SpudFiles

JoergS wrote:Stu,

a smooth hard object works fine, but in a slingshot, you try to avoid anything that weighs. Means, there is little enough power. Also, hard objects are a serious danger in case the rubber tears.

In your design, I don't think the weight matters, and you have put on a shield for your face anyway. So for your gun, this would work.

Covering the knot in rubber would work, but then the assembly is tough because of the friction.

Here is a picture that shows how rubber typically breaks:

Jörg.

Your photo seems to indicate that melting the cord creates a hard zone which is causing the rupture.

Have you tried not melting the knot ends?

Do you cut off an inch and reuse the rubber?

I finished a 1/4" Type L tubing 24" long piston sealing barrel insert yesterday.


I used a short piece of drilled 3/4" oak dowel, a groove and an O ring.

JB Weld Epoxy used to seal the copper to the oak is curing as I write.

The muzzle attachment is very simple and I can slide the barrel to any position within the 3/4" chamber and lock it. I can 'play' it like a trombone.

Testing should be very interesting.

Your chrono tests inspired me to eventually move the crosspiece forward for 400% elongation. (V=aT)


BoyntonStu

Member of the gray beard slingshot fraternity.

Stu,

true, the molten ends can damage the tube. Usually, they are aligned so they never touch the rubber walls.

However, I had already shortened the tubes two times, they are just about finished. It is the draw factor of 550% that kills them quickly.

The silver tubes hold up longer! They just have much thicker walls.

But shooting the green tubes is fun, it is so easy to draw them out.

Maybe you should consider a light "training setup" too, with bands that you can easily cock like a crossbow and not string by string.

Jörg

I shot my first projectile a few minutes ago.

.25" diameter steel slingshot ball, muzzle loaded with a piece of greased cloth.

I aimed at a piece of pine wood about 4 feet away.

A single pair of Silver bands. I can add 4 more pair.

The pellet made a 1/4" deep dent into the wood and it bounced off.

I was impressed at how fast it seemed to travel.

Thwat!


The gun held together fine.

The spoon trigger was very smooth.

It has proven the concept that a 3/4" piston powered by rubber, will drive a 1/4" ball with compressed air.

I need a chronograph.

Next, a video.

BoyntonStu

First test shots.

It works, but I'm still not convinced of the benefits of this system compared to a much more compact high pressure pneumatic that would easily have gone through that board.

Still, this is the concept you've chosen so I'm not going to knock it Some constructive criticism if I may:

One thing you might want to add is some rubber tubing to the breech that will hold the projectile in place, this will allow more pressure to build before it starts moving and should increase power considerably, assuming your barrel/piston setup is airtight.

I would also consider a much shorter piston stroke but with a wider diameter piston and more rubber, to develop the power more quickly instead of over a longosh distance. This would also serve to keep overal length down.

One other thing, while in general longer barrel -> more power, this is not always the case, there is always an optimal barrel length above which you will actually start losing power.

How about adding a burstdisk between the piston housing and the barrel?

iemand wrote:How about adding a burstdisk between the piston housing and the barrel?

A burst disk would be possible.

It could be breech loaded snug against the 1/4" barrel and the 3/4" fixed piston.

My next steps will be to add more rubber tubes and adjust the dead space for maximum performance.


After that, perhaps a burst disk.

Thanks for your suggestion.


BoyntonStu

wait...
I know...
There is no spoon...

Brian the brain wrote:wait...
I know...
There is no spoon...

Do you speak with a forked tongue?

BTW How fast did the marbles shoot from your 1:2 launcher?

Joerg and I are working on a mechanical velocity multiplier.

Joerg has successfully launched a projectile over 400 fps using 300 fps rubber.

I am working on a method of quickly drawing 200 lb of force with little effort.

My goal is to exceed the performance of arrow compound bows.

All of this effort thanks to your marble launching invention.

I wish that the 3 of us lived near each other.

Do you have Skype? I am BoyntonStu.

Brian the brain wrote:There is no spoon...

Quick, to the archives!


It pays for an occasional reminder that Jack has too much time (or that he sometimes wakes up too early in the morning, as he claims). Also, we have to keep the Spudfiles memes alive.

... Look, Elephant!

wish that the 3 of us lived near each other.

I believe you are in Germany right?

Not that far from the Netherlands now is it?

I have never measured the speed of the marbles coming out of the rubber band gun...but I did shoot one halfway into solid wood..

I believe arrows would work much better...more mass..and pointy..

My goal is to exceed the performance of arrow compound bows.

That is well within the realm of possibilities...