Engineering Fit
This discussion is entirely driven by the ANSI/ASME B4.1-1967 and B4.2-1978 standards for "Preferred Limits and Fits for Cylindrical Parts". There are other standards, but this is the standard for American, British, and Canadian organizations up to 20 inches in diameter.
Definitions
Let's start with some definitions.
- Nominal Size
- The nominal size is the designation used for the purposes of general identification. Think of this as when we talk about a 2x4 piece of lumber. That is the general identification, but it is not the actual size.
- Dimension
- A dimension is a geometric characteristic such as diameter, length, angle, or center distance.
- Size
- Size is a designation of magnitude. When a value is assigned to a dimension, it is referred to as teh size of that dimension. Often size and dimension are used largely interchangeably.
- Allowance
- An allowance is a prescribed difference between the maximum material limits of mating parts. It is a minimum clearance (positive allowance) or maximum interference (negative allowance) between these parts.
- Tolerance
- A tolerance is the total permissible variation of a size. The tolerance is the upper and lower bounds of size.
- Basic Size
- The basic size is the size from which the limits of size are derived by the application of allowances and tolerances.
- Design Size
- The design size is the basic size with allowances applied.
- Actual Size
- An actual size is a measured size.
Fits
Fit is the general term used to signify the range of tightness that may result from the application of a combination of allowances and tolerances in the designed mating of two parts.
Actual Fit
The actual fit between two mating parts is the relation existing between them with respect to the amount of clearance or interference that is present when they are assembled.
Clearance Fit
A clearance fit is one having limits of size so that a clearance always results when mating parts are assembled. With a clearance fit, the shaft is always smaller than the hole. This enables easy assembly and leaves room for sliding and rotational movement.
Interference Fit
An interference fit is one having limits of size so that an interference always results when mating parts are assembled. Interference fits are also known as press fits or friction fits. These types of fits always have the same principle of having a larger shaft compared to the hole size.
Transition Fit
A transition fit is one having limits of size so that either a clearance or interference may result when mating parts are assembled. A transition fit encompasses two possibilities. The shaft may be a little bigger than the hole, requiring some force to create the fit. At the other end of the spectrum is a clearance fit with a little bit of room for movement.
Determining Fit Measurements
For both standard and metric measurements, there are a set of standard fits, which prescribed tolerance limits and clearance for a specific type of fit for specific functional goals. For example, let's take a hole and shaft with a nominal diameter of 1 inch that need to meet with a RC1 (close sliding) fit. This leads us to (with tolerance in thousandths of an inch):
| Dimension | Base | Upper | Lower |
|---|---|---|---|
| Hole | 1.0 | 1.0004 (+0.4) | 1.0 (-0.0) |
| Shaft | 1.0 | 0.9997 (-0.3) | 0.99945 (-0.55) |
Thees guides, whether you want standard or metric, are a very useful thing to understand as a starting point when you're working with materials. You can find in-depth discussions, equations, and pre-calculated tables in Machinery's Handbook's "Dimensioning, Gaging, and Measuring".
Note
When working with FFF 3D printing, however, you will find that the tolerances of the machine are so sloppy that you will need to develop your own understanding for your printer and printer settings (see Prusa MINI+).
Comments or Questions?
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