Tolerances

Tolerance is permissible variation in a measurement. For example a tolerance of +/- .005" indicates that measurements can be .005" less or more than the nominal value specified in the design. 

To select a tolerance for a particular line use Line | Machine | Tolerance. If the offered tolerances are not sufficient or you need to indicate the tolerance of a distance between features, the position of a feature or other types of tolerances use Comments to Machinist to specify the needed tolerance (also see GD&T).

To select the general tolerance for a part or thickness tolerance for 2D sheet material use Job | Settings | Specifications: Auto.

Note: Metals allow for parts of higher tolerance than plastics and some plastics can warp over time due to moisture absorption.

For a sense of tolerance values consider the thickness of some common items:

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

Implicit and Explicit tolerancing

Often, mechanical drawings show lines with arrows 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". This is referred to as explicit dimensioning or GD&T.

eMachineShop also supports a system of default tolerancing which is easier and faster for non-critical applications and for users without engineering experience. In this system, explicit dimensions are not required and the job default tolerance per Job | Settings | Specifications is applied to:

• overall part size
• hole positions
• hole sizes
• Z values
• edge locations
• radii

Additionally, angles and perpendicularity are checked against a tolerance of 2 degrees.

Most eMachineShop customers use this approach and have found it adequate.

Advanced Tips

A Datum can be specified via Line | Machine | Datum.

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

Precedence: When raw sheet surfaces are specified to NOT be machined the sheet thickness tolerance overrides the default tolerance. CTM (Comments To Machinist) and specific dimensional tolerances take precedence over default tolerances.

For the Bend machine, tolerance is the permissible variation in distance from datum to any bend line on the flat sheet.

For the Extrusion machine, tolerance is the permissible variation in distance from datum to any feature or size of a feature (e.g. diameter of holes) within the cross-section.

For the Silk Screen machine, tolerance is the permissible variation in distance from datum to any feature or size of a feature. This tolerance takes into account screen tooling, positioning of screen and edge definition.

Tolerances may not be met when impractical due to weak structure of a part - for example, a spiral shape cut from sheet material.