Boring is a Good Way to Go
Add gearmaking to your multitasking machining repertoire
By Jim Lorincz
There are a lot of different reasons to buy large machines. One obvious reason is that industries like aerospace, energy/oil and gas, and agricultural/construction/mining equipment have an insatiable appetite, or so it seems, for large workpieces. Another reason is that shops want to add capacity that separates them from the pack of shops with banks of small- or mid-sized machine capacity. As a result, large boring mills with advanced contouring head technology and multitasking mill turns (or turn mills) with advanced programming and cutting tool technology are being tasked to produce a wide range of traditional workpieces. To the familiar pump housings, valves, and blowout preventers, you might also want to add large gears as they emerge from being strictly a niche machined product to one readily produced on multi-tasking, five-axis machining centers.
When Production Machine & Tool (Wichita Falls, TX) decided to move up to the “mega-part” league of machining, it selected the RT 1250 U rotary-table horizontal boring mill with a powerful milling spindle and contouring head from Fives Giddings & Lewis (Fond du Lac, WI). “We built our business by offering capabilities other shops in our market typically do not, and this acquisition fits our business strategy perfectly,” said Mark McMullen, president of Production Machine & Tool. “There’s nothing else that can match the capability and versatility of this machine for large-part metalcutting. It is the first ‘no compromise’ design that delivers the full reach and power of a boring mill, while integrating a U-axis contouring spindle that uses standard tools with automatic toolchanging,” said McMullen.
“This machine will accommodate parts up to 60,000 lb [27,200 kg], which is 15 times greater than anything we’ve done before, allowing us to tackle the industry’s largest workpieces, such as blowout preventers, complete fluid ends and pump cases, to name a few examples,” said McMullen. Both spindles on the RT-1250 boring mill are powered by a single 56-kW motor, and the milling spindle’s 1-m Z-axis reach is complemented with 2500-mm W-axis travel in the table.
“The large parts we are aiming for require about 90% milling and 10% contouring, and the boring mill platform gives us unmatched metal-removal muscle,” said McMullen. “Currently, there is no single-platform solution that can match the milling capability of this machine and still do contouring with an integrated spindle that employs standard tools. In addition, the live boring spindle can use 50-taper tools up to 750-mm long, as well as programmable boring bars. We believe this machine’s unique capabilities will be the wave of the future for large-part machining, cutting the cycle time and improving the quality,” said McMullen.
The machine’s contouring head slide stroke allows creation of turned features up to 540-mm diameter without head changing or manual intervention, so complex features can be machined in one setup with reduced cycle time and labor. The contouring head can produce features such as bottle bores, valve seats, seal faces, phonographic sealing surfaces, O-ring grooves, straight/tapered threads, chamfers, external profiles and others. It is located immediately above the machine’s main spindle. The contouring spindle has a standard Sandvik Coromant Capto C8 tool interface, which reduces tooling costs, and it loads tools via the machine’s automatic toolchanger for faster processing without operator involvement. Coolant through the contouring spindle at 20 Bar maximum pressure eliminates manual intervention and ensures maximum tool life.
“The integrated contouring head can be used on any part with bored surface/features [ID or OD]. This capability reduces the number of fixed boring tools needed for a job and provides the flexibility to also machining threads, face grooves, and other contoured features on parts like pump housings, valves, and blow out preventers,” said Peter Beyer, Fives Giddings & Lewis director of product strategy and development. “The application of contouring heads on boring mills is not unusual, however, the combination of the features we have incorporated into the contouring make it unique. To my knowledge we are the only company proving an integrated contouring head on a HBM with fully automatic toolchanging of two types of nonproprietary tooling.”
Energy Industry Relies on Boring Mill Technology
“Our large boring mills are used for both subsea or on-land oil and fracking for natural gas applications for machining a variety of wellhead workpieces, including pumps, connectors, high-pressure manifolds,” said Jim Yeakley, southwest regional sales manager, SB Machine Tools (Schaumburg, IL). “With a boring mill you can use the contouring head and the W axis of the machine to power the U-axis head and blend the internal radiuses of mud or fracking pumps as if they are on a lathe, which is a pretty unique usage of a boring mill,” said Yeakley.
“Actually, there are two major aspects for these types of heads. Because you can adjust the diameter of the insert on the cutting end of the boring bar and its stroke, you can do deep pocket work. About half of my customers have gone to the contouring head configuration for oil field work for large parts that don’t lend themselves to being put on a lathe,” said Yeakley. SB Machine offers three Nomura boring mill types with 4 and 5" (110 and 135-mm) quills with table pallets ranging from 45 × 45" (1143 × 1143 mm) for the 4" HBA 110T-R2 model to 72 × 78" (1829 × 1981 mm) for the 5" HBA 135 P-R5 model.
“One of the most exciting new things that I’m working on is a specialized machine called the FBA which has a built-in facing head for machining large valves for drilling, milling, and internal facing. Instead of using a contour head, a built-in facing head with a toolchanging capability is used. The FBA is being tested on turning large-diameter pipe up to 60" [1524 mm] for applications where it would be used as pilings, such as north slope applications,” said Yeakley.
Big Gear Sets Machined on Multitasking Centers
“Many shops increasingly incorporate full five-axis multitasking machine tool technology because it allows them to handle basically any kind of part that comes through the door and do so with one type of machine in single setups to increase productivity. But rarely, if ever, do these shops realize that large, spiral-bevel gear sets are among that wide mix of part-processing capability,” said Mike Finn, process development engineer at Mazak Corp. (Florence, KY).
Mazak has demonstrated that with a larger sized five-axis multitasking vertical machining center, shops can cost-effectively and quickly produce those occasionally required, low-volume spiral-bevel gear sets in-house. In this way, shops can avoid the long turnaround times associated with farming the job out to a specialty gear shop, which may or may not have a machine large enough or available at the time for the job. Loading a model with a proper contact pattern into a CAM system allows the gears to be programmed the same as any other five-axis part. And from this point forward, the machine tool simply cuts to the model.
“A multitasking, five-axis machine tool can cut both ring and pinion gears so accurately and consistently that a shop could, for instance, separately cut five pinion gears and five ring gears,” said Finn. “It could then pair the gears up with one another in no particular order to create five gear sets. Plus, when not machining the occasional gear sets, a shop can use its multitasking machine to cut all of its other parts, including the gear boxes the sets go into and any other associated transmission components, to maximize machine spindle utilization. It is this production flexibility that attracts shops to multitasking machines,” said Finn.
In process-development testing, Mazak machined a spiral-bevel gear set that included a 29-tooth, 22" (559-mm) diameter pinion gear and a 114-tooth, 6' (1.8-m) diameter ring gear on a Mazak Integrex e-1550V/10 multitasking five-axis machining center. The gear set was completed in days as opposed to months and machined complete using only one machine. The Integrex e-1550V/10 is a standard model with a two-pallet changer. The machine performs all required machining processes such as milling, turning, boring, drilling and tapping. Its tilting spindle and turning table allow for machining at any angle or cross cutting position as well as for contouring operations. Also for development testing, Mazak used no special tools to machine the ring and pinion. All tools were standard, off-the-shelf and readily available.
Pinion gears for the sets are machined from solid cylindrical pieces of 8620 steel, while the ring gears are machined from forged 4340 carbon steel rings that have about 0.250" (6.3 mm) of additional stock for machining, but lack any near-net teeth shapes. Both gears are first rough-machined in their nonhardened states, then pinion teeth are carburized to Rc 62, ring gear teeth induction hardened to Rc 55 and both are finish-machined in hard milling operations on the Mazak machine.
For machining this particular gear set, a pinion gear is fixtured on one of the multitasking machine’s two pallets and a ring gear on the other. While one gear is being machined, a machinist sets up the other at the pallet load station. After it is rough-machined, the pinion gear moves out of the work envelope and the ring gear moves in for its rough-machining. Both rough-machined gears are then sent out for hardening. With its teeth hardened, the pinion gear is relocated back onto the pallet fixturing and the machine finish-cuts the gear’s bore as well as hard mills the flanks and edge radii of its teeth to eliminate any secondary deburring operations. The ring gear is refixtured and finish-machined in the same way.
“Of all the challenges involved with machining gear sets, imparting the correct involute form on the pinion gear is the most daunting, and in machining, Mazak uses several proprietary techniques to minimize close out error,” said Finn. In doing so, the company is able to determine the minimal amount of stock to leave on the pinion gear to correct for warping from the heat-treating process, yet also leave enough stock to fulfill the case hardened depth requirement. Removing too much material during the finish-machining operation can compromise the gear’s case hardened depth, thus affecting the gear’s operation and its working life.
Largest Parts Can Pose Special Challenges
The demands placed on the largest machines to meet the increasingly difficult processing requirements for accuracy and productivity are typical of machining large workpieces for the power generation industry. Workpieces like rotors for power generators can be massive, weighing in at the raw stage to as much as 32 t, requiring special machines to produce grooves using HSS side-milling cutters. This was an extremely time-consuming process, because tool penetration depths were limited to only a few millimeters. Processing times of 200–300 hours with another 40 hours required for deburring were not uncommon.
The mpmc 1200 multiproduct machining center from Weingärtner Maschinenbau (Kirchham, Austria) was able to slash processing time to produce the slots on generator rotors. Using special tooling developed by Boehlerit (Kapfenberg, Austria) that is capable of maximum cutting depth of 200 mm with a cutting width of 36 mm, machining time including deburring was done in less than 60 hours. To facilitate machining, the mpmc 1200 is equipped with an opposed spindle with an additional integrated measuring system. This measuring capability is important because the slots are always machined in a continuous flow and the angular position from the initial position to the end position must not differ by more than a few microns.
Considering the weight of the workpiece and the requirement for micron-like accuracy placed special demands on the clamping and mounting of workpieces. According to Weingärtner, commercially available steadyrests are simply not adequate for workpiece weights up to 60 t. So its design engineers developed freely programmable steadyrests with four CNC axes (X1/X2/Y/Z) that can handle workpieces from 100 mm to 1.2 m in diameter with micron accuracy. Freely programmable means in this case that adjustment options for concentricity, depth, height, and length can be made to handle up to 10,000 kg in contrast to the usually available devices which are designed for a maximum of 5 t.
Global Accounts Consider All Possible Solutions
Detlef Streichert is global key account manager for DMG Mori (Hoffman Estates, IL) and, as such, works with customers in energy-related oil and gas, aerospace, construction and machinery equipment industries. “With about 300 different machine tools, we can provide multiple possible solutions with different approaches to large-part machining with five-axis and five-side machining capability. It doesn’t matter if the customer prefers a vertical or a horizontal machining approach. All of our machines are modular in construction so that we can use a vertical machining center with a B axis and a table weight capacity from 1.5 to 10 t, or in the horizontal type of arena, a swivel head for universal five-axis machining capability,” said Streichert.
“Using conventional five-axis machine tools, we are taking gear machining out of its specialized niche and handling gear machining with standard machine tools, very smart programming systems, and new approaches to standard disk-cutting tools,” said Strichert. “We have worked with Sandvik Coromant to develop and use standard disk cutters with inclined forms to machine the involute shape of the gear. With our software on board, we can create the entire machining process, according to the original print part, and the user can choose between different machining approaches. You can imagine when you talk about gears that were created on a blueprint 30 years ago that not every user, particularly the smaller users, has the luxury of very complex programming systems. So we developed the software for gear machining and involute machining to create the gear shape or create the gear program. We just put in the standard drawing dimensions and then the programmer can create and get the result for any size gears. This is of great benefit to users who can replace the large inventory of special tooling with just three different standard disk cutters. In addition we can put a CBN grinding wheel into our machine tools to finish the gear,” said Streichert. ME
This article was first published in the December 2013 edition of Manufacturing Engineering magazine. Click here for PDF.
Published Date : 12/1/2013