It's safe to say that 3D printing has moved past trinkets and toys and onto more interesting things. From large scale objects like cars to functional 3D printed motors, it seems that almost anything can be 3D printed. In order to build these huge or complicated items, one must work around the limitations of 3D printing and make careful design choices. A key shortcoming of 3D printing is the strength of the parts. With this in mind, I pose the question: is it possible to 3D print a powerful full sized crossbow?
Yes, in fact, it is.
Materials
This crossbow is made mainly out of 3D printed parts, but some hardware and other materials are also necessary. A few lengths of 1/2" and 3/4" aluminum tube were used to form the channel for the arrow (or bolt if you prefer). The prod (bow) is made of a piece of 3/4" PVC with two lengths of fiberglass driveway marker inside, and has a draw weight of 85 pounds at 20".
Various bolts and nuts were also used. Everything else is printed in PLA.
Design/Features
Obviously, there are very high levels of stress involved in a crossbow. Combine this with the fact that plastic, especially when 3D printed, is not especially durable, and you have a bit of a design challenge. To prevent the crossbow from breaking, the majority of the stress is placed on the aluminum tubes, which are plenty strong. Also, all of the printed parts are printed in such a way that the forces won't separate the layers.
The trigger mechanism is a three piece system inspired by this video.
Obviously the part that holds the string is taking the most force. By bolting this piece through the aluminum tube, all of the stress from the bow is placed on the tubes, and not the plastic trigger housing.
The other trigger pieces also need to be strong, but they are only under a direct compressive force, and don't place a shearing force on the housing. All connections between the aluminum and the plastic are bolted right through the tube for maximum strength.
The bow has a fairly simple shoulder stock that offers a bit of length adjustment.
There is also a Picatinny rail on the top of the handle and trigger housing to allow for a scope or other sighting device.
Testing
I must admit it was a bit terrifying setting this crossbow up and testing it. There was a lot that could go wrong, and I half expected the thing to explode. Thankfully that didn't happen. I did, however, run into a different issue; when the bow was fired the string would go right past the arrow and essentially dry fire. This was rather frightening, and the first time it happened it sent the bow flying off of the stock. It's sort of amusing when you test fire a crossbow and the arrow stays put while the actual bow goes flying forward.
This issue was fixed by securing the bow more solidly to the stock and by adding a spacer which prevents the string from leaving the trigger mechanism without hitting the bolt (you can see this spacer taped on in the video). Once I made these fixes it worked! Mind you, the trigger is a bit hard to pull (this is due to poor leverage and less than ideal engagement angles), but overall it is a success. I consider this bow to be more of a test than a useful device, but as you can see in the test video, it definitely works! While the 85 pound draw weight is lower than most crossbows, the extra long 20" draw length gives it some extra power, and it hits pretty hard. After about 50-60 shots the trigger system has become less reliable, causing the bow to only cock about 50% of the time.
Stay tuned for my next crossbow, which also features a 3D printed trigger mechanism, but is much more reliable and has a far lighter trigger pull.
Note: The DIY Dudes do not condone any form of violence against people, animals or anything else. The crossbow built in this post is a proof of concept done in the name of science. We take no responsibility for any injuries or property damage resulting from attempts to recreate the subject of this post.