Holding Size with Tight Tolerances

CNC machines produce very accurate workpieces, and they do so repeatedly for two or even 2,000 workpieces. Dimensional tolerances between ±0.002 and 0.005 inch (50 to 127 microns) are easy to achieve initially and hold throughout a cutting tool’s life.

Tolerances between ±0.001 and 0.002 inch (25 to 50 microns) are still easy to achieve. However, holding these tolerances though larger lot sizes will require consideration if tool wear causes surface growth or shrinkage past a tolerance limit before the cutting tool wears out.

Tolerances of less than ±0.001 inch (25 microns) are more challenging, yet many companies regularly hold even tighter tolerances between ±0.0002 and 0.0005 inch (5 to 12 microns). These tolerances require consideration to achieve initially and hold for multiple workpieces. Here are some suggestions for dealing with them:

• Use metric mode. Metric mode has a finer resolution than inch mode. The metric-mode least-input increment for offset entry is commonly 1 micron (0.001 mm). The inch-mode resolution is commonly 0.0001 inch. Note that 1 micron is less than half of 0.0001 inch. If using the inch mode, a 0.001-inch (25-micron) overall tolerance will have 10 settings in the offset register that will place the dimension within specification. Using the metric mode will provide 25 such settings.

This makes it much easier to achieve a tight tolerance and hold it through tool wear adjustments because operators can specify more precise adjustments required to achieve target dimensions.

• Use 80 percent of the tolerance band. With tight tolerances, sizing adjustments will be required during the production run. As tools wear, the surfaces they machine will grow (external surfaces) or shrink (internal surfaces). The tighter the tolerance, the more sizing adjustments will be required.

A common rule of thumb is to target the mean value of the tolerance band when initially sizing in a dimension and when making tool-wear-related sizing adjustments. When you target the mean value, you are working with only half the tolerance band. The related dimension for all machined parts will be on the high side for external surfaces or the low side of internal surfaces. With very small tolerances, your operator may have to make a sizing adjustment every few workpieces.

If you target something closer to the low or high end of the tolerance band (depending on external or internal surfaces), you can extend the time between required sizing adjustments. I recommend making the target dimension within about 10 percent of the tolerance limit. This will double the time between sizing adjustments.

• Minimize the effects of thermal deviation. This point applies more to turning centers but can also impact machining centers. As machine tools warm up, their components grow. As warmed-up machines sit idle, their components shrink. As components grow or shrink, machined sizes vary. Thermally induced machine component variations can wreak havoc when you must hold tight tolerances. Some turning centers, for example, experience as much as a 0.001-inch (25-micron) diameter shrinkage on external diameters while components settle in to their working temperatures.

A common technique used to deal with thermal variations is to have the machine run a warm-up program when it is first powered up and whenever the machine will sit idle for more than a few minutes. It can involve running the program without raw material for the length of time it takes to warm up.

Machine tool builders vary when it comes to size variations caused by thermal characteristics. Some machines are much better than others. This should be an important factor when purchasing new equipment if you expect the machine to hold tight tolerances.

• Use right- or left-hand tooling. This turning center suggestion affects the machine’s longevity. It is especially important for very powerful machining operations (like most roughing operations). Use the style of tooling that throws the force of the machining operation into the bed of the machine. For most slant-bed turning centers, for example, this means using left-hand tooling and running the spindle in the reverse (M04) direction.

If using right-hand tooling for this kind of machine, the spindle will run in the forward (M03) direction, and the shearing action of the machining operation will try to pull the cutting tool away from its direction of support. Indeed, it will tend to pull the turret away from the cross-slide, and the cross-slide away from the bed.

This puts undue stress on the machine’s moving components, causing the machine to wear out (or require rebuild) much sooner than it should. The first evidence of a related problem is holding size for tight tolerances. Tolerances the machine easily held when it was new become more difficult, or impossible, to hold.