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Making HMCs Nimble Enough for Small Jobs

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media

Old ideas about horizontal machining centers need to be reconsidered

Always productive, HMCs—such as this DMG Mori NHX 4000 with a speedMASTER spindle—are now also extremely versatile.

Horizontal machine tools (HMCs) have typically been used for longer run production jobs. But as lot sizes decrease, machine builders and their partners have introduced new technologies that speed setups and generally make HMCs nimbler. So much so that one should probably rethink the role HMCs serve.

Important new technologies range from friendly programming to versatility in tool selection, and more.

Horizontals and Operators

One of the key new functions for tackling short run jobs efficiently with an HMC is a straightforward—yet versatile—human machine interface (HMI) so operators can program quickly on the floor. Quick programming means quick changeovers.

Jeff Wallace, general manager of national engineering for DMG Mori USA, Hoffman Estates, Illinois, said CELOS is his company’s approach to providing a consistent interface. CELOS is independent of the underlying control, whether it is “FANUC, Mitsubishi, Siemens, or whatever we decide, though FANUC is our primary control of choice for horizontals.” Wallace described the CELOS interface as “well suited for the shop floor, but it also fits well” when programming is done offline in the engineering office.

Doosan Machine Tools America, Pine Brook, N.J., takes a different approach to a friendly HMC. As Andy McNamara, director of sales, explained, Doosan consistently uses the FANUC control, but they’re also equipping more horizontals with its Customized User-friendly Flexible Operating Solutions (CUFOS), which opens up the control to third-party software. This has enabled the machines to handle entirely new applications, like involute form gear cutting, plus the implementation of advanced probing software. “We’re machining the gears, and then probing the profiles and outputting corrections right to the machine,” McNamara said. He added that third-party, CMM-type software also enables continuous path probing and verification of complex surfaces beyond gears, to include feedback that automatically adjusts for any errors.

Doosan has also created its own dedicated CAM cycles called “shapes modules.” McNamara explained that the user “simply defines the tool parameters they want to use graphically. And they define the shape, let’s say a gear, and whether they want to skive it, hob it, or do polygon turning. Then it just generates the code right there at the machine. It’s easy to come up with a handful of shapes.”

This echoes DMG Mori’s Technology Cycles, 42 canned routines that complement a user’s CAM software, or otherwise make the machine more productive. ME introduced a few in January’s five-axis machining article, but a different set emerges as better suited to horizontals.

Software advances make it possible to cut a wide variety of gear forms on general purpose HMCs, such as this five-axis Mitsui Seiki HU80A-5X.

Wallace agreed that customers are looking for ways to make gears without specialized machines, and said DMG Mori’s gearMILL Technology Cycle enables an HMC to produce a surprisingly large portion of the gear forms on the market. That includes “large involutes, spline gears, and curvics. … Our gearMILL Tech Cycle is for more complex gears that require G-code to drive a cutter or cutters to generate the gear profile.” He added that DMG Mori offers the ability to mill the rough form and then finish grind on the same machine. “You can produce curvics without having to buy a customized machine for a million and a half dollars. We have machines right now making curvics to those classic curvic grinding machine specifications. Horizontals are extremely well suited for that.”

Cycle Automation Helps

John McDonald, executive vice president of DMG Mori USA’s development center in Davis, Calif., pointed to Machine Vibration Control (MVC) as another Technology Cycle providing faster HMC setup. “Let’s say there is a long overhang on a tool that chatters. The old school method is to speed up or slow down the spindle to find the sweet spot that eliminates the chatter. MVC does this automatically in a setup setting, and then has a nice user interface to update the feeds and speeds in the program. It gets you running production faster.” It does this by taking feedback from an accelerometer in the spindle head and then using stability loads to calculate what spindle speed will achieve a stable cutting zone. But the operator doesn’t have to worry about the math. Once he’s run MVC the G-code has been updated for subsequent parts in that run, or when the part comes back.

McDonald said DMG Mori’s Machine Protection Cycle (MPC) is the most popular and highly valued Technology Cycle among HMC users. MPC uses an accelerometer on the nose of the spindle head to sense any jolt or change in acceleration that might indicate a collision. “If it exceeds the set threshold value, it will stop and reverse the axis within ten milliseconds, minimizing or eliminating damage.” That alone makes the option well worth it, but it also has a tool load limit. “You can set load limits for different tools, so if it exceeds the value, the machine safely and automatically stops, preventing damage,” said McDonald.

McNamara added that the cutting tool libraries offered by Sandvik Coromant and Kennametal are excellent resources anyone can use, yet too few do. And they’re particularly powerful with the new FANUC iHMI, he said. “You can select the cutting tool itself, and the holder, and build the assembly on the tool supplier’s site. … And you can import those shapes into the iHMI. … You can upload a certain tool family, or group of tools. You can create simple stuff at the machine, or create it at the CAD/CAM software side, and upload it anywhere you want.”

Errol Burrell, machine center product specialist for Okuma America Corp., Charlotte, N.C., raved about his company’s long-standing partnership with Caron Engineering, Wells, Maine, and its TMAC product. TMAC (Tool Monitoring and Adaptive Control) automatically monitors and detects tool wear and breakage, maximizing uptime. Plus it learns the optimum power for each tool and continuously maintains a constant tool load by automatically adjusting the feed rate in real-time. That cuts cycle time and lengthens tool life. Burrell said Caron has been improving this product for 15 years and remains “way ahead of the game.”

A semi-circular pallet system around the end of a single machine is the leading automation configuration for many OEMs. This DMG Mori option handles 21 pallets.

Autoloading Diverse Parts

Bill Malanche, chief operating officer of Mitsui Seiki (USA), Franklin Lakes, N.J., observed that some of the features that make HMCs better suited than verticals for larger production runs also contribute to their ability to handle short-run jobs. For example, “most of the verticals that I see have 60 tools on average. But in a horizontal, 120 tools is probably the average. Some HMCs are equipped with multiple tool changers that could have 360 or 480 tools in a setup. If you have that kind of capability on your machines, you have the ability to do lots and lots of different parts. Although you may have had it originally set up for higher productions, using tool life management with backup tools and so forth, now you can be a little more creative and use those same machines to do short run jobs.”

HMCs typically have at least a two-pallet changer, if not more, he added, so the operator can be setting up a different job while one is running. For example, the operator could be setting up a four-sided tombstone with completely different jobs on the outside pallet while the parts on another tombstone are being machined. The challenge, he explained, is keeping things balanced when the variations multiply. Malanche offered several solutions, including scheduling software and using a spindle probe to identify parts. “Everybody nowadays buys a spindle probe with their machine, and they’ll use it to adjust an offset in XYZ because the forging is a little different and it moves slightly in the fixture. That’s well and good; that’s what it’s designed for. But part probes can also be used for part identification or fixture identification.”

One simple approach, he said, is to put a ½" (12.7 mm)bushing in the upper right hand corner of every pallet for Part A, and a ¼" (6.35 mm) bushing for Part B. “Then, at a given point the probe goes in and checks the size of the bushing. A simple if-then statement determines which program to run. If the probe measures a half-inch bushing, the machine runs program A. … It takes the pressure off the operator.” Long term, Malanche thinks vision systems will take over this function. “Basically a camera takes a snapshot of the part and compares it to a library to determine which part it is.”

HMC 4.jpg
Doosan’s new DHF 8000 ST five-axis HMC features a direct drive rotary table that can spin at up to 600 rpm, making it suitable for turning operations.

Mitsui Seiki and other builders are increasingly creating their own pallet systems and writing their own automation management software. Malanche said Mitsui Seiki has also decided to stock all machines such that automation can be added later. So, for example, a simple two-pallet Mitsui Seiki machine could later be coupled with a flexible manufacturing system (FMS). Or, as Malanche put it, “anybody who doesn’t look at a piece of equipment now with the idea that it may have to be expanded into something else, or morphed into something else, either has a crystal ball or a job so big he can throw away his machines at the end.”

A semi-circular arrangement around the end of a single machine is the leading configuration for all the OEMs. Burrell said Okuma’s six-pallet system has been the most popular, but increasing numbers of customers want more so it has a 12-pallet system that simply adds a level. “Rather than take up more floor space, you go up. If they’ve got plenty of real estate up in the air, that’s where a lot of people would much rather be. They’d rather have another spindle with the floor space that they save. It makes a lot more sense.” McDonald said DMG Mori offers rotary configurations with five, 14 or 21 pallets, the two extremes being the most common.

Burrell said that pallets in these systems can carry “hydraulic fixtures, tombstones, whatever you want” and he and the other builders interviewed noted the value of zero-point fixtures. “Even tombstones can have a zero-point location, so they stay in the pallet changer all the time. You can have all the offsets assigned to each fixture in your management system or in your control in advance. The operator only has to call that pallet. Or you can program everything from the office without even going to the machine. Customers do it all the time now, with many doing high-mix, low-volume.” With a magazine of 200-300 tools, they have enough redundancy and rarely touch the machine, he added.

McNamara explained that it takes smart software to make these systems manageable. “Can it check, before you tell it to run a certain part number, that you have enough redundant tooling to do that? And if not, can it select a different job until that changes? Can you set rules for prioritizing jobs? Is it first-in, first-out, or is it just-in-time? We offer several such choices. … Let’s say you’re going to run all weekend. You can tell the software to figure it out, scheduling the jobs based on the available tooling and your general prioritization the best way it works out. Because computers can think through complex combinations much better than you and I can.”

McDonald said DMG Mori’s single-machine Pallet Manager softw

Besides checking part position and setting offsets, spindle probes—such as this Renishaw RMP60 on an Okuma machine tool—can also be used to identify diverse parts coming from a pallet system, saving the operator from this organizational burden.

are offers the same sort of scheduling functions, with the additional “crowd favorite” feature of being fully integrated with the CELOS control. As McDonald put it, running unattended introduces the risk that a problem occurs without being detected. “Let’s say a little drill breaks. Maybe the guy catches that drill in the hole in the morning. Maybe not. The customer that bought the part will definitely catch that broken drill!” So Pallet Manager communicates with the tool breakage detection program and flags any part that may have a problem. “It stops machining that particular part, and then moves on to the next part in the queue,” he said. “It effectively quarantines the questionable part in the pallet pool cell instead of automatically bringing it to the unload station. When an operator goes to unload, they can’t get to the pallet that had an error unless they go over to the control, see the flagged pallet, and physically drag it on the CELOS screen to the unload station.”

For its larger linear pallet pools, DMG Mori offers “job base” software with the ability to logically schedule production for all the components that go into an assembly. Using the analogy of making a car, McDonald said the software would know the line needs to make four wheels, two axles, one engine, and so forth. “I would call that one job. If I call up this particular job and want to make five pieces, I’m going to get two wheels times five, two axles times five, and so on.”

Machine Speed and Design Improvements

There have also been improvements to what you might call the “guts” of today’s HMCs. Burrell recounted how Okuma has “sped up everything,” in part by switching to servo motors and then using their precision to synchronize movements to minimize non-cutting time. For example, the chain-driven tool carousel has been replaced by a servo driven disc. And the tool changer arm and access door are likewise servo driven such that the arm is moving as the door is opening, and it bursts through at the first possible split-second. Coolant flow and chip evacuation have also been improved. They’ve even “increased the working area inside the machine but made the footprint smaller,” said Burrell. “So that’s perfect for a small job shop.”

Malanche of Mitsui Seiki offered a solution for making HMCs more nimble that is perhaps both obvious and bold: Go to five axis machining. “If you need to move quickly from one job to the next, five axis gives you the big benefit of not needing complicated fixtures—a big benefit you don’t get even with four axes. Many jobs can be done in just two ops, and you can hit all six sides of anything in two ops. That adds a lot of capability.” Malanche estimates that over 90 percent of Mitsui Seiki sales are five-axis machines. McNamara reported that Doosan is now releasing the DHF 8000 ST, a new horizontal five-axis with a direct-drive rotary table that can spin at up to 600 rpm. This makes it suitable for turning, which is an excellent way to machine certain features on large parts that are best mounted right on the table. DMG Mori, among others, offer a similar approach.

Wallace summarized the case for HMCs this way: “They’re an economical solution to get a versatile machining platform. There’s not much horizontals can’t do.”

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