Tool runout is an inevitable part of any machining process and cannot be completely avoided. Therefore, setting an acceptable runout for each machining operation and staying within that range can optimize productivity and extend tool life. Lower runout is always better, but machine and fixture selection, reach length, tool travel, and many other factors can affect runout.

Tool Runout

Tool runout is the amount a cutting tool, toolholder, or spindle rotates away from its true axis. This is especially true on poor quality end mills, where the cutting diameter will be accurate when measured when the tool is stationary, but out of tolerance when rotating. The first step to reducing runout is to understand the various factors that contribute to runout. Runout is reflected in the accuracy of each cutting tool, chuck, toolholder, and spindle. Each additional connection point between the machine and the workpiece being machined results in more runout. Each additional connection adds further to the total runout. For each piece of tooling and equipment, steps should be taken to minimize runout to achieve optimal performance, extend tool life, and produce a quality finished product.

Runout Measurement

Determining the amount of runout in a system is the first step in finding a way to deal with it. Runout is measured by the change in diameter as it rotates, using a dial indicator or laser measuring device. While dial indicators are portable and easy to use, they are not as accurate as existing lasers, and pressing on a dial indicator can make runout measurements worse. This is primarily a problem with micro and tiny tools, which should strictly be measured with lasers due to the lack of rigidity of the tool. Most end mill manufacturers recommend using laser measurement instead of a dial indicator when possible. Runout should be measured where the tool is making the cut, usually at the end of the tool or along a portion of the length of the cut. Using a dial indicator in these cases is not feasible due to inconsistent tool flute shapes. Laser measuring devices offer this advantage.

High-quality tools

Cutting tools are manufactured with a maximum allowable run-out, some with a maximum run-out of 0.005 mm or less. This is usually the value that should be achieved. For micro tools with diameters as small as 0.025 mm, the run-out measurement must be controlled to be even smaller. The greater the ratio of tool run-out to tool diameter, the higher the risk of tool failure. As mentioned earlier, starting with the tool with the smallest run-out is crucial to minimizing the total run-out. This run-out cannot be avoided.

Precision Toolholders

The next step in reducing runout is to ensure that you are using a high-quality, precision toolholder. These toolholders are typically heat-shrink or press-fit holders that allow for precise tool rotation. Applying even pressure around the entire circumference of the holder is critical to reducing runout. Screw-on toolholders should be avoided because their uneven clamping force can pull the tool off-center. Collet-mounted toolholders also typically introduce additional runout due to the extra components. With every additional connection in a toolholder system, there are more opportunities for runout. Heat-shrink and press-fit toolholders are inherently better at reducing runout because they have fewer components.

Machine cleanliness is a factor to consider when considering tool holding. Oftentimes, chips can get stuck in the spindle. It is critical to ensure that the tool holder and spindle are clean of chips and debris during every job setup. Toolholder Improvements Many factors other than the equipment itself can contribute to increased tool runout. These factors can include the length of the tool, the rigidity of the machine tool mount, and how far the tool protrudes from the holder. Toolholder improvements and the method used to hold the tool can have a large impact.

Runout is a common problem in the metalworking industry, yet it is often overlooked when it can lead to scrapped parts and unacceptable results. Every component of a machine tool contributes to the resulting runout, so no component should be overlooked.

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