May 2012 / Volume 64 / Issue 5|
By Kip Hanson, Contributing Editor
| Can vertical machining centers go toe-to-toe with horizontals on performance?
It’s a tall order to expect a vertical machining center to compete with a horizontal. Horizontal machining centers are smarter, faster and cheaper than ever before. Many would argue they are the logical choice for a variety of machining work, from low-volume medical and aerospace parts to lights-out machining for job shops and automotive manufacturers. Horizontals have greater tool capacity, are more suitable for automation, and even a commodity HMC offers a built-in indexing table and pallet changer, right out of the box. So why would anyone want to buy a vertical?
There are lots of reasons.
Courtesy of Haas Automation
William Howard, product line manager for VMCs at Makino Inc., Mason, Ohio, said, on average, there are four times as many verticals sold in North America each year—6,500 to 7,000 machines—than horizontals.
Howard said: “Because of this, there are a whole group of folks out there familiar with running, maintaining and programming verticals: setup people, operators, manufacturing engineers and, especially, shop owners. Since more metal- working professionals have experience with verticals, and they’re typically less expensive, a VMC—many times—becomes the easy solution.”
Because of this vertical orientation of the U.S. machine tool market, there’s a perceived complexity when shops look at horizontals. “With a VMC, you can slap a couple of 6 " vises on the table, load some tools and you’re making parts,” Howard said. An HMC, he noted, typically includes an index table, a pallet changer and possibly a pallet pool. A shop will likely have more tools to keep track of, need tombstones and multiple fixtures, machine more sides of the part, and, therefore, have more operations to manage in a single program and far more fixture locations. Also, because the shop is potentially machining a large number of parts in a single setup, inspection times on an HMC could be quite long: Instead of doing first article inspection on a single workpiece, you might be inspecting 20 or 30 of them.
Add to this the relatively high price of an HMC and a vertical becomes an easy choice. According to data from AMT – The Association For Manufacturing Technology, the average cost of an HMC is $375,000 compared to $115,000 for a VMC.
Howard then brought out another long-held market belief: “Verticals are traditionally perceived as being more accurate than horizontals, especially in drilling and boring operations,” he said. “For one thing, you don’t have to deal with spindle droop on a vertical, and VMCs typically use fewer axes, which means an inherently higher degree of accuracy.”
No matter how precise the HMC construction and assembly methodology, there’s going to be minor play in the index or contour table, a small amount of wobble when it spins and a little pallet registration and clamping error, according to Howard. While these factors are generally negligible in most production jobs, there’s a small potential for issues in applications where you’re chasing microns.
“In heavy milling applications on a horizontal—especially high-up on a tombstone—the mechanics and leverage mean you’re essentially trying to tip that pallet over, while on a vertical you’re pushing the workpiece right down into the tabletop,” Howard said. “This gives you excellent stiffness and rigidity.”
Courtesy of Makino
Courtesy of Haas Automation
Can that really be true? Is a lowly VMC more accurate and capable of heavier cuts than a big, beefy HMC costing three times as much? Howard went on to qualify his statement. “When you consider the majority of today’s machines—the quality of the ballscrews, advanced servomotors and spindle design and the highly accurate machine structures available—the performance difference between horizontals and verticals may be small.”
Howard continued: “There’s an appropriate place on the shop floor for all machining technologies, and what’s most important is that manufacturers determine an optimal solution. There’s no doubt that horizontals offer a high degree of productivity and quality; however, given a properly designed machine and a well-executed machining plan, verticals can be advantageous when it comes to high-precision boring, drilling and heavy-milling, especially in complex, 3-D contouring applications.”Go West, Young Man
Bryan O’Fallon, product technical specialist at Haas Automation Inc., Oxnard, Calif., has plenty to say about this subject. “Vertical machining centers are the workhorses of the metalworking industry,” he said, noting that VMCs are versatile, uncomplicated, reliable, easy to set up and change over, affordable, and come in a range of sizes. The larger, more open enclosures common to VMCs also provide excellent part access and good visibility during machining. And table weight capacities on verticals are typically much greater than on HMCs with comparable travels.
“The biggest advantage of HMCs,” O’Fallon said, “is reduced handling and machining time by allowing multiple operations with a single setup.” Properly fixtured, a user can machine a part on as many as four sides in one clamping, he explained. This not only reduces handling time, but also the potential for human error, such as an operator incorrectly positioning parts during a second setup. “It also frees up an operator to do something else,” O’Fallon added.
Courtesy of Methods Machine Tools
Although they are becoming more affordable, O’Fallon noted that HMCs are still considerably more expensive than VMCs with comparable work envelopes.
As an example, O’Fallon specified a set of similarly sized and equipped verticals and horizontals from the company’s product line. The result was surprising. For the same amount of money, a shop could have three vertical spindles pumping out parts vs. one horizontal. Also, according to O’Fallon, those three VMCs would be much more versatile than one HMC, as they could be set up with different jobs, materials and tooling, depending on the shop’s part mix.
“Despite having the same travels,” O’Fallon said, “the reality is that the HMC really doesn’t have the same part capacity as a comparable VMC, not without a substantial investment in workholding.” He provided the example of two 15 "-square plates, which could easily fit on a typical 20 "×40 " machine. “That same part on a horizontal would require construction of a right-angle fixture to mount to the pallet and then additional workholding to mount the plate to the fixture. Loading the part would also be more difficult, as it would have to be supported vertically during the process, rather than simply laying it on the table of a VMC.”Look Mom, No Hands
Steve Bond, national sales manager for Methods Machine Tools Inc., Sudbury, Mass., agreed with his competitors. “Verticals remain a cost-effective machine alternative. True, HMCs are coming down in price, but there is still about a 3:1 price differential once the machines are tooled up and in production.”
Similar to HMCs, VMC technology isn’t standing still. Bond said: “Today’s verticals are versatile, especially for job shops and tool, mold and die builders. They offer higher rpm, feed rates and accuracy than previous versions. And VMC builders are adding even more flexibility with 4th- and 5th-axis tables and automation to allow for more unattended operation.”
Also, VMCs have smaller footprints than HMCs, a real issue for many shops. VMCs are often preferred from the standpoint of operator comfort level, according to Bond. He added that they’re easy to setup and program, allowing them to produce parts faster and at a lower cost.
But for someone doing low-volume, repeat work, a horizontal seems to be the better choice. After all, a palletized HMC allows a shop to setup the job one time and forget it. Large tool capacities, a virtually unlimited number of fixture locations and next-available-machine flexibility are compelling arguments for an HMC.
“There is often a limitation to the number of cutting tools available on a VMC, but customers are getting pretty creative in their approach to cutting a part with the least number of tools,” Bond said. “Even with capacities of 21 tools or fewer, they are still able to achieve high throughput on their VMCs.”
And for customers looking to match or even exceed the flexibility and throughput of a pallet pool-equipped horizontal, Bond has just one word: robotics. “Almost every customer we talk to now looks at automation,” Bond said. “No longer is it only for long production runs or for horizontals with pallet pools. Automation, even for single-part runs, is achievable on VMCs, provided you have the right cell-control software and a robot capable of end-of-arm tooling changes to adapt to a variety of workpiece shapes.”
A robot for low-volume machining? Bond cited one customer who machines various low-volume medical parts on its VMCs largely unattended. “They run lot sizes up to 25 pieces using a 6-axis Fanuc robot, together with infeed and outfeed conveyors. All parts are palletized on a common-platform, zero-point chuck, similar to what you see on an EDM.”
Bond went on to paint a picture of a seemingly futuristic machine shop, one where work is shared among all types of machines—lathes, EDMs, VMCs and measuring equipment—and transported by robots. In this shop, you mount the workpiece on a chuck, rough it on a heavy-duty machining center, turn it on a lathe and send it to a 5-axis VMC, a drill/tap machine or an EDM. In between each machining step, the workpiece can be sent to a coordinate measuring machine for in-process gaging. The CMM, in turn, sends tool length adjustments and CNC program updates back to the machine tools, all without human intervention.
It’s a pretty picture, but who could afford it? Lots of shops, according to Bond. “We’re doing this successfully right now. It’s all feasible, without spending a ton of money.” Bond noted that even a small shop can equip a VMC with robotics for as little as $100,000 and might spend about $180,000 for the whole enchilada, including pallet identification and machine integration (not including the machine tool). “A system like this assures the right program, the right offsets and the right tools,” he said. “It’s a great option for a shop that wants to get into lights-out machining without breaking the bank.”On the Up and Up
One of ways shops choose HMCs or VMCs is by considering the traditional strengths and weaknesses of each. However, according to Ross Clark, vice president of sales for VMC builder Chiron America Inc., Charlotte, N.C., those differences have become blurred. “Horizontals have long been purchased for doing multiple faces of a workpiece in a single setup, while verticals are traditionally used for the top face of the part,” he said. But today, with the growing use of improved 4-axis and 5-axis VMCs, “horizontals and verticals are now sharing a lot of the same work.”
Courtesy of Chiron
One area where the lines have blurred is chip flow, which on a VMC has traditionally been poor. “You take a cut, the chips fall back into the hole, and now you’re recutting the chips, leading to broken tools, poor surface finish and accuracy problems,” Clark said. To counter this, builders began offering through-spindle coolant, with high-pressure options upwards of 1,000 psi, about the equivalent of five elephants standing on the latest John Grisham novel. Add to that a sheet metal design that sends chips straight into the chip basket, without screws or conveyors, and you’re left with a machine that has a chip flow every bit as good as an HMC, according to Clark.
Chiron focuses on high-end VMCs. “Our machines are as expensive as typical horizontals, and in some cases more,” Clark said. “We justify this because we are exclusively focused on high-volume production work. On an HMC, you typically have much slower chip-to-chip time than on our verticals. Horizontal users get around this by loading more parts in the machine per cycle, thus amortizing tool-change time. Consequently, this means higher fixturing costs—sometimes much higher.”
Courtesy of Okuma America
How so? “Figure four faces per tombstone with multiple parts per face, times two pallets,” Clark said. “Every part has two to three clamps, hydraulic seals, cylinders and locating pins. This gets very expensive, and it’s not uncommon to rack up $100,000 in fixturing costs in a production setup. With a Chiron vertical, you have two fixtures. Add to that the fact there’s less maintenance, easier access to the fixtures and much less inspection time with two parts per setup compared to two or three dozen, and you end up with a much lower fixturing cost per piece.”The Golden Shoehorn
VMCs have a long history, and, by one definition, a very long history, according to Jim Endsley, machining center product specialist for Okuma America Corp., Charlotte, N.C., and a student of machine tool history. “Vertical machining centers have been around since Egyptian times,” he said. “It was basically a C-frame design, made out of wood and driven by a waterwheel. It used stone cutting tools and placed edges on swords.”
While technology has changed a lot since then, VMCs still need an operator to stand in front of the machine all day, tweaking tool offsets, loading and unloading parts and clearing chips, unless you’ve gone down the robot road mentioned previously. The downside here is obvious. “If you have to touch a part, it costs you money,” Endsley said.
The good news, according to Endsley, is there are technologies available to mitigate labor costs, such as Okuma’s thermal active stabilizer (TAS). “We embed thermal sensors in the machine casting that feed information back to the control, which can then automatically compensate for thermal deformation,” he said. “Without it, you need an operator there to adjust for machine growth, but with TAS the machine tends itself while the operator is away, allowing him to operate multiple machines. This is really effective on a typical C-frame vertical, where thermal growth can be a concern.”
Okuma has also made its VMCs smaller and faster, Endsley said. “We redesigned the sheet metal, reducing the machine’s footprint. Everyone has their ‘golden shoehorn’ out these days, trying to fit more machines into less space.”
Also, the trend toward high-speed machining, especially in the aerospace and tool, die and mold industries, suits VMCs well—particularly on bridge-style verticals, which move less mass than a horizontal, according to Endsley.
However, only about 30 percent of VMCs offered today are bridge style, while the rest are traditional C-frame designs, he said. “C-frames are less expensive to build, so you can keep the price down for the commodity market, but this style of machine tops out on feed rate because of all the moving mass. But with a bridge-style machine the mass is more evenly split among the different axes. This allows for much higher feed rates.”A Consensus?
Herbert Hou, national sales manager for Chevalier Machinery Inc., Santa Fe Springs, Calif., agreed. “VMCs work very well for 3-D contouring and die-and-mold work, aerospace parts and very heavy parts,” he said. ”These are too tough to load on a horizontal, where fixturing can be a problem.”
VMCs are also a good choice for smaller work, according to Hou. “Verticals are the clear choice for most shops. We have options for pallet changers and robots, but most people aren’t doing anything with automation.” Further, there’s the price advantage. “They always consider the price first, so VMCs are almost always preferred over horizontals,” he said.
Courtesy of Chevalier Machinery
Despite these factors, Hou admitted that HMCs have several advantages. “Chip management and recutting on a vertical are big issues. You always need an operator there to open the door and spray out the chips, make offsets and load parts. Efficiency-wise, verticals aren’t as good as horizontals.”
In the world of machining, purchasing the right piece of equipment for the job isn’t always cut and dried. There are many factors to consider, but the machine tool builders interviewed for this article agree (mostly) on a number of points:
• Comfort level. Because VMCs own the lion’s share of new equipment sales, finding a VMC operator may be easier than finding an HMC operator.
• Ease of use. In many cases, VMCs are easier to setup and run, and are friendlier when it comes to part handling—especially with large workpieces.
• Smaller size, smaller price. Not only will two to three VMCs fit in the floor space consumed by a large HMC, those spindles may cost the same or less than a single HMC.
• Machine design. HMCs have great technology and are easy to automate, but for high-speed machining, moldmaking and machining large workpieces, VMCs have an advantage.
Depending on your workpiece variety, production quantities and budget, a vertical might be a good choice for your operation. In any event, VMCs aren’t going the way of the ancient Egyptians—and for good reason. CTEAbout the Author: Kip Hanson is a contributing editor for CTE. Contact him at (520) 548-7328 or firstname.lastname@example.org.
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