Is there a HAM in your future?

Manufacturing Engineering, Jul 2000 by Mege, Claude Jean

In France, high-agility machine's, or HAMs, lead to HPC, or Highly Profitable Cutting

Today, everyone connected with the machining business talks about "high speed," but few can identify the various aspects of this technique. Even fewer manufacturers understand high-speed machining's implications for processes, its constraints and consequences.

To go back a bit, decades ago researchers into the cutting process discovered that beyond a certain cutting speed, mechanical energy generated heat in a totally different way, depending on the part material. The change took place at a speed ranging from about 2500 ipm (63.5 m/min) for very hard metals like titanium to over 100,000 ipm (2540 m/min) for light alloys like aluminum. When conventional machining proceeds below the lower limit, almost all heat goes into the part and tool. Above this limit, almost all heat goes into the chips, keeping both part and tool cool.

Machine tools of the next generation, and particularly five-axis milling machines-whether dedicated to machining aluminum aerospace parts, steel automotive dies, or resin full-scale models-must deliver highly accurate, smoothly finished complex parts, often very large ones, which often require simultaneous five-axis milling. These machines add rigidity, power, agility, flexibility, reliability, ergonomics, safety, and accuracy to high-speed cutting. At Forest-Line we call them High Velocity Machines (HVMs) or Highly Agile Machines (HAMs).

HSC with HAM. In theory, if you have a special high-speed, well-balanced tool and a high-frequency electro-spindle, you can do high-speed cutting. To take full advantage of that tool and spindle, however, you need a machine with the HAM's dynamic rigidity, acceleration capabilities-meaning jerk management-digital control, and linear scales and rotary encoders to do direct measurement.

The aerospace industry produces parts in quantities from a few units to thousands of pieces. They cut a huge amount of swarf from these parts-often, the weight of the finished part is only 15-20% of the original rough billet, so, on average, 80-85% of the aluminum is reduced to a heap of chips.

The first US manufacturer to buy one of our HAMS four years ago posted a sign on his new "monster" reading: "I eat 400 tons of aluminum per month!" The customer was forced to install a second swarf compactor, and now sells bricks of aluminum chips to recyclers.

HAMS in the automotive business mill either resin or foam models at full scale for design and aerodynamic test purposes or cast-iron or steel press tooling (dies). The dies then go to the presses used to stamp auto bodies from steel sheet.

Since early 1997, Fiat Group (Turin, Italy) has produced all dies and tooling with a flexible manufacturing system. This flexible cell features four roughing vertical-gantry-type machines, one 3-D measuring machine, and three vertical-head gantry-type finishing machines with automatic head changers. A computerized supervisor manages the machines and monitors their status, tracking pallets, parts, tools (including "brother" tools), and part programs. This computer is connected through Ethernet to the corporate network, and thus receives the part program from the CAM system. Fabrication orders, with their multilevel priorities, come from the production ERP.

In milling, quality means both dimensional accuracy and smooth surface finish. Many machine-tool builders boast of their speed, many of them take pride in their quality, and sometimes a vendor will talk of superiority in both areas, but very few can supply both productivity and quality at the same time.

A HAM tool behaves like a Formula 1 race car. Milling complex shapes (like car body dies or aircraft wing structures) means following curves and more curves. Like a race car driver, the machine must decelerate before the curve (but not too early), and accelerate during the curve, thus compensating for physical phenomena (inertia forces like centrifugal force).

A real HAM cuts fast, with very high spindle rpm (currently 15,000 to over 30,000 rpm). Very high axis feed rates (currently 800-2400 ipm [20-61 m/min]) take advantage of the machine's cutting capabilities and let it move as fast as possible during noncutting phases. In order to stick to the exact path, the machine will accelerate very fast

A HAM is a fantastic vibrating machine-a sort of concert grand piano. The world leaders in the production of grand pianos master all the vibrating phenomena inside and around their instrument, so the player can offer the audience the sounds he wants, and no others. On a HAM, the cutting edge hits the part a thousand times a second, a 1000-Hz excitation. Acceleration and deceleration of the axes constitute excitations of the machine structure, and they are acceptable if the natural vibration mode frequency is high enough.

Compare traditional machine tool structures (heavy moving portals or tables with a lot of cast iron). Their excitations are usually around 15 Hz. High-speed machines of the first generation had proper modes between 20 and 25 Hz. The new HAMS, especially those with linear motors, now reach 30-40 Hz.