{"id":12200,"date":"2021-12-28T18:35:34","date_gmt":"2021-12-28T16:35:34","guid":{"rendered":"https:\/\/fractory.com\/?p=12200"},"modified":"2024-01-26T14:11:15","modified_gmt":"2024-01-26T12:11:15","slug":"concentricity-gdt-explained","status":"publish","type":"post","link":"https:\/\/fractory.com\/concentricity-gdt-explained\/","title":{"rendered":"Concentricity (GD&T) Explained"},"content":{"rendered":"

ASME Y14.5<\/a>-2009 outlines 14 different types of geometric tolerances<\/a>. Each of those tolerances specifies a type of control over various part features. For ease of understanding, these 14 types can be segregated into five main groups. These are form, profile, orientation, location and runout.<\/p>\n

Location controls maintain tight control on a feature’s position with respect to a datum. Concentricity, symmetry<\/a> and true position are the controls under the location category. In this article, we shall take a look at concentricity, its various aspects, uses and measurement methods. Let us start by defining concentricity.<\/p>\n

What is Concentricity?<\/h2>\n

Many mechanical parts require a highly accurate concentric design for a satisfactory operation. Parts such as tubes that endure high pressures require a design with uniform wall thickness to prevent any weak structural points. Concentricity is a 3D GD&T callout that ensures that one or more part features are concentric about a datum axis.<\/p>\n

However, in GD&T, concentricity has a slightly different meaning than the literal definition that most engineers are aware of. The function of concentricity callout is to ensure that the midpoint of two diametrically opposite points lies within a specified tolerance zone. The circular feature may have notches, dips or other surface variations but the mass distribution about the central axis should be uniform.<\/p>\n

This balanced mass distribution is important in applications where the part undergoes high-speed rotation and there is a risk of oscillation or uneven wear. But concentricity is a difficult characteristic to achieve and measure<\/strong> during manufacturing.<\/p>\n

In most applications, simpler callouts such as circular runout, <\/strong>total runout<\/strong><\/a>, position or profile<\/strong> can do the job equally well. Wherever possible, they must be used to avoid concentricity.<\/strong><\/p>\n

Concentricity Tolerance Zone<\/h2>\n

\"concentricity<\/p>\n

The zone for GD&T concentricity is a cylindrical tolerance zone. The feature control frame specifies a datum axis that is used as a reference point to develop this zone. The diameter of this cylindrical zone is the permissible tolerance value for the callout.<\/p>\n

In order to ensure concentricity, the actual median axis for the part must be derived by calculating midpoints of diametrically opposed points. When all such median points are connected, we obtain the median axis. All the points on the median axis must lie within the cylindrical tolerance zone for the approval of the part.<\/p>\n

Concentricity vs Other Callouts<\/h2>\n

Concentricity is a necessary callout in many specialized applications where a uniform mass distribution is of the utmost importance. But due to the difficult and costly process involved in its application, it is important to be aware of other callouts that can replace concentricity without compromising the required specifications.<\/p>\n

Circular runout<\/a> and true position (also sometimes known as ‘position’) are the two most closely related callouts that can replace concentricity in many applications.<\/p>\n

Concentricity vs Circular Runout<\/h3>\n

The difficulty in measuring concentricity arises from the need to find the derived median axis of the part. There is no method in which such a calculation can be carried out reliably without the use of a computer.<\/p>\n

On the other hand, the runout of a part can be measured easily from the surface since it is a tangible feature. Moreover, simple instruments such as a V-block and a dial indicator can give reliable runout measurements.<\/p>\n

Runout is popularly defined as the sum of circularity<\/a> and concentricity. If a part is perfectly round, runout tolerance becomes equal to concentricity tolerance.<\/p>\n

Specially designed gauges can also measure runout in a quick (<10 seconds), effective and relatively inexpensive manner. Thus, from all angles, runout is a better alternative to concentricity and must be used wherever possible.<\/p>\n

Concentricity vs True Position<\/h3>\n

True position<\/a> in GD&T is a fairly simple callout that can fix the position and size of different features. In many cases, true position callout can effectively replace concentricity. Standard hole sizes and positional tolerances are better than concentricity when there is no need for a precise mass distribution.<\/p>\n

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