Machinist Drawings

Machinists generally use precision machine tools such as lathes, boring machines, milling machines, and cylindrical or surface grinders to shape materials such as steel, brass, iron, bronze, aluminum, titanium, and plastics and to manufacture components to meet customers' specifications.

A machining detail drawing provides all the information needed to manufacture a specific part and is used to machine the casting into a finished component. Typically only one part is detailed in each drawing. Machining details are generally used when machining a rough part into a finished part; a machining detail drawing will specify the surfaces to be machined, bolt holes and locations, reference points, geometric dimensioning and tolerancing (GD & T), and other machined areas. Machining drawings contain critical information to the traditional or CNC (computer numerically controlled) machinist, which would include angles, reference points, surface finish, etc. Once complete, this machined part should match and fit other machined parts as defined in an assembly/detail drawing. Parts that would not normally need to be drawn are standard parts—those that may be purchased from an external source more economically than it would cost to manufacture. Such parts may include screws, nuts, bolts, keys, and pins. While they do not need to be drawn, they nevertheless need to be included as part of the information on each sheet. The blueprint reader must clearly understand the shape, size, material, and surface finish of a part, which shop operations are necessary, and the limits of accuracy that must be observed from the detail drawing. Figure 7.12 is an example of a typical detail drawing.

Normally, detail drawings contain information that is classifiable into three groups:

1. Shape description: describes and explains or portrays the shape of the component

2. Size depiction: shows the size and location of features of the component

3. Specifications: relates to such items as material and finish

Detail machine drawings should include all or most of the following information:

• Views of the component as necessary to allow visualization

• Material used to manufacture the component

• Dimensions

• General notes and specific manufacturing information

• Identification of the project name, the part, and the part number

• Name or initials of who worked on or with the drawing

• Any engineering changes and related information

Books Detailing Machined Parts
Figure 7.12 A typical machine detail drawing (source: Middle East Technical University).

A complete assembly drawing is a presentation of the product or structure put together, showing the various components in their operational positions. The separate components come to the assembly department after their manufacturing processes are finished, and here they are put together according to the assembly drawings.

Many products consist of more than one part or component. A bill of materials (BOM) or list of component parts is often included on an assembly drawing to facilitate the assembly, as well as necessary dimensions and component labeling (Figure 7.13). A three-dimensional picture of a complete assembled unit would help the reader to understand the final shape of the assembly. Front, side, and top views may be critical to communicate dimensions or shapes to the reader. If the assembly drawing is actually one of several subassemblies, the print should indicate this in the title block or bill of materials. Movements of components in an assembly detail drawing should be indicated with the use of phantom lines.

Machine Drawing Assembly
Figure 7.13 A sectioned assembly drawing with a bill-of-materials table (source: College of Engineering, Ohio State University).

There are various types and versions of assembly drawings including:

• Layout assembly drawings initially used in development of a new product.

• Exploded assembly drawings pictorially showing parts laid out in their correct order of assembly, found in machinery catalogs designed for homeowners or suppliers for ordering parts (Figure 7.14).

• Diagram assembly drawings use conventional symbols and are used to show the approximate location and/or sequence of the components to be assembled or disassembled.

• Working assembly drawings are fully dimensioned and noted. When applied to very simple products, they can act as alternatives to detail drawings.

• Installation assembly drawings are used to show how to install large components of equipment.

As mentioned earlier, an assembly drawing is a drawing of various parts of a machine or structure in their relative working positions. An assembly drawing essentially conveys the completed shape of the product as well as its overall dimensions, relative position of the different parts, and the functional relationship of its components. When all the parts are produced using their respective machining detail draw-

Assembly Order Drawing
Figure 7.14A An exploded assembly drawing typical of drawings found in machinery catalogs showing the various components laid out in their correct order of assembly (source: StoneAge, Inc.).

ings, an assembly drawing provides the information the print reader needs to assemble the components. The bill of materials (BOM), which is basically a tabulated list, may be placed either on the assembly drawing or on a separate sheet. The list gives critical information such as part numbers, names, quantities, material detail-drawing number, and sometimes stock sizes of raw material, etc. The term "bill of materials" is usually used in structural and architectural drawing, whereas the term "part list" is used in machine-drawing practice.

A three-dimensional picture of a complete assembled unit facilitates the reader's ability to visualize the final shape of the assembly (Figure 7.15). Front, side, and top views may be necessary to communicate dimensions or shapes to the reader. If this assembly drawing is actually one of several subassembly drawings, the print should indicate this in the title block or the BOM.

Likewise, a maintenance technician would normally need assembly drawings at the work site to evaluate the best sequence for the dismantling of specific machinery, to locate parts that must be ac-

Flexmaster Snyder
Figure 7.14B An exploded assembly drawing and photograph of a Flexmaster expansion joint (source: Snyder Industries, Inc.).

cessed or attachment bolts of those to be removed, and to provide detail information on components disassembled for repair. Finally, the technician needs to accurately discern the correct alignment of components upon reassembly.

Customers who deal with consumer products such as electronic goods will also normally require the use of exploded CAD drawings to help in understanding the relationships among assembled parts. Exploded drawings are indispensable for a number of manufacturing industries. While generating assembly drawings, critical interference checks are included to ensure that the entire assembly is integrated, thus saving a tremendous amount of time and costs in the prototyping phase.

Computer-aided drafting has proved to be a huge timesaver when an assembly drawing is being produced. Today, there are a large number of sophisticated CAD programs and equipments, and the vast majority of manufacturers now use these programs to recover high initial production costs. Although many assembly drawings do not require dimensions, 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 other can be included. Most important, however, an assembly drawing should be easy to read and not be overloaded with detail.

Utilizing CAD programs likewise allows individual component details to be merged together to create an assembly or working drawing of the component(s). With CAD systems, three-dimensional (3-D) models can be created that makes it possible to superimpose images and to graphically measure clearances. When parts have been designed or drawn incorrectly, the errors will often stand out so that appropriate correction can be made. This improves the efficiency of the drafter and helps to make the details in the final print accurate and the resultant parts function properly.

Information normally required for general assembly drawings includes:

• Parts to be drawn in their operating position

• Part list (or bill of materials) including item number, descriptive name, material, and quantity required per unit of machine

• Leader lines with balloons drawn around part numbers

• Machining and assembly operations and critical dimensions related to the operation of the machine

Steps in creating an assembly drawing include the following:

1. Analyze the geometry and dimensions of the various parts in order to understand the assembly steps and overall shape of the object.

2. Select an appropriate view of the object.

3. Choose the major components—components that require assembly of several parts .

4. Draw a view of the major components according to a selected viewing direction.

5. Add detail views of the remaining components at their working positions.

6. Add balloons, notes, and dimensions as required.

7. Create a bill of materials (BOM).

Assembly drawings can require one, two, three, or more views, although they should be kept to the minimum necessary. A good viewing direction should be chosen that represents all (or most) of the parts assembled in their working position.

Bill Materials Drawing

Figure 7.15 Diagram showing how multiple parts fit together and a bill of material and pictorial view of the assembled object (source: College of Engineering, The Ohio State University).


Figure 7.15 Diagram showing how multiple parts fit together and a bill of material and pictorial view of the assembled object (source: College of Engineering, The Ohio State University).

In mating parts the two main considerations are the surface finishing and the tolerance (especially size and geometry). The surface finishing means the level of roughness of a surface. Its main purpose is to control the accuracy in positioning and tightness between the mating parts. The other objective is to reduce the friction, especially for parts that move relative to other parts.

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