Productive tooling for small machining centers

Modern Machine Shop, Oct, 1990 by Brian Hurd

Productive Tooling For Small Machining Centers

Numerous Forces are at work leading to a growing use of smaller CNC machining centers with 7.5- to 10-hp spindle motors. Cutting tool selection is a paramount consideration in getting the best from these economical units.

As cutting tool producers and their customers look to the future, they see trends that will affect the types of cutting tools produced and sold. One trend is a proliferation of new materials, including composites, plastics and metal substitutes. Another is the emergence of CNC machining and turning centers capable of near complete machining of complex parts in a single setup. Also coming are dramatic increases in the use of computerized process control and the speed of information processing.

Equally significant, however, are two ongoing trends affecting primary metalworking manufacturers. Both are driven by cost pressures and highly efficient global competitors. The first is the increasing use of automated CNC machining systems for high-production and small-to medium-lot batch manufacturing. The second is the continued outsourcing of parts production to both "Tier One" suppliers (major parts manufacturers, now frequently quality-certified) and smaller contract suppliers (job shops). These trends are reflected in a recently published survey which indicates that, in the U.S. today, 68 percent of all metalcutting machines, and 67 percent of all NC machining centers, are in plants with fewer than 100 workers. Just six years ago, the proportion of all metalcutting machines in such plants was 42 percent, and a decade ago only 33 percent.

Smaller CNC Machines Fill A Gap

The automated machining systems increasingly evident among primary manufacturers are usually larger CNC machining and turning centers, often in cell configurations, which handle a wide range of part sizes and materials.

For many outsourced parts, however, the larger CNC machines simply are not needed. One of the solutions now available to contract shops and other parts suppliers is an advanced generation of "small" machining centers, first introduced between three and five years ago, which can commonly be purchased at less than half the price of many large-size machines (ballpark figure: about $80,000). Typically, in the 7 1/2 to 10 continuous-horsepower range, the small machining centers are capable of handling a broad range of smaller parts. At the same time, they provide the cost-effectiveness needed by many parts suppliers to ensure a suitable ROI (return on investment). Since their introduction, these machines have been used mainly for dedicated applications among Tier One suppliers, while job shops have used them to meet a wide range of part requirements.

Even smaller CNC machining centers, introduced about three years ago, are now available. These machines are generally offered in a 5 to 7 1/2 continuous-horsepower range, and are typically fitted with BT-style No. 30 V-flange adapters. Compatible tooling is within a size range of about 0.100- to a maximum 2.5-inch cutting diameter. With these limitations, the very small machining centers are usually restricted to synchronized tapping, end milling and face-milling operations--mostly on softer materials such as aluminum.

Small manual milling machines also continue to have their place in both Tier One and job shop supplier environments. These machines offer reliable performance, along with a price tag typically only half that of the small machining centers. Their use, however, is limited mainly to linear milling, drilling and boring. Moreover, because of lengthy setup times, the manual units work best for high-production runs. By contrast, the capable small CNC machining centers can execute both linear and contouring cuts with a range of rotary tools. They also provide faster change-overs, faster tool changes, and greater dimensional accuracy, repeatability and quickness than manual units.

Some of the same characteristics are also offered at lower initial cost by today's small CNC milling machines, which provide full contouring capabilities and programmed machining cycles. However, these machines are not equipped with an automatic toolchanger and, therefore, require manual tool change. They also are equipped with less powerful spindle motors (typically, two- to six-horsepower duty-rated) than the small machining centers, limiting their overall range of capabilities.

Tooling The Small Machining Centers

To focus on productive tooling for small machining centers, this discussion indicates a class of CNC machines with automatic tool-changers that typically generate continuous-duty spindle power of from 7 1/2 to 10 hp; provide a toolchanger capacity of up to about 21 tools; use No. 40 V-flange adapters; use tooling with cutting diameters ranging between 0.500 and 3.50 inches; and provide a working surface in the area of 17 by 34 inches, with XYZ travel ranges of about 30 by 15 by 20 inches. Such machines can accommodate workpieces within the XYZ envelope to a maximum weight of around 1,000 pounds.