Lecture Week of March 27 Spring 2000

1
LectureWeek of March 27Spring 2000

• Screw Thread Terminology
• Reading Tap and Drill Charts
• General Tolerancing
• Reading Tolerancing Charts

2
Referring to the dimensions shown in the
sectional views of each drawing, how would you
phrase the callout in the contour view? Use all
applicable symbols.
?
?
?
.500
.500
.500
3
.500
.500
.500
4
Referring to the dimensions shown in the
sectional views of each drawing, how would you
phrase the callout in the contour view? Use all
applicable symbols.
?
?
?
5
.500
.500
6
Tapped Holes
How would you call out information in the
following contour view knowing that a finely
threaded, 3/8-inch diameter bolt is to be
screwed into it?
?
1. Determine which twist drill to use
before tapping the hole. Refer to
Appendix 15 in the text or the drill chart
on the following slide. Annotate that
information on the first line of the callout. 2.
On the second line of the callout, annotate the
tap size, screw type, class, and thread
feature using the correct format. Refer to
Appendix 15 in the text or the drill chart
on the following slide.
7
Drill Bit Tolerances
8
READING the Twist Drill Chart
9
Referencing Screw Threads in a Drawing
10
(No Transcript)
11
How would you call out information in the
following contour view knowing that a coursely
threaded, 3/8-inch diameter bolt is to be
screwed into it?
?
12
(No Transcript)
13
Anatomy of a Screw Thread
14
Screw Thread Types
15
Referencing Screw Threads in a Drawing
16
Screw Head Types
17
Standard Set Screws
18
Tolerancing
19
Tolerancing Formats

• Plus-minus format.

• Unilateral format.

• Equal bilateral format.

• Limit format.

• Unequal bilateral format.

20
Tolerance Considerations inWorking Drawings

• Size of Mating Parts
• Location of Features

21

• Tolerance of Size
• Bolt/Shaft to Hole Mating
• Feature Mating

What is the minimum size hole to be drilled so
that a .25-inch shaft fits?
22
Definitions

• Tolerance. The total amount a specific dimension
  may vary. The
• tolerance is the difference between the
  upper and lower limit of a
• dimension.
• Nominal size. A very general specification
  relating to size. For example,
• a bolt whose diameter measures .249 is
  considered as a 1/4 inch bolt.
• Basic dimension. The theoretically exact size
  from which the limits of
• size are determined.
• Maximum material condition. The condition where
  a feature of size contains
• the maximum amount of material within
  the state limits.
• Least material condition. The condition where a
  feature of size contains
• the least amount of material within the
  state limits.
• External feature. A feature of a part whereby
  when material is added the
• dimension increases.

23
Definitions (cont)

• Clearance. The difference in dimension between
  the least material
• condition of an internal feature and
  the least material condition
• of an external feature.
• Allowance. The difference in dimension between
  the maximum material
• condition of an internal feature and
  the maximum material condition
• of an external feature.

24
Mating Fits

• Basic Types
• 1. Clearance Fit. Tolerances are such that
  all mating parts will fit freely
• with no interference.
• 2. Transition Fit. Tolerances are such that
  mating parts may or may not
• fit freely.
• 3. Interference Fit. Tolerances are such that
  mating parts will not fit
• freely without force.

25

• Classes of Fits

Fitting Classes per ANSI
26

• Classes of Fits

Classes of fits per ISO
27
Reading the Tolerance Charts
Reference Systems

• Basic Hole System. The minimum limit of the hole
  is used as the basic
• feature size.
• Basic Shaft System. The maximum limit of the
  shaft is used as the basic
• feature size.

Example A part you are designing calls for a
class 2 press fit for a 1/4 inch pin. What will
be the tolerance limits of the hole and the
pin? See chart on page a31. The chart is
referencing the basic hole system. Therefore,the
minimum limit of the hole is .2500. The holes
maximum limit is the basic size plus .0006
inches or .2506. The minimum limit of the shaft
is .2500 plus .0010 or .2510. The maximum limit
of the shaft is .2500 plus .0014 or .2514.
Acceptable Methods to Express Tolerance
28
Appendix 7, pg A-9
29
Sizing Holes to Accommodate Bolts and Screws

• Example
• What size hole should be drilled to accommodate a
  1/2-inch diameter bolt used in a nonstructural
• application (?BOLT ? )? With reference to
  the hole tolerance on the following drill chart,
• calculate the allowance and clearance.
• Determine the type of application for the bolt.
  Is it structural such that the bolt would
• experience a significant shear stress or is it
  nonstructural where it is subject to small
  forces..
• Rule of thumb The hole size to accommodate a
  fastener in a structural application should
• be roughly 1/64-inch larger than the nominal
  diameter of the hole. For nonstructural
  applications,
• the hole size should be roughly 1/32-inch larger
  than the nominal diameter of the hole.
• Look at the drill Starrett drill chart. Notice
  in the print just beneath the title, Drills
  designated
• in common fractions are available in diameters
  1/64-inch to 1 3/4-inch in 1/64-inch increments.
• Though the drill chart does not give a number
  or letter for size at a 1/2-inch diameter, a
  drill bit with a 1/2-inch
• diameter exists. For a nonstructural
  situation, our hole to be drilled is .500 .031
  .531-inches in
• diameter. See the following slide for drill
  tolerances.

.500 .496
30
(No Transcript)
31
Tolerancing of Linear Features
With a general tolerance of .005, consider the
following mating parts. Are the dimensions
illustrated correctly to insure a clearance fit?
32

• Problems
• Tolerance stack. Chain dimensioning creates a
  stack of
• .015 over the length of each part.
• Transition fit. Depending on how the dimensions
  of each
• part varied during fabrication, the middle
  flange may or may
• not fit into the slot.

33
Solving the Problem 1. Basic Hole System.
Consider the dimension of the slot to be the
basic feature size. In other words, the slot
dimension in its maximum material condition
1.083. 2. Determine the desired allowance.
Lets say we want the flange to fit into the slot
with .03-inches of space to spare. The
allowance is .03-inches. 3. Calculate the
dimensional parameters. Allowance MMCslot -
MMCflange .03 1.083 -
MMCflange MMCflange 1.053 4. Add in and/or
redistribute the general tolerance. The general
tolerance is the difference between the upper
and lower limits .005 - (-.005) .01
The chain method of dimensioning may solve the
transition problem but not the stacking problem.
34
The Baseline format will solve the transition
and stack problem.