ASSEMBLY DRAWINGS


OBJECTIVE

This chapter is related with the formal ways of presenting a part to be manufactured in detail and the structures that is formed by several parts aligned to each others by appropriate assembly elements such as threaded fasteners, keys, springs, etc.

WORKING DRAWINGS NEEDED FOR PRODUCTION

The drawings that are used to give information for the manufacture or construction of a machine are called as working drawings. Working drawings must include all the knowledge for the production of a machine or structure explicitly so that no further information is required to complete the production. The description given by the set of working drawings will include:

  1. The graphical representation of the shape of each part, namely shape description.
  2. The dimensions of each part; size description.
  3. Explanatory notes on the individual drawings, giving the specifications of material, heat treatment, and surface finish.
  4. A descriptive title on each drawing.
  5. Relationships of each part to the others (in assembly drawings)
  6. Part list.

In general, set of drawings include two classes of drawings; detail drawings, supplying the information in topics 1 to 4, and an assembly drawing, supplying the information about the location and relationship of the parts, topic 5.

It must be noted that the working drawings used for purposes of manufacturing are superior to design layouts. Design layouts are prototype assembly drawings from which working drawings evolve. The below figure is an example of a design layout.

A portion of design layout.

If mass production of the parts is concerned, "operation" or "job" sheets describing the seperate manufacturing steps should be prepared. These sheets also indicates the use and kinds of any special tools.

SPECIFIC TYPES OF WORKING DRAWINGS

We will now turn our attention to particular types of drawings that are used in the production of parts. Design layout drawings are part of the initial design procedure.

  • Detail Drawings : A detail drawing is the drawing of a single part that includes a complete and exact description its form, dimensions, and construction. The worker must clearly understand the shape, size, material, and surface finish of a part, what shop operations are necessary; what limits of accuracy must be observed from the detail drawing. Following figure is an example of a commercial drawing.

A commercial detail drawing.

  • Detail drawings are formed by carefully studying the initial design layouts. Use is made of the scale of the design layout, dimensions that may be given, and all notes provided. Approved standarts for the specific company involved with respect to lettering style, dimensioning techniques, position of notes must be included in detail drawings.
  • In general working drawings are checked by an experinced person responsible for any possible error, when it is finished. This step is the final "proofreading" and cannot be carried out by the person who has made the drawing. The necessary information should be preserved for the future use and reference.

 

  • Assembly Drawings : A complete assembly drawing is presentation of the product or structure put together, showing all parts in their operational positions. The seperate parts come to the assembly department after their manufacturing processes are finssihed and in this department they are put together according the assembly drawings. Small machining operations may be necessary during assembly process such as drilling, reaming, or hand finishing. For such cases, assembly drawings include a note explaining the required operation and give the dimensions for the alignment or location of the pieces.
  • Several different methods can be used to produce assembly drawings; the simplest one tracing from the design layouts. This method is inferior to the method that the assembly drawing is produced from the dimensions of detail drawings if the accuracy of checking considered. Of course the second method is very time consuming. Whereas, the Computer Aided Drafting can be a huge timesaver when an assembly drawing is being produced. Nowadays, there are so sofisticated CAD programs and equipments; almost all the manufacturers use these programs to recover high initial costs. Although many assembly drawings do not need dimensions, the overall dimensions and distances between the centers or from part to part of the different pieces to clarify the relationship of the parts with each others. An assembly drawing should not be overloaded with detail.
  • Assembly drawings should include referance letters and numbers representing the different parts. These part numbers usually enclosed by circles with a leader pointing to the piece .
  • A unit assembly (subassembly) is a drawing of a related group of parts and used to show the assembly of complicated machinery for which it would be practically impossible to show all the features on one drawing. To illlustrate; headstock, tailstock, and gearbox unit assemblies should be included in the drawing of a lathe.

    A unit assembly drawing.

  • An outline assembly is used to describe the exterior shape of a machine or structure, so it contains only the primary dimensions. If it is made for catalogs or illustrative purposes, dimensions are often omitted. They are also called as installation drawings.
  • An assembly working drawing includes all the necessary information for producing a machine or structure on one drawing. This requires providing adequate orthographic views together with dimensions.
  • A diagram drawing is an assembly showing ,symbolically, installation of equipment and often made in pictorial form.
  • The bill of material is a tabulated list placed either on the assembly drawing or on a seperate sheet. The list gives the part numbers, names, quantities, material and sometimes stock sizes of raw material, detail drawing number, etc. The term "bill of material" is usually used in structural and architectural drawing whereas the term "part list" is used in machine-drawing practice.

 

  • Tabular Drawings : A tabular drawing, either assembly or detail, is one on which the dimension values are replaced by reference letters, and an accompanying table lists the corresponding dimensions for a series of sizes of the machine or part, thus one drawing serves for a range covered. If parts are produced in a variety of sizes, using tabular system will be logical, but there is a serious risk of misreading the table.

 

An outline assembly drawing (tabular).

  • Standardized Drawings : The difficulties of the tabular drawings are overcomed by making "standardized drawings". These drawings are complete except the actual figured dimensions. By offset printing or black and white reproduction on vellum paper these standard drawings are reproduced and dimensioned seperately for the various sizes. Following figure is an example of standardized drawing.

A standard drawing and a filled-in standard drawing.

  • Set of Drawings : A complete set of working drawings consist of detail sheets and assembly sheets, the former gives all the necessary information for the manufacture of each of the individual parts and the latter showing the parts as assembled as a finished structure or machine. The set includes also the parts list and also may include further information such as oiling diagrams,etc.
Part 1
Part 2
Part 3
Part 4
Part 5
Assembly

Set of drawings.

An example of assembling parts.

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