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I am trying to read a technical 2D CAD drawing of a machine screw.

This is the fastener in question. This is a link to a PDF of this fastener's blueprint.

Here is an image of the PDF:

Annotated blueprint

I added the blue arrows to highlight instances of confusion.

What does it mean if there are two figures provided for the same dimension?

I thought that maybe these two figures referred to a "tolerance" of possible values (that is, a "minimum" and a "maximum"), but I'm not confident in this theory because, in all four instances, the greater number is on top (which seems counterintuitive to how a range is typically expressed).

I tried looking up "ASME B18.6.3" to see if it contained any hints, but it costs $107 to view this document.


Also, if no units are provided on a blueprint like this, how do I know if the figures are measured in inches or millimeters?

Fastenal is a U.S.-based company, so I expected that they use the imperial system, but the figures contain odd numbers, so, on the other hand, they might be using the metric system.

  • 1
    Some (perhaps non-official?) information on ASME B 18.6.3: globalfastener.com/standards/detail_3889.html – Rob Sep 17 '18 at 3:55
  • You can make a good guess at this drawing without the standard because 1/4-20 is the standard pitch for coarse threads (0.25" diameter, 20 turns per inch) which would mean the other measurements are in thousands of an inch. Thousands is the standard measuring system amongst machinists, as opposed to the fractions used by carpenters. Machinists are also why the ranges are counterintuitive; you want the larger tolerance first when you're removing metal to form a part. – Matthew Gauthier Sep 17 '18 at 15:08
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The two numbers represent the minimum and maximum range for the dimensions shown.

The dimension units would normally be specified on a drawing like this. In this case you can make two inferences that the dimensions are in inches as follows:

  1. The stud/thread size is shown as 1/4-20 which is a standard inch size fastener.
  2. The dimensions of the head size for a 1/4-20 fastener fall in line with what you would expect for such a fastener. For example a head diameter in the range of 0.443 to 0.472 inches is logical for a 1/4 inch diameter fastener.

As the OP has noted the reference to the ASME B18.6.3 document is one that specifies dimensions for standardized fasteners. Taking a quick look at this document is problematic because it is a controlled distribution document that requires a sizeable fee just to acquire a license to see it for just personal use. After quite a search I located a web page that showed a snapshot of one page of this document as shown below:

enter image description here (image from: https://www.coursehero.com/file/24235851/ANSI-B1863-Machinery-Handbookpdf/)

Dimensions are in inches as one would expect for an ASME standard.

A screw head size chart from another vendor is shown below which some of the dimensions on the OP's PDF picture from Fastenal as min and max values.

enter image description here

(image from: http://homepages.cae.wisc.edu/~me231/online_notes/dimensoning/fastener_handout.pdf)

  • This sounds correct to me, most of the drawings I have worked with give a value and the tolerances ear specified with a +/- value in the notes in this case they provide the range+. – Ed Beal Sep 16 '18 at 20:13
  • I wouldn’t infer (assume) anything. All the info is in the bottom left corner. Go to ASTM B18 and find the correct material (stainless steel), type (Type 1) and get the proper info...unless you’re putting your outdoor beer bar together. – Lee Sam Sep 16 '18 at 20:23
  • As with most things there are multiple ways that dimensions are specified. Another thing to look for is double numbered dimensions that show both metric and inch dimensions. – Michael Karas Sep 16 '18 at 20:24
  • @MichaelKaras Yes, often multiple ways of dimensioning are shown on a drawing in various ways. However, this drawing is NOT that way. Please look to the bottom left corner and the draftsman has clearly indicated what the dimensions are and how to interpret the dimensioning. You’ll see a note referring to ASTM Standards...no guessing or inferring. – Lee Sam Sep 16 '18 at 21:45
  • @LeeSam - You should post you own answer if you believe you have more or better information than I have posted. – Michael Karas Sep 16 '18 at 23:46

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