How to become a better-informed machine tool consumer.
Obviously, shops in the market for a milling, turning, or multitasking machine want the best system for their money. They also want machines that can handle their applications. But beyond these are several other important considerations — some general and some specific to each machine type. By asking the right questions, shops can better sort through their options to make the best choice for their particular operations.
Price is typically the first thing shops examine. However, any financial evaluation should consider the long-term cost of a machine and not just the initial sticker price. "How much is it?" and "How much will you discount?" aren't good opening questions, advises one turning machine OEM.
Instead the end user should begin by showing the OEM/distributor sales engineer parts and part drawings, if possible. According to Bill Jacobson, president of Romi USA, these drawings provide valuable information to help the sales engineer select the right machine. If the sales engineer is unable to help, the OEM should have technical-support personnel that can. The OEM's ability to provide innovative ways for making a customer's parts is crucial in obtaining optimum machine throughput.
Once a shop sets its sights on a certain model, it should ensure the machine is available. If there is a wait, ask if the machine will be delivered in a reasonable time frame, or take several months or longer.
Regarding service and support, shops should question if an authorized distributor is nearby to set up, service, and support the new machine and if repair/service parts are readily available and affordable. Generally, shops want operator training to be part of the package and free telephone assistance for solving everyday problems in operations and conversational programming.
In addition to these issues, buyers should check into the duration of the machine's warranty and where warranty work comes from. If a supplier is unwilling to warrant its product for a year or more, Jacobson recommends finding one that does.
How should shop owners prepare for purchasing a vertical milling machine (VMC) that best fits their individual applications?
Haas Automation of Oxnard, Calif., suggests shops start by determining the reason for buying the machine. Is it a first machine for a start-up shop or a first step into CNC machining? Is the machine intended to expand shop capacity or improve capability to take on new jobs? After sorting through these questions, it's time to go shopping.
VMCs come in all shapes and sizes, and models range from bare-bones 3-axis machines not much larger than a refrigerator to 5-axis behemoths that occupy a good portion of floorspace. What the average shop owner is looking for, however, probably lies somewhere in between. And for most, a standard C-frame VMC fits the bill.
But, warns Makino in Mason, Ohio, some traditional C-frame machines may incorporate structural and axis configurations with over-hung elements, stacked axes, and thin structural elements. These, combined with long unsupported cantilevered distances, can negatively affect stiffness, rigidity, and dynamic distortion, causing vibration and chatter. Such instabilities, inertial factors, and bending movements during cutting degrade surface finish and accuracy, along with cutting speeds and feeds, depths of cut, cycle times, tool life, and machine productivity.
VMC shoppers need to ask at what Y-axis travel a machine shows noticeable side-thrust deflection. This depends, says Fadal of Chatsworth, Calif., on the tolerance of parts. Open-toleranced parts may not show side-thrust deflection unless it causes extreme lost motion, while tight-toleranced parts show minimal lost motion from side-thrust deflection. Tool chatter is one symptom of side-thrust deflection.
What shops need is a heavy cast iron construction and an axis configuration that provides stiffness, rigidity, thermal stability, accuracy, and full-axis-travel support.
Machine designs should have minimal distances to the cutting edge, no axis stackup and overhang, and a table center of gravity within the guideways for good reach over the table without sacrificing stiffness. Ballscrew size, pitch, and servomotor capability also impact machine accuracy, axis thrust, stiffness, rigidity, and cutting performance.
While shopping for a VMC, shops will inevitably face the age-old boxways-versus-linear-guides decision. Mention linear guides, and fast cutting feeds and low friction comes to mind. On the other hand, boxways conjure up thoughts of slow cutting feeds along with deep, heavy cuts. But this mindset is changing because today's linear guideways are more rigid, and new coatings are delivering faster feeds for boxway machines.
Another key machine-design issue with VMCs centers on chip and coolant containment. Fully enclosed splashguards prevent flying chips and coolant from escaping during high-speed machining. However, a full enclosure must provide easy access to the machine table for loading and unloading. This includes overhead crane accessibility, closeness to the control, and a comfortable table height and reach for manual loading.
VMC spindles also have a great impact on machining quality. They come in a variety of styles, but it's crucial that they be stiff and rigid. Shops want spindles that not only minimize vibration but also control thermal change that impacts spindle growth and accuracy.
Design factors that impact a spin-dle's stiffness, rigidity, chatter, and cutting performance include number, size, type, and location of bearings, preloading techniques, balance, and sensitivity to vibration.
With VMCs, the two most common spindle-taper sizes are 40 and 50. According to Ron Kilgore, machining centers product manager, of Daewoo in West Caldwell, N.J., smaller 40-taper spindles usually come in high-rpm models for lighter faster cuts, while the strong and rigid 50-taper spindles are the choice for heavy cutting. But the line between the two is blurring, he adds.
New face and taper-contact systems make today's 40-taper spindles more rigid for heavy cuts. Conversely, new bearing configurations are boosting the speeds of 50-taper spindles. But how much speed is enough?
While spindle speeds in the 30,000 and 40,000-rpm range are available, the most versatile, says Kilgore, is an 8,000-rpm spindle. These handle heavy cuts and, although not the fastest, they do deliver light, fast cuts.
Above 8,000 rpm, a shop is looking at about a 12,000-rpm spindle, which most likely has ceramic bearings. These spindles offer high rpm, but cannot take the heavy cuts an 8,000-rpm spindle can. The main thing to remember for those with experience on lower-rpm machines, advises Kilgore, is that the higher the rpm, the more delicate the spindle. Also, higher-rpm spindles have limited lives, some as short as 2 yr — and some spindle lives are quoted in hr/run time.
For thermal control, VMCs may incorporate large-capacity heat-dissipating spindle chillers that maintain temperatures of spindle bearings and motor areas to minimize spindle-growth effects on part surface finish and accuracy. Some faster-rpm spindles feature core cooling, under-race lubrication, and closed-loop oil-temperature control.
In addition to spindle speed, VMC productivity features such as rapid-traverse rates, cutting feedrates, toolchange time, and spindle spool-up/spool-down times impact part cycle times. Choosing the best machine speeds depends on the material mix of a shop's parts, tooling required, and its idea of how to best produce the part in the least amount of time or at the lowest cost.
"Removing material faster is usually the goal of any machine buyer," says Kilgore, "But, in some instances, a slower feed and heavier cut could outproduce a lighter, faster one."
"A VMC must be considered relative to the field of application," remarks Mal Sudhakar, general manager at Mikron Bostomatic in Holliston, Mass. "A machine useful for high-speed positioning in an automotive application, for instance, is not necessarily of significance for high-speed contouring in a die/mold application."
As with any other type of machine tool, there are basic questions shops need to ask concerning a VMC control. What type of control is it? Is it widely used, reliable, and easy to operate? And is it powerful enough?
Ease of CNC upgrade, according to Fadal, is also a desirable benefit. Removing an entire CNC can be costly, forcing shops to purchase a new machine to get the speed, features, and/or capability of a new control.
Fadal suggests shops ask if the machine's CNC is upgradeable. If so, what elements of the control change out, how labor-intensive is it, does it take a day or a few hours, and can a local distributor perform the upgrade? What are the measurable benefits of speed increases and programming functionality, and what is the true CPU MHz speed?
Once a shop sets its sights on a particular VMC, the next step is considering options to further increase machine performance and better tailor it to suit specific shop applications. Beyond simply finding reasonably priced options, shops should check if the manufacturer offers packages at reduced prices or requires that options be purchased separately.
Haas suggests VMC shoppers consider these useful options: programmable coolant nozzles, chip-auger systems, 4th and 5th-axis rotary tables, and automatic toolchangers (ATCs).
To evaluate a VMC's ATC, recommends Makino, shops investigate how complicated it is. Also, is it reliable, and what size tools can it handle?
In addition to ATCs, VMC buyers should consider pallet changing, suggests Makino. It says that pallet changing dramatically increases VMC productivity by upping spindle use and recommends that the machine control and automatic-pallet-changer pushbutton station should support setup of multiple pallets of work as well as the actual production operation from the front of the machine.
Tim Rashleger, COO, of Milltronics Mfg. Co. in Waconia, Minn., recommends shops answer some preliminary questions to choose a production-style lathe:
- What swing is required above the bed?
- What is the necessary spindle-bore size and automatic chuck size or collet system?
- Does the lathe need a barfeeder, spindle orient, tool presetter, and parts catcher?
- Should the machine have live tooling — stationary spindle or contouring type — and a tailstock?
Dave Hayes, lathe product manager, of Haas suggests shops focus on the parts they will run to navigate through such lathe-buying issues as horsepower, chuck size, speed, and through-hole size. "A good choice is a machine with the capacity to handle 70% or more of a shop's parts," he says. More casting-type parts require high torque, while shops doing a lot of barfeeding work should zero in on machine through-hole size, spindle type, and speed.
There are basically two styles of production-lathe bases to choose from, conventional slant bed and flat bed (or wedge-style). Slant-bed bases are built up to a slant (45°, 30°, or 60°) that holds a crossrail and turret. A wedge-type lathe's turret and crosslides are mounted at an angle on a wedge block/carriage that moves on rails. "While there are pros and cons with both designs," says Hayes, "they both work."
Whether a slant bed or wedge type, a lathe's base must be a rigid, fairly heavy structure. "Cast iron alone, in some cases," says Richard Lewis, marketing operations manager for Hardinge in Elmira, N.Y., "is sufficient, but in high-accuracy applications, it may not be enough." This is why some OEMs incorporate polymer composites in their lathe bases or reinforce a cast iron base with the material for vibration dampening and thermal stability.
A sturdy machine base also better handles speed. According to Brian Ferguson, turnkey operations manager at Hardinge, high-speed lathe spindles are becoming more popular. While OEMs provide spindles with plenty of torque, a lot of shops are switching from high torque to high speed. The reason is that tooling innovations and through-tool coolant make for increased cutting speeds, so shops looking for a new turning machine may want to consider a spindle that can keep up.
Because lathe spindles hold workpieces, as opposed to cutting tools, they must be rigid, easily adjusted, have good TIR, and be quickly changed over. In addition, spindle designs should not reduce work-envelope space or create excessive overhang.
Lathe spindles secure workpieces using chucks and collets, and the key to increased machine production is a spindle that quickly and easily switches from one to the other. For using collets, most lathes require mounting some sort of adapter in the spindle, but there are those that don't.
In these jaw-chuck/collet-ready spindles, collets seat directly in the spindle and close to the bearings, so there's less overhang. Also, these spindles quickly changeover to accept a variety of workholding collets and chucks. This ability, along with the right tooling and machine-turret style, contributes to how fast and easy a lathe sets up, says Jeff Thomason, an applications engineer at Hardinge. Lathe shoppers must also determine whether or not their machines need tailstocks, and if so, if they should pick a manual or programmable type. Programmable tail-stocks are recommended for shops doing a lot of shaft, end, and steadyrest work.
Programming at the machine is a big issue with turning, and most OEMs have addressed this with features such as 3D graphics/templates, tutorials that walk operators through part programming, and software that that permits machines to operate as either manual or CNC lathes.
Such controls and software are useful for inexperienced operators and those reluctant to try CNC. "In a nutshell," advises Hayes of Haas, "shops should consider a conversational-type control that links manual and CNC programming."
Before heading off to shop for a mill-turn machine, Gerald Owen, product manager of integrated technology at Mori Seiki in Irving, Tex., says shops must have a clear understanding of their production goals. Is the focus on leadtime or mass production? Answering this question is crucial because in some mass-production applications, such as in an automotive shop, a mill-turn machine may not serve overall production needs. And vice versa — shops planning to use the machine as a 2-axis lathe with live tooling are not maximizing the machine's full potential. Therefore, they may want to re-think purchasing an integrated machine.
"A shop owner has to ask himself: Am I setting up for 1 hr and running three parts for a total time of 1 hr, 15 min, or am I trying to produce a million parts at 3 min/piece?" explains Owen. If the majority of time is spent on setup rather than production, then an integrated machine makes more sense, he advises.
Once a shop determines it could benefit from a mill-turn machine, it has to pick the appropriate size, and that is based on as many of the shop's different parts as possible.
"Selecting a machine size based on only one part or job is not always the best way to go," warns Owen.
According to Hardinge's Ferguson, shoppers should also compare capability to machine footprint. For instance, a shop with a part requiring milling and turning wants to complete both sides of the part. So the shop purchases a mill-turn machine that includes a milling head, toolchange system, and other "goodies" for flexibility. However, adding that flexibility may have doubled the size of the required machine as compared to one sporting a turret housing with fixed live tooling. "Keep in mind what a footprint for flexibility is going to cost," advises Ferguson.
However, mill-turn machines with B-axis milling heads do offer one important benefit. With them, mill-turn machines operate as 5-axis milling machines. Because the heads work as a milling spindle and as a toolholder for turning and boring, parts are completed in one setup, and the spindles easily handle off-center milling and compound-angle cuts.
But how does a shop decide whether or not it needs a mill-turn machine with a B-axis spindle head? The decision, according to Thomason of Hardinge, is application dependent. Parts with angled milling, drilling, boring, or tapping are obviously good candidates for a B-axis spindle. In addition, if the emphasis is on overall productivity as opposed to cycle time, a B-axis machine is a wise choice. That's because, while cycle times are longer, cost per part decreases due to less required labor.
If short cycle times are important, most OEMs recommend a 4- axis, twin-spindle mill-turn machine with a Y axis. But keep in mind, these machines don't provide the programmable angular capability of a B-axis head.
Parts with secondary operations and those that run from barstock are well suited for a twin or sub-spindle mill-turn machine. Subspindles deliver matching or slightly less power than main spindles. They sport an additional axis for moving in to meet the main spindle, which allows for machining part backsides and part transfer. Teamed with twin turrets (upper and lower), twin spindles can work on two parts simultaneously.
Lower turrets, says Mori Seiki's Owen, can increase machine productivity as much as 30%, depending on the part. A lower turret with live tooling shortens chip-to-chip times and keeps machines in the cut.
All these turrets and spindles mean more programmable axes. "Some mill-turn machines can have up to nine programmable axes, so programming is a different mindset as compared to a 2-axis lathe," says Owen. He recommends shops deal with an OEM that makes this programming transition easy through support and features such as conversational programming, interference-check software, and CAM post-processors. They should also supply software transfer of programs across ethernet connections to quickly and easily load programs into the machine.
Mill-turn machines let shops change from job to job and quickly adapt to new applications. Going from a 2-axis-turning part to one requiring 5-axis machining-center-type work is no problem for today's mill-turn machines. And their large tool-storage capacities — over 100 tools for some machines — make such job changeovers even faster, so shops can switch back and forth between small-lot/short-run and large-batch/long-term jobs.
Most OEMs agree that prospective mill-turn owners should shop for a machine and tooling at the same time, instead of as an afterthought. This is because mill-turn machine performance is extremely dependent on the right tooling. Shops should consult cutting tool companies that will study the shop's mill-turn applications and recommend tooling that is rigid, quick changing, and applies to a range of operations.
Swiss-Style And Rotary-Transfer Machining
Shops need to look beyond traditional operations of certain types of turning equipment, recommends Tornos Technologies in Brookfield, Conn. Two good examples are Swiss-type machines and rotary-transfer systems.
Swiss-type machines are commonly associated with long, skinny parts. However, they are well suited for many other types. With 3 or 4 axes of movement (including C axes), Swiss-style machines have no problem performing extremely intricate milling, states Tornos.
In addition, small parts that require little turning, but lots of milling, may be difficult to hold using traditional clamping. But on a Swiss-type machine, shops can use the barstock itself as the fixture and then complete the backsides of parts, employing additional axes and tooling available for machine subspindles.
These types of parts are also good candidates for rotary-transfer machines. Such systems not only generate the necessary output for higher-production requirements, but their flexible, modular designs provide sound alternatives to conventional-manufacturing processes. Some systems, for instance, combine Swiss-style turning with rotary-transfer machining by incorporating a sliding-headstock configuration. The result, according to Hydromat Inc. in St. Louis, is excellent part concentricity, precise diameter control, and consistent surface finishes.
AM Manufacturing Co.
Parts for the new job at AM Mfg. are round and octagonal, requiring both milling and turning, so the shop is evaluating whether to purchase vertical machining centers, lathes, or mill-turn machines. Because the parts can be made from barstock, the company is not ruling out Swiss-style or rotary-transfer machines.
AM Mfg. plans to look closely at well-built VMCs with midrange spindle speeds for handling both ferrous and nonferrous materials. The shop also needs to confer with OEM engineers as to the best way to hold, machine, and automatically load/unload round and octagonal-shaped parts on a VMC. The company's other option is to use standard production lathes paired with a VMC — doing all the turning first and then transferring the parts to the VMC for the milling details.
AM Mfg.'s other option is an integrated mill-turn machine with live tooling. The shop is going to look at single-spindle, single-turret machines. However, it believes that a twin-spindle, twin-turret model may better meet part-production needs. But some workpieces require angled milling, so the shop is also considering mill-turns with B-axis spindles.
Daewoo Heavy Industries America Corp., West Caldwell, N.J.