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The most important thing a CNC operator must know

Most companies expect a lot from their CNC operators. Common tasks include:

  • Loading and unloading workpieces

  • Activating and monitoring the CNC cycle

  • Measuring workpieces and reporting findings to an statistical process

  • control (SPC) system

  • Making adjustments required due to tool wear

  • Replacing dull tools

  • Keeping the machine and work area clean

Every one of these tasks, of course, is very important, and it could probably argued that any one of them is the most important – for without a mastery of each task, a CNC operator will not be successful.

Maybe a better question might be “Which task does an operator tend to struggle with the most – when mistakes can lead to wasted time, scrap parts, and possibly dangerous situation?” Again, several of the related tasks may come to mind. But the one we’d like to stress is Making adjustments due to tool wear.

Since turning centers incorporate many single-point cutting tools, they tend to require the most in the way of workpiece sizing. However, machining centers also require their share of sizing adjustments. In any event, a CNC operator must be able to determine whether a measured attribute is within its tolerance band. Since this is such a basic task, experience, shop people tend to take the related skills for granted – and sometimes assume that everyone (including entry level CNC operators) can do them.


The first related skill an entry level operator must master is accurately measuring workpiece attributes with the gauging tools your company uses. This can take a lot of practice, especially with variable gauges that have Vernier (or similar) scales. For this reason, more and more companies use gauges with dial or (better yet) digital displays.

Measuring accurately requires a certain “feel”. Once a person is taught how to use one of your gauges, they must practice to master it. One way to get started is to use known sizes for measuring. For example, use a set of gauge blocks to simulate workpieces. If the operator know the thickness of a gauge block is supposed to be 0.500 inch, for example, they’ll be able to experience the feel of the gauge when it displays 0.500 inch. This is mandatory when measuring actual workpiece attributes.

Evaluating measured values

Assuming a CNC operator can accurately measure workpiece attributes, next think about what must be done in order to determine whether a given workpiece attribute is within its tolerance band. Consider a “simple” 2.0 inch turned (external) diameter machined on a turning center. How is the tolerance specified? It could be done in at least three ways:

  • With a plus/minus tolerance specification (2.000 +/- 0.002)

  • With an uneven tolerance specification (2.001 +0.001 -0.003)

  • With the high and low limits (2.002 / 1.998)

Even for this simple example, it can be difficult to get across the point that, in order for the turned diameter to be acceptable, the operator’s measured dimension must fall between 1.998 and 2.002 inches. Again, this may seem elementary to any shop person, but it will be a new concept for most non-manufacturing people. It will take practice to master.

Remember, we’ve shown a pretty simple dimension and tolerance example. Determining the high and low limits is pretty easy in our case. But consider a dimension and tolerance of 1.8324 +0.0004 -0.003. Even experienced shop people may have to think about this one for a bit before they can determine the mean value and high/low limits. A newcomer may have to use a calculator.

As you begin teaching people how to hold size on CNC machines, remember that this is a task they must be able to perform flawlessly and consistently – based upon the dimensioning and tolerancing methods you use. You can have them practice by giving them problems like this to solve:

  • Dimension is 2.125 +/- 0.004. The workpiece attribute you measure is 2.123. Is the measured value within the tolerance band?

The target value

Next, CNC operators must understand that when a measured workpiece attribute is not within its tolerance band (and often even when it is), an adjustment must be made. Making an adjustment first requires a person be able to determine the target value. That is, the dimension the operator is shooting for with the adjustment. Many companies have their CNC operators target the mean value of the tolerance band – and this may be just fine in the beginning.

(But remember, when you target the mean value, you’re only working with half the tolerance band – and adjustments will be required twice as often. Based upon the direction of workpiece-attribute growth caused by tool wear, many CNC operators will target a value closer to the high or low limit – whichever provides the longer period of unattended operation. This concept, however, may be a little difficult to relate to entry-level operators. The more important point is that they will have to know the target value before an adjustment can be made.)

Based upon your company’s methods, you must ensure that entry-level operators can accurately and consistently determine the target value. Again, coming up with exercises shouldn’t be too tough:

We target the mean value of all tolerance bands. Based upon the following dimensions, what is the target value?

  • 2.000 +/- 0.002

  • 2.375 +0.001, -0.003

  • 1.4378 / 1.4373

When to make an adjustment

CNC operators must know when adjustments are required. Companies vary with specifics, but the concept remains the same. Of course the operator will make an adjustment when a measured workpiece attribute is not within its tolerance band (and the current workpiece is scrap). They must know not to run any more parts until the adjustment is made (don’t assume they know this).

More commonly, CNC operators will be making adjustments as the growing or shrinking workpiece attribute draws close to a high or low limit. Explain that this growing or shrinking is caused by tool wear - and is especially common with single point tools. As tools wear, more and more material will be left on the surface/s machined by the tool. As more and more material is left on the surface, external surfaces (like turned diameters) will grow and internal surfaces (like bored holes) will shrink.

So a turned diameter that starts out at precisely 2.000 inches in diameter will grow as more and more workpieces are machined. After fifty parts, this dimension may be 2.0003 (it has grown by 0.0003 inch). After fifty more parts, it becomes 2.0007. And so on. Eventually the tool will become completely dull and will require replacement, but a lot of growth (or shrinkage) can occur before this is necessary. In most cases, and especially with tighter (smaller) tolerances, the workpiece attribute will grow or shrink out of its tolerance band long before the tool is dull. This means several adjustments will often be necessary during a tool's life.

So, exactly when should the adjustment be made? Again companies vary with what they tell operators to do, and of course, you’ll relate your methods to your new CNC operators. Most have a ten or twenty percent rule-of-thumb. When the surface grows to within ten or twenty percent of a tolerance limit, an adjustment will be made. Better stated, when a 2.000 +/- 0.001 tolerance grows to 2.0008 or 2.0009, an adjustment will be made.

How much to adjust

CNC operators must be able to determine the amount and direction for the required adjustment. Explain that the adjustment amount is simply the difference between the measured value and the mean value. If an operator measures a turned diameter with a target dimension of 2.000 inches as 2.002, the adjustment amount will be 0.002. Again, you can easily come up with exercises for determining adjustment amount.

Polarity of adjustment

Explain that in some cases, the adjustment will be positive and in others, it will be negative. Determining polarity requires an understanding of the machine’s axes – as well as their polarity. So be prepared to explain which way is plus and which way is minus for each axis.

Fortunately, many turning center adjustments are very simple: If a turned or bored diameter is too big, the adjustment will be negative. If it is too small, the adjustment will be positive (for most turning centers). Again, be prepared to explain polarity of each kind of adjustment your operators will be making.

And again, develop exercises to confirm entry-level operators understand:

We target the mean value for all dimensions. Dimension is 2.000 +/-0.001. Measured value is 2.0008. What is the adjustment amount and polarity?

What must be adjusted?

In rare cases, the cutting tool itself must be adjusted. Consider, for example, a boring bar used on a machining center. A mechanical dial controls the precise diameter that the boring bar will machine. If the measured hole size is too small, the dial will be turned in one direction. If it is too big, the dial will be turned in another. And mechanical linkages precisely move the boring bar insert to make a bigger or smaller diameter.

In most cases, adjustments will be made in offsets – so be ready to explain what they are and how to access them. Explain that offsets are referenced by a number – offset one, offset two, and so on. Also explain if your offsets contain more than one value (length and diameter for a machining center for example – or X and Z for a turning center).

CNC operators must know which offset must be adjusted - and if the offset contains more than one register, which register is involved. Explain that most programmers will make the offset number correspond to the tool station number – so if the operator knows which tool machines a surface, they’ll know the offset number that contains the adjustment values.

(Even this concept is difficult to relate. It can be hard for an entry-level operator to determine which offset must be adjusted. For reason, more and more companies are including offset information in the production run documentation that goes along with the job.)

Unfortunately, practicing this can be more difficult. Take newcomers out to your currently running machines and show them how to determine which tool machines each surface and how to determine the related station numbers and offset numbers. Have them tag along with experienced operators to see it done first hand.

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