The more a task is repeated, the easier it is to justify improving it. Most improvement programs begin by looking for the biggest bang for the buck. The benefits provided when improving an often-repeated task will be experienced every time the task is performed, making the required effort to improve it worthwhile. This assumes, of course, that the task is performed in the same manner every time it is performed.

Consider, for example, an often-repeated job that runs on a CNC machine tool. Many companies run a finite number of repeated jobs — over and over again. An improvement made to improve the repeated job will provide the same benefit every time the job is run. But again, this assumes that the job is run the same way every time. A variation can wreak havoc with the expected productivity gain.

Along these lines, companies strive to “qualify” CNC programs for repeated jobs. Once a program is deemed to be qualified, the program should run flawlessly whenever the job is run. Anything that causes a qualified program to fail will be related to some kind of variation. Something is different now than it was the last time the job was run.

For this reason, the act of qualifying CNC programs (or any repeated task, for that matter) is largely related to eliminating variations. This can be a daunting task when you consider how many things can change from one time a job is run to the next.

Workholding variations

The same workholding device must be used each time the job is run. Using a different vise on a machining center or different top tooling (jaws) on a turning center may change the clamping forces used to secure the workpiece, and lead to issues during machining operations. In similar fashion, manually tightened clamps, like vise jaws and fixture clamps, must be secured in the same fashion each time the job is run.

For machining centers, placement of the workholding device on the machine’s table must be consistent each time the job is run. Variations will cause the need for remeasuring program zero assignment values and entering them into fixture offsets.

Cutting tool variations

It can be challenging to eliminate variations with cutting tools from one time a job is run to the next. And admittedly, doing so can conflict with efforts to improve the machining process by employing newer cutting tool technologies. But if CNC programs are to be qualified, variations in cutting tools must be eliminated.

This means cutting tool documentation must be all-inclusive. The person who assembles cutting tools, possibly a person in the tool crib, must be provided with a complete list of components that comprise the cutting tool, and for machining centers, information that illustrates precisely how long each cutting tool must be.

Machine condition

This potential variation is often overlooked. Wear and tear eventually affects how machine tools perform. Be sure your company adheres to a good preventive maintenance program. It will ensure that older machines perform nearly the same as they did when they were new. Examples of machine deteriorations that cause productivity-related issues include gib and way system wear, faulty coolant systems and worn spindle bearings. Additionally, corrective maintenance must be properly done whenever mishaps (crashes) occur to ensure that the machine behaves the same way it did before the mishap.

There are also machine condition issues caused by poor usage techniques. With turning centers, for example, use the style of tooling (right- or left-handed) that causes the force of the machining operation to be pushed into the machine bed — as opposed to pulling the turret away from its direction of support. For machining centers with rotary axes, be sure to apply the rotary-axis clamp in the program (usually specified with an M code) prior to performing any powerful machining operations.

Lot size

A job may run just fine when only a few workpieces are produced. Cutting tools may last for the entire job before showing any signs of wear. But as lot size grows, cutting tools may not last as long as they should — or operators are making an excessive number of sizing adjustments. Indeed, process or cutting tool selections may have to change to accommodate the number of workpieces being machined. Indeed, some companies maintain two or more versions of the program/process based on lot size.

People

The same people will not be involved every time the job is run. Your company likely experiences turnover in workers or multiple people doing the same things, especially during multiple shifts. But as with other variations, varying skill levels of involved personnel can negatively affect productivity. The two general ways to overcome this variation are to improve skill levels through training and/or to lower the skill level required to perform tasks by simplifying to the point that anyone can perform them.

Environment

Even variations in the machining environment, such as temperature and humidity, can cause consistency issues. Consider a shop that is not air conditioned. Jobs run in the winter, when the heat is on, maybe run well when the ambient temperature is around 70ºF. Jobs run in the summer (without air conditioning) may be run at a substantially higher ambient temperature. Operators having no trouble holding size on a job run in the winter may struggle when running the same job during a hot summer.