Tips for Controlling Runout in Toolholding and Machining

Quick take: Runout is often the hidden source of poor finish, short tool life, and unstable drilling or milling. This page works best when it is used together with the holder and collet references that explain where the error actually starts.

Related references: The Secrets of HSK Toolholding, Shrink-Fit Toolholder Fundamentals for CNC Shops, and What is a Collet? Types, Uses, and Design Explained.

Machinists struggle with different problems during the day like materials that are hard to machine, tight tolerances, programs that don’t make any sense and the safety guy who is always finding something wrong. Of course, there are many other things that can go wrong, but most of our machining aggravations occur only occasionally.

Unfortunately, we also face a few constant problems, and runout is one of those that machinists fight regularly.

Runout is rotational deviation of one feature relative to another. Consider the motion of a crank shaft and its crank pins. This motion is a good way to visualize runout. When a crank is rotated about the journals the crank pins swing in a path that rises and falls relative to the center of the journal. Runout in a shaft or a cutting tool has a similar motion.

So why is runout a problem? It can cause alignment issues in rotating components leading to bearing failures, bad seals and out-of-balance conditions. Runout in cutting tools causes tool breakage, out-of-balance conditions, over-size holes, bad surface finishes and machine failures. So, minimizing runout is important.

When creating specifications for new components engineers give a significant amount of consideration to runout and the effects on machinery. That is why we see tight tolerances on drawings. It is common to see a drawing that has a runout requirement less than 0.0005″ between two or three features.

Runout issues are most common on turning because parts made on a lathe are often rotating parts or parts that combine with rotating parts. Therefore, engineers will include runout tolerances to control geometric and rotational characteristics.

The first step to controlling runout at the lathe is building a process that negates the errors that are inherent to turning processes. Although obvious, it must be noted that success requires using a lathe that is well-serviced, operating correctly and accurate enough to hold the tolerances. If the lathe spindle is running out 0.0005″ then the best part runout you can hope for is 0.0005″.

Assuming the machine is accurate and capable the next step is to build a turning process that negates any of the inherent errors in the machine and the workholding. When possible, machine every critical feature without removing the part from the machine. Doing so will ensure the errors that exist in the workholding device are negated. No workholding device is perfect; they all have some runout that will be introduced into the system. This is why you see machinists turning (or grinding) between centers.

Often, parts must be “flipped” over to complete the machining process. So, the third step is selecting the right workholding method. It is impossible to say that one method is better than another. The best workholding is going to depend on the conditions of the job, the machine, the shop, the material, etc. For production quantities on modern CNC turning machines a good hydraulic three-jaw chuck is sufficient for most jobs. Machined soft jaws that match the diameter of the part are often good enough to meet the drawing requirements. Depending on the machine, material and skill of a machinist, 0.001″ to 0.002″ runout can be held with a three-jaw. Once the runout requirements get to 0.001″ or less a collet chuck is the best choice. Consistently holding 0.0005″ runout with a quality collet chuck is not a problem. This is why high precision tool room lathes come standard with a collet chuck and the three-jaw is sold as an option.