by Gary Whitmire, of the DraftingZone.com
Some people may think that the use of CAD systems for engineering drawings has made the step of checking the drawings obsolete. An experienced drafter knows that’s not true. All good CAD systems will check most of the easily defined problem areas, but that may just make the checking process more difficult. You need to look past the common mistakes and find the more difficult problems because the drawing may look correct, but still contain errors.
If there are changes to be made after the drawings have been released and sent to manufacturing or vendors. The whole checking process starts over. This is why checking is so important. I cannot stress enough the need to avoid the related cost and time lost due to revisions.
The Checking Process
The completed drawing, no matter whether it is a detail, assembly, or circuit drawing, cannot be considered finished and ready for release until it has been completely checked by an experienced person.
This person may be the drafting supervisor, engineer, or a professional checker. Not only must the drawing be accurate and complete, it must also agree with the latest standards that apply to the drawing. The copy of the drawing being checked will become part of the stored design history record.
The person checking the drawing should use the following colored pencils to mark-up the print: yellow for items that are correct; blue for comments and errors that need not be changed; and, red for errors that must be changed.
There are three levels of drawing checking. The first level is a format check. The second level is a design check for form, fit and function. The third check is done by a team of experts consisting of Engineering, Manufacturing, Quality Control, Safety, Vendor, Customer, etc. who look at the design feasibility, cost, schedules, etc. The purpose of these checks is to remove all errors so that costly and time consuming mistakes will not show up during manufacture. One small dimension mistake can cause extensive and costly rework, particularly in high volume production.
Level One, Format Check – The format check looks for errors such as spelling, unlisted items, drafting convention errors, poor dimensioning practices, etc. This level also helps in the training of drafting personnel. This level could be delegated to a senior drafter.
Level Two, Design Check – The design check using the three colored pencils needs to be done by an experienced person in the engineering field that pertains to the drawing. For a mechanical field, the checker must check the dimensioning including performing a tolerance stack-up analysis, and checking for related standards and processes. This check may include doing Computer-Aided Design (CAD) enlarged or rotation layouts (descriptive geometry) to check clearances. This level may also include having a stress engineer check the structural requirements.
After the drawing has been corrected, the checker reviews the update and signs off and dates the drawing in the title block under the label “checker.” The checker retains the marked-up prints.
Level Three, Design Review – Government contracts have a preliminary design review (PDR) and a critical Design Review (CDR). The PDR is done when all the drawings are finished and checked. This is where the government team inspects the drawings for compliance with Government drafting practices and checks the design for meeting the Government design requirements. After the drawings have been updated, the CDR is done as a final overview of the changes. The PDR or the CDR will include, as a minimum, the following personnel: Customer team, Engineering, Manufacturing, Suppliers/Vendors, Quality Control, Configuration/Data Manager. When it is not a Government contract, this check is usually done in a less formal atmosphere. This check is done before the drawings are released to manufacturing or the vendor(s).
Guidelines for checking drawings
The following are some guidelines for checking drawings. A drawing must be able to be read without requiring any interpretation. All material pertaining to the drawings being checked should be gathered. This includes catalogs, calculations, prints or reference drawings. I also recommend that checkers have a good reference like Genium’s Modern Drafting Practices Manual available so that they can refer to it when they encounter drawing situations that are new to them or perhaps outside their area of expertise.
When all of this material has been gathered, the checking should be done as follows:
1. The checker must understand the function of the part(s). First start with the simpler drawings, such as those of minor detail parts. These are checked for correct dimensioning, tolerances, references to other drawings, and other information. Dimension calculations should be made and become part of the checking package record. Check the less important dimensions first, such as notes, title block information, etc.; followed by critical information related to other drawings of parts and assemblies, such as dimensioning holes.
2. All drawings and views should be checked for complete dimensioning. Make sure that manufacturing is not expected to calculate any required dimensions.
3. Drawings should show surface finishes, plating and a note that dimensions apply before or after plating. Material specifications should be given in detail.
4. Drawings should include all necessary views and sections in order to clarify the detail or assembly.
5. The clearances between adjacent and moving parts should be carefully checked for interference using computer-aided design (CAD) layouts.
6. Do a tolerance stack-up analysis to check close tolerances and make certain that they are necessary.
7. Assembly drawings must undergo the same critical examination. Mechanical fasteners must be checked for catalog information, material, corrosion problems, and for sizes and lengths.
8. The parts list on assembly drawings should be checked to make sure they specify related drawings, specifications, sizes of parts, etc.
9. Where possible, make suggestions to simplify the design or reduce the cost.
10. If the drawing must meet military drafting standards, then the checker must understand those requirements.
11. The word METRIC or INCH must be stated above the title block in .250 inch high lettering.
There are two types of metric drawings. One type is a conversion from inch to metric. The second type is a drawing in pure metric using only mm. Except for thread sizes, the conversion is a simple mathematical conversion. In rounding off metric numbers (mm), it is not uncommon for the metric to be one decimal place less than the inch dimension. CAD systems can automatically convert from one system to the other. The problem is the way a CAD system rounds off the last digit of a number. To avoid an interchangeability problem, the checker needs to take the converted mm number and convert it back to inches to see if proper clearance is still maintained. The American dimensioning standard no longer uses dual dimensioning inch [metric] on drawings. Instead, dimension the drawing in millimeters and have two columns (inch and mm) both to the same number of decimal places listing the conversion. For example:
25.400/25.578 = 1.000/1.007
3.2 texture = 125
10 K7/m6 = .3937 LN3 (Locational interference fit)
10 N7/m6 = .3937 FN2 (Medium drive fit)
10 P7/m6 = .3937 FN4 (Force fit)
40 H11 (40.000 / 40.160) = 1.575 / 1.581 (Hole size)
40 c11 (39.720 /39.880) = 1.564 / 1.570 (Pin size)
Metric fit symbologies need not be used on drawings staying within the United States.
1. The word METRIC or INCH must be stated above or in the title block with .250 inch lettering.
2. Inch fasteners converted to metric fasteners may not have the same strength or codes. Have Purchasing call the manufacturer for input.
3. Make sure that the third-angle projection symbol is on the drawing for the American projection system.
Schematic Diagrams – Using a yellow pencil and working from the engineering sketch, the checker should check the line-by-line, point-to-point circuit on the sketch and on the drawing. The checker needs to also check any related drawings for continuity such as wiring or harness diagrams.
1. Check for proper symbols, component values, polarity markings, and reference designators. Make sure the reference designators are not close to another symbol. Also keep in mind that industrial schematic diagrams may use different symbols.
2. Do not permit the use of a dot for lines that connect with other lines; each staggered line will terminate individually to another line without a dot. If dots were permitted, a dirty copy machine may inadvertently insert a dot, thus changing the drawing.
Wiring Diagrams – Check the wiring diagram and compare it to the corrected schematic diagram print using a yellow pencil. For tabular type diagrams, check the “to” and “from” designations for the different connectors as well as the color designation of the wires. If possible, any expected harness wiring should be drawn together on the wiring diagram in the same manner as shown on the harness drawing.
Harness Drawings – Check the harness drawing and compare it to the corrected wiring diagram print using a yellow pencil. Use the same point-to-point procedure described above. Care must be taken to locate the exact termination points on both the wiring diagram and the harness print. Check the harness drawing for wire type and color, insulation, shielding and harness ties. Check the length of the harness and the length of each connection lead extending from the harness and their connector if applicable. If a wire list is part of the harness drawing, it should be checked for wire type, insulation, shielding and any other data.
About the Author
Gary Whitmire is the Technical Editor of the Modern Drafting Practices & Standards Manual and moderates the “Ask Gary” technical question forum at www.DraftingZone.com. He is nationally and internationally recognized as an authority on the application and interpretation of ANSI, ASME, and ISO standards. He is a past member of the ASME Y14 Main Committee (serving for 28 years), including Geometric Dimensioning and Tolerancing (GDT). He has also served on the Executive Board of the American Design Drafting Association (ADDA). Gary can be reached at firstname.lastname@example.org.