Bill of Materials
This table on a drawing lists all parts and materials needed to fabricate or assemble a component.
What is the bill of materials?
This block records changes made to the drawing, including what changed and when it changed.
What is the revision block?
These short written instructions appear on or near the drawing when something is difficult to show with lines alone, such as “REMOVE ALL BURRS.”
What are notes?
This is the total amount a part’s size is allowed to vary between its smallest and largest acceptable dimensions.
What is tolerance?
This manufacturer-issued document provides wear limits and conditions like ovality, bow, and twist for parts that slide or rotate.
What is the schedule of fits and clearances?
This number in the BOM matches the circled or ballooned number pointing to a part on the drawing.
What is the item number?
These two pieces of information in the revision block identify who made the change and who verified it.
What are the drafter’s initials and the checker/approver’s initials?
These numbers and letters along the edges of a drawing divide it into a grid, similar to a map, so you can locate parts by coordinates.
What are zone numbers?
For a part dimensioned 2.50 ± 0.05 inches, this is the smallest acceptable size.
What is 2.45 inches?
This word describes when two parts are meant to slide or move over one another and must stay within specified clearances.
What is motion (or sliding fit)?
This BOM column tells you how many of each part or material you need to complete the assembly.
What is the quantity?
This is the term for the letter or number, such as “Rev A” or “Rev 1,” that separates one version of the drawing from another.
What is the revision identifier (revision letter/number)?
This station system runs forward and aft along the length of the aircraft from a defined station zero.
What are fuselage stations (FS)?
For a part dimensioned 0.500 ± 0.010 inches, this is the total tolerance.
What is 0.020 inches?
These marks and symbols on a drawing indicate how smooth or rough a surface must be or that a surface must be machined.
What are finish marks?
In a BOM, this can be used when a different but equivalent part (like another rivet with the same specs) can be used instead of the primary part number.
What is an alternate part number?
Before starting work from a printed drawing, technicians must always verify this about the revision block to avoid rework or unsafe repairs.
What is that they are using the latest/current revision?
These stations run left and right from the aircraft centerline and are used to locate parts sideways across the fuselage.
What are buttock lines (BL)?
This term refers to the intentional difference in size between mating parts, like a shaft and a hole, to achieve a specific class of fit.
What is allowance?
The phrase “REMOVE ALL BURRS AND BREAK SHARP EDGES, RADIUS .015 MAX” is an example of this type of instruction.
What is a finish/notes instruction (surface finish note)?
This BOM entry might read something like “6061-T6, .090 thick” and tells you what the part is made of and how thick it is.
What is the material and thickness (material specification)?
This is what can happen if a mechanic stores old printed drawings and accidentally uses them later without checking the revision block.
What is they may perform work using an outdated revision, causing rework or possible safety/compliance issues?
These reference lines are like horizontal levels on the airplane and are used to indicate vertical locations above or below a reference plane.
What are water lines (WL)?
This type of tolerance allows variation both above and below the nominal size (for example, ±0.05), while the other type allows variation in only one direction.
What is bilateral tolerance?
Although it affects the appearance of a part, this process—such as paint or enamel—is not the same as the machining finish referred to by finish marks.
What is painting/plating/enamel (coating)?