example.
BIG Kaiser’s EWB-UP ultraprecision boring head can be adjusted to 0.00005". “EWB-UP lets you get the fine adjustment, and that is why we bought it,” said Greg Husman, process optimization manager for Applied Engineering Inc., a Yankton, S.D., machine shop. “A lot of these tools come in 0.0005" increments, but when you are dealing with a total tolerance of 0.0003", you can’t be adjusting with a 0.0005" tool. We were recently awarded jobs that required that 0.0003" so we had to purchase the EWB.” Applied Engineering mainly performs high-speed machining of aluminum parts for the aerospace and defense industries.
Courtesy of BIG Kaiser
Many toolmakers also offer digital readout capabilities. “You still have the adjustment piece, but on the other side you have a place to plug in a digital readout,” said Duane Drape, national sales manager for HORN USA Inc., Franklin, Tenn., which offers the URMA fine boring head. “The digital readout is much easier for the operator to read and understand how much they are adjusting the bore diameter.”
Once the correct adjustment is made, a locking system prevents any diameter shift during boring. In normal operation, the operator loosens the lock screw, makes the adjustment with the dial and then locks the screw.
Courtesy of Sandvik Coromant
Courtesy of Komet
Other digital offerings include BIG Kaiser’s EWN 2-54D digital fine boring head, for the hole diameter range of 0.078 " to 2.125 ", which supports spindle speeds up to 20,000 rpm. The boring head features an integrated LCD display with one-button operation.
To provide a display wirelessly, Komet’s MicroKom BluFlex fine adjustment system is equipped with Bluetooth technology. The display is separated from the head making it more convenient to read the data. The user can attach the external display unit anywhere near the machine.Deep and High Speed
As hole depth increases, so does the challenge. “If you get beyond a 4:1 depth-to-diameter ratio, we recommend switching to a reinforced holding mechanism of some sort,” Drape said. “If the depth-to-diameter ratio is more than that, then we work with an overall dampened system to eliminate the harmonics that are sure to be there.”
Most companies offer modular systems for effectively boring deep holes, such as extension adapters to make a long assembly. “We feature Coromant Capto center bolt-style clamping that you use to screw each segment together,” Godfrey said. “It provides huge pull force, which helps reduce deflection and minimize runout.”
For large-diameter deep holes, manufacturers offer aluminum adapters to reduce weight. The head can also be made of aluminum to lighten the load for any depth.
When boring at spindle speeds of approximately 4,000 rpm or higher, balancing is usually necessary to maintain accuracy. “Balancing means correcting the force caused by the imbalance of the cutting edge being moved inside and outside of centerline,” Burley said. “This imbalance causes force that tends to make the hole out-of-round or makes the tool vibrate when you run it at high speeds.”
Most adjustable boring heads feature automatic, integrated balancing. The insert cartridge is on one side and the counterweight is on the other side. When the insert side is adjusted, the counterweight side automatically adjusts.Materials and Inserts
Finish boring operations generally require light DOCs under controlled stock allowances. More than the recommended stock removal could mar the surface finish because of the higher cutting force of the tool and lower feed. Komet’s Pigno recommends removing 0.002 " to 0.015 " of stock when finish boring steel and up to 0.157 " for aluminum applications.
For any finish boring operation, whether it is a relatively soft material such as aluminum or a hard material like tool steel, applying an insert with the correct edge radius is critical. Normally, a sharp cutting edge is necessary. “Sharp inserts are good for finish boring because they create really low cutting forces,” Godfrey said. “And because it is such a light DOC, using the sharp edge is good on just about any material, whether it is aluminum, a heat-resistant alloy or even steel and cast iron.”
For deep-hole finish boring, a sharp insert—small radius—reduces the amount of force on the cutting edge. This creates less tool deflection, which means less vibration.
On the other hand, when finish boring short holes under stable conditions, a duller insert—larger radius—run at a higher feed imparts a fine surface finish while providing long tool life, Burley noted. “In other words, pressed inserts,” he said. “The tool is so short it would be hard for it to get into a vibration pattern, so you can apply more cutting force to it before it starts bending.”
During finish boring, chip flow is critical, particularly for deep holes. The ideal chips are very small and quickly evacuate from the hole with coolant. Some stringier materials, such as stainless steel and low-carbon alloy steels, do not break up into fine chips and can develop into a “bird’s nest.” The nest is usually not problematic if the stock allowance is not too great and it does not attach itself to the tool. The nest tends to move forward and out of a through-hole or comes out with the tool in a blind-hole. If not, a quick air blast will remove it.
Courtesy of Sandvik Coromant
“The worst, though, are the shoestrings, or long strings, which tend to wrap themselves around the tool,” Burley said. “They indicate you need to adjust something—your chipbreaker, stock allowance, feed rate, coolant application.” He added that those chips tend to mar a hole’s surface finish.
One final consideration for surface finish quality is insert wear. The major reason to change an insert is when surface finish quality declines, according to Kennametal’s Mueller. “Because you are not taking a lot of material,” he said, “you don’t see the wear characteristics you see with other inserts.”
[Editor’s note: Jack Burley of BIG Kaiser Precision Tooling served as a technical adviser for this article.] CTEAbout the Author: Susan Woods is a contributing editor for CTE. Contact her by e-mail at email@example.com.
Courtesy of Kennametal
Keeping boring in the loop
BIG Kaiser Precision Tooling Inc.
HORN USA Inc.
Komet of America Inc.
Sandvik Coromant Co.
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