Sooner or later, the topic of how to callout a threaded hole comes up in pretty much everyone’s career in the Mechanical Engineering field. I’ve seen the nature of those discussions be straight forward, but I’ve also seen angst riddled arguements. Though this isn’t a SolidWorks specific topic, it is important to its users. This is because SolidWorks specifies hole callouts differently in different scenarios.
The conventional rule (within ANSI Inch) is that a threaded hole should be called out as a leadered note showing its nominal drill size and depth on the first line, and the thread size, threads per inch, thread series designation, thread class and thread depth on the second line. This is common practice, so most people are comfortable using it.
Example (without use of symbols):
2X .190 DIA .190 DEEP
8-32 UNC-2B .164 DEEP
Of course, this method has flaws, which I’ll get into later.
I’ve seen two extremes as well. At one extreme, the threaded hole callout has the actual drill bit size listed in addition to specification for the tap and drill. I gather it would look something like this:
2X .438 DIA .25 DEEP WITH 7/16 Q DRILL
.438 UNC-2B .375 DEEP
Of course the basic flaw with adding the drill size is that this is a specification of process, which is generally disallowed by ASME Y14.5M-1994. It is equivalent to having the note “FORM THIS PART WITH LATHE MODEL XYB” or even, “JUMP UP AND DOWN THREE TIMES AND SPIN IN A CIRCLE BEFORE USING THE MILL TO CUT THIS HOLE”. Hyperbole aside, this practice is not appropriate.
On the other end, one might find a hole callout that simply states the thread size, such as “TAPPED HOLE” This is a bad case of under-specification. I haven’t seen this method often on formal drawings, but it is very common on preliminary sketches. There just isn’t enough information.
What is just-enough-information for a threaded hole callout? Well, this answer is easy. Thread size, threads per inch, thread series designation (sometimes considered optional), thread class, thread depth, and sometimes drill depth or end condition. The “nominal” drill diameter isn’t actually needed. There’s several flaws with including the drill diameter. First, the actual drill diameter is not based on the callout, but rather the thread itself. It is over-specification. Second, drill diameter is stated as a dimension, so it is not nominal. Because of this, the standard drawing tolerance must be applied to that dimension. Again, this is over-specification because the thread has its own tolerance for its final size. Simply by stating the thread class, its tolerance is called out. Third, because of these other points, specifying the drill diameter is actually a specification of process. Given all that, I always callout a threaded hole as so:
2X 8-32 UNC-2B DEPTH .165
In the rare event that drill hole depth or end condition is necessary to call out, then simply state that specification in the callout, or show it dimensionally on the drawing view itself. How this relates to SolidWorks and the calloutformat.txt file will be discussed in Part 2 of this article.