0003// Different between Diameter and Radius

This week, we will discuss about what is the different between diameter and radius. The concept sound simple, but most of us actually making mistake for it.

Is radius is just simply half of diameter

2R = D ???

In GD&T, this basically is wrong.

Diameter and radius are different in term of tolerance zone shape.

Consider the tab feature above, they are many ways to specifiy its geometric specification. You can specify it with:

=> two dimension for diameter + one dimension for width + one dimension for height

=> two dimension for radius + one dimension for width + one dimension for height

=> and etc.

However, diameter or radius actually give you different meaning for the drawing.

In figure 1, we use radius to specifiy the two half cylindrical features. Based on Y14.5, 2.15,1 Radius Tolerance , A radius symbol, R, creates a zone defined by two arcs (the minimum and maximum radii). The part surface must lie within this zone. Therefore, the tolerance zone as illustarted in figure 1.

In figure 2, we use diameter to specify the two half cylindrical faetures. The part surface must lie with two concentric circles (not the cylinders, because it control actual local size). Therefore, the tolerance zone as illustrasted in figure 2.

All in all, decision to select diameter or radius to specify the part's geometry specification must base on its functional requirement. So, next time remember don't mix up between diameter and radius again!!!

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