Showing them exactly what to measure helps ensure execution through the higher level assembly processes. Giving that person a heads-up on the requirements is the point of the whole document. The care put into the dimensions and the drawings will be used at the end of the line by an inspector to determine whether to accept or reject an entire lot of boards. It is already doing the best it will do given the CAD data. It does not care how you dimensioned anything.
The CNC machine that cuts out the board outline isn’t capable of distinguishing anything other than the nominal values. Labeling it for First Article Inspection (FAI) ensures that the measurement is taken and recorded when introducing the initial run of the product.ĭatum, or direct, the geometry is captured either way. If the width of the slot is a defining feature, a dimension from one slot edge to the other will be more useful. Labeling (the dimension) for First Article Inspection (FAI) ensures that the measurement is taken and recorded when introducing the initial run of the product.
This is called a tolerance stack-up, and it allows twice as much slop as a linear dimension of the slots two edges. The far edge also has the same allowance based on the common datum. The tolerance from our origin allows the near edge to be anywhere within the standard tolerance, say +/-0.1 mm. Image credit: Author - Datum dimensions based on the board originĪn example that comes to mind is where there is a slot in the board that is designated for a nice tight fit with a mating connector. Datum dimensioning by itself, without the use of geometric dimensioning and tolerancing, can lead to undesirable outcomes in certain circumstances. If you have a simple outline and bolt pattern, you’re good to go. A rule of thumb is that a non-plated hole makes the best datum on a PCB. The Datum method takes the least amount of effort and uses the least amount of space. Extension lines with location values for the horizontal and vertical features from a fringe around the layout. The easiest method is called datum dimensioning where all you do is establish the center of the universe at some point and call it zero-zero. Notes From the Board Room - On Dimensions Dimensioning isn’t Hard
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