Cover concerns.

We in the engineering department take issue with the December cover. Showing equal tolerances of +/-0.003 on dimensions that are depicted at 1, 2 and 3 decimal point precision is not the norm. While we applaud the idea of teaching designers the importance of proper tolerance techniques, it is a disservice to use such a poor pictorial image. Typically dimensional tolerances are expressed as a value based on the precision of the dimension. For example:

x.x +/-0.020"

x.xx +/-0.010"

x.xxx +/-0.005"

Another item of concern on the cover is the practice of double dimensioning. It is not necessary to show the board's width and length more than once on a fabrication drawing.

Unmentioned in the article ["Preventing Mechanical Interference," pg. 26] was hole size tolerances. Too often, fabrication drawings have hole size and quantity, plated or not, but no mention of the tolerance for the finished hole sizes. A general note that all holes are +/-0.003" unless otherwise specified can save unnecessary phone calls and delays. In many cases via holes are toleranced such that they can be plugged, for example; 0.010" via; +0.003" -0.010".

Nick Meeker 

Tech Circuits Inc.

Ed.: Mentor Graphics, which supplied the image, says it was created by a former designer who probably used defaults for tolerances to create it more quickly.