Outdated secondary-operation machines were strangling the productivity of a Chicago-area screw machine shop's impressive battery of CNC screw machines. Now, CNC drilling and tapping machines and turning centers are providing secondary machining efficiency that has raised the firm's performance to a level few competitors can match.

When you drive past the building, with its concave front and narrow, close-set, floor-to-ceiling windows, you would guess an insurance firm, or an engineering or architectural firm, or perhaps an association office. In fact, the distinctive exterior pictured in Figure 1 houses the operations of General Automation, Inc., one of the most impressive screw machine shops in the country.

General Automation specializes in, among other things, Swiss screw machine parts, that is, workpieces with high length-to-diameter ratios. The firm produces them on numerous Swiss-type cam automatic screw machines, arranged like spokes on large wheels, bar feeds toward the center. (The arrangement makes the most efficient use of floor space and makes it easier for the operators to load the machines and tend their operation.)

Although cam-operated screw machines have been around for generations, they remain a very competitive method for producing workpieces in large quantities. They can produce workpieces in less time (a shorter cycle) than by most other methods.

But cam automatics are inefficient when the job involves making a few prototypes, or a small quantity of workpieces, or when machined part dimensions must be accurate to one or two ten-thousandths of an inch. Such jobs are best handled on CNC screw machines.

Advantages Of CNC

Screw machine shops are aware of the advantages of CNC screw machines; many have one or two in order to remain competitive on prototype work, short run jobs, and jobs where fast turnaround and/or high quality are critical. However, few shops are in a position to invest in enough CNC screw machines to comprise a separate department.

General Automation is one of the few. The firm operates some 63 Nomura CNC Swiss-type screw machines--the largest concentration of such machines in the U.S. The CNC Swiss-type screw machine offers a number of advantages over the older cam automatic screw machine. As its name implies, the cam automatic's cutting tool movements are controlled by a set of cams specially made for the workpiece being produced.

The CNC screw machine needs no cams. Tool movements are automatically determined by data from the dimensions of the part, fed into the machine's CNC unit. The workpiece can be programmed on the CNC right at the machine. Or, as is more often the case, the program can be prepared on a programming system located off the shop floor, and loaded in the machine's control when required. The control's "memory" is capable of storing dozens of workpiece programs; preparing the CNC screw machine to produce a different workpiece simply involves calling up the program for the next job, minimizing machine downtime between jobs.

Because CNC machine tools handle job change-overs much faster than manual or automatic machines, they are usually thought of as "short-run machines." By contrast, cam automatics, which are noted for their very rapid cycle times, are considered "production machines," ideal for producing large quantities of workpieces over long periods of time.

However, General Automation's CNC screw machines are faster than its cam automatics, so the company benefits not only from fast, easy machine setups, but from fast workpiece cycle times as well. According to General Automation president Max Starr, the CNC Swiss-type machines are frequently the most economical choice for complex workpieces, regardless of the length of the run.

Other advantages of the CNC screw machines are their greater accuracy and piece-to-piece consistency. Both are important to today's product manufacturers who demand parts machined to finer dimensional tolerances--and that those parts be within tight statistical process control parameters.

Another big advantage of the CNC screw machines--important to any job shop concerned about controlling costs--is that operator responsibilities are reduced to monitoring the machines and keeping their bar feeds filled. This enables General Automation to use less skilled operators at lower hourly rates.

A Weak Link

General Automation's CNC screw machines, added to the firm's other single- and multispindle automatic screw machines, give it a primary turning capability that few, if any, shops can match. But they called attention to a serious shortcoming in another part of General Automation's operation.

The firm's machining requirements typically include milling, drilling, tapping and other hole-making operations, in addition to secondary turning. The secondary turning requirements were met by a number of precision, small-part, turning centers, most equipped with automatic part-loading devices to permit untended operation over long intervals. But hole-making operations (drilling, tapping, and so on) were performed on older manual or automatic machines that were dragging down production times on jobs and compromising dimensional accuracies initially provided by the CNC screw machines.