Ratchet and Pawl Mechanism

servo-driven ratchet and pawl

I've been working on a project that requires some precision motion control, rotating an object 360 degrees in 12 steps. At first, I tried using a stepper motor but it drew too much power, ran hot, and was too heavy.  Worst of all, it wasn't strong enough to turn my load. A servo would be a better solution but most servos are only capable of 180 degrees of motion. How do you get 360 degrees of rotation from the limited throw of a servo? A ratchet and pawl looked like it might work.

drafting the ratchet

I looked online for an existing design but I couldn't find anything that would work for me. So I drew one up in 123D Design. It was pretty easy; simply draw two circles to define the top and bottom of your teeth. Sketch one line from the center point to the outer edge of the circle. Then use the Circular Pattern tool to replicate that line around the circle as many times as you need (once for each tooth). Then use the 3 Point Arc tool to draw the back edge of the tooth and replicate that arc just as you did the lines. Lastly, just extrude and remove any unwanted parts. The arm and catch aren't nearly as critical, just sketch them to fit the profile of the teeth.

the finished design

With the design finished, I printed the parts in ABS at 230°C with a brim. Since the appearance isn't critical, I printed with a 0.2 mm layer height at 80 mm/s. I mounted the pieces to a scrap piece of lexan and used some small springs to tension the pawl and arm against the ratchet. Using an online calculator I determined that for a 30° arc, the servo arm needed to be about 19 mm long to give me the necessary linear motion from the ratchet arm.

servo connected to arduino

I used an arduino to test the mechanism. I simply modified the Sweep code from the Servo library and shortened the sweep from 180° to 30°. To my surprise, it worked on the first try. I modified the code a few degrees to dial the servo motion in perfectly. This is a nifty little design that should come in handy in the future. Next I'll print a hub to mount this to the part I need to rotate.

Files are available on Thingiverse and YouMagine.

Pen Holder

I like to collect all-metal pens. I have a small collection that I would like to display on my desk rather than have them hiding in a drawer. I figured I could print a nice one but I was not able to find a design that I liked. A few days ago, I came across a design by Karas Customs on Kickstarter. I loved the look of it, but I couldn't justify the $75 price tag just to hold my pens. So I decided to draw up something similar in CAD and print it.

design in 123D Design

Drawing the holder was pretty easy. It is just a cube with 9 holes in it. But I wanted to add a custom touch, my company logo embossed on one of the sides. With my limited CAD experience, that turned out to be a bit more difficult than I expected. I am sure there are easier ways to do this, but here's how I did it.

sketching over stl object

First I opened a .png of the logo in Cura and created an .stl object just as I described in this post. Then I opened the new .stl file in 123D Design. The file had way too many faces and I couldn't manipulate the object to extrude it. So I lowered the object below the plane and manually traced a new sketch on top of it. Then I deleted the object and just saved the new sketch. Next I opened the pen holder file in 123D and inserted the new sketch.

sketch of logo with holder

Finally, I just moved the sketch into position, scaled it to fit, and extruded it into the pen holder. Then I exported the file as an .stl so I could open it in Cura and print. It sounds easy enough but it took me several tries to get the workflow correct. Again, there is probably a more efficient way to do this, but this worked for me. Like the original, I added some recesses in the bottom for rubber feet.

final design in Cura

I printed it out at the normal quality defaults. After 19+ hours, I had a really nice holder for my pen collection. It's  not as sexy as the original but I really like it and the logo adds a nice personal touch.

Replacement Nuts

cracked nut

Summary: Using 3D printing to fix things around the house.

Today I noticed one of the nuts was missing from our floor mop. Upon closer inspection it became apparent that the other nut was broken. I could easily have grabbed a pair nuts from my shop and used those as replacements, but where's the fun in that?

I quickly designed a replacement in 123D Design. I changed a few measurements to try and make the new nuts stronger. Then I printed out a pair on the UM2 at normal quality.

What do you know, they fit! There were a few design challenges. I tried to size the hole so the screws would cut threads into the nuts. The size of the screw hole was critical. Too small and the nut would crack. Too large and the threads wouldn't catch. I also added a chamfer at the bottom of the hole to make it easy to start. I guess I got it right because the screw is on nice and tight without cracking. I needed pliers to screw it on the first time, but after that I could do it by hand. I also needed to make the nut strong. I meant to print it solid but I forgot. But luckily it looks like the 20% infill was sufficient.

problem solved

The new nuts work perfectly. And as a bonus, the print lines give the nuts a no-slip texture! Maybe this wasn't the most practical solution but the blue plastic knobs look much nicer than a pair of steel hex nuts. And who wants to use a wrench on their mop anyway? I wonder what else needs fixing around the house?...

The files are available on YouMagine and Thingiverse.

Printing Airgun Targets

printed target in blue

Summary: First print of original design. Low quality print setting needs work!

If you read my other blog, you will see that I love shooting airguns. One of my favorite targets are Daisy Shatterblast disks. They are biodegradable plastic disks that explode into pieces when you shoot them; much more fun than punching paper. Last weekend as I was shooting, I managed to hit one of the stakes that holds the disks and broke it. No big deal, I can always print a new one I thought. Then it occurred to me, Shatterblast targets are biodegradable. PLA is biodegradable. Why not print my own disks in PLA? I tend to shy away from shooting the Shatteblast disks because I have to mail order them and once you add shipping, they get a little pricey. Especially considering that you can only shoot them once. So my supply is limited. If I can print my own, I can get more disks any time I want.

I took some measurements from one of the disks and went to work in 123D Design. The design is simple, just a bunch of extruded circles. It only took a few minutes to draft the design. Once the model was complete I saved the .stl file on my computer and loaded it into Cura. I don't need these things to look nice considering I am just going to break them, so I printed at the low quality setting. It took about 17 minutes. I also tried printing without glue for the first time. The print stuck just fine and I think I would have had a hard time removing the print without damaging it if I had used glue.

low quality print

Low quality? They weren't kidding. The top layer wasn't nearly complete and there was so little material that the disk is flexible. I am going to have to ask on the UM forum if this is normal or if I should expect better results from the low quality setting. Based on this, the LQ setting looks pretty useless. Let's try that again at the normal quality setting. The normal quality print took 46 minutes.

normal quality print

Now that's better. The normal quality print came out great. The dimensions are spot on and the disk fit perfectly in the target stake (see first photo). But how does it work as a target? I reluctantly shot the printed target with my Crosman 1400 .22 air rifle at 3 pumps. The pellet punched a nice hole in the disk but the disk did not shatter the way the Shatterblast targets do.

both targets shot with same rifle

Hmmmm. Despite the brittle nature of PLA, it holds up better than I expected. It could be because of the 20% infill. It looks like the infill lattice almost acted as a frame to isolate the impact from the shot. Perhaps if I printed it with 100% infill the disk will shatter? But that would take too much material and take too long to print. Not going to try it.

rear view of hole

This was a fun little experiment but I don't think 3D printing is an efficient way to make breakable targets. Maybe I can make targets that can withstand an airgun shot without damage? Perhaps with ABS? Then 3D printing might make sense for targets. But that's a project for another day.