PMTH began some time early in December, when Alex and I decided we should try to build a battlebot. We didn't really know what we were doing, and the project turned out to be mostly a failure, but it was a great introduction to battlebots. We began with some ideas and material.
Ideas
Material
At first, we were planning on building some sort of automatic flywheel powered flipper, but we would later scrap the idea due to complexity and lack of space/time. We used many parts from Alex's class project, which was supposed to be a speaker, but actually was a battlebot drivetrain in disguise.
Almost all of the material shown came from the Stata loading dock. On December 8th, I found some sort of giant old industrial motor controller with some large heatsinks. As far as I can tell, they were machined from solid 6061 aluminum.
Controller board on floor of Stata dock
Cabinet with microwave
After lots of CAD, we decided we'd have more fun if we designed the robot as we made it, so we threw our designs in the trash and got to work making parts.
On January 21st, we went to MITERS to begin construction of the robot. We clamped the heatsink to the table of the Bridgeport, and removed the fins.
Heatsink on mill table
Heatsink with some fins removed
The next day, we made some motor mounts. They have little slots milled in them to fit around the plastic gearbox housing. We also made shafts from the really awesome 1/2" 7075 Hex shaft from Vex Pro.
The shaft is in the background.
After that, we made bearing blocks for the wheels that weren't driven by the motors. I started by making a rectangle
Shiny!
then bored a hole for two bearings.
There are two bearings in there.
I goofed and didn't get a good press fit, so I added some bearing retaining screws.
Two bearing blocks on MITERS bench
The next step was to cut up the heatsink plate and put holes in it.
Victor Mill with lamp and aluminum plate
We ordered sprockets from Vex Pro, which were received by a man(?) named MICHEEEL
The plates also needed to be modified to let the chuck of the drill fit.
Cutting Fluid removes Sharpie, so this was a bad idea.
Suddenly, it began to look like a robot.
I should have used a flycutter.
Our next challenge was to create the weapon drive. We looked in the large bearing drawer at MITERS and found some gross looking bearings that seemed suitable. At first, we couldn't even get the bearings to turn, but we were able to get them rolling after a bath in acetone.
Questionable Bearings.
Finding a suitable shell was difficult too. All the woks were flimsy, so we went with a really small cast iron skillet. Unfortunately, it is very small, so getting everything to fit will be a real challenge.
Robot Soup.
We also encountered an extremely unusual left-hand end mill.
WHY?
To drive the weapon, we are using an NTM Prop Drive 50-60 380KV motor with a bored out skateboard wheel.
The motor is rated for 2600 watts
At first, we didn't support the shaft, but we later created a little block that held everybody's favorite 8mm ID 22mm OD skateboard bearing.
Another questionable bearing.
To make an axle for the weapon, we used a 304 stainless steel pressure vessel we found at the loading dock. 304 is not one of the free machining stainless steels, and part temperature quickly became an issue on the MITERS lathe.
We needed to remove almost 2 lbs of material.
The chips were beautiful shades of blue, gold, and purple.
I finished the axle on one of the lathes in the FSAE shop that has flood coolant and a more powerful spindle. They currently use only one type of insert, which is terrible for stainless, so I dug through the collection of damaged/used up inserts and found something good for stainless. Running flood coolant, I could get around .100" off the diameter in each pass. This was the limit for this lathe - any more and the servo drives would give errors.
Shiny!
I put a 3.25" diameter bolt circle in the skillet by clamping it to the table of the Bridgeport.
This wasn't the first time somebody has machined a pot at MITERS.
Cast iron is cool.
We needed a way to attach bearings to the pan, so I made a set of bearing blocks. Looking back, these are probably the two best parts made in this project, and I'll likely find some better use for them in the future. The first operation was to bring the stock to size
I used the flycutter this time...
I started to create the bearing bore using the boring head on the mill, but this was too slow, so I switched to the lathe.
I gave up.
Dialing in the part was a massive pain.
Shown without bearings or bolts
On the final night before leaving, everything was very rushed. We added an idler wheel for the shell,
With delrin wheel
added some holes in the plate for mounting the weapon shaft,
Add caption
made a pocket in the bottom of the weapon shaft (I don't remember why the larger pocket is there, but the bigger one is to hold the nut from turning),
Shown from the inside of the weapon shell
chamfered the inner bearing block and removed the square part to avoid interference
it was still scarily close!
and finally put the whole frame together!
We tested it:
and brought it to Motorama, where we tested it again:
Unfortunately, PMTH was a terrible failure of a battlebot, losing both of its matches at Motorama. The drive system was absolutely terrible and the center of gravity was too high, making the robot difficult to control. It would also destabilize and dip into the ground, causing it to tip over and roll around. The 'shell' was made of cast iron, which cracked and chipped with every impact. Currently, I don't plan to build another robot for the next Motorama, but there's always the chance that Alex and I will be inspired for the next Motorama and we'll build another.
My friend builds nuclear reactors, and didn't want to spend a
small fortune at MIT's Central Machine Shop getting parts modified, so I
offered to help him out. You can read more about his crazy projects
on his blog. I
started this project with access to none of MIT's machine shops, so I could
only work at MITERS, the only shop on campus that's open to absolutely anybody.
All the other shops required safety training, or thought the parts were too
complicated.
The reactor chamber is made from a 304 stainless 6" diamter
hollow hemisphere with .125" thick walls. My job was to put 5 holes,
each 1.500" in diameter, in the hemisphere. The end product
should look like this:
I didn't have access to a boring head or a chuck large enough to
hold the sphere, so I had to get clever with a rotary table. The first
step was to align the axis of rotation of the rotary table with the center of
the rectangular stock. To make it easier, I put a collet block with the same
thickness as my stock in the vise and dialed that in. The vise didn't
quite fit on the rotary table, so one bolt (not shown) goes through a slot in
the middle of the vise with a bunch of washers.
Once the block was in the right spot, I used an indicator in the
spindle to set my zero on the DRO as the center of the collet block.
The fixture I made has a circular edge that rests on the inside of
the hemisphere and a tapped 1/2-13 (same as typical Bridgeport hold down) hole
for bolting the hemisphere down. This single bolt attachment was a little
sketchy, but I never had any trouble with the part moving.
I first tried to put a hole in the center of the hemisphere with
one of the MITERS center drills. Unfortunately it was very dull and
caused work hardening even with a lot of force – I was worried I would break
the tip of the center drill if I pushed harder. In the end, I used a 3/8”
3 flute carbide end mill with a small corner radius in the lathe tailstock, but
a sharp center drill would work too.
Next, I bolted the hemisphere to the fixture and set it up in the
vise with a 45 degree angle block. Indicating the sphere was frustrating
because the hemisphere was not perfectly spherical. When rotating around
any diameter of a sphere, the indicator should not move, but I was seeing .01”
maximum deviation near the edges. I also didn’t see the typical high/low
pattern that normally appears with a spherical/cylindrical part being turned
eccentrically, meaning the sphere must have some weird distortion. The
hemisphere appears to be formed, so this isn’t too surprising. I found it
useful to put a 1/2" diameter steel rod in the drill chuck and line up the
sphere so that the plane of the bottom of the rod is tangent with the sphere to
get an approximate location of the sphere before using the indicator.
-Notice the bolt holding down the vise!
The hemisphere is a terrible shape for ringing and chatter, so I
had to go quite slow. I found that at a certain feed rate the part would
make a pleasant ringing noise similar to a tuning fork. I was using one
of my end mills – MITERS had nothing that could cut stainless. I used a
½” carbide six flute coated end mill. I was taking .05” to .10” wide
passes at 1100 rpm and around 3 ipm or so. The chips created were sharp
and got stuck in my hands, which was very annoying. The finish left
behind was quite good.
Once I had one hole done, I walked in to the Edgerton Student
Shop, which has nice equipment, and asked if I could continue machining the
part there. After showing the instructor there that I was able to make one of
the holes, they were much more willing to let me work there. The guy there did
not like my fixture, so he suggested that I use a very small end mill
to reduce cutting forces. I used a 3/16” 3 flute carbide end mill to make
a .01” undersize hole first using a CNC ez-trak Bridgeport. It was one of
the fancy 3 axis ones that had a canned helix cycle, so I set it up to
continuously ramp down as it cut the circle. I used a .04” deep cut at 6
IPM. It was completely happy slotting at this depth, but sometimes
chattered before the cutter was fully engaged. As I did more holes I
cheated it down so the cutter would be fully engaged sooner. I got a
great surface finish with a .005” wide finish pass at full depth and 8
IPM.
