Before I get to the good stuff (or boring for some, I doubt you would be reading this if you were in this group) I wanted to announce that we at J&Q will be changing the way we blog. Rather than having the blogs be the responsibility of one, we decided to rotate who writes. We hope this keeps things interesting as you will be getting drafting insight from different point of views. We all use CAD efficiently and professionally. We will be releasing blogs on a regular schedule covering various topics and drafting tips.
In this blog I will share some information on ASME Y14.5-1994 standards, different unit systems, and tolerancing.
Drafting standards often vary between industries, but they all essentially rely on the same principles. The standards I will be referring to are those published by the American Society of Mechanical Engineers, ANSI Y14.5. ANSI or the American National Standards Institute was formed in 1918 and is known for establishing drafting standards that are used throughout the United States.
Let’s begin by defining a dimension, typically referred to as a value that is used to define the size, shape, or location by any given feature. Dimensions are typically displayed in 1 of 3 systems IPS (U.S.), MMGS (S.I.), or Dual.
The IPS system, typically used in the U.S. stands for inch, pound, seconds. Length is defined by inches(in), mass by pounds(lbm), and time by seconds(s).
The S.I, or International System of Units, uses millimeters to define length, grams to define mass, and seconds to define time.
Dual dimensioning displays both the U.S. and S.I. systems. The S.I. units are found above the U.S. units in parentheses.
Since dimensions are used to build parts it is important that enough information is provided on the drawings. It is also important not to provide too many dimensions (double dimensioning) as the drawing can become cluttered and difficult to read. Occasionally, double dimensioning is required to make a drawing clearer, these dimensions are called reference dimensions and should be placed in parentheses.
When dimensioning in IPS two decimal places should be used as a standard (.xx) this states that a dimension accuracy within .01”(.02??) is expected. For more precise dimension requirements (machining) three or more decimal places can be used. (.xxx) defines a tolerance of .005” and (.xxxx) defines a tolerance of .0001”
There are various types of dimensions that can be used in drawings and their different views (drawing view will be a whole other topic). The main dimensions that depict the overall size of the object are length, height, and depth. Two of the three are often shown on the main view with the third being defined in the auxiliary view.
A few tips to consider when dimensioning:
- Try to avoid dimensioning inside an object
- Do not over dimensions (avoid duplicate dimensions)
- Keep dimension spacing uniform
- Place smaller dimensions inside larger dimensions
- Try to avoid dimensioning to hidden lines
- Do not cross dimension lines
When dimensioning the arrowhead between dimension lines should sit between them if space allows. See below for examples of preferred arrowhead settings.
Tolerances define the amount of variation that is acceptable in any given dimension. Tolerances have a great effect on the production cost of any component as it defines the types of process required to make an acceptable part. The most common tolerance standards are those defined by ANSI and ASME.
Drawings can show both General and Local tolerances. General tolerances can be found in the title block and are what every dimension should be held to at a minimum. Local tolerances will override the title block dimension with the tolerances attached to a defined dimensions. (we will cover an in depth discussion on GD&T soon)