What does it mean if there are 2 different figures for a dimension on a blueprint?

by Fil   Last Updated September 16, 2018 20:21 PM - source

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.

Tags : fastener

Answers 1

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.
Michael Karas
Michael Karas
September 16, 2018 19:45 PM

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