Milling on Lathes from 1914 to Today's Mill-Turn Centers | Manufacturing Engineering | advancedmanufacturing.org

Contributing Editor

Milling on a lathe is nothing new. Old-timers reading this might still have a 1968 operator’s manual for Brown & Sharp automatic screw machines tucked inside their toolbox, which explains the use of slotting and slabbing attachments. And closer to Kipling’s era, the 1914 edition of “How to Run a Lathe” from South Bend Lathe Works illustrates how to set up attachments for milling and keyway cutting.

Precision Product Development is using its Willemin-Macodel 408MT seven-axis, multi-process machining center to produce orthogonal parts like these in a single operation.

These, and many other examples over the past century, have made it abundantly clear that performing as much machining as possible in a single operation improves part quality, increases throughput and reduces costs. Today’s mill-turn lathes and multitasking centers (lathes with a machining center-style milling spindle and ATC, or automatic toolchanger) take these benefits to the extreme, often completing even very complex parts in a single operation.

And yet, those looking to leverage this technology should be aware of several limitations, the first being that the milling attachments used on CNC lathes—though very capable—do not offer the same power, speed or longevity as the milling spindles found on CNC machining centers. Koma Precision Inc. of East Windsor, Conn., a well-known supplier of these live-tool attachments under the Alberti brand, carries a broad selection of straight, right-angle, dual output and adjustable-angle “universal” heads for the most popular brands of CNC lathe—some boasting spindle speeds up to 12,000 rpm.

With a live-tool capable lathe, even oddly-shaped parts like this one are now candidates for single-operation machining.

The company has also introduced a “Smart Change” line of quick-change live tools for select machine models, promising “quick, easy and precise” three-point toolholding. The system promises to allow manufacturers to preset offline and swap tools in less than 30 seconds, opposed to stopping the machine, replacing the cutting tool and manually touching off—an all-too-common task that can easily consume a couple of minutes or more.

“An increasing number of shops want to maximize their machine uptime, or OEE, so these types of quick-change systems are becoming quite popular,” says Andrew Esposito, vice president of sales. “And if you’re short of stations, they’re fast enough that some customers will pause the program and drop in a completely different tool. It gives you a lot more flexibility.”

CJ Abraham has a unique perspective on mill-turn machines. A one-time technical marketing specialist at Autodesk and product manager for Fusion, he launched his own machine shop—Los Angeles-based Precision Product Development LLC—about two years ago. His first machine? A Willemin-Macodel 408MT seven-axis, multi-process machining center, which he uses to produce Nano-D and Micro-D connector shells and similar components via an automated, single-piece flow.

“The term ‘lathes as mills’ is kind of vague,” he says. “There’s a spectrum of machine tools, with lathes that do some milling all the way to mills that do some turning. A lathe with live tooling is obviously one of the former, but sitting at the other end are machines with proper milling spindles that happen to hold material in a chuck or collet, like the 408MT. That’s why whenever I describe my machining services, I don’t say mill-turn; I say bar-fed milling.”

Abraham explains that milling is usually associated with vises, clamps, dovetails, etc.; but in bar-fed milling, the bar itself is the workholding and can be thought of as “a consumable fixture.” He seconds Autodesk’s Dan Pacific’s point about longer parts and rigidity considerations but says these can be overcome by supporting the bar with the sub-spindle, or in the 408MT’s case, a sub-vise.

“Connectors and other good candidates for bar-fed milling are traditionally made at high quantities on pallet fixtures or from material strips,” Abraham says. “Producing them with a one-piece flow from bar stock has a few advantages over a part-dense pallet, starting with much lower fixture development and manufacturing costs and the elimination of loading/unloading labor.

“Further, milling vs. turning part features is interesting from an automation perspective—milling chips are easy to evacuate from the machine while turning chips can clog things up,” Abraham continues. “So if the milling spindle is capable of high material removal rates as with the Willemin-Macodel, the process actually becomes more reliable due to improved chip flow. That’s been my experience.”

Even if live tool attachments have limited life and capabilities relative to a machining center, Esposito reasons that the ability to machine complete parts in a single handling easily offsets the lighter cuts and additional passes that might be required when CNC lathes assume the role of milling machines. As with other live tool manufacturers, Koma stresses the importance of routine maintenance.

“It’s a mechanical device,” Esposito says. “There are specific break-in and lubrication procedures to follow, and just like your car, you need to have the bearings and seals replaced periodically, more often if you’re running them wide open for hours at a time.

“Yes, you can push live tools pretty hard,” he continues. “Just be sure to service them before they reach the point of no return. That’s why I tell customers to always have a spare on the shelf—you don’t want to keep a $200,000 machine tool sitting idle while a $2000 attachment is out for a rebuild.”

Brian MacNeil supports many of the cutting tools used in these attachments. A milling products and application specialist with Mebane, N.C.-based Sandvik Coromant US, he says there are big differences between the toolholders used in mill-turn machines vs. multitaskers, but emphasizes that quality tool holding is critical for any successful machining operation. Rigidity, the least amount of runout and the shortest possible projection should be the goal, MacNeil adds, noting that most multitasking lathes have dedicated milling spindles and are usually capable of higher metal removal rates, although these rules apply to both styles of machine.

“Regardless of what machine is driving the tool, always have a good understanding of the power and torque available at the rpm needed for your tool,” MacNeil says. Also, temper your expectations regarding the depth and width of a milling cut—if you don’t have your machine’s power and torque graph available, call the machine tool distributor.

If you need to know how much power and torque the cutting tool will pull in a given material, users can check Sandvik’s website or call their representative for help. “That small investment in time upfront can save you a lot of aggravation later,” MacNeil says.

It’s often the simpler things we forget when starting a job on a lathe that requires live tooling, he explains. For starters, there typically is a limited Y-axis travel, which can be further impacted by larger diameter workpieces, restricting tool selection.

The CoroChuck 930 high-precision hydraulic chuck is designed for milling, drilling and reaming operations where precision and high pull-out security is required.

And, as Esposito pointed out, live tool maintenance needs to be diligently tracked. “Many times customers run beyond the recommended time limit before maintenance is required and are surprised by live tool failures during a job, which are expensive to repair,” MacNeil says.

Another area of concern is coolant filtration, concentration and quality. Sometimes, live tools are cooled using the machine’s cutting fluid, so installing five-micron filters will prevent toolholder passages from being plugged and blocked.

Like any machining operation, milling on a lathe involves many considerations, and tooling should be chosen based on specific needs. MacNeil suggests that solid round tools will always be more productive in smaller diameters, but depending on the feature, operators may be able to get away with a more economical indexable tool when machining small flats or shallow features. This also helps to reduce tool inventory.

“There is no right or wrong solution for tooling on a lathe,” MacNeil says. “If the machine has the power to securely drive a cutter and it satisfies your cost per piece, use it.

“As for programming,” he says, “most CAM systems and conversational controls do a great job. However, proper cutter position is important to achieve good finishes and tool life, and to reduce vibration in less-than-stable components. Your local tooling representative should be able to help you make these calculations—just be sure to ask them to leave a calculator for future projects.”

Sandvik Coromant’s Capto interface—a common feature on many multitasking lathes—is a rigid dual-contact, taper lock connection. The system provides “the best rigidity” you can get from a machine connection, according to Randy McEachern, the company’s product specialist for holemaking and tooling systems in Canada.

It also adds flexibility and quick-change ability when used with live tool attachments, McEachern says. “Coromant Capto has a program called MACU (Machine Adapted Clamping Units), specifically designed with the bolt patterns to fit most of the industry’s lathe turrets, and the clearances needed for the various working envelopes.”

“Milling on a lathe has been around for quite a while,” notes Jamie Ochs, applications engineering regional supervisor at DN Solutions’ Pine Brook, N.J., headquarters. “The main difference between that and milling on a machining center is a lathe’s use of bolt-on spindles, which provide a cost advantage in terms of rebuilding, as well as a certain level of flexibility.

“You can also perform some pretty heavy milling on the larger machines,” Ochs says. “But it’s important to remember that the main function of any lathe is turning, and the milling feature is primarily for secondary operations.”

DN Solutions manufactures numerous mill-turn centers and multitasking machines, some of which are available with Coromant’s Capto interface. There are numerous other benefits of mill-turn and multitasking machines, according to Ochs, including:

In the case of large-capacity multitasking centers, an ATC with upwards of a hundred tools allows shops to set up dozens of parts and run even one-piece orders on demand.

Mark Christopher, DN Solutions’ applications engineer and five-axis machining specialist, says this helps explain their growing popularity. “We have many customers who have traded in their lathes for mill-turns and multitaskers. Our Smart Manufacturing Experience series, for example, has a milling head with 12,000 rpm and 240° of B-axis rotation.

“These machines allow you to perform operations that would otherwise require a trip to a machining center, which means additional labor and fixturing costs, longer lead times, and so on.,” Christopher says. “And as Jamie said, shops will often set up a bunch of jobs and just leave them in the machine, making changeover quite fast.”

Yet mill-turn lathes are expensive, and multitasking centers are even more so. Some shop owners might argue they can easily buy a conventional CNC mill and lathe for the same price—why put all your eggs in one basket that might break down, bringing production to a halt?

It’s a fair question, yet Ochs argues that if one of those legacy machines breaks down, you’re not getting throughput anyway. “I understand the concerns about machine cost, but scrapping parts is also quite expensive.”

Because there’s no risk of misalignment as with traditional machining—where an operator moves parts between fixtures and machines—milling-capable lathes come as close as you can get to “guaranteed” part quality, Ochs explains. Multitaskers take this concept one step further. With a spindle-mounted probe that’s stored in the tool changer and out of harm’s way, shops can securely perform automated in-process part measurement, opening the door to lights-out operation.

Christopher seconds this point. “I worked with a customer recently who was taking parts off his machining center, sending it to the inspection lab, and then if something wasn’t right, had to put the part back in the machine, relocate it and recut the feature. That’s fairly typical.

“A multitasker with its swiveling head allows you to probe pretty much anywhere on the part, eliminating all this wasted effort while greatly reducing the chance of making a mistake. They’re just a lot more flexible,” Christopher asserts.

Multitask machines are also easy to program, despite their greater complexity. That’s according to Jason Taylor, Advantech product manager for Mazak Corp., Florence, Ky., who says the company’s Mazatrol conversational control allows even non-programmers to quickly develop very complex toolpaths. “Let’s say you need to mill a pocket on a workpiece face. Depending on the lathe, you might cut it with the X and C axes simultaneously or use the Y-axis if the machine is so equipped. Either way, you just program it in the XY plane like you would on a machining center, and the control takes care of the axis movements in the background.”

That said, Taylor quickly clarifies the common misperception that Mazaks are limited to conversational programming. Since 1984, he says, all Mazatrol controls have accepted industry-standard G-code, and shops wishing to program offline on a CAM system can do so as long as they have the correct postprocessor. He also reiterates the many benefits of mill-turning and multitasking discussed earlier, adding that manufacturers everywhere struggle to find skilled labor. Making parts “done-in-one” helps mitigate this problem.

“People don’t want to touch parts more than is necessary,” he says. “They want to do whatever they can in one operation and remove a finished part, preferably by a robot. This explains why even the smallest shops are beginning to pursue minimal handling strategies, and since it doesn’t cost all that much more to add milling capabilities and maybe a sub-spindle to a lathe, it’s within reach of practically everyone.”

In addition to making their machines easy to program and operate, Taylor notes that Mazak provides standard tooling packages with each of its new mill-turn lathes, including static toolholders and rotating attachments. “Our philosophy is that when the machine shows up, you should be able to start cutting the same day.”

While programming milling operations for a lathe is no more difficult than “regular” mill programming, it does present some unique challenges. So says Dan Pacific, the Fusion product management engineer for manufacturing at San Francisco-based Autodesk Inc., one of the many Mazak-friendly CAM systems.

Smart Manufacturing Experience machines are equipped with larger chuck sizes than PUMA MX machines but deliver the same high performance and impressive flexibility.

“Part stick-out length is probably the biggest variable, and with that comes the possibility of deflection and chatter, and whether you can support the part with the sub-spindle or a live center,” Pacific says. “Of course, you typically have greater flexibility with a mill-turn or multitasker. Will you rotate the C-axis to generate a part feature or lock the spindle in place and use the Y-axis? How about using polar interpolation? What’s going on with the second turret, if so equipped? Will you attempt to machine simultaneously on the sub-spindle? These and other questions are now part of the programming process—how you answer them depends on machine capability, the cutting tools you’re using, and personal preference.”

Greater complexity aside, Pacific says that Autodesk has worked hard to automate much of this process, and to provide highly accurate simulation tools through its 2020 CAMplete acquisition. Such capabilities will only grow as CAM providers begin to leverage artificial intelligence and machine learning in their software.

“These technologies could be especially helpful with toolpath optimization, where you’re trying to remove as much air-cutting and wasted time as possible,” Pacific concludes.

Autodesk Inc.

415-507-5000 | www.autodesk.com

DN Solutions America

973-618-2500 | www.dn-solutions.com

Koma Precision

800-249-5662 | www.komaprecision.com

Mazak Corp.

859-342-1700 | www.mazakusa.com

Precision Product Development LLC

650-605-5544 | www.ppd-llc.com

Sandvik Coromant US

919-563-5008 | www.sandvik.coromant.com

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Contributing Editor