Shop Doc – Call of Confusion

Today’s Machining World Archive: March 2007, Vol.3, Issue 03

Dear Shop Doc,

We are a job shop manufacturing turned parts on screw machines and CNC lathes for a variety of customers and industries. On parts with multiple diameters some customers call out a concentricity specification. Others will use run-out and some show a position requirement. Aren’t they really all asking for the same thing? Why do they say it three ways, and can they be used interchangeably?

Call of Confusion

Dear Call,

Concentricity, run-out and position are all methods that can be used to control the “coaxiality” of part features. “Concentricity” is still a term commonly used by designers and machinists when describing how much deviation is allowed between coaxial diameters.  There are however, significant and sometimes critical differences between the three terms as described in the current standard ASME Y14.5M-1994. This standard has been adopted by the Department of Defense and the majority of companies in the pubic domain. It is generally recognized as the last-word regarding dimensioning and tolerancing issues in the USA.

Regarding “concentricity,” the standard specifies that the median points of all diametrically opposed elements are on the axis of a datum feature. The standard does not say that the features have to be circular or that there is any rotation around the datum axis. This means that this control can apply to any shape feature such as an ellipse, rectangle, square, etc. The measuring method used is generally very complicated and usually not necessary. In most cases concentricity may be what the designer is asking for but is not the appropriate control for turned parts.

The “run-out” definition states that there is “rotation about an axis.” It also mentions that the tolerance can be expressed as “full indicator movement.” From the designer’s perspective, run-out is usually the proper control for rotating applications such as a pulley on a shaft. From a manufacturing perspective, run-out has the advantage of being checked with simple and low cost measuring equipment. When you are rotating a part on a pin or in a collet fixture and sweeping a diameter with a dial indicator, you are in fact checking run-out, not concentricity.

“Position” is usually the least used of the three controls but has many advantages in both design and manufacturing. When defining position, the standard talks about the “actual mating envelope” of the features. This means that the size of the feature can also be considered in the equation. A good application for position would be two or more internal diameters that assemble to a mating part with outside diameters. If the design intent only requires the part to assemble without interference, this control is a good choice. A position control allows you to use a simple “hard” gage that simulates the mating part. It also allows you to play with the relationship between size and coaxiality. It is then possible to take advantage of the things your process does well (size control on a CNC) and get some relief on the things you have less control over (run-out between two chuckings). If you understand the function of the part and have a dialog with your customer, suggest position as a possible alternative to concentricity or run-out for these applications

Ken Sojda
Eagle Comtronics, Liverpool NY

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