Lego Car 2

With the extreme slowness of our first car in mind, we constructed another, smaller car. We initially wanted to use treads and a 1:15 gear ratio, but the treads were much too high-friction. We also encountered the problem of the 40-tooth gear being too large to be on the same axle as the wheel itself.

Next, we decided to try to change the wheel diameter in order to keep our 1:15 ratio. However, any larger wheels would collide with the overhanging axles of the gear train. This problem could have been solved by lengthening the axle of the wheels, but we were already using the longest available axle and were loath to connect smaller ones together due to the threat of bending.

Instead, we moved the motor to the top of the car, with the belt running diagonally down to the underbelly, and installed a 3-gear gear train diagonally (8-toothed to 40-toothed to 24-toothed, for a total ratio of 1:3 among the gears alone).

The diagonal gears of our 1:9 car

This arrangement, with a total ratio of 1:9, moved very fast without weight, but only very reluctantly and with a slow start would it move the weight. We noticed that the belt was kind of loose in attaching the motor, and after moving the motor one unit over, the belt was much tighter and the car started without any problems, even while carrying a weight. We timed this configuration and had a fast run of 6.2 sec!

In the official race, the car got a time of 6.5 sec. If we could change something, I would probably want to try lengthening the axles by connecting multiple ones together, despite the weakening, and use larger wheels.

The car featured a festive tropical theme

Windlass Outcome

Today we demoed our windlass. Unfortunately, the string winding along the rod pushed the outer bushing off the rod and broke the windlass. If we had encountered this problem earlier, we could have used the same piano wire attachment that we used on the crank arm to connect the outer bushing to the rod, which would have solved the problem.

Material use:
Delrin: 19x26cm (494 cm^2)
Rod: 9 + 17 cm (26cm)

Lego Car Version 1

Task: Build a Lego car that can carry 1kg for 3m as fast as possible, using one motor.

Cabrina and I started by making some simple gear trains, not connected to a car. We experimented with larger and smaller gears, and tried connecting the motor. We had a lot of trouble properly aligning the motor to any gears, because we were using an 8-toothed gear on the axle of the motor. After Amy told us about the belt mechanism, it became much easier to position the motor and have it be able to turn our gears. From here, we made our first car, which was extremely large and slow. It was able to carry the weight easily, but was very slow.

The gear ratio was 75:1

We then decided to decrease the gear ratio, to increase the speed, and keep decreasing it until the car would no longer run carrying 1kg. Our next iteration had a 45:1 ratio:

45:1 car

This was slightly faster, but took 26 seconds to cover 3 meters. We noticed a lot of bowing in the axles; moving the wheels closer to the chassis resulted in a 2-second improvement in the car's time (from 26 seconds to 24). 

We decreased the ratio to 27:1 and tested again. This time, the motor could not carry the weight and the gears slipped instead of turning, resulting in an awful din and no movement. We decided to move back to the 45:1 ratio. 

This is a very misleading picture. The front and back trains were not actually connected at the same time; this picture was taken in the process of switching back to the 45:1 gear train after the 27:1 failed. The front-axle train in the picture is what the car was actually running on when we tested it (i.e. the gears were actually touching, unlike in this picture).

Back on our 45:1 gear ratio, we significantly reinforced the car's body, which had been bowing in the middle under the weight. We added crossbeams across the bottom and reinforced the entire center seam of the top platform with another plate at right angles to the first two. After these reinforcements, the time again improved by 2 seconds (down to 22 seconds to run the course).

But this car couldn't be made any faster without radically changing everything about it. The size of the wheels and body, and the friction of having three gear changes, prevented us from having anything under a 45:1 gear ratio. To get a time under 20 seconds, we needed to start anew, which is what we did. 

Well Windlass: SolidWorks and Construction

We now had a satisfactory design and a scale drawing of the windlass. We first decided how we would attach the plastic parts. We considered using piano wire on the perpendicular joints, but decided with such a large piece it might be unwieldy to use the drill press. Instead, based on playing with the model of notches, we decided to connect all our perpendicular joints with tight notch fittings. From Amy's suggestion, we decided to attach the crank arm to the rod with piano wire, to make it strong enough to turn even with a weight on the string, and decided to connect the string to the rod with a tight bushing.
Now that we had a good enough idea of what our parts should be, we built them in SolidWorks.

The sketch for our first part - the "bridge" part

We ended up with five unique parts in total: the "bridge;" the top brace to hold the bridge parts together; the bottom brace, wider than the top, to stabilize the structure and keep it from toppling; the crank arm; and the bushings. Initially, we drew our notch holes to correspond with what we had measured with calipers on the sample notches, since we had not been able to test our own notches yet. 

Then, we put our parts together in an Assembly. We learned a lot about how to "mate" components and, in the end, were able to make an assembly where the crank arm was constrained just enough that it can rotate in the assembly just as it would in real life. 

Our SolidWorks Assembly

We thought we were ready to print and assemble our windlass, but we first had to test that our notches would be a tight fit. We made another part that consisted of two squares, one with two holes and one with two teeth, to see if the notches would fit tightly. 

They didn't. Since at the time we didn't know that the Delrin sheets varied in thickness, we carried out three more tests of progressively narrower holes, none of which fit tightly, without using the same sheet of plastic every time. We also tested our bushings, and the second iteration was a tight enough fit to not slide along the rod, and was also the right size to hold the string onto the rod. This was the moment when the laser cutter decided to break for the first time, and our design was delayed until we could cut our test notches from the previous assignment.

Julie and I came in over the weekend to work on our project, and I was finally able to print out my test notches. From this test, since I knew what the SolidWorks input was for the width of the holes and did not need to rely on calipers for this measurement, it became clear that the notches would fit tightly if the holes were .2mm narrower than the measured thickness of the Delrin. Armed with this knowledge, we found a sheet of plastic, measured it, and adjusted the holes to be small enough to ensure tight notches. After one last printing of our notch testing part, finding it was a good fit, we were ready to print the whole thing. 

We tried to make a SolidWorks drawing that resembled the original graph paper arrangement of the parts, but were unable to make multiple copies of each part or to orient some of the pieces upside down. We ended up sending five separate drawings to the laser cutter and having each one cut a certain number of times: two "bridges," two of each crossbar, one crank arm, and four bushings. 

All of our Delrin parts, fresh from the laser cutter. 

The notches worked great and the pieces literally snapped together, almost like Legos. Now we just needed to add the rod. Our first attempt at piano wiring the crank arm to the rod failed, as the wire got stuck halfway into the hole and our attempts to push it through cracked the plastic of the crank arm. We tried again, using the other hole of the arm, and were successful in pushing the wire through because we used continuous pressure instead of trying to hammer the wire as we had the first time. 

We added the rod, bushings, and string to the piece and tested it. Though it wobbles slightly before the string becomes fully taut, when there is tension in the string it is quite stable and very easy to crank; the bottle does not feel heavy at all. There is also another length of rod that can optionally be attached to the other end of the crank arm. I find it easier to wind with this extra rod; Julie prefers not to use it, so we kept this one detachable. 

Some of the initial wobbles come from the fact that some part of the plastic is slightly warped: though the parts have right angles in SolidWorks, the opposite corners of the bridges are slightly off from one another. I also noticed that the cut of one of the upper crossbars is not perpendicular to the plane of the plastic: the plastic must have been resting at a slight angle in the bed of the laser cutter, which would explain why the overall assembly is not more stable (or none of the tables are flat?). 

The windlass complete. 

We will be demoing the design on Wednesday, hopefully it goes well. 

Well Windlass: The initial design

The well windlass has been a long saga, and we will finally display our design on Wednesday.
It started with the assignment and constraints: The device must lift a bottle out of a 12cm gap between tables, raising the top 10cm above the table, using only 500 square cm of Delrin and 50cm of rod.
First, my partner Julie and I individually brainstormed possibilities.

First page of planning sketches

Second page of planning sketches

We brought our ideas together in a meeting. We decided that a design with a triangular profile, while it seemed strong, would be too difficult with our attaching methods, which work at right angles. Instead, we chose the last design shown on the second page of notes: an arch-like design that would place two sheets vertically, so that the force of the bucket would be pushing parallel to the sheets of Delrin instead of perpendicular, allowing them to support more weight. We wanted a crank attached to a Delrin rod on which the string would wind, as well as a ratcheting gear to hold the bucket if you stopped cranking in the middle of raising the bucket. After our meeting, I sketched the parts we would need on graph paper, as shown below.

Sketch of parts, 1 square = 1cm

In sketching the parts, I used a scale of 1 square = 1cm to make sure that all the parts would fit within 500 square cm. In the end, I could fit everything in a rectangle 19 x 26 cm, or 494 square cm - barely fitting! Due to the constraints, the width of the "bridge" could only be 18cm, which, if the feet of the bridge are 2cm wide, leaves only 1cm between the feet of the bridge and the edge of the well. Also, the height to the rod is only 11.5 cm. We decided on a width of 6cm between the two bridge pieces, trying to keep it narrow so that the rod would not bend under the weight of the bucket. 

With these worries in mind, we constructed a cardboard model using the dimensions in the drawing. The model seemed quite stable (for being held together with tape) and gave us a good idea of the size of the device. The crank arm and the ratchet gear were especially hard to make out of cardboard and didn't really add anything to our understanding of the device. 

Cardboard model of the windlass

Julie suggested using two cranks and rods, to disperse the weight of the bucket and prevent the rod from bending. However, we abandoned this idea because it would take a lot of force to raise the bucket  high enough between two rods (to have 10cm of clearance above the table in our design, the bottle has to get within 1.5cm of the level of the rod(s)), and when the bottle is close to the rods, pulling from two places would require a lot of tension for a relatively small vertical force on the bottle. Therefore, we returned to the idea of having one crank in the center of the "bridge." When we tested the strength of the Delrin rod, we determined that a 6cm length would be stiff enough to hold the bottle without bending, which alleviated our fears. 

We also abandoned the idea of having a ratchet to hold the bottle up, since it seemed much to complicated to achieve in the time we had available and would not significantly improve our overall design. 

We then got our design approved and proceeded into SolidWorks!

Rotational to Linear Motion

I found the "Multiple Straight Line Drive" very interesting out of the mechanisms on the site. It takes the rotation of a large disk and ends with a piston moving up and down. The small gear spins faster than the disk because of the teeth around the edge, and the small gear itself, while rotating, moves in a circle. Because the small gear has half the diameter of the disk, its edge is always at the center of the disk. The piston attaches to the small gear so that it is tangent to the small gear, and the piston moves up and down while the side-to-side motion of the gear is compensated by the arm between the piston and the center of the small gear. Therefore, the piston follows the up-and-down motion of the gear but not the side-to-side motion, changing rotational to linear motion. I especially liked this mechanism because it is able to create a constant motion without sudden changes in speed, which some other mechanisms had.

Notches (for real this time)

I finally had a chance to cut my notches on Saturday, after learning that the notches need to be adjusted based on the individual piece of Delrin. I adjusted the dimensions for a certain piece and made the notches. I found that the best tight fit resulted from a hole that is .2 mm smaller than the measured thickness of the Delrin. Using this strategy, Julie and I constructed our windlass with tight notches that snapped in to place!

Fastening and Attaching

1) Heat Stake
Used for joining perpendicular sheets edge-to-middle
Pros:

• Strong perpendicular bond
• Peg and hole measurements need not be completely precise
• Very permanent

Cons:

• Time consuming to assemble
• Easy to accidentally make a joint where the two pieces are not completely flush with each other

2) Bushings
Used for attaching rods to sheets, and spacing along rods.

Pros:

• Choice between sliding fit or firm friction fit
• Enable combination of rods and sheets in a construction
• Enable moving joints

Cons:

• Limited strength, cannot be made tight enough to completely resist turning
• Require very precise sizing, can easily be too loose/tight
• Special, stronger bushings with set screws are expensive

3) Piano Wire

Usually used for joining perpendicular sheets edge-to-edge, also possible in other applications such as edge-to-middle or sheet-to-rod connections.

Pros:

• Very strong attachment
• Choice between stiff and moving joints
• Interlocking measurements need not be completely precise

Cons:

• Requires careful drilling
• Wire "tails" could be an issue
• Wire must be able to go all the way through - joint must be whole width of the piece

4) Notches

Used for attaching perpendicular sheets edge-to-middle. At this time my notches have not been printed, but Julie Barron and I have tested several size iterations of notches for our well windlass as well as reviewing Ashley's working notches.

Pros:

• Variable strength joints
• Detachable and reattachable
• Can attach a perpendicular sheet anywhere on a sheet, not just the edge

Cons:

• After being reattached a few times, the joint loses strength
• The dimensions must be extremely precise
• Cannot attach edge-to-edge with this method

Project 1, The Bottle Opener

The Task: Make a functional bottle opener using Delrin cut on the laser cutter.

The Process:
First, my partner, Ali, and I sketched as many bottle opener designs as we could think of. They started falling into categories: the hook-over-top kind, the half-moon kind, the mostly-circular kind. After we had found all three of these basic designs, we started trying to find ways to make one of these basic designs cute. We chose the hook-over-top style and started brainstorming interesting shapes (mostly animals like fish or a running fox).

After the brainstorming process, we narrowed the selection to four: Gaping Fish and Tail Fish (two different locations on a fish shape for the opening mechanism), Smiley Face (a half-moon style opener), and the basic circle. Using a Pugh chart, we narrowed the options on the basis of Strength, Comfort, Attractiveness and Size, and ended up settling on the Gaping Fish model.

At this point we had to quantify the design: how wide should the mouth be? How curved? After measuring the bottle cap itself, we came up with a sketch and transfered it to foam core.

From the foam core, we proceeded to SolidWorks! We had some trouble trimming our circles and curves to meet at points (getting rid of extra interior curves), but after we had mastered that, and figured out how to "fix" lines so that the body of the fish would be the same no matter how we modified the mouth, it became much easier. The "fillet" tool was extremely helpful to keep the lines of the fish smooth and organic.

Finally, we could print our bottle opener. Unfortunately, the plastic was chewed by the edge of the cap, and the fish was ineffective even after modifying the nose to be larger (to make it a better lever point). In order to have a working model, the fish was abandoned and replaced by Ali's design of a half-moon shape with a filed edge, which was the final model.

Personal Goals

My main reason for taking this course is to see whether I should take engineering courses at Olin or MIT. Will the engineering process appeal to me as much as science does? I'm not sure. I have done some "engineering" type projects for Chris A's research lab, including using SolidWorks to design parts, but this mostly ended in frustration. I want to work on problem solving skills and creative thinking, which overlaps with the course goal of applying an engineering design process. I'm also especially excited about creating models and simulations, a topic that has always fascinated me. I look forward to the hands-on approach and I can't wait to start building things!

PS the title of the blog means "the woman devising" in Ancient Greek (I happen to be a Classics major).