Drill and Tap (~Part 3)

I previously discussed threaded hole callouts in the context of SolidWorks and its calloutformat.txt files (Part 1 and Part 2). As mentioned before, there is a tendency for some to callout threaded holes with too much information. Often, the thread callouts include the drill size. As argued before, including the drill size usually over-defines the threaded hole because the specifications of the thread itself identify the drill size. It also attempts to specify manufacturing processes, which is not allowed by ASME Y14.5M-1994. In fact, including the drill size within a thread callout may actually provide incorrect specification in many cases.

This is particularly true in the case of threads that are in blind holes. These are usually made with forming taps (roll taps). The diameter of the drilled hole for a roll tapped thread is bigger than it is for a cut thread. For example, for a 10-32 roll tap, the drill size is .1762, while a 10-32 cut thread drill size is .159. Once formed or cut, the specification for the ID of the thread is .156 to .164.

On drawings where customary units (inch) are used, the number of decimals places in the dimension usually determines the tolerance for that dimension. Stating a drill size as a decimal dimension applies the standard drawing tolerances to that dimension unless some general note is added.  This means that the tolerance for the drill callout likely differs with that required by the thread.  So, if the drill size is called out, drawing may be providing the wrong information to the machine shop.

Author: fcsuper

As a drafter, mechanical designer and CAD engineer, I've been in the mechanical design field since 1991. For the first 8 years of my career, I was an AutoCAD professional. I utilized AutoLISP and many other AutoCAD customization features to streamline drafting activities for 6+ drafters and designers. I authored several custom functions, one of which was published in the March 1997 issue of Cadalyst Magazine. Since 1998, I've been used SolidWorks non-stop. I've worked to utilize the SolidWorks' user environment to simplify drafting and design activities for 20+ engineers. I've created this website to provide current information about SolidWorks from a variety of contributors. More recently, I am now employed by Dassault Systemes as SOLIDWORKS Sr. Product Definition Manager to improve drawing, annotation and MBD related areas. View all posts by fcsuper