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!