Karen's trebuchet project

In May 2005, I received an email from someone that had used my plans for trebuchet #1 for a school technology project.  I was delighted to get some photos of the treb, it was beautifully built.

I shan't blither on too much, as there's already plenty of preamble on my pages.  Instead, I'll hand you over to Karen and her father, Ed.

Karen wrote:

Hi!

My social studies teacher gave a ancient technology project.  He also gave a challenge; "Only a few people have built a working trebuchet inside the time limit."

I took the challenge.  Then I had to find instructions or plans.

Google was used to find a whole bunch of useless and confusing stuff.  But buried in that there was your excellent site.

I had modified your plans so I could use sheet plastic, but I ran into many problems.  The copper weight would be thicker, but that would mean the arm had to be thicker. (Ed: triangular plastic pieces pivoted on brass tubing eliminated lots of precision cutting and worked well with the wooden arm!)

I also took a bunch of pictures.

The overview is the whole trebuchet put together with my projectile.  My projectile was a Zome construction kit connector that weighs 1 gram.

Weight shows a close up of the weight and how I built it.  The green and black stuff around the pennies is tape to hold the penny-cylinders together.

In sling there is a picture of my sling and the Zome ball.  They are hanging from the end of the Throwing arm.

The attachment method is shown in joints.  I used brass tubing and screws for my axel.

I also had to write up a report for the project and it is attached as a .pdf. (Andy's note, pdf is not on this site).

Sincerely,
Karen

Here are the photos that Karen sent me, I was impressed.

I asked Karen some questions about her project, she responded:-

Hi!

> What did your teacher think of your rising to the challenge?

My teacher actually expected me to rise to the challenge because I always try to do the hardest option, in this case a trebuchet.

> How much time were you given to finish the project?

I had about four weeks to make the project.  Spring Break was in the middle of the project.

> How far does it throw?

It throws the Zome ball about 13-14 feet, a 2 gram grape sliver about 25 inches, a 3 gram grape about 20 inches and occasionally backfires, a 4 gram grape about 16 inches with more often backfires (it hit me in the nose!!), a 5 gram marble straight up.  The grapes were wrapped in foil to keep them from sticking to the sling, because some were cut to get that weight.

I have attached a small movie of the treb firing the Zome ball.  We (Dad and I) calculated the speed of the projectile and it was:5.625m/s.

Sincerely,
Karen

And here's the movie...

After Karen had mailed me, Ed wrote:

It seems our young lady is off and running with her email correspondence...

Well, OK, I did tell her that it was -her- project, so -she- should send information back to you.  We had a talk about "doing science", how she used your information in her project, and why she should pass on what she learned: so someone else could take it further.  Her first lessons in the Open Source philosophy...

I showed her how to drive the camera, transfer pictures to the PC, resize them with GIMP, and then turned her loose.  She didn't show the results to me before she sent them to you... I had to do a bit of sysadmin snooping on that PC to find out what happened.

Her trebuchet was both the smallest and the best-done project in the class, which probably made everyone think Daddy did it.  In point of fact, she's getting pretty good with shop tools; a few photos show her in action.  I provided advice & techniques, let her make some mistakes, and helped patch up the consequences... but she did all the layout and cutting and filing and sanding and gluing.

My collection of stuff includes telescoping brass tubing, miniature brass machine screws, and suchlike, so that's what we used for axles and spacers.  The weird stubs are from a bag of cutoffs I got a while ago; it's far easier to find two matching pieces than to cut them yourself!

The counterweight worked out really well.  I do have some lead sheet, but figured life would be simpler without trying to get that hideous toxic metal (sigh) into school.  It turns out that the space between adjoining US pennies is -exactly- the right size to clear a 4-40 machine screw -and- a 100-gram collection is just the right thickness for your trebuchet!  She glued the coins together with dabs of urethane, then we clamped the stacks in a V-block to hold them square while the glue cured.

Having some thin acrylic sheet around, it was only natural to show off those pennies...

The shutter was 1/125 sec and the frames are 1/30 second apart.  We measured the length of the blurred ball by picking a round pen in GIMP that matched the width, then stepped off the length; I think it was three diameters long.  Then we measured the actual ball, multiplied by three, divided by the shutter time, and got the speed.  The frame-to-frame distance/time is in rough agreement with that, so I think her number is within spitting distance of being right.

We'd be delighted if you could add our construction to yours, as that way people can find all the info in one spot.

Carry on!

-- Ed

Here's Ed's photos.

And here's a test fire, frame by frame.