Représentation graphique du réel
Principaux ajustements
Filetage
A. Chevalier, Guide du dessinateur industriel: pour maitriser la communication technique, Hachette, 2003.
Geometric Dimensioning and Tolerancing
Geometric dimensioning and tolerancing (GD&T) is a way to provide tolerances on the geometry and fit of mechanical parts in order to describe the engineering intent of a part; it allows engineers to describe the size, form, orientation, and location of features in a way other than simple maximum and minimum dimensions (limit dimensions). The international standards for GD&T are ASME Y14.5-2009 and ISO1101(e)-2004.
Tolerances and features in GD&T are called out using symbols. A tolerance on a feature is often called out in reference to a datum, which can be a line, point, axis, etc. established by a feature on the part. Datums should be chosen to adequately constrain the part and establish a datum reference frame consisting of three mutually perpendicular, intersecting datum planes. It is from the datum reference frame that the location or geometric relationship of another feature can be defined. Datums should be chosen based on the function of the part. Basic dimensions are those that define a true position of a feature but are not toleranced. A tolerance zone, a zone within which a feature can vary, is then located with a basic dimension. |
| Field Guide to Optomechanical Design and Analysis |
The maximummatterialcondition(MMC).
Call out refers to a feature of size that has the maximum amount of material but remains within the size limits. Examples include the largest possible diameter of a pin or the smallest possible diameter of a hole.
Positional tolerances are used to define features of size. A feature of size is defined as an object that has opposing points (see left figure). Some examples of objects that are not features of size a flat surface or line element (see rightfigure).
Conversely, the least material condition (LMC) callout refers to a feature of size that has the minimum amount of material but remains within the size limits. Examples include the smallest possible diameter of a pin or the largest possible diameter of a hole. Material modifiers allow the size of the associated tolerance zone to be adjusted based on the measured size of the feature.
Pictorially demonstrated below is the envelope principle, or Rule #1. Given a (a) toleranced part, when that part is at MMC, it must have (b) perfect form. (c) Variation in form is permitted as the part varies from MMC. There is no requirement on the form of the part at LMC, but (d) the maximum variation that occurs at LMC must not exceed the boundaries set by MMC.
 |
| Field Guide to Optomechanical Design and Analysis |
On a drawing, individual tolerances and material conditions are called out in a feature control frame. First, the geometric symbol is called out, then the specific tolerance and any material conditions are stated, and finally, the primary datum from which the tolerance is referenced is listed. Sometimes secondary and tertiary datums are also included.
 |
| Field Guide to Optomechanical Design and Analysis |
Aucun commentaire:
Enregistrer un commentaire