ME/EE 72: Electronics

The Frog (our amphibious robots) electronics were one of my personal favorite parts of the project. We didn’t get both robots quite set up by the competition itself, but designing the system was a ton of fun. Working with water was a particularly interesting challenge.

At a high level, the system works as follows. Each robot is powered by two batteries, one per side, that are shared between the land and water drive systems. The batteries are connected by relays to distribution boards, allowing the main drive power to be disabled between matches (or to reset the motor drivers). The motor controllers are then controlled by an Arduino nano through a PCA9685.

Note: This project / write up was actually completed several years ago. It’s just taken me a little bit to get around to posting it.

Our prototyping board during first term. Very convenient, but not exactly waterproof…

Motors and Drivers:

We decided early on that we wanted to use brushless motors for the mobility systems. Partly that was driven by a desire to learn about brushless motors, but mostly we were hoping for a speed advantage.

We selected our motors by first determining the speed we needed each to run at (dictated by the cavitation speed of the props for water, and by our desired top speed for land). Then, once we had a KV rating for each motor type we selected the most powerful model we could find under about 30 dollars. Since this project I’ve learned quite a bit more about brushless motor control and would recommend actually computing the required torque’s/currents, but I will say that that method worked quite well in practice. The motors we selected can be found below. Note that for the water motors, we choose different motors for the second robot based on what was on sale. The land and water motors both performed well (though the water motors were massive overkill) and I plan to re-use these motors when I have similar torque and speed requirements in the future.

Notable Parts:

• Land Motors: Turnigy G32 600kv motor [Link]

• Water Motors: Turnigy XK-4074 2000kv [Link]

• ESCs: Turnigy 70A marine ESC [Link]

On quirk of the ESCs we used is that they require fairly specific inputs on startup, or they will fail to arm. This is common on a lot of hobby ESCs as a way to prevent crashes or injuries on powerup. Our eventual solution to to this problem was to have the arduino reprogram each ESC to our desired settings whenever the power relays were cycled. This was probably slight overkill for normal operations, but was very handy for debugging and I will certainly include similar features on future robots.

The ESC’s we used were nominally water cooled. We designed out first robot to use a tap off the jets for cooling the water ESC’s, and a closed water loop (with a heat sink chamber milled into the rear plate) for the land ESCs. In practice we discovered that no water cooling was needed for our current draws, so we dropped the jet taps and heat-sinking geometry from our second robot.

Ball Handling:

The ball handling electronics ended up being a rush job. Each of the Frogs had a pair of custom made linear actuators driven by a brushed DC motor. We did not have time to develop a driver board, and so instead we selected a Polulu board (model unknown) from a bin in the shop. This worked fine on the bench, both under central control, and when set up to run off a separate receiver, but proved unreliable in the competition when exposed to water.

Waterproofing:

Our electrical systems had two lines of defense against water. First and foremost, all of the core electronics were housed inside sealed waterproof containers. The batteries were also housed in a waterproof 3D printed container, with the distribution boards acting as pass-through bulkheads. This worked well on the whole, although some of our systems required more tuning that was practical day-of. Secondarily, all of the critical components were waterproofed with nail polish on a board level. That was largely unnecessary, but also proved highly effective during testing.

I would, unfortunately, be remiss if I did not note that during the actual competition some of our components were not yet enclosed in waterproof boxes, and failed as a result. In particular, the drive system for the ball handing was external to the main control module, and proved particularly vulnerable to water.

Power supply:

The control module had it’s own internal quad of AAA batteries to power the display, driver, and nano. Those batteries were connected to the main PCB through a normally open reed switch. This let us enable and disable the robot by connecting a magnet to the outside of the control housing. Under normal (no wifi) use the control module batteries had about 12 hours of life. To make it a bit easier to tell when we were getting into the danger zone an ESP8266 with OLED screen was added to provide a voltage readout.

Each side of the robot shared a singe 4s battery and distribution board between the land and water mobility systems. The distribution boards feature a single XT60 input port on their lower (dry) side for power input. The current is then run through an automotive relay and out to the connector’s on the output side of the board. The automotive proved useful, because it allowed us to safe and power cycle the robot remotely without having to worry about plugging or unplugging anything. In addition to the drive systems, our other subsystems (ball handling and the pumps) were distributed between the other two boards wherever made the most sense from a wiring perspective.

For our second robot, we added another set of relays to automatically cycle the pump motors every 10 seconds. This really helped with priming, but was unfortunately not ready in time for the competition.

Control and radio:

The RC receiver was housed inside the control cylinder along with the primary micro-controller (an arduino nano) and the power monitoring ESP8266. Command signals from the receiver were processed by the Nano, and then corresponding servo control signals were generated with a PCA9685 and sent out of the interface bulkhead. Several nano pins were also exposed directly through the bulkhead PCB to provide feedback, Serial, and general purpose I/O to the rest of the robot if needed (several of these were re-purposed to run pump priming control on the second robot).

ME/EE72: Hull Panels V2

The left outer panel shortly after machining, but before the bronze bushings had been pressed.

Our second round of hull panels was largely the same as our first round. In part because we were satisfied with our first robot’s performance, and in part because we wanted to maintain parts compatibility between robots. With that said, there were a few substantial improvements to the design and fabrication process. In articular, we expanded our use of issogrids to substantially reduce part weight (without reducing part stiffness), switched to fly cutter surfacing, and made a few process tweaks to improve overall stiffness.

Overall I am very pleased with our second round of panel fabrication. We were able to cut the parts to tighter tolerances, with no scrapped parts, in about half the time.

IsoGrid:

One of the big focuses for the second generation panels was weight savings. In order to accomplish this, we replaced the previous (much wider) pocketing pattern with a true issogrid. This reduced the weight from 1.25'lbs to .8lbs, and brings the outer panels to just under 50% volumetric metal removal.

We cut the iso-grid in two stages. First, the triangles were all roughed using a 1/4in endmill. This removed the majority of the material. Then the corners, and some of the smaller pockets, were finished using a 1/8in endmill. Both operations used HSMworks 3D adaptive operation with no stock to leave. The second operation was configured for rest machining and also removed the 20thou radial stock to leave from the first operation. Each operation took about 1:20 with fairly conservative feeds and speeds.

One very substantial time saving for this round of panels was the switch to all carbide tooling. For the 1/8in endmill that meant a nearly 6x increase in volumetric metal removal over the HSS endmill we used for our first round of panels.

Although the operations themselves were fairly time-comparable to the larger weight reduction pockets from our first robot, I should note that the CAM was significantly more time intensive. In particular, selecting all of the faces to be cut caused significant slowdowns and periodic crashes in Solidworks.

Cutting the iso-grid did not seem to increase warping. If anything it may have contributed to a reduction in warping relative to our much larger lightning panels from the first round of plates.

Overall I will certainly re-use the pattern when weight budgets are tight.

Surface Finishs:

Thoughts:

For this round of outer plates, we decided to experiment with fly cutting rather than using the 1/2in endmill for facing. Overall I am very pleased with the results. Both the inner and outer faces seem cleaner, and the shinier surface really makes the engraving pop (see image below). Cycle times were also reduced by around 70%, even using multiple passes.

We got some smearing with all of the recipes we used (see results right), but even so I like the results well enough to use the superfly for future projects. Of the recipes we tried, I think 5 was the best overall.

For the inner panels, which were faced with the same 1/2in endmill we used on robot A, we decided to try a brushed look. The parts were cut exactly the same way as last time, but were then run over a medium Scotchbrite pad. The results were definitely not as shiny as an unmodified machined surface, but I think the look was a bit cleaner overall. The brushed look did seem to hide subsequent scratches slightly better than the flycut surface we used for the outer panels.

Experimental Results

All cuts taken with a 2.5in Tormach SuperFly, with a 2” stepover, running 2500RPM and 30IPM.

• 7 thou, 3 thou, engrave.

  • Looked good, slight smereing, crisp lines, raised a small bur.

• 7 thou, 3 though, engrave, .5 thou

  • Looked good, slight smearing, crisp lines, no burr.

• 7 thou, 3 tthou, engrave, 0 thou

  • Looked okay, significant smearing, no burr.

• 10 thou, engrave, -.5 thou

  • Looked Poor, significant smearing, smeared lines, some burr.

• 10 thou, engrave, .5 thou

  • Looked Great, minimal smearing, no burr.

The robot’s outer surface after fly cutting and engraving. I think this combination really pops well, and will certainly re-use it where visual is a top priority. One thing to note is the sharp vertical lines between passes. Those were reduced on the Fadal (which is better trammed), but still suggest that running passes long-wise might be better for future parts.

The right inner panel after machining and “brushing”. The machining marks are still clearly visible, but I think the effect is a bit more subtle than a straight machined surface.

Tolerance Improvements:

Surface Flatness:

Our last round of hole panels was plagued by flatness issues (on some parts we were out by as much as 15 thou). For this round we made a few critical changes. First, we re-designed the surface plate to allow for machining the entire top surface of each blank. This allowed us to fully remove the skin from each part which seems to have helped reduce twist by around 60%. Second, we planned in 0.030in of surfacing. This gave us considerably more wiggle-room for facing both sides flat.

Our results were still not perfect, but we held to our 5 thou flatness and thickness tolerances on all 5 panels (3 thou on thickness, and 5 on flatness, measuring with a height gauge on the surface plate). For the most parts the solutions we implemented were things we knew about before starting the first batch, so I think the lesson here might be that it pays to start off with a better process when possible.

Hole Tolerances:

Not really a full investigation of boring as an alternative to drilling. Did some inspection with gauge pins just to see what we are getting, and I’m going to leave the results below for future reference.

• Counterbores - Rated: .2188 Actual: .221-.222, Variance: Minimal

• Rear pulley hole - Rated: .1590, Actual: .160 - .161, Variance: N/A

Getting a vertical dial indicator really helped improve our process for surfacing the parts.

Being able to “see” the thickness variations and bowing made it much easier to decide where to spend our extra material.

Fabrication Pictures:

Taking a shot at chamfering both sides without a relief cut. This had the unexpected effect of actually raising a larger burr than would otherwise have been present. Not a great idea as-implemented but maybe an area for further experimentation.

Sometimes you need a tool extender… Sometimes the best tool extender in the shop is a TTS collet holder… It worked?

The Haas’s tramming error is particularly notable during facing operations. In practice we did not find the ridges compromised water-sealing so it was largely a non-issue.

As zeroing setups go I can’t say this is my favorite, but the 1-2-3 block is only out by a few tenths, and it did work well.

The ME shop’s vertical bandsaw was down so we used the student shop’s instead.

A blank after being cut and milled to size. The sharply definitely helped cut down on fixturing errors.

ME/EE72: Fixturing Improvments

Summary:

Before beginning the second round of CNC fabrication, we fabricated two locating blocks, and added counterbored holes to our side panel fixture plate. This made the plate much easier to set up, and dropped our fixture setup time from around 30 minutes to around 5. We also added set-screws to the unused 1/4-20 holes. This didn’t really improve setup times but did make cleaning easier.

Update 4/17/2019: Adding the tramming blocks and flush mounting bolts was a fantastic ideas. I have used this fixture for easily 7-8 medium-low tolerance parts and consistently held 1-2 thou with no tramming.

Traming Blocks:

To make Traming a bit faster, I made up two blocks to locate the fixture plate against one of the T-Slots. The idea being that with the blocks pressed against the T-Slot, and the fixture plate pressed against the blocks, the entire setup would be trammed without any additional alignment. In practice, the plate tends to move by 4 thou or so when being tightened down so there is still a bit of tapping in required. Still faster than without though.

Fabrication:

In the first setup, the stock is clamped in the middle of the vise by the lowest 1/8in of the stock. The top geometry, outer profile, and engraving are then cut with a 1/2in endmill and 20 thou ballmill. The part was the flipped and placed with the retention ridge against the edge of the parallels. The top 130 thou was then faced off to produce a squared part with all 6 (or 10 depending how you count) sides faced.

Reflections:

Overall, not huge time savings. Learning how to tram properly (tighten one corner than tap the other) made a much bigger difference. Still, it was a good way to turn low value time at the beginning of term into high value time later on. Would do it again in a similar situation.

ME/EE72: Hull Panels V1

Overview:

The right rib partially assembled.

This post outlines the fabrication process for our side panels, which run the length of the ship and provide mounting geometry for most of our land mobility systems. The side panels are made from 6061-T6 aluminum, and all four were cut on one of the school’s VMCs using our fixture plate from earlier in term.

Making the side panels was easily the most time consuming portion of our fabrication process for Frog A, requiring over 70 hours of reserved CNC time. With that said, the majority of that time was spent checking cuts, tweaking setups, and repairing/replacing errors in prior runs. All told the 4 plates only have around 8 hours of actual “cut time” so I am hopeful that we will be able to reduce fabrication times substantially moving forward.

All four of the side panels prior to assembly.

The right rib, post assembly.

Cutting the Faces:

The fixture plate and prepared blank on our Fadal VMC15 before the first operation.

Setup:

Each side panel was cut out of a 19.5” by 5” by 0.25” blank of extruded aluminum bar stock. The stock was purchased from online metals in 6’ lengths (to reduce shipping costs) and then cut to rough size on the shop’s horizontal bandsaw. One 5in edge edge was then finished on a manual mill to provide the two reference corners for the part. The part edges were then deburred on a Scotchbrite wheel, and the two reference corners were marked with Sharpy.

Tabs helped increase process reliability and came off easily using the bandsaw.

Fixture:

We set the work coordinates for this part to be the top rear left corner for the first operation, and the top front left corner for the second operation, with the part about it’s long axis. This meant that we were using the same physical corner as our zero for both faces, thus eliminating the risk of errors due to irregularly sized stock. The tools were measured from the center of the workpiece using the shop’s 4in height guage*. In some cases it was necessary to lengthen the tools slightly to ensure that they were able to reach the bottom of the parts given our short fixture plate. We also found that some of the operations raised burrs on the non-part portions of the stock. We solved this by sanding them down, but I think moving to facing the entire top of each part would be a good idea for round two.

The blanks were held using our 5in fixture plate, and tensioned with 1/4-20 miteebites, spaced with some copper shims as needed to accommodate thickness variations in the stock. This seemed to work well, and I would certainly re-use a similar fixture for future panels and flat parts in general. We did find that with the thin (1/8in) edges on some of our parts it was necessary to add tabs since the foil tended to tear when thin enough to be convenient. Tabs about 1/8in wide and 40-50 thou high seemed to be more than sufficient, though we did use several on the longer faces of the panels. These seemed to cut better than fewer larger tabs.

*Note Post Build: Our fixturing setup for round two was vastly better, I would suggest copying that if you plan to cut parts like this in the future.

Faces:

The first operation on all of the parts was to face the simpler side (inner for inner ribs, outer for outer ribs). With a slightly dull HSS endmill machining marks are inevitable (though they are less pronounced in person than in the images below). To make the best of things, I decided to use a contoured spiral to cut the outer faces. This added about 10 minutes to each outer plate, but left us with a cool pattern vaguely reminiscent of a wale. That said, it did make it a bit hard to read the engraved “Undescided” (our team name, yes that is the correct spelling) on the outside of the panels though so I plan to move to fly cutting for the next batch.

On a more practical note, the facing operations did a good job of underlining just how warped our stock is. For most of the parts we ended up having to take multiple passes to the tune of 15thou per side in order to get a clean surface. It is not clear to me exactly how much of this has to do with our fixturing/cutting methods and how much has to do with the actual stock, but it is concerning in any case.

Lightening Pockets:

Both the inner and outer panels had significant lightening pockets cut out to reduce the part weight. The inner panels had through-cut lightening holes, while the outer pockets were cut with our rough approximation of an issogrid. Removing as much material as we did proved very time consuming. The inner panels took around 45 extra minutes to cut, while the outer ones gained around 75 extra minutes. That said, we ended up being quite tight on our weight budget, so the extra fabrication time was more than worth it.

The pockets were first roughed using a 1/4in endmill, and then finished using a 1/8 in endmill to get into the smaller pockets and tighter corners. We used a conventional 2D pocketing strategy for both passes, and then cleaned up the walls with a 10 thou finishing pass. These cuts would likely have benefited from an adaptive approach, but the FADAL lacks the requisite block processing, so we stuck with simpler machining strategies.

These cuts are one of the parts of our process I am least happy with. Although the pockets turned out well in the end, we got a lot of chatter and resonance in the part while cutting. I suspect this was due to a combination of HSS tooling and long tool stickouts. I plan to use much more conservative stickouts, as well as carbide endmills, for the next set of plates.

Note: The weight budget ended up being tighter than we expected. With that in mind, the inner panel lightening holes were sufficient, but the outer panel pockets were not. I plan to remove significantly more material from the next set of plates.

Tapped Holes:

Tapping the top holes, all of the CNC work for the edge tapping was done on the TM1.

Manually tapping some of the 6-32 side holes.

4-40:

Not a setup I am exactly proud of… it did work though.

A tapped lower standoff hole. These provided the spacing and structure for the ribs

One of the more interesting parts of this design, were the vertical mounting holes. The top and bottom edges of each panel have a number of tapped 4-40 holes used to connect the plates to the hull-bottom, and hopefully used to connect the ball hardware to the plates. With several hundred holes to drill, we decided to approach them as a CNC operation, and successfully roll-tapped all of the holes using the Haas TM1. Since the first set of plates varies in thickness by 15 thou, we found it necessary to keep track of which edges were “reference edges” so that we could drill and tap the the holes relative to those edges. This required a large number of fixtures. We plan to use custom soft jaws for the next batch.

The other set of 4-40 holes found on the robot were are the standoff mounting holes. These are used to connect the inner and outer panels of each rib, and are tapped about 100 thou deep. Since we do not have a bottoming roll-tap, we chose to tap only the first turn of the threads and then chase the rest out using a 4-40 screw. This worked well and will be replicated on future builds.

6-32:

In addition to the 4-40 screws used to tension our standoffs, we also have a number of 6-32 tapped holes used for mounting the gearbox assembly’s as well as the ball handling systems (possibly). Since these interfaces had not been finalized when we began fabrication, the holes were drilled as 4-40 clearance holes and then finished by hand during assembly.

10-24:

There is a single 10-24 screw used as a shaft for the lower, rear, belt rollers. It was drilled to size on the CNC and then tapped manually at the tapping station.

An angle block for drilling the bow holes.

Test fit-up of the jig for our bow holes.

Teflon strips were used to prevent scratches.

Chamfers:

We used three sizes of chamfe on the side panels. see below for a summary of each. The pocket and perimeter chamfers were cut with a 1/4in, 4 flute, spiral, carbide chamfer mill, and the hole chamfers were introduced with the 5/16 HSS spot drill during the initial spotting operation. All chamfers were 90 degree.

Drilled holes on the part were all furnished with a 10 thou spot-drilled chamfer. That seemed to work well, and I would use it again. The milled pockets (lower) were milled with a 1/4in carbide endmill and given no chamfer.

The test-part was given a 40 thou chamfer on both sides. This was by far our prettiest option, but ended up cutting into the available edge space a bit too much.

The 4 production parts were given a 15 thou chamfer on all edges. This worked well and looked good. Although, as you can see, we did get some chatter towards the inside of the part.

ME/EE 72: Gaskets

Summary:

My favorite part of our design for Frog B* is our waterproofing method. Instead of using some kind of adhesive sealant, or chalk, we use laser cut silicone gaskets placed between our various hull plates. The gaskets are simple, cheap, and (once we got the method down) relatively easy to make.

Note: I want to applaud Jack for his willingness to assemble and disassemble the robot 4 or so times while we were figuring out waterproofing.

*Our first robot, it’s a long story…

Application:

Installation: We specified gaskets between all of our hull panels, and between every jet-hull-nozzle interface. We found that in-order to get a good seal, some amount of consistent pressure was required along the gasket. In general, we solved this problem by putting clearance holes through our gaskets and securing the two plates (whatever they might be) with 4-40 screws. This did by and large work, but we still got some leakage though areas with more curvature or less pressure. To address this, we dipped each screw in marine grease, and applied a layer of marine grease to each side of the gaskets.

Results: Our initial tests sealing acrylic to the bottom of a machined cylinder were extremely promising. We had good sealing with no leakage. The actual robot proved a bit more finicky to get working right, but once we fully implemented the compression we were able to get to a no-leak condition.

The edges of our structural plates were all tapped with 4-40 holes to facilitate tapping. Corresponding clearance holes were then cut (slightly undersized) in the gaskets.

From Jack: (Who did the overwhelming majority of the seal installation)

• Take the Silicone and coat both sides liberally with marine grease.

• Coat the metal or PLA with marine grease.

• Align by hand, put grease on a few screws, and insert them to keep the silicone positioned.

• Coat remaining screws with grease and insert them lightly.

• Tighten all screws until most grease has been pressed out and the silicone begins to extrude.

• Back all screws off 1/4 turn (6.25 thou).

• Test water seal and tighten leaking areas as needed.

• Repeat previous step until entire seal is good.

• Remove extruded grease as needed.

Lower jet gasket and associated grease

Corner gasket without pressure applied.

Corner gasket with some pressure applied.

Fabrication:

Laser cutting gaskets on our 40 watt machine, that ended up working less well.

We made our gaskets out of 1/16 “High-Temperature Silicone Rubber Sheet” from Micmaster. It comes in strips of varying size, we got 4”, and ended up costing about .07$ per square inch.

To cut the gaskets, we used an 80 watt laser cutter set to 60% power, 6% speed (ambiguously 10in/min) and 500 PPM. This is just about perfect to cut a deep scribe line. The parts can then be torn out by hand to produce beautiful crisp lines.

We did experiment with ways to cut all the way through using increased power or multiple passes, but had limited success. Increasing the power did not seem to have much of an effect beyond a certain point. Perhaps because the powdered silicone was blocking the beam. Taking multiple passes was intermittently successful, but for larger parts the silicone seemed to move on the second pass producing less precise lines.

Immediately post-cut, the gaskets tend to have some white powder and other residue along the cut edges. This did not have much of an impact on actual performance, but it is aesthetically sub-optimal, so we cleaned up the gaskets by scrubbing them with simple green and then rinsing them in water.

Our sealing test, once tuned it held water for the better part of a week with no leakage. This part was also our first test of CNC tapping.

We originally used 24 screws (see cap below) but found that it sealed better and was easier to tune with only 8.

Using 24 holes let us try lots of different spacings to see what worked. As for the frog… got to test engraving somewhere right?

ME/EE 72: Fixture Plate

Summary:

Completed fixture plate, clamping test blank.

This is a fixture plate for clamping flat pieces of 6in stock. It is designed specifically to accommodate pieces 19in and 11in long, but can be used for any length larger than 3in, so long as proper precautions are taken. You will likely see this fixture used in a number of other projects, primarily relating to ME72.

Purpose:

Our ME72 robot, has a number of very large components cut out of 1/4in plates. Rather than fixture them in a double-vise setup (too slow), use super-glue fixture (not allowed in the shop), or cut them on the water jet (not high enough accuracy), we decided to fabricate an aluminum fixture plate. Since all of our 1/4in parts are cut out of the same 1/4in by 6in stock, we only made provisions for clamping 6in pieces. However, the design could be easily expanded to hold other sizes, or even non-square stock if need be. Note, that although the X axis clamps are nice, they are not required for proper clamping.

Design:

The general design is fairly simple. The parts index off of two rows of 1/8in dowel pins pressed into the plate, and are held in place by a row of Mitee-Bite hexagonal fixture clamps. The plate has a machined reference surface along the left (x) most edge, and two machined reference slots along the rear (y) edge. Additionally, there are two 1/4in dowel pins sticking out of the bottom of the plate which index against the T-Slots on our Fadal (tram is checked using the reference slots).

Note: The tramming pins did not end up working out the way we would have liked. Either because of T-slot issues, or because we installed them poorly the default tram is off by 20 thou. This is unfortunate, but only ads about 10 minutes in tram time.

Design Files: [Click Me]

Fabrication:

Fixturing:

Our first step (after milling a reference flat onto the left edge) was to install two vises in the Fadal VMC15. This let us securely grip the stock with minimal vibration. The following process was used:

• Wipe down table and apply protective layer of WD40

• Install first vise and tram.

• Install second vise using steel bar to rough-tram.

• Tighten close side of second vise and tap vise into tram.

Setup:

Next, we dialed in the tool offsets, and set up the fixture coordinate system. For the Y axis this involved the standard Fadal process using an edge finder. For the X axis, where we were travel limited, we created a “virtual” 6.5in fixture setting tool, and measured off of a 1-2-3 block clamped against the reference edge of the part.

This was not an ideal setup, but it seems to repeat to within a thou or so.

Cuts:

The piece was then cut as follows:

• The majority of the top was faced with a spiral tool path.

• The edges were faced with custom tool paths designed to remain within allowable travels.

• Reference flats were cut along the +Y side.

• All holes were spotted and then drilled.

• Each hole was then drilled or taped as required.

Pins:

Finally, we pressed the pins in place using locktite retaining compound to ensure the 1/8in dowel pins wouldn’t pull out. In order to ensure consistent heights, we pressed the pins through a 3/16 plate. This both ensured the pins would be below the top of our parts, and helped us guarantee the pins would go in straight.

The tramming pins were pressed in without the benefit of retaining compound on the theory that we might at some point want to remove them.

MISC Photos:

Copper shims help compensate for undersized stock.

The fixture plate after cutting our first few parts.

Using a 3/16 piece of stock to press each dowel pin to a consistent height. The hole used was drilled undersized and reamed to a slip-fit.

The spiral tool paths worked very well, and proved easier to contain within allowable travel.

ME/EE 72: Airboat Proof of Concept

Purpose:

The goal with this build was to prototype a flat bottomed RC boat with as little investment of resources and engineering time as possible. To that end, this boat features a 3D printed hull (low time commitment to fabricate), with propulsion hardware scavenged from a team member’s quad-copter, and a controller borrowed from the shop. Although less polished than most of my projects, the payoff in terms of [lessons learned] / [time invested] was fantastic.

ME72: This is the first in a series of posts about ME72, a design capstone course in which 5 person teams compete to build 3 robots then can to fit that year’s competition. This year, the course is amphibious in nature, with a strong emphasis on ball manipulation.

Design:

I choose an air-boat for our first prototype, largely on the grounds that it is by far the simplest and cheapest motion system to build. The thrust-tower is designed to support at most an 8 in prop, with provisions made in the model for increasing the height as needed. This provided adequate power for hull-testing, and is most likely what I would use for future hull-tests as needed.

Using a half-height (4in) steering fin reduced print time, and made the model easier to assemble. However, it did result in a noticeable decrease in overall control authority relative to a full-height fin. This was most noticeable at high speeds, but was an issue throughout the power curve. In the future, this could be addressed with a taller fin, or by increasing the maximum turning angle of the fin beyond it’s current 10 degrees.

Although the hull doesn’t have any water-tight compartments, it does feature a sealed deck. This allows it to swamp, but not sink, when taking water over the sides. Although this proved unnecessary in normal operation (no water was taken over the stern), it did prove useful when a friend accidentally drove the vehicle through a fountain.

The control fin (see below) is actuated by a single servo, and has a 10 degree travel arc. This is sufficient for general locomotion, but would likely be insufficient for operation during the game. The fin pivots on two 1/4in steel dowel pins pressed into the motor mount and motion tower base. Both pins were heated and then inserted into nominal size holes. The fin’s pivot hole is 25 thou above nominal, and produced an adequate low friction fit after some working back and fourth.

The thrust and control tower without linkages. Overall it performed well, with the sacrificial motor mount working as expected.

The completed assembly in Solidworks.

3D printed hull:

In the interests of time, and as a bit of an experiment, I decided to 3D print the entire hull as a single piece. This was largely successful, with a few fairly major caveats. The print took about 100 hours on one of the library’s CR10s. The lower hull was printed 5 layers thick, the deck was printed 3 layers thing, and the internal ribs were printed 2 layers thick. The boat was printed with layers perpendicular to the spine, with 105% flow and a perimeter speed of 30mm/s.

The first major issue I ran into was under-extrusion. Even running at 30mm/s, this print really pushed the CR10’s limits. As a result, the hull came out a lot weaker than I would have liked. Slowing down the print mid-way through helped, but seemed to deteriorate even more late-print. I am not clear on why, but plan to do some exploratory disassembly to find out.

The bigger issue, is that as a result, the boat leaks leaks like a cive. This was most pronounced in the early and late print where the under-extrusion was worst. Rather than printing a second hull, I decided to seal this one with spray paint. That required 4 general cotes across the whole hull, and 3-6 more in the worst areas. This fixed the leaking, thought it did nothing for the structural integrity of the hull more generally.

The control fin, motor mount, and thrust tower were printed without issue on my own machine.

Lessons:

• 3D printed parts cannot be assumed to be water-proof unless the filament is significantly (and consistently) over-extruded.

• Spray paint makes a good sealant, but requires a large number of layers and has a poor $/in^2 ratio.

• Slant sided, flat bottomed, hulls are very vulnerable to weight imbalance.

Project materials:

• Solidworks: [CLICK HERE]

• STLS: [CLICK HERE]

Video: