PUPS 1: Pliers

For this problem set, I considered a basic pair of pliers and emphasized durability and robustness in use for the design.

Free-body diagrams and error motion identification for pliers
FRDPARRC Table for a pair of pliers

One piece of feedback that I received from the peer review discussion (see below) was that the clamping force exerted by the jaws and the amount of torque that can be transmitted are related functional requirements. While this is true, my original rationale for separating the two was to try to capture the different things people do with pliers. In the ensuing discussion, I thought of another possible loading configuration when one tries to pull a nail or staple by gripping and twisting. This induces bending and torsional loads on the jaws and introduces error into jaw alignment. In fact, I now remember that I have done this to a pair of cheap pliers I own (see below). This is a reminder that we need to consider not just intended loading configurations, but also configurations that result when the machine is abused/misused!

Permanently sprung plier jaws; this permanent deformation adds to the misalignment caused by pivot clearance! (Photo by Shien Yang Lee, CC-BY-SA 4.0)

SNG 1: SOG PowerLock Multitool

SOG PowerLock Multitool (Photo by Shien Yang Lee, CC-BY-SA 4.0)

The star of this week’s Seek and Geek is a multitool I own. It uses a patented configuration of levers to achieve greater mechanical advantage. Compared to conventional pliers, which are made up of two class 1 levers pivoting about the same point, the handles of this tool pivot about a pair of pins (with formed heads) attached to the horizontal link in the image and attach to the jaws via the partially obscured button head socket cap screws.

Sprockets (Photo by Shien Yang Lee, CC-BY-SA 4.0)

As a 5-bar linkage, this mechanism should have 2 degrees of freedom according to Gruebler’s Equation. But the designers have incorporated a pair or sprockets that couples the motion of the handles, thereby eliminating one degree of freedom. I think this is to maintain the familiar usage pattern that people have come to associate with pliers and to facilitate the relatively fine manipulation of small parts that needlenose pliers often get used for.

I have noticed a couple of issues from using this tool. The greater mechanical advantage obviously comes with the side effect of requiring larger hand movements to move the jaws by a given amount. “Twice the cutting and gripping force” sounds great — it was partially why I bought it in the first place — but I have since come to realize that the limiting factor with using pliers is usually not how hard you can squeeze but how far you can open the jaws with one hand.

Another annoyance is the backlash introduced by the additional joints between handles and jaws. Normal pliers, even ones with worn pivots, primarily have out-of-plane free play. These have noticeable backlash around the actuation axis, which can be frustrating when trying to fine-tune gripping force on compliant or fragile parts.

Planar Exact Constraint Toy Fabrication

I intend to make this toy out of plywood and wooden dowel pins. A combination of the Makerworkshop being closed for the week and the desire to get some peer review feedback before building the final version led me to fabricate a works-like prototype from cardboard.

Planar Exact Constraint Toy Mock-up (Photo by Shien Yang Lee, CC-BY-SA 4.0)

One potential issue that this mock-up highlighted to me was the undesirable out-of-plane tipping caused by the additional weight of the fixtured object moving the system’s center of mass forward. In this configuration, the object is essentially held on by friction against the pins, which is non-ideal. The magnitude of this effect will be smaller in the final version since the fixtured object will have a much smaller mass relative to the plywood board. I think I will be able to correct this tipping in the final version by calculating the moment contributions of the board and the fixtured object, and offsetting the screw eye attachment point backwards.

Planar Exact Constraint Toy Design

The idea behind exact constraint design is to build things that are simple to analyze. When an object is constrained by exactly the same number of elements as there are degrees of freedom, it occupies a tranquil middle ground between the complexities of motion and elasticity. This obviously makes the design engineer’s life easier, and hopefully encourages more analysis and less “let’s just build it and see how it turns out”.


“DSCN9828” by mtneer_man is licensed under CC BY-ND

The design of this planar exact constraint toy is inspired by the pegboards used to organize tools. Most pegboards are mounted securely in workshops and hipster lairs, where the walls don’t move appreciably. But what if you wanted to design a pegboard for use on a boat? or just someplace prone to earthquakes? This toy allows you to explore how far a particular peg configuration would allow you to tip the board before your object falls off.

I plan to make my toy from a small sheet of plywood in which I will drill a 1″-pitch grid of 3/8″ holes. The user tests out different support configurations by inserting supplied dowel pins into these holes. A square block of wood will serve as the planar object to be fixtured. The board can be suspended by the attached screw eye and rotated about the pivot point to vary the direction of the gravitational force.

I wrote a simple MATLAB script to model the stability of a planar object supported by three pins. The script takes the coordinates of the pin contact points as well as the orientation of the weight vector (theta) as inputs and returns the reaction force developed at each contact point. The values of theta that correspond to sign changes in the pin reaction forces should be interpreted as the limits of stability for the support configuration. My MATLAB model neglects friction, both between the object and support pins, as well as between the object and the underlying board surface, so one can expect the analysis results to vary slightly from actual system behavior.

Standing Desk White Paper

Coke can standing desk (Photo by Shien Yang Lee, CC-BY-SA 4.0)

This is a (non-adjustable) standing desk I built my sophomore year of college from empty soda cans and a shelf borrowed from my kitchen cabinet. It cost me virtually nothing to built and was extremely portable. When it came time to move out of my apartment, I simply removed the gaffer tape, recycled the cans, and returned the shelf. It, therefore, addresses two major problems I have with commercially available standing desks — cost and portability. However, its non-adjustable nature meant I was subjected to the tyranny of being forced to stand all the time. Healthy and fun when I was working; less so when I was trying to watch the latest edition of Top Gear or call my friends on Skype.

I am going to take Precision Product Design as an opportunity to build a standing desk that is not only adjustable, but programmable, so that it can (sometimes) have a mind of its own and gently enforce some healthy proportion of standing time.

Another interesting functional requirement I have specified is portability. I will be moving back to Singapore after completing my program at MIT in August, and I would like to be able to bring this machine with me. My vision is for the core drive and locating elements of the mechanism to be separable from bulky but easily replaceable structural elements like the tabletop. This requirement is also pushing me towards building a table-top appliance (e.g., Varidesk Pro Plus) rather than a standalone desk.

 

FRDPARRC table in notebook (Photo by Shien Yang Lee, CC-BY-SA 4.0)

Here is a FRDPARRC table outlining some additional functional requirements. The plan is to keep evolving this document for the next few weeks as I get feedback, and eventually to consolidate it in an electronic spreadsheet.