Milling's many paths
- Published: April 1, 2012
"Machining centres are, by definition, milling machines, so they inherently have milling capabilities," says Scott Rathburn, marketing product manager with Haas Automation. "Obviously, different types of machining centres have different capabilities, and those capabilities are determined by a host of factors."
The list of those factors is long. A partial tally includes machine size, horizontal or vertical spindle, spindle taper, horsepower, torque, spindle speed, geared-head or direct-drive, axis speed, table size, and type of control.
"Some manufacturers do build specific machines for specific types of machining," says Rathburn. "Haas Automation, however, produces more general-purpose machines, with capabilities designed to meet the needs of a wide range of customers, rather than a specific few."
That said, Haas has an extensive product line, and the company has certain machines (or groups of machines) that are targeted to more specific markets or industries, such as its VM series VMCs for mould making, and its Super-Speed machines for high volume production. These are still general-purpose machines, but they have features and options tailored more toward those industries or markets.
"Essentially, though, Haas machining centres can be used for pretty much any type of milling, regardless of what â€˜type' they are," says Rathburn. "And each machine can be customized with a host of options to boost productivity, such as through-spindle coolant, chip removal systems, 4th and 5th axis capability, high capacity tool changers, high speed spindles, probe systems, and more."
That's an impressive range of features. A company like MC Machinery Systems Inc., a wholly owned subsidiary of Mitsubishi Corp., is also positioning itself to have a bigger presence in milling. About six months ago the company introduced a general milling line of VMCs.
"We have the Roku-Roku for high accuracy graphite and hard milling, but that only covers a small percentage of the market," says Dan Haight, MC Milling's product manager. "This is a more general purpose milling line at 8,000 -20,000 RPM with a variety of sizes including your standard 40 in. x 20 in. (1,016 mm x 508 mm) table size."
The advantage with the new line is that MC Milling can establish itself in more general-purpose shops, covering almost all aspects of manufacturing in order to cover their entire supply chain of products.
Many turning centres also have some milling capabilities. A good example is Hardinge Inc.
"We have turning centres that offer milling capabilities," says Andy McNamara, director of sales and marketing North America for Hardinge Inc. "The benefit is that the machine can do the turning operations while also completing the secondary milling work."
When milling is the primary concern, Hardinge has its Bridgeport GX line, which can deliver a lot of power in the cut and high metal removal rates.
"The GX machines are Bridgeport's performance range of machining centres," says McNamara. "They compete in the high volume machining centre market all over the world."
McNamara says Bridgeport's GX line differentiates itself from the competition through features like standard high speed milling software to ensure accurate tool path at high feed rates.
"We also have dual contact Big Plus 40-taper spindles," he says.
Big Plus has a simultaneous fit of taper and face, which reduces vibration and enhances the stiffness of the cutter in the spindle.
"That helps ensure improved tool life, better surface finishes, and higher overall part accuracy at higher rpms."
Because speed and accuracy are crucial, select capabilities with the right software controls can make all the difference. Everyone wants their milling applications to run fast and clean, with increased depths of cut and better metal removal.
"Tooling costs are about 3 per cent to 5 per cent of an operation," says Bill Fiorenza, product manager for Ingersoll Cutting Tools, in Rockford, IL. "But direct costs for machining hours are five to six times that. Speeding up your machine a relatively small percentage can make a big difference."
A high speed approach to machining requires excellent control capabilities and a software interface that an operator can understand. Sometimes old machines can be retrofitted with new controls to improve precision and the depth of cut, as well as removal rates. Either way, finding the optimal tool route brings benefits all around.
"If you are making the same part day-in and day-out, then controls and processes can be optimized," says Fiorenza. "But you need to know your market niche. Some people think they want high speed, to take it to the Nth degree, but what they really want is high volume."
In the die and mould industry, for example, 3D geometries can create a lot of machining variables in the corners. Spending time up front to assess areas of high load, and high engagement, can result in great benefits over the long haul.
"Identifying machinability problem areas up-front is not done enough," says Fiorenza. "You need to address those areas that will cause high tool loads, chatter, and other problematic tooling conditions."
Milling looks ahead
Milling can hit the wall when it comes to tool wear and work hardening, but one of the hidden challenges is simply tool path efficiency. High contour 3D parts require the most advanced command system. The best tool in the world can't do much if it doesn't know where it's going.
"Look ahead is very important," says Fiorenza from Ingersoll. "A lot of controls can see 1,000 blocks ahead."
As a result, particularly in 3D machining, high speed milling needs not only the right software, but also the processing capabilities to keep up with the very large amounts of NC commands being processed.
"A lot of CAM softwares allow tolerance control of the 3D model geometry and the tool path within the CAM system," says Fiorenza. "By manipulating these tolerances for the application at hand you do end up with larger or smaller NC file sizes. Doing this will affect the finish quality, reduce control limitation, reduce programming calculation time and more. But In most cases a control capable of a 1,000 block look ahead is more than sufficient for most end-users, gobbling up the NC code data like Pac Man."
In this regard, Hurco's newest software innovation, UltiMotion, an addition to its WinMax control software, is impressive.
"UltiMotion is a new and different motion system and it brings a lot of new features; some are industry leading such as double arc smoothing" says Joe Poulin, Canadian sales manager for Hurco North America.
"It slightly adjusts data points within tolerance to compensate NC round-off error. Tangential arcs replace lines, resulting in smoother motion and gradual arc curvature changes, with less vibration. Consequently, a better surface quality can be obtained."
UltiMotion also uses Hurco's patented "Dynamic Variable look-ahead." It doesn't require a fixed number of block look ahead like conventional controls; instead, the look ahead varies dynamically depending on the geometry and feed rate, but always guarantees there is enough to make optimized manoeuvres. The result is a proven increase productivity of up to 30 per cent.
"Not only does it improve efficiency, but the machine is also smoother," says Poulin "Our advanced model compensation includes velocity, acceleration and jerk compensation."
By converting a hard corner into a smooth transition, the result is smaller impact and tool path deviation compared to a conventional arc transition. Among other benefits, this then delivers higher accuracy, improved surface quality, higher throughput, and improved machine life.