Much progress was made today. I will need to make a trip to the local plastic supplier to get the stock for the wheels and new front blocks, place an Online Metals order for the armor plates, and place a McMaster order for new shoulder bolts and a few other things. In the meantime, however, I got to work machining the new drum internals and side plates. I also need to work on machining the blades, which I have been avoiding for several months, but I am going to continue putting that off.
Before I do a spin up test with the blades, I'll need to make a testing box. It could be extremely dangerous if one of the blades comes off with the drum at full speed. They are sharp enough to shave hairs off of my arm.
The first things on the table are all of the 3/8" UHMW parts. I have taken to doing my CNC work in large sheets. Here is a sheet containing all of the parts that need to be machined. I will need to make fixturing pieces to do the finish machining on the new front blocks, but that is a project for another day.
CNC Sheet of changed parts
If anyone is curious, I use the HSM plugin for Autodesk Inventor for writing G-Code. My CNC router is running Mach 3.
I tried a new endmill for these parts. It is an Amana Tool 1/8" plastic cutting bit, which I tacked onto a re-stocking order of 1/8" aluminum cutting bits made by the same company. The finish was slightly better than the old 1/8" straight flute mill that I have used in the past. I used twice my normal cutting depth and it worked well. In the future, I think I will use a slower feed because the finish is a bit rough.
After finishing the new UHMW pieces, I went back to the drum and machined a new idler axle. Originally, I was going to make a press fit for the axle, but I decided that there are several situations that may warrant removing the axle, so I enlarged the hole slightly so that it is just a tight slip fit.
Idler shaft and motor in drum
The bearing system on the idler side of the drum is just the aluminum shaft resting in a hole in the frame member. I used a reamer to bore out the hole, which left a good finish. I then added some spindle oil. This resulted in a surprisingly smooth bearing, though I am concerned about frame deformations causing it to lock up. I think I'll leave it for now, at least until I have the opportunity to do some testing.
I have finally un-killed myself from the FRC season.
My first priority on Event Horizon is to fix the misalignment issue I saw after spinning up the new drum for the first time. It seems that I will need to make a solid connection from the idler shaft to the motor shaft in order to make a continuous solid body. My first idea was to make a 3/8" aluminum shaft that presses onto the motor shaft. I bought a reamer for this purpose and re-designed the drum internals slightly.
New idler shaft
Then, I looked over the last revision of the design, and found a bunch of things that I was not particularly happy with and re-designed them. I was going to need to re-make the side pieces to accommodate the new idler shaft design on the drum, so it seemed reasonable to make more changes. The first of these was the serpentine belt run. I mostly used the belt run to reduce space and weight while still maintaining high traction wheels. After thinking about my design more, I concluded that I did not actually care about traction. If I ever get into a pushing match, I am probably going to lose that match anyway. As such, I got rid of the BaneBots wheels and replaced them with pulleys, which do not use serpentine belt runs and make everything thinner.
New wheels and bottom view of armor modifications
I then decided that the way I had mounted the front braces might result in stress concentrations near the weapon bearing mounts. I switched the mounting to pins, which resulted in interference between the pins and mounting screws. Thus, I offset the side plates and rounded the front corners of the braces. This is actually beneficial for a few reasons: 1) It makes it less likely that an undercutter blade will catch the edge of the armor plate 2) It lets me switch from plastite screws, which tend to strip out, to bolts.
Pinned front joint
Inside of the finished front block design
Outside of the finished front block design
It looks like I pretty much redesigned everything... I guess I have some machining to do.
In the past few weeks, I have overhauled almost every aspect of Event Horizon with the goal of making it more robust (Silent Spring proof). Hopefully, this will give a good overview of what I changed.
Drum
The drum is where most of the issues from the original design originated. Here are the changes made:
Added pin between sleeve and motor mount piece: This should prevent these two pieces from rotating in unfortunate ways and shearing the wires.
Added internal steel nut strip: If the teeth get ripped out, it won't trash the drum (as much). The nut strip also acts as a keyway for the piece attached to the output of the motor.
Removed clamping piece for motor output and replaced with a plate bolted to the face of the motor: The primary motive for this was reducing weight, but it also leaves more space inside of the drum for other modifications.
Moved idler bearing from drum to frame: Making those press fit pieces was a pain.
Added retaining rings and washer to idler shaft: This constrains the bending of the frame by putting the drum in tension when the two main frame pieces are pushed apart. I believe this will help prevent Silent Spring-induced explosions.
Switched to a shielded bearing for the motor side: The open bearing got filled with dirt and other crap, which made it very stiff. It is best to avoid having the bearing filled with dirt.
Added a nub in the frame piece that matches the wire cutout in the motor mount sleeve: This constrains the rotation of the drum and prevents it from pulling its own wires out as in the Gemini match at FI.
Switched to a continuous steel blade for a tooth: It is inset into the drum and should be much tougher. I may not bother with this if I don't have enough time to do the machining.
Washer added to the idler shaft
Keyway nub to prevent rotation
The internals of the drum
Full render of everything
Drive Base
Most of the changes to the drive base were aimed at weight reduction. Changes made:
Back wheel is no longer direct drive: This improves the space situation and some reduction is probably necessary for the new motors.
Front and back wheels are now different sizes: Improves packaging.
Aluminum hex hubs are switched for nylon: Weight savings.
New wheel configuration
Frame/ Armor
I made a lot of changes to the frame aimed at improving strength.
Re-worked front plate mounting: Should be less prone to screws ripping out, etc.
Extended belly pan forward: This will brace the frame further toward the front, helping to prevent the main plates from buckling.
Switched from finger joints to pins to locate center plate: Fewer giant cutouts = less bad things.
Moved belly pan holes: I moved holes to places that do not have as many holes, rather than just ignoring their existence. This should reduce the likelihood of catastrophic failures of the frame members.
Reduced angle of side armor: I did not find myself needing it at all at FI and the weight savings are probably worthwhile.
Removed middle armor support: The fewer holes the better and, again, I didn't really need the side armor.
Switched to aluminum top plate: It should be stronger (and look better) and I think I have the weight.
The new system for the front corners
What the inside looks like now (I didn't bother modeling the motors)
Extended belly pan and holes strategically placed to avoid other holes
Other Thoughts:
Hopefully I will be able to manufacture this over winter break. While there are changes that I probably will not get around to, I suspect this version will be much more competitive than the last. Some experimental things, like the brushless motors may end up not working, in which case I will switch them out for more known quantities.
I have been looking to compete in a combat robotics competition for a while, and my schedule and robot state finally lined up for the Franklin Institute NERC event (FI) this year. I spent the week before the event on the CNC and lathe making spare parts in anticipation of some likely failure modes.
My father and I took a train from Boston to Philadelphia Friday night before FI, and arrived at 4:30 am Saturday morning in a fairly sleep deprived state. After waiting around in the train station for a few hours, we walked to the venue (about a mile) and checked in.
Event Horizon went through safety inspection without issue and weighed in at 3 lb. 0 oz. Over the course of the next few hours, the pit area filled up completely. All in all, around 80 bots were present in the various weight classes.
Match 1: Silent Spring vs. Event Horizon
When the bracket was posted, I found that EH was against Jamison Go's Silent Spring for its first match. Silent Spring is an exceptionally well built and high energy undercutter and for all intents and purposes, it is indestructible. I knew this would be a hard match, and that it would likely result in a significant amount of damage.
The match started with a few very violent hits that sent the bots to opposite sides of the arena. During the first of these hits, Silent Spring's disk sliced through the bottom of EH's left front armor brace, leaving it touching the ground, and eliminating my ability to drive straight. Despite this, I was able to keep the drum facing Silent Spring for two more large hits. Looking back on the match video of this match is interesting as there is a clear difference between the hits where the majority of energy came from EH's drum and the hits where the energy came from Silent Spring's disk.
Damaged front armor brace
In the end however, Silent Spring hits the corner of EH's front right armor support plate, ripping it off. This spins EH slightly, and Silent Spring's disk tooth connects with the inside of the now unsupported side plate, causing it to buckle at the point where the first screw connects the bellypan to the side plate, which lines up with a large cutout for the center armor plate support, causing a huge concentration of stress. It appears that the screw that holds the drum to the side plate came loose due to a combination of vibration and a small number of threads being engaged, so when the side plate buckles, the drum is unseated. The still-spinning drum then hits the floor or Silent Spring's blade, causing it to jump back, dig into the frame cross brace, and be shot from the robot due to its large stored energy. I was quite confused about exactly how this happened until watching the video repeatedly in slow motion.
While this looks like fairly complete destruction, I found that many vital parts of the robot, such as the motors and electronics, were still fully functional. Since I had CNCed an entire spare set of frame members and brought the old drum, the repair process was actually fairly straight forward.
Silent Spring damage
Aside from the obvious complete disassembly, the drum itself sustained a significant amount of damage, with one screw ripped out, one shattered, and one bent at a severe angle. Moral of the story: Aluminum is too soft for this application.
Matches 2&3: <3 Winning by Default
I was able to complete the reassembly process before my next match because of the large number of bots at the event and the resultant long break periods. It turned out that this did not matter much as EH's opponents for its second and third fights did not show up, so it won by default.
Match 4: Hard Drive vs. Event Horizon
My next actual match was against Hard Drive, a bot with some sort of low power vertical spinner and a wedge.
Hard drive has exposed wheels and I was able to knock one off fairly quickly. Before I could knock the second one off, however, the set screw that holds on the back (driven) wheel on one side came loose, and left me without one drive side. EH won the match, but not as spectacularly as I would have liked.
After the match, I put Loctite on the set screws and then put the drive train back together.
Match 5: Gemini vs. Event Horizon
After beating Hard Drive, EH was up against Gemini, a double sided wedge bot. The match started off good, although I accidentally spun up the weapon too fast and flipped the bot over. I was able to get one good hit before the screw that holds the weapon on came loose and the wires spun around, pulling one of the connectors out and turning off the weapon. I tried to push Gemini around after that, but the robots are pretty much matched in pushing power and EH lost to a judges decision.
Thoughts
EH's issues in the last two matches were relatively minor and can be easily fixed by proper fastening methods (pins) in the next version.
The structural issues uncovered by Silent Spring are more troublesome as they necessitate a significant redesign of the frame. Since the bot is at weight, this is difficult.
The drum easily has enough energy to be effective and might very well have been the highest energy drum at the event.
The frame did a good job of taking impacts dealt by the drum.
The robot did not drive particularly well. It was not fast enough and the turning was too sensitive.
The drum tooth system needs a major redesign to make it multi-use.
Planned Changes
I am going to make the frame much stronger by eliminating stress concentrations and move the drive base to brushless motors to free up more internal space. This should make it possible to reduce the length of the bot and thereby reduce weight. If I can, I will use an un-geared brushless motor mounted on the outside of the frame rails to maximize internal space for electronics. I will also extend the belly pan forward on the sides to provide additional bracing.
Regardless of what I eventually do with the motors, I am going to move all wheels to dead axles.
I also need to add full front armor to better protect against spinners with sharp teeth.
On the drum, I plan to add nut strips on the inside to anchor the screws and switch to alloy steel flat-heads. The nut strip will also double as a keyway to prevent all drum pieces from spinning. The motor hub piece will be pinned to the frame so that it can not rotate.
Rather than re-build the drum in the same configuration that failed initially, I decided to modify the motor mounting to make it more robust. This was achieved by replacing the giant stock mounting plate with a mounting piece that is actually the right size.
Now that I have a large lathe, my design of simple round pieces has changed from Computer Aided Design to Lathe Aided Design.
The final motor mounting piece.
This piece is tapped and has the mounting pattern of the motor. I drilled the mounting holes using the old mounting plate as a guide, which seems to have been effective.
This slips inside a sleeve, which has a slot for the wires and is the right diameter for the bearing and frame holes. A single screw tightens it onto the frame, which should be sufficient to take the relatively minimal torque that this will see. Best of all, however, the sleeve is large enough diameter to cover the heads of the screws attached to the motor, which should prevent them from falling out.
The motor mounted to the frame side.
Everything fit together nicely, and I was able to spin the drum up successfully. The 3/4" bearing seems to have picked up some dirt (because it is open), which has added friction. I will probably replace it with a shielded bearing eventually.
Event Horizon, mostly reassembled.
As you can probably see, I have not yet re-machined the drum to take care of the screw that was ripped out. This picture also shows the new fastening system for the drum motor (the screw sticking out of the frame). If this design does not self-destruct, I probably will not bother making the hub motor that I was considering previously.
One of the single greatest factors in making a combat robot successful is durability.
With this in mind, I decided to do some testing on Event Horizon. I spent some time looking for large and relatively solid objects to hit, and finally settled on the EH 1.0 carcass with a large chunk of scrap aluminum in it to make it heavier.
I found that running into EH 1.0 slowly with the weapon at full speed produced only small impacts, which would not be sufficient in an actual match, so I rammed it at full speed. The EH 1.0 frame flew three to four feet, which is much more in line with the power that I want from the EH 2 weapon (Sorry, no video, I can't drive and take video at the same time).
Upon initial inspection, EH 2 appeared not to have sustained damage in the impact, but after looking closer, I noticed that one of the teeth (1/4-20 flat head cap screws) had been completely ripped out of the 1/4" thick aluminum drum. Also, the drum did not spin freely anymore, and the wires for the motor had been partly sucked inside of the drum.
I took apart the drum.
... and was confronted with a scene of absolute destruction. The motor mounting plate was completely deformed, and the wires for the motor (all three of them) had been completely sheared off. Interestingly, all of the screws that hold the motor mounting plate to the hub were missing, and the threads on the hub were completely intact, showing no signs of an impact. Thus, we reach the title of this post: As far as I can tell, the screws fell out at some earlier point, meaning that the drum motor had been solely held in place by its wires. When the drum hit the EH 1.0 frame, it seems that the motor flexed somewhat and the mounting plate caught on the inside of the drum somehow, bending the plate and shearing the wires. Using loctite and/ or jam nuts might have prevented this failure. Thankfully, this is an easy fix since I have another motor.
Yes, that hole was round once.
As a backup, I am also going to start development of a hub motor using the parts from the broken motor. More on this later.
The machining of Event Horizon started with a large amount of work on the CNC router. I cut all of the UHMW pieces first, then cut the aluminum side plates and the bottom plate. UHMW cuts very easily on the router. All UHMW was cut with a single flute 1/8" endmill. Aluminum pieces were cut with an aluminum cutting router bit (one of these).
Preparing to cut the side pieces
Some of the pieces completed.
I had not actually cut 7075 aluminum on the router before, and it basically consists of dumping lots of flood cooling onto the piece continuously and hoping that nothing breaks.
The bottom plate after machining
I assembled all of the router pieces, which generally fit well. The grooves in the back piece were not quite wide enough and I had to file them out slightly. This is because the UHMW stock is actually slightly thicker than 3/8", which is a common problem with UHMW.
Initial assembly of the frame
Next was the machining of the drum pieces. I did most of this with the big lathe, which I attached a quick change tool post to. The first drum piece was the drum itself.
I started by cutting the stock roughly to length using the horizontal bandsaw, and then faced off the ends to the correct length.
Drum end being faced down
Drum stock cut to length and polished with fine grain sand paper
I initially planned to drill the holes on the mill with the small rotary table, however, after fixturing the piece initially, I realized that the small rotary table does not even come close to holding large parts straight. Additionally, the locking mechanism that allows the chuck to tilt is not strong enough to allow me to drill the holes farthest from the chuck jaws without slipping. I thought of three possible solutions to this: 1) put a block under the drum piece at the end 2) buy a lathe chuck for the big rotary table 3) drill the holes with the drum piece held in the vise. I decided to go with option 3 since blocking the piece up would not solve the issue of the chuck being off center, and buying a lathe chuck for the big rotary table would be expensive. I used a height gauge and the small rotary table to mark the end of the drum every 60 degrees and then clamped the drum piece in the vise, using a parallel block as a stop. The parallel block doubles as a point of alignment for the marks on the drum, allowing me to drill the holes at 60 degree intervals with good accuracy.
Drum piece clamped in the vise
After the completion of the drum, I made the two bearing rings and the motor mounting hub. This required several hours of machining on the lathe since all three pieces have the majority of material removed from the original piece of stock. I also machined the axle piece for the free-spinning side of the drum.
Completed drum piece and motor mounted to motor hub
I managed to break not one, but two 4-40 taps off in the bearing ring for the 3/4" bearing, which means that I have to leave two of the six screws out. This is fine for now, but I will probably re-make the piece at some later point.
Frame with drum attached
At this point, the majority of work was done on the bot and I just had to finish up a few minor parts. The first of these parts were the armor support blocks, which back the front half of each aluminum armor plate. I cut these pieces from some leftover EH 1 frame stock and beveled them on the mill.
My solution to putting beveled edges on things
Next, was finishing off the drive train. I finally got around to generating G-code for cutting the pulley blanks (the pulleys have a hex bore and I don't have a hex broach), and CNCed four, which I then added grooves to on the lathe. I also machined hubs for the front wheels from some pieces of aluminum hex shaft. The hubs were a little rough while spinning on the shoulder bolts, so I put a little spindle oil on the shoulder bolts, which solved the issue. I also made the disk for connecting the motor to the drum, which I cut from the same piece of UHMW sheet that the frame came from.
EH with completed drum, side armor supports, and front wheels attached
Aside from the top plate, the bot was mechanically done. I weighed it with electronics, and found that it was ~5-6 oz. over weight, which is a lot. I removed the side armor braces, drilled holes in the frame side plates and enlarged some of the holes on the drum (also useful as vent holes for the motor). The side armor pieces should be unnecessary against all robots but powerful horizontal spinners. Hopefully, the angle of the armor will be sufficient to deal with these types of robots. After looking over the electronics system, I realized that the drive motor controller that I weighed the robot with is actually an ounce heavier than the two Vex Motor Controller 29's that I was planning to use, which helped as well. These changes brought the weight down to a more manageable 3 lb. 2 oz, so I started working on the electronics.
I have not yet mentioned the control system for this bot, so here it is:
Drive controllers: modified Vex Motor Controller 29 (2x)
Weapon: HobbyKing 40A ESC
Radio: HobbyKing 6 channel radio
Battery: E-Flite 3S 1300 mAh LiPo
Power Switch: FingerTech Mini Power Switch
I started by removing the casing of the Motor Controller 29's and cutting off the connectors. I then removed the casing from the ESC and soldered the V_in wires of the 29's to the ESC board where the power wires are attached. This allows me to eliminate the ground wires running to the receiver from the drive motor controllers, saving some weight and reducing the number of wires cluttering the inside of the bot. I cut down the output wires of the motor controller 29's, and soldered on the bullet connectors, then taped everything together in a small block. Before competing, I will make a second block for a quick replacement should anything in the electronics system decide to spontaneously combust.
Electronics block soldered together and mostly completed
I finished the electronics system by adding the battery connector, PWM connectors, and power switch, then stuffing everything into the rather limited space between the motor and the inside frame member.
Electronics system mostly installed
With the electronics system now complete, I finished the top plate and assembled the full bot.
Completed bot
In all, the weight came out to 3 lb. 1.3 oz. I should be able to cut the remaining excess weight by changing the top and bottom plates to garolite and polycarbonate, and if necessary, lightening the middle armor support blocks substantially.
And, of course, here is some testing footage:
EH is more maneuverable than it looks in this video. I am just bad at filming and driving at the same time.
I noticed a few things while testing today that could be problematic:
The weapon can spin up fast enough to flip the bot over on startup, then send it shooting across the floor. This is why you see me spinning it up slowly in the video. I might be able to use this for self righting, which would be very useful.
The turning speed is very limited when the weapon is at full speed because the gyroscopic forces cause the bot to tip and ground out on the side armor.
One of the front wheel axle bolts came loose. I will need to loctite everything in place before competing.
