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Checking Mechanical Drawings

The notes, on this page, are a collection of key points that can be usefully applied to a checking process at virtually any level.  However it is recognised that there are many very small companies that produce high quality engineering components without the need for highly formalised drawings.   "Measure twice cut once" is checking at its most basic level..



Introduction

Engineering drawings are used to facilitate the production of all engineering components and systems.   An engineering drawing is only one document of a suite of documents used including calculations,design specification, data sheets, material specifications etc. etc.   There is a strong economic motivation and often a vital (mandatory) safety requirement that the drawings are fit for their purpose.   Standards including the ones referenced in BS 8888 provide guidance on the preparation of drawings.   Unfortunately drawings are produced by humans or human substitutes (computers) and often include errors.  The checking function is an important function enabling the reduction of errors to an acceptable level.   Checking can be completed at various levels as listed below:

  • Signing the check box to get the drawing through the system - useless
  • Simple scan to check for neatness and presentation -virtually useless
  • Detailed check of ones own drawing - Useful but not substitute for independent check- One is often blind to ones own errors
  • Detailed dimension check by the designer of drawings produced by subordinate draughtsmen.  Useful but design errors may be overlooked
  • Detailed dimension check by independent engineer- Very useful but fundamental design errors may be overlooked
  • Detailed design & dimension check by independent engineer- Very useful with some cost penalty
  • Design review by diverse design team with follow up detailed independent design /dimension check- extremely useful -expensive

Engineering production includes an intrinsic self correcting facility in that in the manufacturing process drawings are used by number of downstream departments each using the drawing for their own purpose. Each responsible person in these departments seeing the drawing is able to feed back to the designer any errors noticed.   In the various production stages drawing errors may result in obvious errors in the components produced resulting in scrap and then consequent correction of the offending drawings.  The inspection of finished components often identifies drawing errors which are communicated to design department.  The assembly and testing of produced components often uncovers problems resulting in correction of drawing errors.  The end user of the component will also have some impact on the design when complaining about its low reliability.   All of these feedback routes have progressively increased cost penalty.

Preparation for Checking

For a drawing check to be completed efficiently and correctly the following items must be available.

  • Checking pens/pencils three colours [ OK, Delete, Amend ]
  • Approved design specifications
  • Information on the service conditions if not included in above
  • Approved process /mechanical data sheets
  • Reports /minutes of relevant design review meetings
  • Company Drawing Standards /Relevant standards e.g. BS 8888 referenced standards
  • Company Material/Equipment Standards /Relevant Standards/Directives etc e.g Machinery Directive.
  • All relevant component literature. e.g. Bearing design data, gearbox information etc
  • Drawings of existing similar successful operating systems
  • Full set of completed drawings to be checked

Checking guidance

New Designs
The checker should make himself fully aware of the design requirements by reviewing all documents prepared prior and following production of the drawings.   The checker should, at this stage confirm that these documents are approved.

Layout /Arrangements
The layout /arrangement drawings should be checked for the following features
The drawings should clearly :

  • Illustrate the complete system design with no ambiguity
  • Position the system in the relevant world.e.g. Datum positions and Direction arrow pointing North
  • Identify the space required by the system.
  • Identify the access required for and to the system.
  • Ensure that all sub-assemblies and details are correctly referenced
  • Indicate floor static / dynamic / full/ empty loadings
  • Indicate important lifting provisions
  • Identify the relevant installation drawings.

The checker should, at this stage review in outline that the system will operate satisfactorily and is aesthetically acceptable and that there are no obvious strength problems.  The checker should check the design against any similar successful systems.

Installation/ Assembly /Detail Drawings

Document presentation:
The drawing should be neat with bold outlines and clear hidden/centre lines The checker should ensure that the drawing is unambiguous and conforms to the accepted company standards ( Better to BS 8888 ref'd standards).
The title block should be correctly completed and the title should be clear and provide the correct key words in the correct arrangement.
The text size should be in accordance with the relevant standard and should be clear without undue style
All elevations and sections should be made to show most clearly the shape of the items and the machining production processes to be completed.  The drawing should clearly show the the complete shape and all necessary dimensions of the item.
All sub items should be completely identified against a component schedule.   The drawings should include sufficient notes to clearly identify all production stages, and surface coatings..
If needed sub-views should be used to show parts not easily shown in the main elevations.

Strength:
All parts should be checked for strength and rigidity by confirming that suitable strength calculations have been completed and the drawing conforms with these calcs.   The check should include comparing the items with similar existing successful parts.
The check should confirm that the design has included consideration of fatigue. If operating conditions include repeated/reversing loads it is important that sources of stress concentration have been minimised and calculated strength levels have been suitably adjusted

Materials:
All materials specified should be suitable for the functional requirements and the operating environment..   In the UK, at this time, material specifications seem to be in continuous flux.   There is still a tendency to specify materials such as BS 970 EN8.   This is probably acceptable as it is definitive but it is much better to use the latest material specification and the material should be specified as recommended in the relevant standard.
The checker should provide an input on the suitability of the material form if a significant design improvement or cost reduction results.i.e forged, cast, welded.

Dimensions
Ensure the drawing includes all dimensions and that the dimensions are correct.
Dimensions should be checked manually to confirm they are to scale.  If any dimension is out of scale ensure that, if it is correct, then it is marked appropriately..
Overall dimensions should be clear and unambiguous and machining allowances should be identified.
Reference dimensions provided only for information should be so identified
All extension lines should go to the correct points on the drawings
All arrow points should point to the correct witness points
The geometric and linear tolerances and limits should be checked to ensure correct clearances and tightness
Ensure there are no strings of dimensions where errors can accumulate
If the machining process includes a requirement for co-ordinate positioning the parts should be dimensioned accordingly

Checking for assembly
Ensure all parts can be conveniently assembled with the associated parts
Male parts filets/chamfers and female part radii/ fillets to be sized so they do not interfere on assembly .
Ensure bearings fits are correct to allow relative motion
The parts should include features to enable dismantling if required

Castings
It is important when checking items made from casting that the requirements of the casting process are considered e.g. the method of supporting the cores, vent provisions, and pattern split lines.
Cast items should be of uniform thickness - no adjacent thick and thin sections
Generous fillets should be used
Ensure that machining allowances are included for core movement and swelling.
Ensure sufficient machining allowances are provided for distortion of the casting

Machining
Ensure that the correct machining symbols are used and they are located against the correct surfaces or witness lines
When specific machining operations are required ensure that the requirements are identified
Confirm that the correct drill, reamer,and tap sizes are included on the drawing
Ensure that all features required to ensure convenient machining are provided i.e. clamping points
Ensure that where required suitable raised faces are included to allow clear machining operations
Ensure clearances are provided for machining and fastening spanners

Handling
Ensure that all details include the weights
All heavy items than say > 300N should include provision for lifting
All lifting points should be clearly identified
Specific lifting features should include requirement for factory testing -e.g a note
The checker should confirm that the necessary calculations for the the lifting points have been completed and approved

Identification
All components and main assemblies should be clearly identified either by an code number which cannot easily be removed or a fixed on label

Heat Treatment
When needed the drawing should identify clearly that heat treatment is required and should also identify the relevant specification
The checker should confirm that the specified heat treatment is appropriate

CAD drawings.

It is desirable that within a company all CAD drawings are produced to a common CAD standard.  This aspect of the drawing process is generally checked as a separate process

Checking CAD drawings includes confirming that the following tasks have been completed correctly..

  • Separating parts of the drawing onto separate layers (title blocks, Line Types, etc)
  • Using standard fonts
  • Using Designated line types
  • Allocating colours to drawing features
  • Using correct title blocks / labels /borders
  • Drawing to scale when plotted at designated size
  • Blocks correctly inserted and unused blocks purged
  • Associated dimensions used

To Be continued



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Last Updated 19/01/2013