Linear and angular dimensioning

The latest ISO standard concerned with dimensioning is ISO 129"1985. However, it is known that a new one is soon to be published which is ISO 129 Part 1 (the author sits on BSI/ISO committees and has seen the draft new standard). It has been through all the committee approval stages and has been passed for publication. However, it is being held back, awaiting the approval of a Part 2 so that they can be published together. The BSI estimates the publication date will be 2003, hence it will be ISO 129-1:2003.

There have been versions prior to 1985 and each has defined slightly different dimensioning conventions. Needless to say, the 2003 convention gives a slightly different convention to the 1985 one! Throughout this section, the 2003 convention will be presented so that readers are prepared for the latest version. Figure 4.3 shows a hypothetical spool valve that is defined by 14 dimensions in which 12 are linear and two angular. The valve is shown using the ISO principles of line thickness described in

Chapter 3. Note that the valve outline uses the ISO type 'A' thickness whereas the other lines (including the dimension lines) are the ISO type 'B' thin lines. The outline thus has more prominence than the other lines and hence the valve tends to jump out of the drawing page and into the eye of the reader. The valve dimensions

follow the dimensioning convention laid down in the future ISO 129-1:2003 standard. Tolerances have been left off the figure for convenience. In this case there are two datum features. The first is the left-hand annular face of the largest cylindrical diameter, i.e. the face with the 30 ~ chamfer. Horizontal dimensions associated

with this datum face use a terminator in the form of a small circle.

The other datum feature is the centre rotational axis of the spool valve represented by the chain dotted line. All the extension lines touch the outline of the spool valve. The dimension values are normally placed parallel to their dimension line, near the middle, above and clear of it. Dimension values should be placed in such a

way that they are not crossed or separated by any other line.

Engineering Drawing for Manufacture

by Brian Griffiths

Publisher: Elsevier Science & Technology Books

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Cold Slug Wells

At all runner intersections, the primary runner should overrun the  secondary runner by a minimum distance equal to one diameter, as shown in Figure 39. This produces a feature known as a melt trap or cold slug well. Cold slug wells improve the flow of the polymer by catching the colder, higher-viscosity polymer moving at the forefront of the molten mass and allowing the following, hot, lower-viscosity polymer to flow more readily into the mold-cavity.

The cold slug well thus prevents a mass of cold material from entering the cavity and adversely affecting the final properties of the finished part.

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The "shaft basis" and the "hole basis' system of fits

In all the examples given above, the discussion has been concerning 'shafts' and 'holes'. It should be remembered that this does not necessarily apply to shafts and holes. These are just generic termsthat mean anything that fits inside anything else. However, whatever the case, it is often the case that either the shaft or the hole is the easier to produce. For  example, if they are cylindrical, the shaft will be the more easily produced in that one turning tool can produce an infinite number of shaft diameters. This is not the case with the cylindrical hole in that each hole size will be dependent on a single drill or reamer.

The right-hand diagram in Figure 5.10 shows the situation in which the shaft is the more difficult of the two to produce and this is referred to as the 'shaft basis' system of fits. In this case the system of fits is used in which the required clearances or interferences are obtained by associating holes of various tolerance classes with shafts of a single tolerance class. Alternatively if the shaft is the easier part to produce then the hole basis system of fits is used. This is a system of fits in which the required clearances and interferences are obtained by associating shafts of various tolerance classes with holes of a single tolerance class. In the case of the shaft basis system the shaft is kept constant and the interference or clearance functional situation is achieved by manipulating the hole. If the hole-based system is used, the opposite is the case. The appropriate use of each

system for functional performance situation is thus made easier for the manufacturer.

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Engineering Drawing for Manufacture

by Brian Griffiths

Publisher: Elsevier Science & Technology Books

Forging Simulation

According to the difficulties which have just been presented, the design problem often results in trial and error. It is both time and money  consuming, as the shapes of the dies are complex, difficult, and expensive to produce. In order to lower these costs, several simulation strategies have been developed to replace the actual trials. Easy to form model materials such as lead, plasticine [52], and wax [64] can be substituted to the current metal, making it possible to reduce the forming force. These materials, and more particularly plasticine and wax which are more widely used today, exhibit nearly the same behavior as the metal under hot isothermal conditions. One of the main restriction of this approach is the temperature dependency of the model materials, which is quite different from the behavior of the metal.

However, for most of the processes, the forging is so fast that the heat exchanges with the dies is small and that the heat generated by the material deformation is not enough to significantly modify the material flow. Thus, this technique provides an easy way to study the material flow, to predict the major defects such as folds [42] and insufficient pressure in the die details, and to estimate the forging force. Moreover, it is possible to study the material deformation and the fibering by mixing model materials of several colors. With some mechanical models, the total strain can be computed out of these pieces of information [52]. By mixing model materials with different properties, or by adding some other components, the behavior of the model material can be slightly adjusted to the behavior of the studied metal. As often for metal forming processes, the friction phenomenon is difficult to simulate. However, some kind of model materials for the lubricants can be proposed.

The main shortcoming of this simulation approach is that although the dies can be produced into a less expensive material and without heat treatment, they are as complex as the actual process, which requires significant time and energy to produce them. Moreover, as has been mentioned earlier, the thermal effects cannot be taken into account.

Nowadays, the numerical simulation provides much faster results as the toolings are only virtually designed. Moreover, these results are more accurate and more varied, such as the flow at different times of the process, the velocity field, the strain, the strain rates, the stresses, the temperature, the tool wear, and the tool deformation. Several softwares have been marketed, such as the well spread FORGE2 and DEFORM2D (initially called ALPID), [47] for axisymmetrical and plane strain problems.

They are actual tools for designers and their industrial use is ever increasing. Indeed, they make it easy to quickly find the main shortcomings of the studied design, and then to test several modifications to improve it. For expensive safety parts, or for very large single parts, they also provide a quality insurance as they give an estimation of the mechanical characteristics of the part which should otherwise be obtained by destructive testing. Regarding the true three-dimensional problems, automatic remeshing difficulties, as well as  computational time and memory requirements, have long hindered the industrial use of these softwares. 

Nowadays, both the software and hardware progresses have made these computations possible. They are used in forging companies, for instance to understand the development of a fold and test whether a new preform design can remove it [22]. The tool shape discretization required for the numerical simulation, often a finite element mesh of the tool surface by triangles, can almost directly be obtained from the CAD design. It reduces to rather insignificant times the specific numerical design required by the finite element simulation. FORGE3 and DEFORM3D [66] are such available softwares

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COMPUTER-AIDED DESIGN,

ENGINEERING, AND MANUFACTURING

Systems Techniques And Applications

VOLUME V I

Editor

CORNELIUS LEONDES

Boca Raton London New York Washington, D.C.

CRC Press

MANUFACTURING

SYSTEMS PROCESSES