{"id":8750,"date":"2020-12-09T15:42:45","date_gmt":"2020-12-09T13:42:45","guid":{"rendered":"https:\/\/fractory.com\/?p=8750"},"modified":"2024-01-26T15:08:47","modified_gmt":"2024-01-26T13:08:47","slug":"true-position-gdt","status":"publish","type":"post","link":"https:\/\/fractory.com\/true-position-gdt\/","title":{"rendered":"True Position (GD&T)"},"content":{"rendered":"

We know that manufactured products can never match their theoretical drawings perfectly. The actual size varies from the intended design. The goal of GD&T<\/a> is to hold this variation within set limits.<\/p>\n

True position is a GD&T callout for specifying the position of a feature. It is more correctly referred to as “position”. We use this concept in GD&T to control the variation of a specific feature from its desired position. This is important for mating parts to ensure a seamless assembly.<\/p>\n

For example, the screw holes on a cylinder head cover must match the screw holes on the engine casing in size as well as position. In the event that they don’t, the two parts will not mate, and the lubrication oil for the valve assembly will leak out, defeating the purpose of the cover. The cylinder cover is useless, and a different piece must be used.<\/p>\n

To prevent this from happening, we use position tolerances as guidance for CNC turning and milling services<\/a>.<\/p>\n

The position symbol is an extremely useful one but its application may be a bit complicated. In this article, we shall learn the basics of this feature callout and how to apply this tool to different material conditions.<\/p>\n

Definition of True Position<\/strong><\/h2>\n

The true position tolerance in GD&T informs us of the maximum allowable deviation of a feature (e.g. hole, slot) from its true position. By true position, we mean the ideal position of the feature according to design.<\/p>\n

The idea is that, for a feature, there is a true position that we desire. We define datum planes and axes as references to define the exact placement.<\/p>\n

Then, we mark the true positions of the features of size using basic dimensions. We take into account any material condition modifiers if needed.<\/p>\n

Position is a 2D\/3D tolerance in GD&T that defines tolerance zones depending on the feature. For cylindrical features, it creates a cylindrical tolerance zone around the true position of the feature within which the axis of the feature must lie for all manufactured products.<\/p>\n

For other features, parallel planes are defined within which the centre plane of the feature must lie for approval.<\/p>\n

Difference Between Position and True Position<\/strong><\/h2>\n

In the ASME Y14.5 standard<\/a>, the true position callout is referred to as just “position”. “Position” is the correct way to term it as we know that true position actually refers to the theoretical point on the surface with basic dimensions which is impossible to replicate in actual products.<\/p>\n

However, in CNC machining vernacular, the term true position is more popular when referring to the feature\u2019s position on the actual product.\u00a0<\/p>\n

Therefore, when we use the term \u2018true position\u2019 in the article, more often than not, we actually mean the position of the manufactured feature in relation to the actual true position and not the true position (exact dimension or nominal value) of the feature.<\/p>\n

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