Selecting the wrong datum surface significantly affects the production cost of the optical element, as well as the optical system quality. On a spherical or chamfer surface, several more complex variants are possible. There are multiple ways of defining the datum surfaces. Need assistance designing a custom optic or imaging lens ? Learn more about our design services here. In our case, this is the surface centering (see “How To Read An Optics Production Drawing” – “Table of parameters area” – “Column with surface parameters”). The cylinder formation axis is selected as a datum axis, and all other geometrical requirements are defined relative to it. In the drawing above, while the lens front and back face are spherical, the side face is a cylindrical surface. Note: technical standards prohibit using letters I, O, and Q for datum features. It can be placed on the feature surface outline or the extension line, marking the surface as a datum surface. The datum surface symbol consists of a capital letter in a square frame and has a leader ending with a black triangle (shown in the image below). The datum surface symbol looks like a black triangle with a letter inside a square near the triangle apex in optical drawings Selecting the datum surface For this reason, in optical engineering, the key function of the datum surface is defining a “reference point” for geometrical lens requirements. Since optical engineering products heavily rely on the surfaces and precision, GD&T is widely used. Thus, datums are indispensable when characteristics such as parallelism, perpendicularity, or profile surface are required.
Even though some of the geometrical characteristics (cylindricity, roundness, flatness) do not require datum, it is needed when defining positions between the edges and surfaces.
#DATUM IN DRAWING ISO#
Datum definitionĪs defined by both ISO and ANSI technical standards, datum marks part entity (i.e., axis, surface, point), allowing us to apply geometrical requirements and tolerances (GD&T). One of the parameters frequently found in optical drawings, “Datum Surface,” should be covered more closely.
#DATUM IN DRAWING HOW TO#
It appears that in your case the designer has used both approaches which I would personally just put down to bad practice.Our recent post described how to read optical production drawings. However, it then goes on to say that the drawing can be simplified by only referencing a single letter or by referencing the entire group as B-B. The standard says that for common datums there should be as many datum references, separated by hyphens, as there are features, so in this case there should be B-B-B-B-B. According to ISO 5459-2011, your datum B callout is defining the pattern of 20xØ13.5 holes as datum B. You need to get clarification from whoever issued the drawing but I believe in this case B and B-B mean the same thing.
You could have a cylinder and a plane or, as in this case, a pattern of cylinders and there are many other examples also. Although two concentric features combining to form an axis of revolution would be one allowable case, it is not the only valid application.
A common datum is a collection of features that, when combined form any of the allowable invariant classes.
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