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Mechanical Drawings



Introduction

There are a number of drawing types associated with the mechanical engineering design process.

A list of the Drawing Types covered by these notes is provided below

  1. General Arrangement Drawings
  2. Arrangement Drawings
  3. Assembly Drawings
  4. Detail Drawings
  5. Fabrication Drawings

These notes also include comments on item identification identification



General Arrangement Drawings

This drawing shows overall views of the equipment and provides all of the information to produce transportation, layout and installation drawings.   The drawing includes a list of the arrangement drawings.   The drawing includes overall dimensions, installation details, overall weight/mass, weights of sub systems, and service supply details.

The general arrangement drawing includes references to the design documents.   The drawing often also identifies relevant internal and external contract numbers.   An example of a typical general arrangement drawing is a roller conveyor system comprising a number of conveyors with independent drives and guards.

The drawn separate assemblies and parts will be identified with leader lines to balloons which include the arrangement reference number linking to the list of arrangement drawings.


Arrangement Drawing

Arrangement drawings represent self contained units used to make up the system drawn on the general arrangement drawing.    Examples of arrangement drawings include drawings of assembled conveyers, drive systems, elevating units etc. The drawing should show in, at least three orthographic views, clear details to show all of the components used to make up the equipment items and how the component parts are located and fastened together.  

Arrangement drawings include a table (parts list) identifying assemblies, fabrication drawings, detail drawings and proprietary items used to make up the equipment.    Arrangement drawings include overall dimension, the weight/mass of the equipment drawn, the lifting points.   All information needed to construct, test, lift, transport, and install the equipment should be provided in notes or as referenced documents.

The arrangement drawing may be a standard internal drawing which is repeatedly called up on different system general arrangement drawings.

The drawn separate assemblies and parts will be identified with leader lines to balloons which include the item reference number linking to the parts list.


Assembly Drawings

The assembly /sub-assembly drawings are drawings of discrete sub-systems showing in some detail how the component items fit together.   Typical assembly drawings include gearbox drawings, roller drawings, guard system drawings.  

The assembly drawing will generally include at least three orthographic views with sections as needed to clearly show all of the details and their relative positions.   Overall and detail dimensions will be shown.  The weight/mass of the assembly/sub-assembly will be noted.  The drawing will include a parts list identifying all of the component details with quantities and materials and supply details.   The assembly drawing will include a list of reference drawings and notes identifying the relevant codes and specifications and testing requirements.

The drawn separate items will be identified with leader lines to balloons which include the item reference number linking to the parts list.


Detail Drawings

All individual items required to produce mechanical equipment need to be described in some detail to ensure that they are manufactured in accordance with the designers requirements.   Proprietary items are selected from technical data sheets obtained from manufacturer /supplier.    Items manufactured specifically for the application need to be made to detail drawings which include the geometry, material, heat treatment requirements, surface texture, size tolerances, geometric tolerances etc.

The detail drawing should include all of the necessary information to enable procurement, manufacture and should identify all of the relevant codes and standards.   The item weight/mass should also be included for reference.

Depending on the level of detail, a detail drawing can comprise one drawing on a sheet or a number of separate drawings on one sheet.   It is sometimes possible to combine the detail drawings onto the assembly drawing.  The detail drawing must cross reference, both ways,to the parent assembly or arrangement drawing.


Fabrication Drawings

The fabrication drawing is a specific type of detail drawing.   Some fabrication drawings are virtually assembly drawing e.g. when a number of items are assembled together as a fabrication.    The fabrication drawing generally includes a material parts list identifying all of the materials used to build up the fabrication.  All weld details are included using the standard symbolic representation of welds as shown in BS EN 22553.   All of the materials should be identified in accordance with the relevant standards and codes.

The fabrication drawing should clearly describe in notes or in referenced documents the heat treatment and stress relieving requirements prior to, during and following the completion of the fabrication processes.  The dimensions and relevant linear and geometric tolerances should be indicated.

A fabrication drawing sometimes only includes the fabrication details, the final machining details are then shown on a separate drawing.   It is equally acceptable to show all manufacturing information on one drawing.

The items used to make up the fabrication will be identified with leader lines to balloons which include the item reference number linking to the parts list.   The listed items on a fabrication drawing do not identify items which can be disassembled, as on assembly and arrangement drawings.   The numbering system should reflect this difference. Methods of numbering items on fabrication drawings include using lower case alphabet letters e.g a,b,c or optionally as sub units of the fabrication item number e.g 1/1, 1/2 1/3 ... or 1/a , 1/b, 1/c...


Item Identification

The method of identifying the parts must be clear and unambiguous. The equipment as represented on the general arrangement drawing and the sub-assemblies as shown on the arrangement and assembly drawing should be clearly identified with plant item numbers. The relevant drawing numbers are obtained by reference to the plant items list.   Plant items are annotated by leader lines to a double balloon.

Typically a conveyor may have a plant item number e.g.H1040 and be shown on a drawing e.g. drawing number A0 12500.

The detail drawings are sub items of the arrangement drawings and are identified on the arrangement and assembly drawings.   Typically an item say a conveyor frame may be identified from the conveyor plant item number e.g. H1040/3 . Optionally it may be identified using the arrangement drawing number e.g. A0 12500 /3.   The frame will also have a discrete detail drawing number e.g A2 12503

The fabricated items which are based on sub-parts welded together should be identified as details but the individual sub-parts should be identified in a different way to avoid ambiguity.    One option is to number the fabricated sub-parts alphabetically e.g a, b, c ...or as a combination of the fabrication detail number and the part number i.e 3/a , 3/b....   These sub-parts do not need to be identified as separate parts because following fabrication they will not exist as separate parts.   If the sub-parts are complicated shapes or machined items and they cannot be described in sufficient detail on the fabrication drawing they should be drawn as separate detail drawings but still identified as sub-parts of the fabrication detail.



Links Providing information on Drawing Diagrams and Symbols
  1. Volvo Drawing Standards......For access to document 101-0005 on European-Standard-based drawing procedures
  2. The Design Process...A review of the design process including drawing examples.( Not BS 8888 )
  3. Drawing Tools...Notes on Drawing using CoralDraw
  4. Cadalot...Lots of Advice and Lessons from a Drawing expert

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Last Updated 13/04/2010