Dimensional tolerance

AWS) has issued specifications covering the various filler-metal systems and processes (2), eg, AWS A5.28 which appHes to low alloy steel filler metals for gas-shielded arc welding. A typical specification covers classification of relevant filler metals, chemical composition, mechanical properties, testing procedures, and matters related to manufacture, eg, packaging, identification, and dimensional tolerances. New specifications are issued occasionally, in addition to ca 30 estabUshed specifications. Filler-metal specifications are also issued by the ASME and the Department of Defense (DOD). These specifications are usually similar to the AWS specification, but should be specifically consulted where they apply. 

Plywood requirements—includes wood species used, synthetic repair requirements, veneer grades, veneer layers and thicknesses, panel grades with respect to end-use, adhesive bond requirements, panel constmetion and workmanship, scarf and finger-jointed panels, dimensional tolerances, moisture content, and packaging and loading... 

Metals andMllojs Aluminum Association 900 19th Street, NW Washington, D.C. 20006 Standards for wrought and cast aluminum and aluminum alloy products, including composition, temper designation, dimensional tolerance, etc. 

Hot pressing produces compacts that have superior properties, mainly because of higher density and finer grain size. Closer dimensional tolerances than can be obtained with pressing at room temperature are also possible. Hot pressing is used only where the higher cost can be justified. It has been usehil in producing reactive materials. One use is the combination of P/M and composites to produce hot-pressed parts that are fiber reinforced. 

Electronic-Grade MMCs. Metal-matrix composites can be tailored to have optimal thermal and physical properties to meet requirements of electronic packaging systems, eg, cotes, substrates, carriers, and housings. A controUed thermal expansion space tmss, ie, one having a high precision dimensional tolerance in space environment, was developed from a carbon fiber (pitch-based)/Al composite. Continuous boron fiber-reinforced aluminum composites made by diffusion bonding have been used as heat sinks in chip carrier multilayer boards. 

The dimensional tolerances of parts formed by plastic deformation are typicaUy not as good as those ia parts formed by dry powder pressiag. 

Impact and Erosion. Impact involves the rapid appHcation of a substantial load to a relatively small area. Most of the kinetic energy from the impacting object is transformed into strain energy for crack propagation. Impact can produce immediate failure if there is complete penetration of the impacted body or if the impact induces a macrostress in the piece, causing it to deflect and then crack catastrophically. Failure can also occur if erosion reduces the cross section and load-bearing capacity of the component, causes a loss of dimensional tolerance, or causes the loss of a protective coating. Detailed information on impact and erosion is available (49). 

Construction. The parts, dimensions, and dimensional tolerances used in manufacturing of screw conveyors are highly standardized. Standards for dimensions and minimum service requirements are available (24—26). These are accepted by most of the industry. 

Sand Casting. Sand casting is the most popular method of molding for cast copper alloys. It is the least expensive near net shape process, and pattern costs are low. Dimensional tolerance capabiUties vary widely and are the least accurate when compared with other methods of casting. 

Specifications include dimensions of length, width, and depth, in that order (Fig. 21-40 ). When boxes are set up and closed by automatic equipment, dimensional tolerances become critical. Cartons are shipped knocked down to the user from plants located in all industrial centers. Because order lead time is 4 to 6 weeks, inventories of empty boxes require considerable space. A useful booklet describing all aspects or corrugated box designs and materials is the Fiber Box Handbook available from The Fiber Box Association, 2850 Gulf Road, Rolling Meadows, IL 60008. 

The second approach, that of surface coating, is more difficult, and that means more expensive. But it is often worth it. Hard, corrosion resistant layers of alloys rich in tungsten, cobalt, chromium or nickel can be sprayed onto surfaces, but a refinishing process is almost always necessary to restore the dimensional tolerances. Hard ceramic coatings such as AbO, Cr203, TiC, or TiN can be deposited by plasma methods and these not only give wear resistance but resistance to oxidation and... 

The tolerance grades are interpreted using standard tables (BS EN 20286, 1993) for conversion into dimensional tolerances. However, the tolerance grades do not take into consideration different materials machined or the complexity of the component being processed. 

Figure 2.40 shows the initial detailed design including the tolerances required on each component in the stack to achieve the 0.2 mm assembly tolerance, (not included is the dimensional tolerance on the fuel port block of 12 0.05 mm which is set by the... 

The inadequacy of the worst case approach to tolerance stack design compared to the statistical approach is evident, although it still appears to be popular with designers. The worst case tolerance stack model is inadequate and wasteful when the capability of each dimensional tolerance is high > 1.33). Some summarizing comments on the two main approaches are given below. 

BS 6615 1985 Dimensional Tolerances for Meta and Meta Alloy Castings. London BSI. 

Because of the above properties, together with other features such as the ability to mould to close dimensional tolerances, low warpage, low shrinkage, low moisture absorption and good surface finish, polycarbonate-ABS alloys have become widely used in the automotive industry, for electrical applications and for housings of domestic and business equipment. 

Good dimensional stability (and low moulding shrinkage)—thus allowing the production of mouldings with close dimensional tolerances. 

An alternate method involved pre-machining the core detail to contour while it was stabilized in a frozen block of water or other medium. This entailed the expense and time to freeze the block and keep it frozen during machining as well as the risk of contaminating the core bond surfaces. In addition it was difficult to maintain the dimensional tolerances necessary to match the spar dimensions to the core adequately. Both spar and core details had machining tolerances and significant hand sanding of the core was often required to match the details. 

Nickel is also widely used as an electrodeposited underlay to chromium on chromium-plated articles, reinforcing the protection against corrosion provided by the thin chromium surface layer. Additionally the production of articles of complex shape to close dimensional tolerances in nickel by electroforming —a high-speed electrodeposition process —has attracted considerable interest. Electrodeposition of nickel and the properties of electro-deposited coatings containing nickel are dealt with in greater detail in Section 14.7. 

Design drawings Detail or fabrication drawings Materials requirements including composition, quality standards, and minimum structural properties Fabrication requirements and standards, including dimensional tolerances, allowable defects, and minimum structural properties Requirements for prototype and quality control tests and procedures Shipping and handling... 

Fortunately, there are many different types of plastics that can provide all kinds of properties, including specific dimensional tolerances. It can thus be said that the real problem is not with the different plastics or processes but rather with the designer, who requires knowledge and experience to create products to meet the desired requirements. The designer with no knowledge or experience... 

Table 3-6 Examples of dimensional tolerances for injection molded TP products... 

The dimensional tolerances with the more conventional single-mold system are generally 0.6% ( 0.35% for close tolerances). With female molds 0.5% ( 0.3% close) with male molds under 3 ft., 0.8% ( 0.4% close) and with male molds over 3 ft., 30% ( 10% close) for wall thicknesses. 

Basically, with the higher pressures it is possible to develop tighter dimensional tolerances with higher mechanical performance, but there is also a tendency to develop undesirable stresses (orientations) if the processes... 

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