Machine tool considerations come to the surface - Cover Story

When all the design, NC code generation and fixturing are complete, it's a machine tool that gets down to the business of contoured surfaces: cutting. Here are some things to consider about the machine you choose for the job.

A trip through any supermarket provides more than enough evidence to explain why machining of contoured surfaces is a growing segment of manufacturing. For example, each of the thousands of uniquely shaped plastic containers, found along the miles of aisles, gets its shape from a mold. And the mold gets its shape from a machine tool. It's estimated that 60 percent of all parts made today--a percentage that's growing--are made from plastic.

But there are more contoured surfaces to machine than just molds, although as our supermarket tour illustrates, they do make up a large chunk of the surface machining universe. Manufacturing is applying contoured surface machining technology across many industries such as automotive, power generation, aerospace, die and mold making, and health care.

Design considerations that take into account form as well as function increase the demands that are placed on manufacturers for contoured surfaces. Ergonomics--the physical interface between people and equipment--is also a force behind smoothing the square edges of many products. These design directives are showing up in virtually all manufactured products, whether they are made fromThis article is about machine tools that make such design a reality--specifically, machine tools that sculpt contoured surfaces--in metal. The end product of that sculpting may be a stainless steel mold and core for a plastic bottle, a medical implant or a highly contoured titanium spar to strengthen an aircraft fuselage. But without machine tools that are capable of efficiently performing such complex machining, the molds and spars themselves--as well as the end products that rely on them--would be much more costly.

To get a sense about the most important equipment-related issues, we spoke to LeBlond Makino (Mason, Ohio) and Cincinnati Milacron (Cincinnati, Ohio) about their surface cutting machines. While each builder approaches problems associated with machining surfaces somewhat differently, their customer goals are identical--machine more accurately to reduce labor and time spent doing non-value-added benchwork, thereby increasing throughput and quality.

Machining Surfaces 101

Probably the most basic requirement for machining surfaces is a machine tool that can adequately manipulate a cutter to impart the desired shape onto a raw workpiece. In other words, it needs the ability to perform simultaneous axis moves. While there are techniques for doing surfaces with less than three axes, we're going to concentrate on contour machining using at least Cartesian coordinates (X-Y-Z) and up to five axes--all capable of independent and simultaneous movement--linear as well as rotary.

The machining process for cutting a contoured surface is complicated not only by the rise and fall of the surface but also by the relatively small-diameter cutting tool that's used. On a 12-inch-wide flat surface, for example, two passes of a six-inch face mill will machine the surface. A 12-inch contoured surface, using a 3/4-inch ballnose end mill, may take 98 passes to cover the same area, because the ballnose design cuts a width that is a fraction of the tool's 3/4-inch diameter. And generally, surface machining is further divided into two operations: roughing and finishing.

In mold and die shops, roughing accounts for about 15 percent of the total machining time of a workpiece. While roughing may only use about 15 percent of machining time, it removes the majority of material, leaving just enough stock for the second operation--finishing.

Finish machining on a surface doesn't take up the other 85 percent of cycle time for producing a surface. Actually the percentage is closer to 50. Of the 35 percent that's left, 25 percent of that is hand machining (benchwork) needed to finish the surface. The last 10 percent is called tryout in the mold and die industry, which equates to measurement or verification in other surface applications.

Many shops perform roughing operations and finishing operations on different machines. Historically, a big beefy machine tool that didn't move very fast but sure could hog metal was the roughing machine. For finishing, the workpiece, mold or die would be moved to another lighter, more nimble, machine tool to remove the remaining stock.