SC QUALITY GUILDDrawing title block tolerance reader
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Turn each dimension on a drawing into upper and lower limits, using the general tolerance block or the tolerance written on the dimension, and check revision, scale and projection.
ON THE EXAM · CQI III.A.1, III.A.2, CQT IV.A.1 · FREE · NOTHING YOU TYPE LEAVES YOUR BROWSER
Q-BotClick for an explanation of this tool.For educational use. This page is a study aid. The method behind it has far more depth and detail than one page and one tool can cover, and what you see here is not everything the method offers or everything this site offers. For full understanding and application, use the governing standards and texts, and seek further education, training or qualified advice. Results depend on the data you enter, so check them before relying on them.
The title block and the general tolerance block
The title block identifies the drawing: number, title, revision, units, scale, projection, the dimensioning standard and the approvals. Next to it, most drawings carry a general tolerance block. It states the tolerance for any dimension shown without its own, by the number of decimal places written. With .XX ±.010 and .XXX ±.005, a dimension written 2.50 means 2.490 to 2.510, while 2.500 means 2.495 to 2.505. On an inch drawing the trailing zero changes the tolerance, so read the dimension exactly as written.
Ways a tolerance is stated
- Bilateral: equal plus and minus, such as 1.250 ±.001.
- Unilateral: variation in one direction only, such as .750 +.000/−.002.
- Unequal bilateral: both directions but not equal, such as 1.000 +.003/−.001.
- Limit dimension: the two limits themselves, such as .6245−.6250. No arithmetic needed.
- Single limit: MAX or MIN, for depths, radii and similar.
- Reference dimensions in parentheses carry no tolerance and are not inspected for acceptance. Basic dimensions in a box are exact; their tolerance comes from a feature control frame.
A tolerance written on the dimension always overrides the block.
Revision, units and scale
Inspect to the revision the order calls for; a mismatch is a stop, not a note. Confirm the units before reading any number. The scale tells how the views were drawn, never a way to measure: do not scale the print.
Third-angle and first-angle projection
In third-angle projection, used under ASME in North America, the top view sits above the front view and the right side view to its right. In first-angle projection, the ISO practice in much of Europe and Asia, the views are placed on the opposite sides: top view below, right side view to the left. The projection symbol in the title block tells you which; misreading it puts features on the wrong side of the part.
On the exam
CQI questions give a title block and a dimension and ask for the limits, or ask which view appears where in third-angle projection. Count the decimal places, apply the matching line of the block, and remember that reference dimensions are not inspected.