Tolerances

Precision micrometer

Tolerance is a specification for how much variation from ideal measurements you can tolerate for your parts. You must specify a tolerance in menu Job | Settings. If you are new to thinking about small tolerance values, consider the thickness of some common items:

• Copier paper ~.003"
• Business card ~.014"
• Credit card ~.030"
• House key ~.080"
  

Note that metals allow for parts of tighter tolerance than plastics. 

You can specify tolerances in three ways:

1. The general default tolerance is specified via Job | Settings. 
2. Specific local tolerances are specified via Line | Machine | Tolerance or ...
3. To specify more advanced GD&T tolerances use Comments to Machinist.

Implicit and Explicit Tolerances

Often, mechanical drawings show lines with arrows and other symbols that indicate distances and other measurements that must be held to a certain tolerance. For example one might indicate that the distance between two holes must be 3" +/- .005" - meaning that the distance can range from 2.995 to 3.005. This type of dimensioning is explicit dimensioning. 

eMachineShop CAD offers the ability to rely mostly on implicit dimensioning which is easier and faster to use. Implicit dimensioning is supported via the general tolerance at Job | Settings | Specifications.

Simplified Rules

The linear tolerance value you specify at Job | Settings | Specification is referred to below as "T". In simple terms, the value you select for T is used approximately as follows:

• The size of features such as circles, slots and outer shapes can vary by +/- T.
• As measured from datums, the center of circles can be in a circular region of radius 1.4* x T.
• As measured from datums, the position of single straight edges can vary by +/- T.

Precise Rules

Below is the complete definition of how default tolerance is used at eMachineShop, requiring an understanding of GD&T and ASME Y14.5. The following is used as the basis for determining whether your part conforms to your specification: 

Additionally, tapped holes shall be perpendicular to the originating surface within +/- 2 degrees.

Bend angles shall be +/- 2 degrees. For bends, the general tolerance applies to bend line positions on the flat sheet.

Following is the order of precedence of tolerances with highest precedence first:

1. Tolerances indicated via Comments To Machinist.
2. Local tolerances from the Line | Machine dialog.
3. For 2D parts cut from sheets Job | Settings | Auto: Specifications | Material Thickness Tolerance - applied to sheet thickness only.
4. General tolerance per Job | Settings | Auto: Specifications | General Linear Tolerance.
   

For parts made by extrusion the general tolerance applies to the cross-section.

Tolerances may not be met when impractical due to weak structure of a part (e.g. a spiral shape cut from sheet material).

Measurements are done at at 20C +/- 1 or 68F +/- 2.

* The value 1.4 (SQRT(2)) and 2.8 (2 * SQRT(2)) are derived from the GD&T equivalent of older style square zone tolerancing where +/- T is applied separately to each axis.