Sunday, November 30, 2014

Servo Spline Adapters

My transmission design for the Mini Combat Robot uses large servo gears. Unfortunately the output from these gears is a spline shaft. I had to come up with a way to easily adapt to the spline without making a sketchy connection with the servo horns that are meant for those servo gears. My first technique for making a servo spline can be done with a milling machine. It's a fairly straight forward and doesn't require special tools.

I put a "blank" shaft into a collet block. The collet block isn't required, however holding a round part vertically in a vise can be difficult to align. It also has a good chance of slipping which could break tools and ruin the part. The first thing I did was drill a set of starter holes. I used a small carbide ball end mill because it was the only tool that could make a mark smaller than the drill bit I wanted to use. Each hole corresponds to one of the teeth in the spline. I designed the holes so the outer edge would meet up with the tip of each spline tooth.

Once I made all of the starter dents, I proceeded to use the final size drill bit. Each hole was very close, but none of the holes intersected. If the holes intersect the drill bit will likely drift and break. The final step was to mill out the center. Milling out the center creates the inner part of the spline. It is important to design the geometry such that the leftover wall between the drilled holes fits between the spline teeth. I simply plunged and endmill down to the desired depth and let it swirl around to the correct diameter. This opened all of the holes drilled to the center.

This is the final shaft. The part fit snugly on the spline and didn't seem to damage the spline even under loading conditions. I used this part on the Mini Combat Robot until a design iteration forced me to a 3D printed design (I no longer had access to a mill). Any mill with CNC or even a digital readout can produce splined holes.

This is a 3D Printed replacement for the metal adapter. I needed a different pulley, but I couldn't use the metal spline shaft for the upgraded design. I decided to 3D print an adapter instead. I used the same geometry as the metal spline and simply printed a new adapter. It also slipped right on and worked first try.

Here is the 3D printed servo adapter as well as a splined shaft that copied the original servo spline. I doubt an FDM type 3D printer could produce the details required for this spline to work, so I'm glad I went with the SLA type 3D printer (Form1+). Making giant servo splines led me to test miniature servo splines.

The first part to my process to make a servo spline is getting a picture of the spline itself. I use this picture and one reference dimension (the outside diameter of the spline) to trace the spline profile. This seems to be pretty reliable and is able to get details that my calipers can't measure.

Here is a screen shot from the CAD I used for the servo spline. I get the spline dimensions by tracing the profile that comes from the drawing. It is surprisingly fast to CAD this way. The camera image reveals a lot of details that my measuring instruments won't capture.

Here is a servo spline made for a micro size servo (9g servo). The printer was able to handle the small details required to make the spline.

The gear fit perfectly onto the servo first try. I attempted to strip the spline, however I only managed to cut myself with the 3D printed gear teeth. I was unable to get the spline to skip on the servo.

For future projects using large servos I will attempt to make more metal shafts and splines. I find them more durable than the 3D printed parts. I also know the servo gears will strip before the metal spline slips. The 3D printed splines for smaller servos are too awesome. I'm still amazed the printer can handle details like that. The cool part is these splines can be put into any 3D printed part. It doesn't matter whether the part is round, square, or even a hexapod leg!

Saturday, November 29, 2014

Mini Combat Robot Update... again

I decided to use my new 3D printer to solve any engineering problems with the mini combat robot. The first problem was making the motor controllers fit in the space I left for them.

Here is the motor controller mount as it came from the 3D printer. The part wasted a lot of material in the supports. I designed the part to fit the motor controllers perfectly into the robot frame. This part might be machinable, but I would avoid making this part if I didn't have a 3D printer.

The new motor controller mount allows me to fit the wires in to the available space. It may look messy, but I'm just happy I could make the motor controllers fit into the frame. I knew the motor controllers would fit into the frame, but I didn't account for the wires or connectors. These left the space a bit too cramped to properly mount any of the components. The new 3D printed mount allows me to securely mount the controllers without risking damage during combat matches.

The robot looks pretty slick. I was able to replace the old 3D printed parts either with properly machined components or parts printed with my new 3D printer.

This underside view of the robot shows how the motor controller fits into the robot. I still need to replace the top and bottom plates with real armor instead of thin acryllic.

I didn''t quite mange to finish the robot before losing machine shop access. I've been able to replace most of the poorly 3D printed parts with either higher quality plastic components or metal components, however there is still a fair amount left to finish the robot. The biggest thing the robot needs is a spinner.

Saturday, November 1, 2014

Quick Project - Evil IronMan

I got invited to a Halloween party about two hours before the party started. The party had a mandatory costume policy. Being an engineer and a new owner of a 3D printer I figured I HAD to 3D print my costume. The first thing that came to mind when thinking of 3D printing and Halloween costumes was an arc reactor. I've seen a bunch made on the internet and even a few for various Halloweens in the past. I remember that I had a box of LEDs. Unfortunately, I discovered that I only had two blue LEDs. Luckily I also had eight red LEDs... so the obvious costume became evil IronMan. I spent about 30 minutes drawing up the part in SolidWorks and then sent it to the printer. The print time was a little under two hours (it's cool to be slightly late to parties). While the part was printing I had to make the wiring.

I pushed each of the LEDs through a 1:1 drawing of the frame. I soldered all of the LEDs together with some solid wire to give some structure to the electronics. I also used two bent spiral wires for the + and - connections to the battery. I tore away the paper to release the electronics for the final assembly. I didn't use any current limiting resistors or a proper current driver because the battery voltage was lower than the rated voltage for the LEDs. I also figured that the small coin cell battery wouldn't be able to supply enough current to fry six LEDs in parallel.

Here's a top side view of the arc reactor. Each of the LEDs stuck out from the frame to ensure maximum brightness. It also saved some material and print time.

This is the back side of the arc reactor. I have paper shoved between the battery and the contact to prevent the LEDs from turning on. I didn't have a switch so the back has to be unscrewed to connect and disconnect the battery.

This is the arc reactor shining through my shirt. The camera doesn't do a very good job showing how bright the LEDs looked shining through my shirt. I used lots of tape to hold it in place.

Surprisingly the arc reactor stayed lit up all night. I didn't even have to break out the spare battery and hex key that I kept in my pocket. The design wasn't too great, but im pretty proud of it considering it went from concept to finished product in a little over two hours!

Saturday, October 25, 2014

New Machine - Form1+ 3D Printer

After thinking it over for a few months I decided the usual FDM type 3D printer I was building (the kind that squirts out plastic like a hot glue gun) wasn't going to meet my needs. I just moved and no longer have access to mills, lathes, or other metal working equipment aside from my trusty file and hand drill. I decided a 3D printer was the only way to continue my projects without making a mess in my apartment. After looking around at the different available 3D printers I came across the Formlabs Form1+ printer. This printer uses SLA technology. SLA printers use a photosensitive resin that solidifies when exposed to a light source. The resin in this printer cures with a UV laser.

I need a printer than can make parts with fine details, high accuracy, and decent strength. Although I'd love a printer that could make metal parts, however I'm a few orders of magnitude away from being able to afford one. I decided to settle with a printer that makes plastic parts. The FDM printers (like the one I am/was building) are nice because they have become very popular. The popularity has driven down the material costs which makes the printers very affordable. There is also a large number of users who have lots of great tips and tricks for getting high quality parts. Unfortunately the nozzle design in an FDM pritner limits the minimum features size. The fusion of layers through heat also tends to leave voids which causes very different properties depending on a part's orientation in the printer. This can make "engineering" grade parts difficult to make.

The SLA printers have only recently become available to hobbyists, so there is not a very large number of users. The material is somewhat proprietary, so it costs 3-5 times more than an FDM printer for a given volume of material. The quality from the SLA printers can't be beat by the FDM printers. The laser curing technique is able to make much finer details in the parts. I expect to be printing gears that are 32 and maybe even 64 pitch. The parts are also solid, so the material properties are going to be the same regardless of the part orientation. There should also be fewer voids in the parts, which will make "engineering" grade parts easier to produce.

Here's the printer in all its glory. I personally find the printer aesthetically pleasing. It also fits well on my bookshelf.

Here's the setup for my first prints. These parts are shock spring mount replacements for my rc car. The original ones no longer fit properly due to my machined replacement parts not being the exact same geometry as the original components.

The printer readout is pretty cool. It tells me the remaining print time and the number of completed layers. The remaining time is exact and can help me schedule my time around the printer so I can be as productive as possible.

This printer is upside down compared to FDM printers. The table rises out of the resin to grow the parts. It's a pretty neat concept and seems to work well.

Here are the final parts after removing the excess resin with isopropyl alcohol. They came out perfectly first try. I'm sure more complicated parts or parts with tighter tolerances will require some test parts, but parts like these take nearly zero effort to make.

Overall I'm pretty happy with the printer. I have more project ideas than ever now that I can make parts without labor. I'll be running test parts for things like gears and press-fits. For now I'm not going to finish the 3D printer I started making. The FDM printers can't make the parts I want to make. It is a good XYZ CNC platform, so I should be able to use it for another project some time in the future.

Sunday, July 20, 2014

New Project - RC Car

I've wanted a real RC car (not one of those tiny indoor only RC cars) since I was a little kid. Now that I'm a big kid I can just go out and get one. In this case I mail ordered a relatively cheap 4WD 1:10 scale buggy. The frame came as a kit. I had to pick out the motor, motor controller, radio, and batteries. I already had the radio from the mini combat robot, which cut the cost of the car. The reason I decided to buy a RC car instead of building one was that I didn't want to build something and then decide it was boring. I figured a cheap RC car would require repairs and could have an opportunity for many custom parts.

Here's a sweet action shot of the car before parts started breaking. I put a "somewhat" larger motor and battery in the car than suggested. The car's peak speed is over 40mph! (timed by driving on a football field) It probably has more power than I can control... I flipped it and hit things a full speed many times as well as taking jumps that brought the car around head height in the air. Needless to say the cheap plastic parts started breaking real fast.

Here's the machining of a new shock absorber mount. The original part didn't break, but it bent ~45 degrees. I decided a metal version would be a good first upgrade for the car.

Here is the aluminum part next to the original (which I bent back). I removed some of the extra "adjustment" holes that were in the original part. I also remade the pockets to make the part considerably stronger. I don't expect this part to break before any of the other parts on the car.

After a few more jumps I busted the front A-arms. I didn't have a spare axle so I was forced to bend this one straight again. Thankfully the axle was made from cheap steel and I have decent pliers.

Here is the A-arm that broke. The plastic split at the axle. This part is not a simple 2D part like the shock mount.

I started the part from flat aluminum plate. I forgot how slow it is to mill with a 1/8" endmill...

I wanted to save material so I did a REALLY good job orienting the stock on the mill table.

Here's a before and after picture of the side machining. The parts required a total of 3 sides of machining each.

My trusty mini-vise did a great job holding the parts. I also tried a set of carbide drill bits. They seemed to drill much better than my sketchy old drill bit set.

The new A-arms look pretty nice on the car. The shock spring mounts rub the aluminum a bit. I'll need to make some replacements to prevent any possible damage to the shocks.

Here's the whole car with its cover attached. The rear wheels are pretty worn down. Those will have to be replaced soon as well.

Here's the car without the cover attached. The electronics are fairly compact. I've been very impressed with the motor and motor controller combination. Neither has melted yet!

Overall I'm glad I bought an RC car instead of making one from scratch. I really wouldn't be able to beat the price I paid for the frame. When a part breaks I can have fun making a replacement. Eventually the whole car will be custom!

Monday, March 24, 2014

Mini Combat Robot Update

I tend to prioritize class work rather than my own projects, so I haven't made too much progress on the Mini Combat Robot. Hopefully I'll find the time to finish this project before the semester is over and I lose access to the nice CNC machines at school. 

The frame started to come together nicely. I quickly held the unfinished parts together with a few clamps just to see how the frame looked in person. I have quite a few 3D printed parts on the robot. These parts are low quality compared to nicely machined parts. I want to see the robot assembled as quickly as possible, so I decided to 3D print a number of non critical parts.. The 3D printed parts should hold up well enough to drive around , however almost all of them will need to be replaced before the robot goes into combat.

The 3D printed transmission mounts might stay... They seem to be strong enough to handle the torque output from the transmission. They're also green which matches the motor's color.

Here is right after I finished drilling and tapping all of the cross bars. this is the first time the robot has actually looked like the final product in the CAD. The robot uses "tank treads" which are made from timing belt. They took less space than wheels and seemed like a simpler, more reliable way to make the robot move.

These are all of the frame parts. Although they aren't the simplest shapes, they can all be made with a standard milling vise setup. They took longer to machine than I had estimated, but I guess that is always the case with machining. Cutting the 45 degree angles took some patience, but they shoud be worth it if there are any other robots with spinners. Hopefully the beveled edges will help the robot deflect any kind of spinner weapon.

This is the state of the robot for now. It is by no means ready for combat. It still needs a spinner, replacement parts for the currently 3D printed parts, electronics mounts, as well as top and bottom armor plates. I'll try to finish this project before the semester ends, but I can't make any promises...