Projectile accelerator (concepts)

A project log for Stickvise Crossbow

Every electronics bench should have a crossbow. First crossbow project on

alex-richAlex Rich 12/20/2015 at 17:123 Comments

"Why did you spend four nights on the computer just to make a pencil do this? (drops pencil straight down onto the ground)"

-Alex's Wife, 2015

I was close to submitting this project to the tip line for fails of the week, but then I decided I had to try a bit harder to make it work. Here are the things I am fighting right now:

  1. Too much energy wasted fighting inertia (m*a)
  2. Not enough spring travel (x)
  3. Not enough spring stiffness (k)

here are some ways to improve the situation:

1. Reduce the amount of energy wasted fighting inertia.

This sounds self explanatory, just reduce the mass of the moving parts right? Wrong! The Stickvise movable jaw is not something I can easily change so I am going in the other direction -- I am going to try reducing the acceleration of the heavy parts. In other words, I want the movable jaw to gradually move forward never reaching a very high speed.

You might be wondering how this will increase projectile speed...

Well in my previous log, I showed that we are moving 58 grams of mass: 55 grams is in the movable jaw parts and only 3g is in the pencil! What happens if the jaw top speed approaches zero and I dump closer to 100% of the spring energy into the pencil?

This is easy to calculate using the conservation of energy equation again, but this time just use 3g (.003 kg) for the mass of the pencil (recall k = 2700 N/m and spring travel is 0.00948 m).

The previous velocity was 2 m/s so we have an increase of 350% if we use all the spring energy on the projectile rather than on the movable jaw parts. To do this I need to implement a clever mechanism to keep the heavy stuff moving slow and the projectile moving fast. Imagine stepping on a garden rake and having the long handle swing up and hit you in the face. That's what I want the movable jaw to do to the projectile.

2. Increase spring travel

Spring travel is limited by the shoulder screws shown below, the current ones are 20mm long, McMaster sells longer ones so I am going to buy some 45mm shoulder screws for a couple bucks and see what that does. Of course this does add mass, but theoretically this won't hurt me if I can keep that mass from accelerating too much.

3. Increase spring stiffness

Stiffness goes at odds with spring travel, the longer a spring is the lower its spring constant is. To make sure this isn't a problem, I am going to order some ridiculous die springs with stiffness of 10,507 N/m (60 lbs/in) each for a total of 21,000 N/m in parallel. Of course these cut into my spring travel because they don't compress very far, but I think that tradeoff will work out ok. See below for a comparison of new springs with the standard Stickvise springs. They might break my trigger mechanism but that's ok, I'll cross that hurdle when I get there.

The final measure I am going to take if necessary is to add a third spring to the center shaft. This will bring the stiffness to 31,500 N/m which is 11.7 times the stiffness of the stock vise (2700 N/m)

Theoretical results:

Assume we have a clever mechanism that puts all energy into the projectile, and assume upgraded springs and spring travel, the velocity works out to:

This is 55.5 miles per hour, not bad! At this speed a projectile would travel over 29 meters horizontally before hitting the ground. Of course this is the upper limit, we'll see how it goes.


Alex Rich wrote 12/20/2015 at 23:25 point

Thanks Martijn!  Actually I have tried something similar, but it involved wrapping an elastic band around the fixed and adjustable jaws, then if you loosen the wing nut the trigger will release the whole adjustable part of the vise.

The problem with this is the weight of the adjustable side, it's too heavy to get good speed.  We'll see where this goes, maybe I will make a version that is more classical, just releasing a bow string.  Honestly it sounds like it would be fun to build a regular crossbow, not all that difficult either.

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Martijn wrote 12/20/2015 at 22:02 point

I like your crossbow design, it's really nice. I was wondering , could you try reversing the adjustable jaw, then you could use the spring action to pull a string looped round the two front corners.

It would mean you'd have to redesign your crossbow though.

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Alex Rich wrote 12/20/2015 at 23:25 point

ah I never remember to use the reply function

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