Drill and Tap; and calloutformat.txt (Part 1)

This entry is part 1 of 4 in the series Hole Callouts

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.

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.

21 thoughts on “Drill and Tap; and calloutformat.txt (Part 1)”

  1. Nice post. I think some of the confusion comes when we try to combine two types of hole wizard features. How does SW handle that? I want to combine a simple counterbore hole and then tap the smaller diameter with a 8-32 thread.


  2. Steve, caution is definately needed when combining hole types. SolidWorks can handle c’sunk holes pretty well, but c’bore might be an issue. Let me look into this. In the meantime, do you have your own experience to relate?

  3. Good posting. I concur, as it bothers me that Solidworks also calls out drill size. That is already defined by the UN threading callout, and if a deviance is appropirate for the material, that is for the machinist to decide, not me. All I (as an engineer) should care about is that the screw fits in the hole afterwards.

  4. The company that I work for doesn’t seem to be interested in the way the rest of the world does things. They have their own standards which have been in place since 1963 and they have an “If it’s not broke, don’t fix it!” attitude. Drives me crazy. Anyway, they require what you’ve referred to as an “extreme” hole note requirement. As an example, a tapped hole in a square-tube with 1/4″ walls would be as follows.

    Drill #21 (Ø.159), 3/8″ (.375) Deep
    Tap #10-32 UNF – 2A, 3/8″ (.375) Deep
    Four (4) Places as shown

    Their reasoning is that the machinists aren’t the most intelligent folks in the world, and everything must be spelled out for them. Oh my God!! One one drawing I tried to pass through the following.

    2X 1/4-20 UNC – 2A Thru

    They ACTUALLY THOUGHT that they needed to multiply the size of the hole somehow!

    I can’t fight them forever, but I am trying to get them to see things in a new light. In the meantime, how can the hole note templates be modified (if at all) to satisfy these ridiculous and archaic requirements?

    (name withheld for obvious reasons)

  5. BigKahunaFL,

    I’ll address editing calloutmethod.txt in a later part of this article. Maybe in a couple of articles, since this topic is getting bigger than I expected. There are limitations that likely will not give you what you seem to need.

    As far as your fight goes, it may be time to invest $80 in ASME Y14.5M-1994. Also, consider ASME Y14.100-2004. It’s easier to argue about the wording of a standard that everyone knows, rather than to debate whether or not “People are stupid” (I hope).

    The question to ask that might help is this: “What do we callout for an external thread?” Do they require that the OD be shown along with the thread callout? Do they callout the lathe machine used to cut that OD? Just an idea to help break down the arguement.

  6. Hi Matt, good article. I started in tool & die in 1977. I am now not on the bench. But I will agree with 99% of your call out. We never call out the class of the thread. No tool makers pay attention to it. They pick up a tap and start tapping. I don’t think in my whole career that I ever made a hole other than a 2B. And thats thousands of holes in a myriad of materials at many shops. The only time I could possibly see anyone using other than that could be maybe a rocket, aerospace or maybe a lunar module. All others are 2B.

  7. Matt,

    Great post! I too have noticed this topic popping up a lot recently.

    We too use the format that you use (i.e., 2X 8-32 UNC-2B DEPTH .165) for calling out tapped holes. Anything else is more information that what is needed, and just clutters up the drawing.


  8. I respectfully disagree with this post. The reason the depth of the blind tap drill hole should be called out is because it leaves a geometric vestage in the part that often effects the function of the part. Presumably, if the tapped hole COULD be a thru hole, it should be – the fact that it’s not a thru hole means either the backing material is too thick or there is something below it that you’d rather the machinest not drill into. It’s not up to the machinest to figure out whether he’s going to into some cavity of the part you’d rather him not. Left to my own devices, as a machinest, I’m going to drill the deepest tap drill possible to avoid breaking to tap when chips collect at the bottom. There are plenty of times when I’m forced to use a bottom tap because there is insufficient clearance for a deeper tap hole. The designer has ready access to this information. The machinest does not and will have to gues.

  9. Dave, If end condition is important, then it should be stated. Please notice this is addressed in the article. However, for the normal thread, the callout of the drill dia itself is still redundant to the thread callout itself.

  10. Dave,

    You have a right to your opinions, but it’s not a black or white issue. Even when tap drill depth is important – unless I need a specific tap drill depth, I just add a DO NOT BREAK THRU note to the hole callout.

  11. Rob,

    I use the note “DO NOT BREAK THRU” or “DO NOT DRILL THRU” myself when when depth isn’t important, but I don’t want a through hole either.

  12. My employer wants me to change hole callouts to
    something like this:
    (diameter symbol) .375 X 1.500 DEEP
    CBORE (diameter symbol) 1.000
    X .250 DEEP
    I have made changes in the calloutformat.txt in such a way
    that the hole callout is like the one above, but the quantity (number of holes) of the original SolidWorks callout still appears
    like this:
    2X (diameter symbol) .375 X 1.500 DEEP
    CBORE (diameter symbol) 1.000
    X .250 DEEP
    I was able to add the second 2X by typing in the
    format for counter bored holes. How do I remove the first
    quantity? There is no in the calloutformat.txt.
    Thanks in advance.

  13. Pingback: fcsuper
  14. I have designed changes in the calloutformat.txt in such a way.
    2X (diameter symbol) .375 X 1.500 DEEP
    CBORE (diameter symbol) 1.000
    X .250 DEEP

  15. The saga continues, specification of process???? Is this not what we are trying to do when creating a document? So what is so bad with being specific?

    Realizing that a sesoned or new machinest may have every possiable “pre-drill” size mimerized for inch and metric is not likely.

    The reason for including pre-drill depth and dia is to take the guess work out of the operation. And save the machinest time, also this helps the machinest verify that the pre-drill is the correct size.

    Many times the stamped size on the bit is warn off, and must be measured to confirm. If the machinest is not told then any size that is close may due.

    So the next time you have a leaking port or bolts falling out of your machine, think about what specification of process that was missed.

  16. PSTEW, if the product is designed correctly and the product is made to specifications, there won’t be leaking ports or bolts falling out under normal operation conditions. What you are referencing is more along the concept of quality control, which is handled by processes such as those required by ISO, not individual drawings. Also, the process information added to “take the guess work out of the operation” could be wrong or unnecessarily limiting. What if the best process is formed threads instead of tapped? Also, drill sizes for tapped threads vary based on the part’s material. Or, what if the process is a waterjet operation instead of a punch on a sheet metal part? Tooling and setup is completely different for the same results. The ASME standards state that processes do not belong on the drawing. The drawing represents the final product. If you are the manufacturer, you may wish to create process drawings, but these are not the contract between a vendor and customer. They are internal manufacturing instructions, which is a completely different purpose.

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