3. Representing Tolerance Values
∙ Tolerance is the total amount a
dimension may vary and is the
difference between the maximum
and minimum limits.
(A) Tolerance = .04
(B) Tolerance = .006
∙ Tolerances are represented
as Direct Limits (A) or as
Tolerance Values (B).
4. Important Terms of Toleranced Parts
A System is two or more mating parts.
Nominal Size is used to describe the general size (usually in fractions).
Basic Size – theoretical size used as a starting point for the application of
tolerances (written in decimals).
5. Important Terms of Toleranced Parts
Actual Size is the measured size of the finished part after machining.
Limits – the maximum and minimum sizes shown by the tolerance dimension.
The large value on each part is the Upper Limit, the small value = Lower Limit.
6. Important Terms of Toleranced Parts
Allowance – the tightest fit
between two mating parts.
(The minimum clearance or maximum interference).
7. Important Terms of Toleranced Parts
Maximum Material Condition (MMC)
The condition of a part when it contains
the greatest amount of material.
Least Material Condition (LMC)
The condition of a part when it contains
he least amount of material possible.
8. Important Terms of Toleranced Parts
Piece tolerance
The difference between the upper and lower limits of a single part
(.002 on the insert in this example, .004 on the slot.).
System tolerance
The sum of all the piece tolerances.
For this example (.006)
9. Fit Types:
A Clearance Fit occurs when two toleranced mating parts will
always leave a space or clearance when assembled.
An Interference Fit occurs when two toleranced mating parts will
always interfere when assembled.
10. Functional Dimensioning
Functional Dimensioning begins with tolerancing the most important features.
Then, the material around the holes is
dimensioned (at a much looser tolerance).
Functional features are those that come in contact with other parts,
especially moving parts. Holes are usually functional features.
11. Occurs when dimensions are taken
Tolerance Stack-up from opposite directions of separate
parts to the same point of an assembly.
Dimensioned Dimensioned
from the from the
left. right.
AVOID THIS!!!
12. Avoiding
Tolerance
Stack-up
Tolerance stack-up can
be eliminated by careful
consideration and
placement of dimensions.
(Dimension from same side).
Better still, relate the two
holes directly to each other,
not to either side of the part.
The result will be the best
tolerance possible of ±0.005.
13. Today’s Lecture - Week 10:
Working Drawings
Generally, a complete set of Working Drawings for
an assembly includes:
1.) Detail Drawings of each non-standard part.
2.) An Assembly or Subassembly drawing showing
all the standard parts in a single drawing.
3.) A Bill of Materials (BOM).
4. A Title Block.
15. Assembly Drawing of a Piston & Rod containing 8 parts.
An assembly drawing normally consists of the following:
1. All parts drawn in their operating position
2. A parts list or Bill of Materials (BOM)
3.Leader lines with balloons indicating all parts.
4. Machining and assembly instructions
16. Detail Drawing of the retainer ring used to
fasten the rod to the piston.
20. …contain...
Title Blocks
A. Name & Address of Company
B. Title of the Drawing
C. Drawing Number
D. Names and dates of drafters,
checker, issue date ,
contract number, etc.
E. Design Approval
F. Additional Approval Block
G. Drawing Scale
H. Federal Supply Code for
Manufacturers
J. Drawing Sheet Size
K. Actual or estimated weight
L. Sheet Number
21. Parts Lists
The information normally included in a parts list is as follows:
1. Name of the part.
2. A detail number for the part in the assembly.
3. The part material, such as cast iron or bronze.
4. The number of times that part is used in the assembly.
5. The company assigned part number.
6. Other information, such as weight, stock size, etc.
26. Fastening Devices
Fastening is a method of connecting or joining
two or more parts together, using devices or
processes.
Mechanical Fastening –Process that uses
manufactured devices (Nuts and Bolts)
Bonding – Using material (Glue, Welding)
Forming – Using component shape itself (HVAC,
Tupperware, Velcro)
27. Threaded Fasteners
First Application of a screw thread was
developed by Archimedes to lift water.
1800’s Joseph Whitworth – English Standard
Screw Threads
1864 – US Screw Thread Standard
1946 – ISO Develops Metric Standard
1948 – US Develops Unified Standard
35. Machine Screws:
Machine Screws are finished with flat bottoms instead of chamfered corners.
A ½” Hex Head Machine Screw is used on this week’s assembly drawing.
