The Electronic Pencil Sharpener










By:  Kyle Hallenbeck

Instructor:  Shad

Date:  8/3/06








            A pencil sharpener, what is its purpose?  The dictionary may define a pencil sharpener as “a device in which a pencil is inserted into and is sharpened within a finite amount of time”.  The main mechanisms in the particular pencil sharpener that has been modeled for this project are a set of gears that are rotated by a motor.  These gears work together to rotate a blade holder which is strategically placed in such a way that when the blade holder rotates, the blade itself rotates.  This causes a double rotational blade that enables the pencil to be sharpened quickly and evenly.  Throughout this write up, a short discussion on how each part was made as well as problems faced with each part and the total assembly of the sharpener will be given.  A discussion on a few kinematic measurements as well as a list of calculated mass properties will also be shown in the paper.


































Solid Modeling and Detailing


          Each component of the electric pencil sharpener was modeled using approximate millimeter measurements using a ruler.  A description of how each part was modeled as well as any difficulties faced with modeling the part follows.



          The blade was the first component of the sharpener modeled.  First, a simple circle was extruded and then a helical protrusion was rotated around the cylinder making the sharp edge.  The protrusion was patterned to dill the cylinder with sharp cutting blades.  The gear type protrusion on the top was then created by extruding a circle upwards and cutting out the shape of the gear, and patterning it around the circle.  A hold was cut through the length of the blade for a rod to go through.  Colors were assigned accordingly.  There were no difficulties in modeling this particular component of the sharpener.  A simple rod was created by extruding a circle a measured distance.  The purpose of the rod is to connect the blade to the blade case.



            The blade casing was the next component created.  The purpose of this component is simply to hold the blade and create the secondary rotational motion for the blade.  The modeling of this component could take a very long time to describe, so it will be simplified.  A base cylinder was created and through lots of extrudes, cuts, and revolves the component was created.  The main difficulties in creating this blade case were simply the detail of the cuts and extrudes.  This problem was easily solved using multiple datum planes and references and choosing the proper cuts and extrudes.



            The “blade rotor” was the next component modeled.  The purpose of this piece is so the blade with the blade casing can rotate about the gear cut outs.  This piece was no problem to model.  A simple circular extrude with some extra circular cuts as well as a gear shaped cut was all it took to finish off this component.











The next three components modeled were the three gears necessary for the motion of the sharpener to work properly.  The first gear shown is the gear that attaches to the blade case.  This is the gear that rotates the blade case which in turn rotates the blade.  This piece was easily modeled by extruding a circle and then cutting the pattern of the teeth.  The cut was patterned around the gear and then the attaching piece was extruded from the center of the gear.  The second gear, shown in black, is

one of the two gears in the sharpener that translate the rotational motion to the blade.  This gear was created exactly the same way as the first.  The third gear, shown in white, is the second gear translating rotational motion to the blade.  This gear was also created the same.  There were no problems in creating the gears except for matching the teeth shapes to fit inside one another.  This was done through trial and error.








          The next component modeled was the motor that actually gives power to the system.  This piece was extremely simple to make.  A simple cylindrical extrude along with a small circular extrude on top was all that was needed.  Two electrical connecting posts were extruded off the back for small detailing.





            This component is the gearing that attaches into the motor.  This is the starting gear powered by the motor that turns the rest of the gears.  Another two cylindrical extrudes followed by a gear cut was all that was needed to complete this component.  The gear cut was then patterned around the cylinder to complete the piece.



          Another simple, decorative component created was the power connector.  This was just a simple rectangular extrude followed by another of the same.  A small circular cut was made with a smaller cylindrical extrude inside of it to make the connection area.  No problems were faced in making this component.





          One of the most difficult components modeled in this project was the casing.  This required a blended protrusion of a previously sketched outline of the casing.  The sketch was brought into the blend and then the same sketch was brought in again except scaled to 0.9.  It was then strategically places and the blend was completed.  From this cuts could be made to create the hollow inside.  Normally the shell command would have been used but because of the geometry of the casing it couldn’t be used with ease.  Multiple cuts were made instead to create the needed shelling of the housing.  Extrudes were then used to create the screw placements.  A cut was used to create the decorative groove.  Rounding on the edges of the housing was used to make the casing look sleeker and more aerodynamic.  Other decorative features were created by extrudes and cuts.  The housing was then mirrored to make a full case and a cop was saved.  The copy was opened and the half that was already created was cut off to create the opposite half of the housing.  The main problems with this piece were solved with plenty of datum planes and references.




            The shavings holder was next on the list.  A sketch was created from one half of the housing and saved as the outline of the side of the shavings holder.  This sketch was then brought into a new model and extruded the length of half of the housing.  Proper cuts were made as well as a cosmetic grove.  The holder was then mirrored to create the full version of the component and rounds were used to give it the sleek and aerodynamic look.






            The screws were the last components created.  These were easily created using a cylindrical extrude with a circular extrude for the heads.  A helical sweep cut was used in order to create the cosmetic/decorative threads in the screws.  No problems were seen in creating these screws.  The use of the screws in the model was simply for decoration reasons.




            Drawings of certain components can be seen in Appendix A.



            An assembly of the blade, blade gear, blade casing, and the “blade rotor” was created separately from the full assembly for simplification.  First the blade casing was brought into the assembly and fixed.  The blade was then brought in and aligned with the axis that runs through the two holes in the blade casing.  This was aligned using a pin connection.  A surface alignment was then used in order to stop rotation of the pin connection.  The blade rod was then aligned on the same axis on the blade casing, and then mated with a surface on the casing to be folly constrained.  The blade gear was easily assembled by aligning two axis together and mating surfaces.  A new assembly was created and the “blade rotor” was brought in and fixed.  The previously assembled blade, blade casing, and blade gear was then brought in.  A pin connection was then made by aligning the center axis of the blade casing and blade gear with the center axis of the “blade rotor”.  The connection was then finalized by aligning the surface of the blade casing to the inside surface of the “blade rotor”. 



The application was then changed to mechanism in order to apply the gear connection and motion.  The gear on the blade was chosen as the first gear with the gear cut out on the “blade rotor” as the second gear.  The pitch diameters were chosen accordingly by approximations.  The application was then set back to standard and the gear connection was tested.  A drastic problem in the gearing in this project is that the pitch diameters were hard to match up.  The only solution to this problem was guess and check.  Eventually the gear worked properly completing the sub-assembly.


            A new assembly was started and one half of the housing was brought in and fixed.  A simplified description will be used to talk about the final assembly of the sharpener because a detailed description would be annoying and monotonous.  The basic assembly required many, many datum planes and datum axis.  A visual of how many datum references were needed can be seen in the fifth picture down.  The pre-assembled blade and blade casing was brought in and was aligned to the axis through the pencil hole in the housing as well as a created datum.  This fixed the blade to the housing.  The two working gears were then brought in and aligned as needed to created datum axis’s and created datum planes.  The motor and motor gear was aligned and fixed the same way.


 The assembly was then brought into the mechanism application and the gears were connected accordingly.  A motor was placed on the axis of the motor component and tuned accordingly.


  The second part of the housing was then brought in and mated to the other side closing the housing around the working components.  The screws were then brought in and aligned to the axis of the holes and mated to the surfaces of the housing.  Finally, the shavings holder was aligned and mated into the housing completing the assembly of the entire pencil sharpener.


            A movie of the fully assembled model in motion can be seen in Appendix B.














Mechanism Modal Analysis


            All of the components modeled in this project were modeled as plastic (polyurethane) except for the blade, gear and blade rods, and screws which were modeled as stainless steel.  The density for the plastic pieces used in the mass properties is 1 x 10-7 kg/mm2 and 8.03 x 10-7 kg/mm2 for stainless steel.  A model analysis was done in ProE and the mass properties of the entire model with respect to the center of gravity coordinate system created were calculated.  Below is a list of the mass properties calculated.

VOLUME =  4.4612143e+05  MM^3




SURFACE AREA =  2.1878462e+05  MM^2




AVERAGE DENSITY =  1.2220971e-07 KILOGRAM / MM^3




MASS =  5.4520372e-02 KILOGRAM




CENTER OF GRAVITY with respect to CSYS_COG_120 coordinate frame:

X   Y   Z     0.0000000e+00  0.0000000e+00  0.0000000e+00  MM




INERTIA with respect to CSYS_COG_120 coordinate frame:  (KILOGRAM * MM^2)



Ixx Ixy Ixz  9.7060353e+01  0.0000000e+00  0.0000000e+00

Iyx Iyy Iyz  0.0000000e+00  1.6202420e+02  0.0000000e+00

Izx Izy Izz  0.0000000e+00  0.0000000e+00  1.6817488e+02




INERTIA at CENTER OF GRAVITY with respect to CSYS_COG_120 coordinate frame:  (KILOGRAM * MM^2)



Ixx Ixy Ixz  9.7060353e+01  0.0000000e+00  0.0000000e+00

Iyx Iyy Iyz  0.0000000e+00  1.6202420e+02  0.0000000e+00

Izx Izy Izz  0.0000000e+00  0.0000000e+00  1.6817488e+02





I1  I2  I3   9.7060353e+01  1.6202420e+02  1.6817488e+02





       1.00000        0.00000        0.00000

       0.00000        1.00000        0.00000

       0.00000        0.00000        1.00000




ROTATION ANGLES from CSYS_COG_120 orientation to PRINCIPAL AXES (degrees):

angles about x  y  z   0.000          0.000          0.000





R1  R2  R3 4.2193109e+01  5.4514312e+01  5.5539397e+01  MM



























Mechanism Kinematics


            The main kinematic analysis done for this project was angular velocity measurements.  There were three angular velocity analysis done one for the blade it self, one for the blade casing, and one for the actual motor gear.  The reason for these analysis was to find out the rotational speed of the blade and blade casing with respect to the speed of the motor with the given power to it.  Below are plots of the angular velocity of the blade, blade casing, and motor.








            From these plots it can be seen that the angular velocity with the step input from the motor for the motor gear is a constant1700 deg/s.  This makes sense in that the gear is very small with a lot of power behind it.  The plot in blue shows that due to the motor gear the blade spins at about 53 deg/s which makes sense because the gears that spin before it are much bigger than the motor making the system a reduced gearing.  Finally the blade casing rotates at about 18.75 deg/s.





















Appendix A:  Drawings


Appendix B:  Movie File