2.77 Seek and Geek #2: Branch Cutter

Seek and Geek #2: Branch Cutter

I found this garden cutter in my living group's backyard and it has a few design features that I find interesting.

It claims it has Power-Lever® Technology, and cuts 2X easier. I ask, 2X easier than what, exactly? A normal lever? We'll find out!

The handles have their own labels to show off their ergonomic design, presumably to catch the eye of the passerby at Home Depot. 

The handles are indeed ergonomic, as the whole thing was comfortable to hold with one hand (My hand was constraining forces and moments and still felt comfortable! )

This can cut branches up to 1.75" in diameter. 

A more detailed look at the mechanism at the end shows off this Power-Lever® Technology. There are three linkages in this system: the left handle/right blade, the left blade shown in black, and the right handle. There is a pin joint holding the left blade and the left handle/right blade together, and a pin joint holding the left handle and the right handle together. The left blade and the right handle are connected with a slotted prismatic joint, which I believe is the cause of this 2X Easier claim. 

Here is an underside view to get a better look at the slot. This is a planar three-bar linkage, with two pin joints (each allowing 1DOF: rotation) and one prismatic joint (allowing 2DOFs: translation in one dimension, and rotation). This resultant mechanism has only 1-DOF, coupled through all 3 linkages. As the handles close together to cut, the right handle linkage (shown here on the left) pivots about its joint with respect to the left handle/right cutter linkage (shown here on the right). The left cutter (in black) is pulled along for the ride through the slotted pin joint with the right handle, but is able to slide along the pin instead of moving completely.

Through this nonlinear motion, mechanical advantage is achieved, and the black left cutter ends up moving about half as fast as the handles do with respect to each other, like gearing. And, like gearing and because power is conserved, this offers about 2X the cutting force as you would have with a regular scissor-type linkage. Cool! 

In addition to the linage design offering mechanical advantage, the leverage you can get on this thing from the grips is quite large, and the ellipsoidal tubes are quite stiff while beasting this thing against a tree branch. It feels solid and sturdy, even though there is a length of about 18" from your hands to the pin joints. 

The pin joints are held down and supposedly preloaded by nylon locknuts. 

Here's a closer look at the chopper. The pressure area on the black left cutter is quite small considering the mechanical advantage this tool provides, but steel is used for the blades, which can handle all that force. The branch will be effectively sheared by this cutter. 

The other side, the right cutter, does not have a small pressure area, but is flat in order to provide a groove in which the branch can sit as it's being cut. 

The difference in pressure area between the black left cutter and the right cutter ensures that only the black cutter is seeing the highest stress, and so if one were to sharpen this tool, only the black one would need TLC. 

2.77 Seek and Geek #1: Apple Lathe

Seek and Geek #1: Apple Lathe

I discovered this wonderful tool in my living group's kitchen. It's used to simultaneously peel and core an apple. 

The lathe can be attached to any smooth surface using a suction base. By turning the handle, a channel underneath the lathe pulls up and creates negative pressure that keeps it against the surface. 

Here, my girlfriend models the machine in action. The apple is held by three skewers at the end of a screw. The screw is actuated by a handle at the back end. By turning the screw, the apple both spins and moves forward, enabling the spring-loaded cutting tool to peel the apple, and the blade at the far left to remove the core. 

This spring-loaded tab disengages the screw and enables it to be pushed back without having to turn. 

By letting go of the tab, the screw engages and must turn in order to move linearly. 

St. Venant's principle in action! The screw is supported by two linear bearing blocks. The distance between them is equal to at least 3-5 screw diameters! No jamming occurs, the motion is smooth. 

The blade at the end that removes the core also allows the apple flesh (the desired output of this machine) to spiral out in one long piece. 

The cutting tool for peeling is normally spring-loaded against the apple to apply cutting force. In order to insert the apple, however, it must be positioned out of the way. There is a small tab near the bottom, with an adjustment screw, to block the cutting tool from coming down as you insert the apple. 

Pushing it aside once the apple is inserted brings the cutting tool to its ready position. 

The cutting begins!

The pitch of the screw matches the width of the "chips" coming off of the apple peeler. 

Such lovely chips! The apple begins to pass through the blade that will remove the core and remove the flesh in a spiral. 

Nearly done with the operation! The spring-loaded cutting tool compensates for a wide range of apple sizes and diameters by being constantly loaded onto the apple with a spring. Note that proper alignment of the apple on the skewers is vital for removing the core cleanly. This is the only failure mode I've seen, sometimes the apple will be pushed off of its skewers if sufficiently misaligned. The blades seem to be removable for easy replacement or cleaning. 

The completed apple spiral! My girlfriend ended up making apple rings with these. 

2.77 PUPS #1

PUPS #1

(See below for peer-review version.)

Peer Reviewed Work

MIT MechE deFlorez Competition Entry: Underactuated Robot Gripper

An Open-Source Low-Cost High-Strength Rapid-Prototypeable Underactuated Robot Gripper

by Daniel J. Gonzalez - dgonz@mit.edu

MIT MechE deFlorez Competition Entry

(Hi, Professor Hover!)

 Here is a solid model of my gripper. All of the components can be readily found on McMaster, Trossenrobotics, and other online retailers for under $500.00 total. The parts can be fabricated using only a waterjet, or using online waterjetting services such as bigbluesaw.com. With the solid model released as open-source, just about anyone can put this thing together for use in their robot manipulation project: no expert machining experience required!

The design of this gripper is inspired by the WillowGarage Velo Gripper (Work by Matei Ciocarlie): 

and the design optimization work of Prof. Aaron Dollar of Yale and Prof. Robert Howe of Harvard:

(link to paper: http://biorobotics.harvard.edu/pubs/2010/journal/Dollar_IJRR2010.pdf). 

The design of the drive mechanism is underactuated. Specifically, one motor drives both fingers, each of which has 3 DOFs. Passive compliance between each DOF allows the finger to envelop any given object that fits within its grasp without having to perform complicated grasp planning calculations in realtime. 

As of this writing, the fabrication process is nearly complete, and it should be 100% functional by the middle of next week (~4/23/2014). An Arduino will receive a grasp command from a host computer (or a button for demonstration) and command the servos to either grasp or let go of an object. The Dynamixel servo is torque-controlled and provides torque feedback, so we can apply the proper amount of pressure for a given object.

Here is an example demonstrating the underactuated closing behavior of the gripper that allows for asymmetric grasps. Using constraint tendons, the gripper acts as a standard parallel gripper until the proximal link encounters resistance. The two distal links then close around the object for a snug grasp. 

Here is a cutaway showing the direct-drive spool of the Dynamixel MX64 servo (which can output 7+Nm of torque), the cam lock mechanism actuated by an EMax digital hobby servo, and the main drive cable (Yellow). 

Here is a detailed view of the various active and passive tendons and other components within one of the fingers. Each finger is identical. 

Thank you for considering my project in the MIT MechE deFlorez Competition! 

-Daniel

EtekChopper: Tearing 'er Down and Cleaning 'er Up. Also Battery Mounting & Paint Thoughts...

I've left EtekChopper outside in the rain and cold for the past few months while I was busy doing Senior things like classes and thesis and grad school apps and stuff. Poor EtekChopper! It was under a tarp, but still! Now that some of my own hardware has started to rust, and now that I know it can move around on its own power, it's time to strip it down and clean it up for a final rebuild. No rust, no janky mounts, no zipties, nice insulated electronics, etc. 

Let's begin!

I'm very pleased to say I've joined the MIT Electric Vehicle Team (EVT) and they have graciously allowed me to work on the motorcycle into their laboratory space!!! Now I can disassemble the bike without the risk of people from outside stealing parts. Plus, there's a bit more room to leave parts in a corner than there is at MITERS (which is conveniently located downstairs). 

Things I want to do over the next few weeks:  

• Strip all parts off
• Remove ALL rust from the frame parts
• Design and weld steel tabs to the frame for mounting and protecting
• Motor
• Batteries
• Controller
• Charger
• Other components
• Re-paint or powdercoat (YAY!) frame parts. 
• Reassemble bike
• Fix front brake
• Lower the bike to fit my short body. I'm only 5'8" and this is a pretty large bike. 
• Raise front fork as much as possible (lowers front of frame)
• Adjust or replace rear suspension (lowers back of frame)
• Clean the rest up. 

First things first. I've started to remove everything off the motorcycle, and first comes the seat, gas tank, rear fender and light assembly, front light assembly. 
Then came out all the peripheral electronic components, Cycle Analyst, Throttle, Sevcon controller, wiring...

Then came the chain and rear wheel. And the batteries. And the motor.
The frame is SO LIGHT NOW! I can move it around without much effort. The wheel assemblies on this thing really add on the weight. 

I removed the rear shocks, footpegs, and swingarm. I'm going to have to probably get shorter shocks or make/buy my own lowering blocks. With the CNC mill working, I may just let it make the lowering blocks for me!

I decided I would inspect the front brake (currently non-functional). I opened up the oil reservoir to find a lot of gunk. Following this guide (http://www.youtube.com/watch?v=0X6BX05JAo0) I cleaned it out. I decided to leave it empty (and drain the line and caliper as well) until I re-assembled the whole thing. 

Here I realized that the clamps for the front levers were where the mirrors mounted to! this rusted and sheared piece of bolt was the old mirror! I went to town trying to remove it. I Dremel-ed a slot on the top and tried to use a flathead screwdriver to remove it. I ended up breaking the screwdriver! Even after torching the area (The aluminum clamp would expand more than the steel bolt) and wailing on it with WD40, the thing would not come out. 

I ended up drilling a hole into the bolt and using a screw extractor, which got a little out of hand when the thing wouldn't budge, until it finally...

CRACKed, taking a piece of the clamp before it would move. I used a Dremel to remove as much bolt material as I could (I feel like a surgeon now). I then squeezed the sides of the bolt together and pulled it out. The threads seemed mostly intact, though...

And sure enough, I could get the mirror onto there! I can see behind me! Turns out there are two axes of rotational adjustment: The angle of the brake lever clamp on the handlebar (adjusts it vertically) and the angle of the mirror on the clamp itself (adjusts it horizontally).

Now if I can only find where I put the clutch clamp, which can hold my left-hand mirror... :x

 Now it's time to take apart the front fork. First comes the wheel...

Then the forks and handlebar. Easy enough.

At this point, I need to finalize where I need to weld tabs that will hold my batteries, controller, fairings, DCDC converter, and provide additional bracing for the motor. 

The motor mounting tabs were easy enough to design/make. I just cut some steel L-channel to size, and used a grinding wheel to shape the channels to fit the frame tubes and other bits. These will come out really nice with a MIG welder. 

Now, the batteries. I've decided to NOT use these two 12S8P packs, even though they have a nice BMS and pakage. The truth is, these batteries are unsafe in this cramped mounting position. If I move it far back enough to clear the front wheel and fender while the fork suspension is fully compressed, I cannot mount the plastic rear fender (which protects the components in front of the rear wheel). If I move the batteries far enough forward to mount that protective plastic fender (I have to squish the plastic battery case a bit against the steel frame), the batteries come dangerously close to the front wheel and fender. With a max front shock compression of ~5.5", there's no way the battery can clear the front suspension. Something has to change. 

Enter the smaller 12S4P pack that is available to me via an extremely generous donation. 4 of these packs is equivalent to the two 12S8P long packs I've been testing with, but they are each in a package that is half the size as well. This leads to more (and better!) options for storing the batteries on the motorcycle frame. 

After playing around with them, I found I was able to fit eight (8!!! Double the original capacity!) of the 12S4P battery packs in the front of the motorcycle in a clean and consistent space. Look at how elegant that is! 
I will only be using 6 packs though, for a total of 24S12P. (150% original capacity, 288 cells, 720 Ah, ~ 45 miles of range!)

To fasten these packs I need to design steel mounting brackets to be welded onto the frame. I'm going to add about 0.5" of padding/buffer/armor to each side of the battery to provide some cushioning and protection from rocks/shrapnel that may penetrate the battery pack and short something. I can't design the specifics of this until I have the 6 battery packs, so I will simply design it for a box of 3x2 of the 12S4P packs, with an additional 0.5" on each size. 

I plan to cut cardboard into a box of this size, then design mounting channels in the frame using steel L-channel to allow that box to easily slide into the channel from one side, then clamp down using webbing or some other type of clamping method. 

An interlude: I have to thinking about the painting process. The problem with the process is many things have to happen quickly after eachother, or the frame will oxidize or the primer/paint will not hold corrctly. I'm going to follow these basic steps:  

• Sandblast frame to remove rust+old paint (start the oxidation clock)
• MIG Weld tabs to frame in proper areas
• Move frame to a dry, warm, and ventilated location and suspend frame 
• Clean entire frame with acetone or denatured alcohol or mineral spirits. 
• Coat frame with primer (RustOleum automotive primer for metal)
• Wait the specified primer recoat time (about an hour acc to the label)
• Spray LIGHT coat of color paint (RustOleum Semi-gloss black enamel). Wait recoat time. 
• Spray main coat of color paint. Wait recoat time. 
• Spray final coat of color paint. Wait recoat time. 
• FINELY sand/polish the frame to make shiny. 
• Coat with a clear gloss paint (RustOleum Clear Gloss Enamel). 
• Final surface polishing. 
• Leave in warm, lit area for some time (on the label). Maybe use a hot lamp?

All that must happen with very minimal interruption, especially the painting portion. Oh boy, what did I just get myself into... But first I need to finalize my battery mounting.

See you next post!