One of my students wanted a straight barrel door bolt printed for his door, because his dog kept on opening it. I designed one up in FreeCad in a few minutes and printed it out with an Anet A8 3D printer during today’s CoderDojo class.

The design is available now on Thingiverse.

The design was deceptively simple. Even though the shape of the thing looks difficult, it’s actually quite easy.

First, we make the base-plate, which I set as 4mm thick for strength. Instead of making three separate pieces, I opted to build it all as one long one. You’ll see what I mean.

Next, I put a cylinder right along it. This will be the outside of the barrel. Notice that I have it overlapping 2mm into the base. The walls of the barrel will be 2mm thick.

And to strengthen the barrel’s connection to the baseplate, I added a cuboid reaching down as a tangent from the cylinder.

I grouped those together using the Union tool.

Next, I worked on the bits that I needed to cut out of the barrel.

First, there’s the cylinder for the bolt. I made sure the leave 2mm in the near end so the bolt didn’t just come right out of the lock when opening it.

Then there’s the channel that the bolt handle slides along.

When locked the bolt handle needs somewhere to slide down into. It needs to be the same width as the channel from the previous step.

Finally, I separated the barrel into three pieces. First, by cutting a 1mm gap to cut off a chunk that connects to the door jamb.

And then a slice that has two uses – first, to let us put the bolt into the barrel in the first place (we’re not printing the bolt inside the barrel, so need a way to put it in). It also acts as a place for the bolt handle to rest when the lock is open. Its width is the same as the earlier two channel cuts.

Lastly, I needed to add 8 cones where the screws should go.

I grouped all of those together then did a cut of that group from the original group I made, which resulted in this:

The bolt itself was next.

First, create a cylinder that is .1mm in radius less than the gap in the barrel and long enough to reach the jamb end from the near end of the main body of the barrel.

Next, add a cylinder for a handle. Its radius should be .1mm smaller than half the width of the channels it slides in.

And then a sphere which serves no real purpose but it looks nice.

Finally group the bolt together into one object, and pull it back in the lock to make sure it fits. If it doesn’t, then adjust whatever you need to!

When we printed this out during class today, the print was difficult to remove from the glass on the hot-bed afterwards. I found it much easier to remove later on when I printed another of them for myself. The difference was that in the class, the hot-bed was still about 60°C, while at home, I was busy with other things and didn’t notice it had finished printing for a while, so it was more like 40°C. So, if you have difficulty removing a part from your 3D printer, maybe just let it cool.