More Testing - 3D Printed Gears

I finally needed to print a functional gear. Driving my RC car in the snow was a bit too much for the main reduction gear (My 3D printed A-arms seemed to hold up fine this time). Snow got into the gear (due to a very poor design that left the gear exposed) and was turned to ice by compression from the pinion gear. The built up ice seemed to push the motor out of the correct meshing distance. The motor then proceeded to grind away most of the teeth.

The snow was deeper than this in most spots. I took this picture after the car had already stripped the teeth from the main reduction gear.

I put the car in the bathroom to wait for the snow to melt. The electronics avoided the water for the most part, so the only damage  done to the car was the stripped gear.

This is the slot in the bottom of the chassis that leaves the gear exposed. The replacement gear is already in the car at this point. I managed to get a face full of gear teeth after a second or two of run time. I didn't have the car on the ground, so the forces on the gear teeth shouldn't have been too high. The gear teeth seem to have shattered. I think this is due to warpage in the gear after it originally printed. The portion of the gear teeth touching the support material had significant warpage. I found this was a problem in my early gear tests, but I decided to try running the gear anyways. I increased the spacing between the pinion and the reduction gear to compensate for the wapage and prevent binding. I also post-cured the gear, which increases the strength and hardness. The post-cure also made the gear more brittle which would explain the shattered teeth.

It's hard to get a good image of the gear without removing it from the car which takes a while. I didn't want to take apart the car until I had a suitable replacement ready to test.

I tried printing with my black resin instead of the clear resin. I had hoped the black resin would have less warpage than the clear resin.

The gear in the bottom left is the original gear. The bottom right is my new replacement gear printed in the black resin. The top two gears are spares that I printed, but didn't release from the support material.

I used a machinists square to visualize the warpage on the gear teeth. The majority of the gear has gear teeth that are square to the faces of the gear, however a small portion (~20%) has slanted teeth like these. This makes it impossible to have a proper gear spacing without binding. I will keep trying different orientations and possibly different gear geometry to avoid this waparge, but for now I may have to order a replacement set of gears for my RC Car.

RC Car Replacement Parts

This time the rear A-arms on the RC car broke. I may no longer have access to a milling machine, but now I have a 3D printer. I modified the CAD for the front A-arms and had a set of rear A-arms printing in a few hours. These parts took a mere 5 hours to print. They could have taken less time, but I decided to try printing with the high layer resolution setting (0.025mm per layer). I also tried printing the parts directly on the table. Technically this is frowned upon with this printer, but the parts came out mostly fine. It saved a ton of support material, so I can live with any defects caused by printing directly on the table.

The car looks pretty slick with all these custom parts!

I also added the shock spring mounts. These prevent any binding between the shock and the replacement A-arms.

Unfortunately... I went too hard with the car shortly after making the new parts...

My replacement shock spring mounts took up more space than the originals. This prevented the shock from reaching its end stopper. The end stopper is an internal piece of rubber that prevents the piston from bottoming out and smashing into hard plastic. The car hit a hard landing after a jump and slammed hard plastic into hard plastic. The 3D printer parts can be post cured (which I did). The post curing greatly hardens the plastic, but also makes it more brittle. I won't do that with a future set of A-arms.

I'm starting to learn the material limitations from the 3D printer. The resin cures to be a somewhat brittle plastic. I have been post curing all of my parts (exposing them to additional UV) to make them harder. The parts that come out of the machine can be somewhat flexible and have a soft surface. The gears I've made need to be hardened or they will wear out quickly. This application of the plastic is better soft and slightly weaker. The added flexibility lowers the forces on the parts and prevents shock loading. I'll also look into making certain portions of the A-arm thicker. Holes - especially tapped holes become stress concentrators. These create points where a crack can propagate and make the part considerably weaker.

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.

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!