John Sharkey
Hernan Joglar
Page1
MIT 2.875
Mechanical Assembly
2.875
Mechanical Assembly and its Role in Product Development
Fall 2001 Semester
Project Report No 2
Pencil Sharpener
October 31, 2001
Hernan Joglar
John Sharkey
John Sharkey
Hernan Joglar
Page2
MIT 2.875
Mechanical Assembly
1. Selected Subassembly
Motor-gearbox subassembly. See Exhibit 1 for detailed description.
2. Function
The primary function of the motor-gearbox subassembly is to transmit rotational
motion from the motor to the cutter unit in an adequate combination of torque and
velocity whenever a pencil is fully inserted into the sharpener.
Secondary functions include (i) transmission of external loads applied from
pencils inserted into the sharpener to the sharpener base; (ii) transmission of
motor reaction torques into the sharpener base; and (ii) de-activation of the motor
safety switch whenever the pencil shavings cover is removed from the sharpener.
3. Key Characteristics
a) Motor shaft concentricity with hole in gearbox housing, and motor shaft
perpendicularity to the internal surface of the gearbox housing.
b) Pitch circle of the motor pinion gear tangent and coplanar to the pitch
circle of the driven gear input (large diameter).
c) Pitch circle of the driven gear output pinion tangent and coplanar to the
pitch circle of the intermediate gear input (large diameter).
d) Pitch circle of the intermediate gear output pinion tangent and coplanar to
the pitch circle of the output gear.
4. Root for the DFC of the Motor-Gearbox Subassembly
The root of the DFC is in the main frame of an assembly fixture that must assist
the mating of the motor and the gearbox housing. This fixture constrains all 6
degrees of freedom (dof) of the motor during the assembly motor-gearbox
assembly process. Note that subsequent to this assembly process, the motor
translation is unconstrained along the insertion axis (local z-axis), as no effectors
are employed to retain the motor within the sub-assembly. During final
assembly, the motor housing is fully constrained by the battery compartment
housing. The motor-gearbox thus comprises a Type 2 assembly process.
Note also that the completed motor-gearbox sub-assembly retains the following
two rotational degrees of freedom, even while inserted in the assembly fixture:
(i)
rotation of the motor rotor about the local z-axis, which drives
a 3-stage speed-reduction gear train;
John Sharkey
Hernan Joglar
Page3
(ii)
MIT 2.875
Mechanical Assembly
rotation of the safety lever about the local z-axis.
5. Datum Flow Chain
All of the KC’s mentioned above are delivered through features in the gearbox
housing and gearbox housing cover. A highly simplified, top-level DFC is
provided below. Due to the concentration and complexity of assembly features
embedded within the plastic, injection-molded gearbox housing, the detailed
integrated DFC (figure (e) in Exhibit 2) is excessively complicated and unwieldly.
It is therefore more constructive to use a sequence of DFC’s representing each of
the above four KC’s separately, as provided in Exhibit 2 figures (a) – (d).
Gear
All 4 KC’s
Gear
Gearbox housing
cover
Motor
Motor and gearbox housing
Fixture
6. Explanation of the DFC
Exhibit 1 provides cross-section drawings of the motor-gearbox subassembly, along with
digital photos of key details on various parts. Exhibit 2 provides a detailed DFC for each
key characteristic. Figure (a) in Exhibit 2 conveys the DFC for the mating of the
gearbox housing to the motor. Three distinct features are employed to fully locate (and
constrain) the gearbox housing relative to the motor. A small boss encompassing the
motor shaft, designated as the motor rim (item 2 in Exhibit 2-a; fig. 9 in Exhibit 1),
locates the X and Y coordinates of the gearbox by virtue of a centerline hole (item 5; fig.
5). Surface contact between the larger circular surface on the motor face (item 3; fig. 9)
and the mating hole in gearbox housing (item 6; fig. 5) locates the Z, X and Y positions.
Finally, the two holes in the motor face (item 4; fig. 9) locate the Z axis of the gearbox;
this peg-and-hole feature also serves as an anti-rotation device to resist motor backdrive
torque during operations.
As mentioned in section 4 above, a fixture is required to assemble the motor-gearbox
subassembly. The fixture represents the base of the DFC, and is required to fully
constrain the motor during assembly. In addition, a contact feature between the fixture
and the gearbox housing must be utilized to prevent excess loads from being imposed on
the motor/gear-train combination; the edge guides on the sides of the gearbox housing
(see fig. 4) are suitable candidates for this purpose.
John Sharkey
Hernan Joglar
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MIT 2.875
Mechanical Assembly
Figure (b) in Exhibit 2 next conveys the DFC for mating the plastic, driven gear to the
motor brass pinion gear and the gearbox housing. The driven gear is located within the
gearbox via two co-located features on the housing (figs. 1 and 7). The axle for the
driven gear (and the axle for the intermediate gear) is assumed to be pre-installed in the
gearbox housing, and is not an element of the assembly sequence. The combination of
the axle and the top surface of the axle stand (items 12 and 13 in figure 1) thus comprise
a peg-and-hole assembly feature for the driven gear, constraining the X, Y, Z, X and Y
coordinates. Note in particular that the driven gear must fully seat on the axle stand top
surface in order to engage the motor pinion gear and satisfy KC(b). The driven gear is
thus completely located. The gearbox cover then retains the driven gear on the axle stand
by virtue of contact between the driven gear block on the cover and the gear upper
surface(figs.1 and 6); the driven gear block (item 2 in figure 1) has a relatively large
clearance diameter.
Figure (b) also conveys the DFC for assembly of the gearbox cover to the gearbox
housing (figs. 1, 4, 6, and 7). The gearbox cover is located in the horizontal plane (X, Y
and Z coordinates) by edge contact between the cover outside rim and the housing
inside edge. The vertical position (Z axis) and in-plane rotations (X and Y) are then
constrained by a combination of features across the two parts. For example, the threaded
stanchion used to fasten the cover (subsequent to assembly) couples with the squared-off
contact end of the housing (figs 3 and 4) to constrain rotation about the Yaxis. The cover
contact end also locates the z coordinate, and prevents rotation about the X axis, by
surface contact with the housing.
Figure (c) in Exhibit 2 next conveys the DFC for mating the plastic, intermediate gear to
the pinion gear on the driven gear and the gearbox housing. The intermediate gear is
located within the gearbox via two co-located features on the housing (figs. 1 and 7) in a
fashion similar to that used for the driven gear. Note, however, that the gear stand height
is smaller for the intermediate gear, to allow engagement with the driven gear pinion.
This gear engagement is assured by fully seating the intermediate gear on its axle stand; a
small degree of gear rotation may be required to meet this condition. The intermediate
gear is retained by a large-diameter contact in gearbox cover, similar to that used on the
driven gear.
Finally, figure (d) in Exhibit 2 conveys the DFC for mating the plastic output gear to the
pinion gear of the intermediate gear and to the gearbox housing. The output gear is
located within the gearbox in a manner different from the driven and intermediate gears.
Note from figures 4, 10 and 11 that the driven and intermediate gears are identical parts
with center holes which mate to axles, whereas the driven gear incorporates a selfcontained axle shaft. The output gear axle shaft mates to the gearbox housing in a “pegand-hole” arrangement as shown in figure 1. The gearbox cover then retains the gear via
a large-diameter contact block (item 4 in figure 1).
The DFC in Figure (d) also denotes an electrical contract spring attached to the gearbox
cover (see fig. 4). This provides an activation switch which applies battery power to the
motor, when sufficient force is transmitted by a pencil through the contract switch. The
John Sharkey
Hernan Joglar
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MIT 2.875
Mechanical Assembly
“free” end of the electrical contact mates with the upper surface of the output gear. This
is accomplished by pre-loading the metal spring contact along the z-axis during the
assembly process.
7. Identification of Any Over-Constraints (or Risks of Over-Constraints)
The above description concerning the pre-loaded electric contact switch between the
gearbox cover and housing (more accurately, the output gear) comprises an overconstrained condition. Other areas at risk of being over-constrained include:
(i)
(ii)
(iii)
The motor-to-gearbox housing, in which the small diameter, inner peg-andhole mate (motor rim) has the potential of conflicting with the large diameter,
outer “peg-and-hole” combination (see fig 5);
Each of the gear-to-gear interfaces has the potential to become overconstrained if the gears are not fully seated on the axle stands;
In addition, the motor “anti-rotation” peg-and-hole features have the potential
to conflict with the motor concentricity features (motor rim);
8. Candidates for Redesign and Improvement


The motor-gearbox subassembly could be performed without use of a fixture if
minor modifications are made to the battery compartment. This would require an
alternate approach for completing the wiring circuits between the batteries and
motor, rather than using solder joints.
The motor-gearbox subassembly, as well as the final assembly of the whole pencil
sharpener, would be much improved by including an effector to retain the motor
mate with the gearbox housing. This could be accomplished by a variety of
means, such as retaining clips or fasteners.
John Sharkey
Hernan Joglar
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MIT 2.875
Mechanical Assembly
Exhibit 1. Gear and Motor Subassembly Description
3
5
2
1
6
4
13
17
10
14
9
18
8
Z
11
15
12
7
16
22
24
19
X
20
21
23
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Gear housing cover
Driven gear block
Intermediate gear block
Output gear block
Top lateral stopping peg
Top central stopping peg
Motor Location pegs
Gear housing
Motor gear
Driven gear input pinion
Driven gear output pinion
11.
12. Driven gear stand
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
Driven gear pin
Intermediate gear input pinion
Intermediate gear output pinion
Intermediate gear stand and hole
Intermediate gear pin
Output gear
Output gear stand
Bottom lateral stopping peg
Bottom central stopping peg
Top part of motor compartment in gear housing
Hole in gear housing for motor location
Motor
Figure 1a. Open view of the Motor and Gear Subassembly
John Sharkey
Hernan Joglar
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MIT 2.875
Mechanical Assembly
Figure 2. Exploded view of the Motor and Gear Subassembly.
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Hernan Joglar
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MIT 2.875
Mechanical Assembly
Figure 3a. Open view of the Motor and Gear Subassembly, cover half the way
closed.
Figure 3b. Open view of the Motor and Gear Subassembly, cover in closed position.
John Sharkey
Hernan Joglar
Page9
Motor rim
MIT 2.875
Mechanical Assembly
Gear housing Cover
Output gear
Driven gear
Motor
Intermediate gear
Motor and gear housing
Figure 4. Photograph of motor and gear subassembly, exploded view
Motor compartment
top surface
Output gear hole
Motor location hole
Motor location pegs
Figure 5. Bottom view of the gear housing
John Sharkey
Hernan Joglar
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MIT 2.875
Mechanical Assembly
Top Stopping pegs
Edge
Output gear block
Intermediate gear block
Edge
Driven gear block
Contact end
Figure 6. Gear housing cover
Rim
Contact surface
Figure 7. Top view of the interior of the gear housing
John Sharkey
Hernan Joglar
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MIT 2.875
Mechanical Assembly
Pitch circle of driven gear
input pinion
Bottom Stopping pegs
Contact surface
Rim
Pitch circle of motor gear
Gears top surfaces
Figure 8. Top view of the gear train
Motor holes
Motor rim
Figure 9. Top view of the motor
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Hernan Joglar
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MIT 2.875
Mechanical Assembly
Pitch circle of intermediate
gear input pinion
Gear holes
Pitch circle of driven gear
output pinion
Gear bottom
surface
Figure 10. Bottom view of driven gear and intermediate gear
Output gear
shaft
Figure 11. Output gear
John Sharkey
Hernan Joglar
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MIT 2.875
Mechanical Assembly
Exhibit 2. Detailed DFC’s
a) Motor shaft concentricity with hole in gearbox housing and its
perpendicularity to the internal surface of the gearbox housing.
z
Z, y , x
4
Motor
Perpendicularity KC
KC
3
1
8
2
6
Gearbox
Housing
Concentricity KC
X ,Y
X
(6 dof)
Key
Fixture
1
2
3
4
5
6
7
8
9
7
Motor shaft centerline
Motor rim
Motor top surface
Motor holes
Hole in gearbox housing for motor location
Motor compartment top surface
Motor location pegs
Motor base
Gearbox edges guides
5
9
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Hernan Joglar
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MIT 2.875
Mechanical Assembly
b) Pitch circle of the motor gear tangent and coplanar to the pitch circle of
the driven gear input pinion.
Motor gear
pitch circle
Gear hole
Driven gear
Pitch circle
driven gear
input pinion
Motor
Top surface
Gear hole
Tangency & Co-planarity KC
Bottom
surface
(6 dof)
Z ,  y ,  x X ,Y
X Driven gear
(6 dof)
Driven gear
stand
Fixture
Bottom stopping pegs
and contact surface
Driven
gear
Top Stopping pegs
block
and contact end
Z, y , x
Edges
Gearbox housing cover
X , Y , Z
pin
Gear
Housing
Rim
John Sharkey
Hernan Joglar
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MIT 2.875
Mechanical Assembly
c) Pitch circle of the driven gear output pinion tangent and coplanar to the pitch
circle of the intermediate gear input pinion.
Intermediate
gear
Pitch circle
driven gear
output pinion
Pitch circle
intermediate gear
input pinion
Gear hole
Driven gear
Top surface
Gear hole
Tangency & Co-planarity KC
Bottom
surface
Top surface
Bottom
surface
Z, y , x
X ,Y
X Driven gear
X
Z, y , x
Bottom stopping pegs
and contact surface
Intermediate
gear block
pin
Intermediate Driven gear
gear pin
stand
Intermediate
gear stand
Driven
gear
block
X ,Y
Top Stopping pegs
and contact end
Z, y , x
Gearbox
Housing
Rim
(6)
X , Y , Z
Motor
(6)
Edges
Fixture
Gearbox housing cover
John Sharkey
Hernan Joglar
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MIT 2.875
Mechanical Assembly
d) Pitch circle of the intermediate gear output pinion tangent and coplanar to
the pitch circle of the output gear. In this case, the output gear is overconstrained in the Z direction.
Intermediate
gear
Pitch circle
intermediate gear
output pinion
Output gear
Pitch circle
output gear
Tangency & Co-planarity KC
Top surface
Gear hole
Gear Shaft
Bottom
surface
Top surface
Bottom
surface
Z, y , x
X ,Y
X
Z, y , x
Intermediate
gear pin
Intermediate
gear block
Bottom stopping pegs
and contact surface
Electric
Z, y , x
contact
Top Stopping pegs
lever
and contact end
X
Output gear
stand
Intermediate
gear stand
Z
X ,Y
Output gear
hole
Gearbox
Housing
Rim
(6)
X , Y , Z
Motor
(6)
Edges
Fixture
Gearbox
housing cover
Output gear
block
John Sharkey
Hernan Joglar
Page17
MIT 2.875
Mechanical Assembly
e) Integrated Datum Flow Chain for the Motor-Gearbox sub-assembly
z axis pre-load
Motor
Gearbox
Base
Flex
Contact
Bored
Stanchion
Output Gear
Upper Surface
Output Shaft
Center
Hole
Bored
Stanchion
Output Gear
Outer Pitch Circle
Bored
Stanchion
Squared
Step
KC #3
Inner Pitch Circle
Outside
Perimeter
Center
Hole
Bored
Thin Stanchion
Stanchion
Intermediate Gear
Integrated
Datum Flow Chain
for Motor/Gearbox
Sub-Assembly
Outer Pitch Circle
5 dof
KC #2
Inner Pitch Circle
z , x , y
Hole &
Inside
Perimeter Low
Stand
x , y , z
Squared
Notch
Peg on
Stanchion
Thru Hole
5 dof
Driven Gear
5 dof
KC #1
Axle &
Med Stand
Axle &
Tall Stand
Centering
Hole
Threaded
Stanchion
Safety Lever
Center
Hole
Cylindrical Base
With
Dimples
5 dof
Outer Pitch Circle
Pinion
& Rotor 5 dof
Top
Centering
Base
Boss
x,y
Centering
Boss
Top
x , y , z , z
Surface
W/Holes
Edge
Guides
Motor/Gearbox Housing
6 dof
Box
Guides
Note: 5 dof = { X , Y , Z, X , Y }
Base
Center
Hole
Assembly Fixture
Motor Case & Rotor
w/pinion gear
John Sharkey
Hernan Joglar
Page18
MIT 2.875
Mechanical Assembly
Exhibit #3
Pencil Sharpener Assembly and Parts List
18
23
4
17
6
20
19
10
1
5
11
7
8
9
12 11
3
13 14
Figure 13. Pencil Sharpener taken apart
15
2
John Sharkey
Hernan Joglar
Page19
MIT 2.875
Mechanical Assembly
Function of Each Part
P/N
Qty
Name
1
2
1
1
Sharpener case
Electric motor
3
1
4
1
Gear subassembly
cover
Base
5
1
Safety lever
6
1
Shavings receptacle
7
8
1
1
Output Gear
Intermediate Gear
9
1
Driven Gear
10
1
Pencil guide
11
12
4
1
13
14
15
1
1
1
Base Screws
Gear subassembly
cover screw
Blade nut and bolt
Blade
Cutter unit
16
1
17
1
18
19
20
3
8
1
21
1
22
2
23
1
Gear and motor
subassembly housing
Battery compartment
cover
Cables
Contacts and rivets
Battery Compartment
housing
Lubrication (red jelly
on gears)
Rubber feet
(Underneath of base)
Gear subassembly
housing
Name Function
Provides structural support and protection
Transforms battery energy into rotary
movement
Provides protection and structural support to
gear shafts, and contacts
Provides protection and structural support to
internal parts
Prevents operation when the shavings
receptacle is not in place
Storages wood shavings that come out as
pencils are sharpened
Transmits rotary movement to blade stand
Transmit Rotary movement from driven gear
to output gear
Transmit Rotary movement from Electric
motor to intermediate gear
Guides pencils into the hole where they are
sharpened
Fasten the sharpener base to the sharpener case
Fasten the gear compartment cover to the gear
compartment
Attach the blade to the cutter unit
Sharpen the pencils
Provide structural support and rotary
movement to the blade
Provide structural support and alignment to
gears and electric motor
Provides protection to batteries and supports
contacts
Transmit electric signal
Transmit electric signal
Provides support to batteries
Reduces friction and noise in gears functioning
Provides support to the unit and necessary
friction so that it doesn’t slip.
Provides structural support and protects the
gear train and electric motor