Concepts I
Tolerances, Limits, Fits and
Surface Finishes
Tuesday 6th Week
Kogan #2 Redo Feedback
50% of those eligible redid assignment
n Those who did not continue to make poor decisions
All but one person of those attempting
assignment again raised their grade to a C
– excellent.
1
Tolerances
(In week 2 we spoke briefly about Tolerances)
No feature of a component can be perfect (i.e., no
surface flat, no hole round etc), because of the manufacturing process
Thus, when dimensioning any feature
n Basic nominal dimension
n Permitted variability (tolerance)
n Dimensions should be given in as large a tolerances as possible without interfering with the function of the part to
reduce production costs.
20,15 20 + 0,15
19,99 - 0,01
Limits Bilateral
Limits
Example
§ Journal bearings are designed to operate at high rotational speed
§ If the clearance between the inner and outer diameter is to small the bearing will seize
§ If the clearance is to big the shaft will vibrate
§ Limits on the size of the shaft and hole provide correct operation
Clearance
Nominal diameter = 20mm
Close running fit = H8/f7
f7: Shaft = 19,980 to 19,959
H8: Hole = 20,000 to 20,033
Clearance = 20 to 74 micron
f20,00 H8/f7
2
lerances
poke briefly about Tolerances)
e., no
he manufacturing process
es as possible without interfering with the function of the part to
20 ± 0,01
Unilateral
Limits
tational speed
ter is to small the bearing will seize
ect operation
Clearance
2
Limits (continued)
Example
§ Spool valve has a shaft that translates
§ This time the clearance should be a sliding fit
Clearance
Nominal diameter = 20mm
Sliding fit = H7/g6
g6: Shaft = 19,993 to 19,980
H7: Hole = 20,000 to 20,021
Clearance = 7 to 28 micron
f20,00 H7/f6
Ranges in Tolerances
Range of tolerances are a function of:
As IT # increases so does the range
As nominal size increase so does the range
§ Range of measuring tool accuracy
§ Range of manufacturing processes
§ Range of Limits and fits
§ Range of material
Process Tolerance range
Lapping IT4 – IT 5
Reaming IT6 – IT10
Drilling IT10 – IT11
Casting IT9-IT11
Nominal size range ISO Tolerances in microns
Over Up to
IT1 IT2 IT3 IT4
-- 3 0,8 1,2 2,0 3,0
3 6 1,0 1,5 2,5 4,0
6 10 1 1,5 2,5 4,0
3
ued)
Clearance
al diameter = 20mm
fit = H7/g6
aft = 19,993 to 19,980
le = 20,000 to 20,021
nce = 7 to 28 micron
s
3
Fits and Tolerances
Clearance fit [provide similar running performance with suitable
Interference fit [create an internal stress that is constant through
size ranges]
n Locational interference
n Medium drive fit
Transitional fit [use for location purposes]
[See Figure 5.11 in text book]
lubrication]
n Loose running fit
n Free running fit
n Close running fit
n Sliding fit
Geometry and Tolerances
Occasionally geometry itself needs to have tolerances set
n Flatness = a surface is flat when all elements are in one plane
n Straightness = a surface is straight if all it elements are in a straight line
n Parallelism = a surface or a line is parallel when all of its points are equal distance from a reference plane
4
Geometry and Tolerances
(continued)
Labeling
n Tolerance box = contains datum label, or information about the tolerance
n Leader line = touches feature or datum and tolerance box
n Datum = some geometries require a reference surface (e.g., parallelism) we label these reference surfaces as a
datum
10
20
Surface Finish
Surface texture is the variation in the surface including
roughness, waviness, lay and flays
Roughness describes the finest irregularities in the surface
Zoom in Roughness
Average
n
Ra yi
n i1
5
eometry and Tolerances
(continued)
abel, or information about the tolerance
datum and tolerance box
e a reference surface (e.g., parallelism) we label these reference surfaces as a
n in the surface including
flays
st irregularities in the surface
Roughness
Average
Measures:
Ra = average peak height Rp = largest peak height Rv = lowest peak valley
Rt = peak to valley distance
5
Surface Finish (continued)
d
Labeling
a
b
x c
10 f e
0,3 A
Milled
U Ra max 3,1
L Ra 0,9
20
a = 2D parameter 1
b = 2D parameter 2 c = 2D parameter 3
d = manufacturing method e = lay pattern
f = allowance
x = not allowed
