Checking
Drawings
by Gary Whitmire, of the DraftingZone.com
Introduction
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.
Mechanical Drawings
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.
Metric Drawings
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.
Electronic Drawings
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 whitmireg@draftingzone.com.
