Dimensions can be tricky to interpret, especially when a drawing uses unconventional references or omits key details that require the reader to infer or calculate dimensions. Drawings are supposed to communicate all necessary information clearly and concisely according to ASME Y14.5 standards. Not all manufacturer’s follow exact guidelines at all times which can lead to confusion.
Example Drawing
Part number NOR1835-ND (189-H15-513R577) has a datasheet with drawings that may take a while to figure out due to which dimensions they do and don’t call out. They also leave some room for interpretation with pin spacing. Here are some tips and details I picked up while going over this drawing.
Faint Center Line Reference
The top front facing drawing does have a very faint center line drawn. This is not called out in this drawing, but it is there for reference.
Center lines are supposed to be the very center of the entire drawing. All other dimensions should be centered around that reference line according to drawing practices. The bottom drawing does call this out more clearly. This bottom drawing is specifically showing the recommended PCB layout.
You may notice there isn’t a dimensional reference for how far the center pins are horizontally from the larger mounting holes on the sides. This is a little harder to determine, but it can be calculated. The pins are all off-center to the left by .01 inches according to the dimension at the bottom of the page. This has to be accounted for. You can then use the info in this next marked up drawing to determine the purple distance:
Take 0.492 subtract the offset of .01 and two pin distances of .090. The purple distance on the left should be 0.302 inches or 7.67mm. You may not need that since the center pins are all shifted based on the overall distance of the outer mounting pins though.
Duplicate Edge Measurement?
One might think the drawing is over defined because .350 is listed twice from the PCB edge. However, this must be referring to both the distance between the edge and larger holes as well as the center of the smaller holes to the edge. That means the bottom five pins are in line with the larger holes. I confirmed this by taking an image editing program to measure out the pixels to see if they were on the same line. This is often a trick I use if I need dimensions that seem to be missing by mapping a pixel count to the smallest measurement on the drawing. It’s not a perfect method, but it gives me a good estimate of a missing dimension.
Vertical Distance of the Small Pins, Which One is Repeated?
You will also notice there are only two distances called out between the vertical spacing of the smaller pins. The top one says .1 inches [2.54 mm] and the other is .090 inches [2.29]mm. Here is my logic: The other vertical spacings must be 2.54mm because both measurements are listed once. The outlier is 2.29mm. How can I be sure? If the other vertical spacings were also 2.29, it would be called out again. 2.54 is a far more standard measurement, so the manufacturer only thought it necessary to call out the standard measurement once and the non-standard measurement once. Again, I used my image editing trick to confirm distances. The more standard measurement is repeated all other times.