Choosing a Surface Finish ost, and aesthetics for

Choosing a Surface Finish ost, and aesthetics for

Choosing a Surface Finish

ost, and aesthetics for criteria. Out of these three components function has the greatest significance followed by both cost and appearance equally contributing. By e
this logic to determine the finish for each surface.

Did you know that surface finish is
measured in micro-inches, thats in-6 ?
In other words when someone uses
the lingo of a 32 surface finish that
means 32 micro-inches is the max
allowable distance between the
varying surface finish.

Got the surface
finish blues? Follow
the flow chart.

Sad Student

Dr. Doom

The surface finish availability for the University of Idaho Machine Shop is 250 micro-inches to 4 micro-inches.

Function
Part Use

Component
Strength

Is a smooth surface
required?
(Examine if/what the part
surface might be in contact
with, how does this affect
the component?)

YES: Use a Finer
surface finish. (i.e. 324in)in)in)

Tolerance
Considerations

Determine How Surface
Finish Will Effect The
Materials Strength

Unmodified Endurance Limit:
Se =0.504*SUT if SUT <= 200kpsi / 700MPa Se=100kpsi if SUT > 200kpsi
Se=700MPa if SUT > 1400MPa

Specify Material &
Define Ultimate
Strength: Sut

NO: Can use a
coarse surface
finish.
(i.e. 32-250in)in)in)

Determine New Endurance Limit
with Surface effects:
Se=Ka*Se
Where: Ka-Surface Effect
Se-Modified Endurance Limit
Se-Unmodified Endurance Limit

Determine Ka: Equation
Method
Ka=a*Sutb
Values for a and b Table 74.
See Below.

Cost $$
How Much Money Do You
Have??

Determine Which Type of Tolerance
Applies
Unilateral Tolerance: Tolerance in one
direction.
Bilateral Tolerance: Tolerance is applied
above and below.
Total Tolerance: The difference between max
and min.
Examine if the component surface needs
to be flat or flush with another surface.

YES: A Finer surface
finish will provide a
more flat surface.

Determine Ka: Graphical
Method
Use Figure 4.15 where:
Ks=Ka
See Below.

O
R

Analyze how surface
finish affects Material
Strength. If the Modified
Endurance Limit is not
sufficient choose another
surface finish method.

Aesthetic
s
Appearance can
contribute to surface
finish criteria as much as
cost.
Is this surface going
to be seen by eyes
that matter?

If youre Super Rich:

Choose any Surface
finish you want!!!

Use Figure 13-3
(below) for Surface
vs. Tolerance
Criteria

Feel, observe, and compare
template samples attached to
conceptualize various surface
finishes: How does 32 compare to
250?

If you live in the Real
World:

Using the cost figure
below,
find the best surface
finish that your cost
constraints will allow.

Yes: A finer surface finish
will be needed; cost may
come second to
appearance in this case.

References:
Mischke, Charles R., Joseph Edward Shigley. Mechanical Engineering Design. McGraw Hill. p. 278, 283, 2002.
Fatemi, Ali, Henry O. Fuchs, Ralph Stephens, Robert Stephens. Metal Fatigue in Engineering. Wiley Inter-science. p. 81, 2001.

Poster Done By: #8, #19, #44, #52

NO: Surface Finish
can be coarser,
because tolerance is
not an impeding
factor.

NO: This allows for an
opportunity to save on cost
and choose the cheapest
finish that still fits the function
constraints.

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