Geometric Dimensioning and Tolerancing and Allowable Variation Essay

Submitted By crcorrow
Words: 738
Pages: 3

Sterolithography (is also known as optical fabrication, photo-solidification, solid free-form fabrication and solid imaging) it is a 3D printing technology used for producing models, prototypes, patterns and production parts.
Stereolithography is an additive manufacturing process which employs a vat of liquid ultraviolet curable photopolymer "resin" and an ultraviolet laser to build parts' layers one at a time. For each layer, the laser beam traces a cross-section of the part pattern on the surface of the liquid resin. Exposure to the ultraviolet laser light cures and solidifies the pattern traced on the resin and joins it to the layer below.
After the pattern has been traced, the SLA's elevator platform descends by a distance equal to the thickness of a single layer, typically 0.05 mm to 0.15 mm (0.002" to 0.006"). Then, a resin-filled blade sweeps across the cross section of the part, re-coating it with fresh material. On this new liquid surface, the subsequent layer pattern is traced, joining the previous layer. A 3-D part is formed by this process. Once a part is built, they are immersed in a chemical bath in order to be cleaned of excess resin and are subsequently cured in an ultraviolet oven.
Stereolithography requires the use of supporting structures which serve to attach the part to the elevator platform, prevent deflection due to gravity and hold the cross sections in place so that they resist lateral pressure from the re-coater blade. Supports are generated automatically during the preparation of 3D Computer Aided Design models for use on the stereolithography machine, although they may be manipulated manually. Supports must be removed from the finished product manually, unlike in other, less costly, rapid prototyping technologies.

An advantage of using this method is its speed , parts can be manufactured as soon as a day. The length of time used to create a part depends on the parts size and the complexity of the project it could take a few hours or a couple of days. Most machines can produce parts with a max size of about 50x50x60 cm and some, the Mammoth stereolithography machine (210x70x80cm), are capable of producing a part of more than 2m in length. The prototypes that are produced are strong enough to be machined and can be used as master pattern for injection molding, thermoforming, blow molding, and various metal casting processes. The cost of using this technology is its biggest downside; photo-curable resin ranges from $80-$210 per liter, and the cost of the machine range from $100,000 to more than $500,000.
Geometric Dimensioning & Tolerancing (GD&T) is a quality control method for three-dimensional solid models that describe nominal geometry and its allowable varation. It is a standard code used by people who design tools and parts, to inform machinists and manufacturers what the proper dimensions of a given part must be. GD&T is used to define nominal geometry of parts and assemblies, to define that allowable variation in form and possible size of individual features, and to define the allowable variation between features. The following are Geometric dimensioning and tolerance specifications:
Dimensioning specifications define the nominal, as-modeled or