Reverse Engineering a Screwdriver
This project was completed during my sophomore year for the ‘Introduction to CAD and Machine Components’ course, and consisted of deconstructing a Black + Decker Li2000 screwdriver in order to model the gearbox in Creo Parametric. The project deliverables shown in this page include a product structure diagram, gearbox analysis, CAD model drawings of the parts and assembly, and finally an analysis of both the design for manufacture and assembly and the mechanical systems used.
Deconstruction
The image below shows the deconstructed screwdriver with all its components laid out, as well as the product structure diagram showing the organisation of the screwdriver’s components.
Gearbox Analysis
The gearbox component of the screwdriver is what makes the head turn. In this design, an epicyclic gear train was used. A model animation of the epicyclic gear train used in this screwdriver can be seen above.
This mechanism was used for multiple reasons. Since this type of gear train can fit within a small space with ease, it allows for a compact and portable design to be used. In addition to this, the gears are relatively cheap to manufacture.
In terms of power, the use of an epicyclic gear train makes sense as it can be used to produce a higher torque with a smaller supplied power. This means that the gear mechanism is energy efficient as it uses relatively little energy from the battery while creating a larger power output. This aids in both usability and durability, as it ensures that the battery lasts for a long time.
Another aspect of the epicyclic gear train that aids in durability is the fact that the load applied to the screwdriver is distributed along the different gears in the mechanism (6 planetary gears total). This prevents the quick wear and tear of the gear teeth, prolonging their usability.
Below are calculations analysing the gear ratios and pitch diameters of the epicyclic gear sets:
Gearbox CAD Drawings
The following are PTC Creo Parametric drawings of the gear train components. These were modeled based on physical measurements and observation and were used for the assembly shown in the video.
Design for Manufacture and Assembly (DFMA)
DFMA is a design approach that aims to simplify a design for it to be assembled efficiently, quickly, and with a low risk of errors. The B+D Li2000 screwdriver design shows evidence of DFMA.
There is a specific pattern of indents along the outer diameter of the motor casing that matches the pattern found in the inner diameter of the gear train casing. This ensures that there is only one possible way to connect these components, as the patterns must match for the pieces to fit.
In addition to these ridges, there are also two pairs of holes that line up on the gear train casing and the motor casing. These have to line up in order for a U-shaped securing pin to secure both pieces together. Since there is only one pair of holes on each casing, there is only one way these three pieces can be properly attached, preventing assembly errors.
The sun gear has a protrusion in its hole, while the motor axle having a matching cut. Thus, there is only one way they can be attached. This serves both a manufacturing and functional purpose, as not only does it ensure proper assembly, but also that the sun gear turns alongside the motor axle (at the same rate) rather than around it.
Explanation of Mechanical Systems
There are a few notable mechanical systems found within the screwdriver in addition to the epicyclical gear train. In the next few paragraphs, these will be explained and demonstrated through diagram analysis.
One of the mechanisms found in the screwdriver is the locking of the handle at a specific angle. The user can press a lock button to prevent the handle from turning. This works as the button has a tooth that is thicker than the rest, and also does not extend to the full depth of the lock button. Thus, when the button is released, the thick tooth does not make contact with the handle joint. However, when the user wishes to lock the position and presses the button, the thick tooth prevents the movement of the joint as it is now jamming the handle joint teeth.
The direction the screwdriver turns in can be reversed in order for the user to both screw and unscrew screws. This is done by reversing the connections of the motor terminals to the battery terminals. When the switch is turned to the left, motor terminal A connects to the negative end of the battery, while B connects to the positive end, making it spin. However, when the switch is turned to the right, this causes the opposite effect with motor terminal A connecting to the positive end while B connects to the negative. This causes the same spinning motion but in the reverse direction due to the reversed connections.
There is a white gear at the very end of the gear box as shown in the image to the right. This is not part of the epicyclic gear train, instead being used to switch between automatic and manual mode. Automatic mode works by allowing the gears to turn freely in the train, while manual mode locks the planetary carrier in place, thus preventing movement. In automatic mode, the white gear is not connected to the carrier, allowing the carrier to move. However, when manual mode is switched on, the white gear is moved upwards, causing it to interlock with the planetary carrier and preventing it from moving. This prevents the train from functioning by locking it in place, allowing the user to manually use the screwdriver.
Project Takeaways
This course was my first in-depth exposure to CAD software, so I learnt quite a bit regarding modelling and simulating parts and mechanical systems in Creo Parametric. In addition to this, this project also led to me developing a better understanding of certain design aspects that may go into the development of mechanical systems. Through analysis of the DFMA aspects as well as the mechanical systems themselves I was able to understand why certain parts were designed and assembled in specific ways, ensuring proper assembly and reducing the need for troubleshooting erroneous configurations while still fulfilling required performance metrics. Generally, it made me more aware of why products may be built a certain way from a manufacturing perspective. I will certainly take what I learnt from this course and project and apply it to future designs.