Introduction: Trebuchet of the Large Variety (a Work in Progress), 4.28.07 Update
This is my second wooden, hanging counterweight trebuchet and the first well-documented one. It will be bolted together from many self-designed and self-constructed components, each about six feet in length. Hopefully, that strategy will allow for some degree of siege mobility and make fixing broken sections of the device far easier. Oh yeah, a trebuchet is a medieval-era siege weapon capable of hurling a variety of things great distances, inducing feelings of pleasure (in me anyways).
I want to hear your trebuchet tips! This project is just starting and will be updated gradually as things progress; I'm sure there's some latent treb-knowledge in the heads of many Instructablers. Let me know what you think of current plans and where you'd like to see the trebuchet go. Favorite arm-ratios, clever trigger systems, winching mechanisms, sling designs...I'd love to try implementing them. I've never attempted a large scale floating arm trebuchet, but if you're convincing enough, I just might. If you'd like to be a collaborator, let me know and I'll try to add you on.
Also, I'm an enormous SketchUp fan. I'll try to present screenshots of any models I use but viewing and helping with this Instructable will be easier if you just download the (free) software here.
This page will also carry the breakdown of the trebuchet's cost (see the Excel file below). As of my last expenditure on April 27, 2007 I've spent $315.
This here Intro page will be the resting spot for the most current model of the entire trebuchet.
A warning to fellow siege engineers: This project can be considerably dangerous, given the gravitational energy of these things and the fact that it is a weapon. Exercise great caution if you decide to build a trebuchet of any size--smaller models (like this) should be attempted first to get a sense of their operation.
Last note! The Sourpuss Trebuchet is a major inspiration to this project; a final product half as good as their device would be incredibly pleasing. The Dr. Seuss Treb should also be cited for it's poetic awesomeness.
Step 1: Base: Design (I)
This step will cover my design for the long end of the base (the entire base will be rectangular in shape, I think; the short ends will be covered in part II). The plans for one of the long ends can be seen in the SketchUp file. As each long end of the rectangular base is identical, this design will just be built twice.
I wanted a slot in the middle of the base to help align the main vertical support when it is put in place (this whole thing is modular, remember). Have no fear, more bracing than 5.5" of pine will be made for this vertical support later. The total length is 18' and I decided to go with 2x6's (which are in fact 1.5" x 5.5", just to make that clear) just cause they seemed pretty sturdy... These long lengths of the base are 3 2x6's wide (aka, essentially invincible).
The screenshots of the SketchUp file below give the whole picture (it's pretty simple). Each section--red, blue, and purple--will overlap by 16" and be bolted through to secure all three pieces into one component. It helps later to remember that every offset made in this part is 16". Note that the red and purple section are identical (whoa!). The blue section looks kinda tricky but just check out the dimensions, don't forget about the 5.5" slot, and remember the overlap/offset 16" rule.
Step 2: Base: Construction (I)
Let's build that base design (I) thing! Hah, I borrowed a little two-door Saturn to get to Home Depot and had to make three trips to get all the wood...sigh. It was fun though; I bought 16 2x6's to do the job (each is 8' in length). You are strongly encouraged to be more resourceful and find some scrap wood or make things less thick, whatever you'd like (that's going to be a recurring, cost-saving element in this Instructable, "be more resourceful than me").
Square the ends of your wood before you mark them (just cut off an inch or so from end, then measure from that freshly cut end). Mark the lengths and slice it up!
A quick cutlist (all parts come from 8' 2x6's):
legend: quantity of 2x6's should be cut to the given length
8 at 6'
4 at 4' 8"; 4 at 2' 9 1/4" (make these two pieces off of the same 8' 2x6 and repeat four times)
4 at 7' 4"
It may be handy to have the previous step, Design of base (1), open as you read this. I used 10d nail fasteners (they're 3" long) and some carpenter's glue to put the pieces together.
Construction of blue/purple sections (remember, blue = purple...I'll stop saying that) is as follows. You'll use a 7' 4" piece, a 6' piece, and a 4' 8" piece. Lay the flat ends of the pieces atop each other with the 6' piece in the middle. Go to one end of this component and align all the ends at the end..an awkward repetition of words but press on! If you align them, the other side of the three boards will become staggered at 16" forming something vaguely step-like. Fasten those suckers together; I put a good layer of glue and then drove two nails every 6" or so. Clean up the glue that comes out of the seams with a warm paper towel. That makes one component; repeat this three more times (you'll have four of the components at the end).
The middle, blue section with the slot will require two 6' pieces and two 2' 9 1/4" pieces each (we'll be making two of the blues). Start by putting a 2' 9 1/4" piece 16" in on one end of a 6' piece. Mark where this piece is going to be located before fastening so you can put the glue down carefully--this thing has a slot and glue should be kept out of that area opposite the 16" offset. Fasten that and then find yourself a scrap 2x6, there should be several somewhere... The 5.5" slot is the width of this scrap; set that in place then butt up the other 2' 9 1/4" piece against it, down the length of the 6' piece. Put down your glue and, with the scrap still in place and the small piece tight against it, nail it all together (er, but no nails in the scrap piece). The remaining 6' piece will go 16" in from the first 2' 9 1/4" in piece you put in during the last sub-step. The entire thing will form a sort of "jagged parallelogram." Hah, you'd likely be better off looking at the design photos than listening to my ridiculous descriptions. Repeat the blue section twice.
You should now be able to create an 18' long section of the trebuchet's base by aligning two of the "step" pieces with the slotted, "parallelogram" piece in between. If it's a bit off and things don't quite line up, it's not a big deal as the bolts used to hold the pieces of the component together can be placed anywhere in that overlap.
I'd love to hear some suggestions for improvement to the techniques in this section--I'm a half step above "novice carpenter" so I have a feeling there are better/more accepted ways to do some of this.
Step 3: Base: Construction (II)
If you recall, the plan was to connect all of these base pieces (like the rest of the trebuchet) with bolts. I somewhat arbitrarily chose 5/16" diameter bolts. They're 5 1/2" which was the minimum length to fasten three 2x6's face to face to face. Washers and nuts corresponding to the 5/16" diameter were also purchased. I initially thought of putting four bolts through every 16" overlap (totaling to 8 bolts per joint). In retrospect that qualifies as excessive--it'd require 32 fasteners all in all. I ended up buying half that and two extra (18 total) for two bolts per connection plus spares. On to drilling!
It'd be nice if any of the red/purple base pieces could go with either of the blue pieces. Part labeling would not be required and assembly would likely be simpler. To attempt this Herculean task I made a little drilling jig out of a spare 2x6. I would use four holes in this piece to align the bit when drilling the red, purple, and blue pieces. This ended up failing for two reasons. (1) I didn't measure the holes in the jig. They would need to be exactly symmetrical about both axes for any red/purple piece to fit with any blue middle section. (2) None of the red/purple (should I just start calling them rurple?) pieces were "exactly" the same meaning the jig aligned on them differently. Things won't fit if they're off by more than 1/8" or so, so another method was chosen. Press on!
Giving up on interchangeable components, I started drilling one part with the jig (let's call this part A). The piece intended to fit with A was then drilled using part A as a template for the second. All pieces were labeled.
Final note (apologies for the wordiness of this step): Though my bolts were 5/16" in diameter, I drilled holes for them with a 3/8" bit (1/16" larger). This made actually putting the bolts in far easier.
Step 4: Main Support: Design and Construction
What I call the "main support" is the vertical piece that goes in the center of the base; it will hold the treb's axle. Each of the two supports is designed to break down into three components. When the pieces are bolted together, the main support is about 15' 6" tall. Cool.
I needed 18 8' 2x6's for these; you can get fewer boards if you buy 10' lumber but the costs were actually about the same and using 8' pieces gives a lot more scrap (useful later). Here's the cutlist for both of the main supports:
(cut this many 8' 2x6's) at (into these measured parts for each 8' 2x6)
2 at 4', 2' 5 1/2" (thus cut one 8' piece into 4' and 2' 5 1/2" pieces and then repeat with the other 8' piece)
2 at 4', 2'
2 at 4'
2 at 5' 4"
4 at 6' 8"
4 at 4' 4"
2 at 6' 1 1/2"
The SketchUp file will be most helpful in construction--the same techniques that were employed in the base construction were used here (16" overlaps, nails, glue, music). Once your pieces are made, align them and drill holes for the bolts to come.
I actually held off assembling the very top pieces (blue) of the main support. The axle will pass through this part and I wasn't sure of how to drill the axle hole such that the axle is level. This'll be posted in the questions section and mulled over. A response and my course of action will eventually go here.
As stated earlier, save your scrap! There's going to be a lot of extra wood and future pieces of the trebuchet will make good use of them.
Step 5: Initial Assembly
With the base and main supports both about two-thirds of the way done and no lumber to work with last weekend, I decided to put this thing together. There's not much instruction here, it was just nice to see how the project was progressing. The diagonal supports that can be seen are temporary. Things bolted together nicely and it was, on the whole, very intimidating...
Step 6: Base: Design (II) and Construction (III)
The two 18' long base pieces need some method of attachment to one another (besides the treb's axle). Enter the base ends! Each base end will consist of two nearly identical components that are bolted together. The long ends of the base will slip into the channels and some sort of rod (or a realllly lengthy bolt) will attach both components. The vertical pieces you see are for diagonal braces of the main support piece; one end of the diagonal attaches here (at the base section's end) and another attaches up at the top of the of the main support.
The SketchUp file posted contains the measurements of these pieces.
Assembly time! Let's start with a cutlist (suggestions for ways to improve my cutlist presentations would be great...I'd love to use a table or something but don't I don't think Instructables has that format yet and I don't want to make people download another file):
(cut this many 8' 2x6's) at (into this quantity of these pieces)
4 at 4' 1/2", 2' 8 1/2", 8"
2 at 2x (2' 6"), 2x (1' 1 1/2") this means take an 8' long 2x6 and make two 2' 6" pieces and two 1' 1 1/2" pieces. Then repeat with another 8' long 2x6
2 at 2x (2' 3"), 2x (1' 4 1/2")
2 at 4x (1' 4 1/2"), 2x (10 1/2")
I ended up using only 8 2x6's (the cutlist above calls for 10 total) because of the large amount of scrap left over from making the main support.
The assembly of these pieces ended up being the most complicated part of the build to date. I was fortunate to escape with only one mistake (gluing/nailing two pieces that should not have been attached). For those following the plans, be extra careful in this step to check that you're attaching the right parts--many things look very similar but some components are mirror images of one another or one piece is slightly longer, etc.
It's also tricky because the nailing has to be done in a specific order due to the length of nails. Allow me to explain...first checkout the SketchUp file. The design can be aptly described as "screwy" so look at it carefully. The specific construction notes are listed in the notes on the images below--read carefully. Questions about any part of this in the comment section or a private message are completely understandable.
Step 7: Current Questions
A list of things that need to be resolved (I'd love to hear your thoughts!):
(1) The main support pieces need to have an axle hole drilled clean through them. I'm not sure how to do this precisely enough such that the axle is level when in it's in place. The difficult comes from the fact that pieces are bolted together and have a little bit of give. The wobbling of the pieces is very slight and may be insignificant...I'm really not sure.
(2) A good counterweight material is needed. High density is a must but something that comes in manageable pieces is also important. I've done concrete blocks before and it worked alright--would adding metal to the concrete mix be beneficial?
(3) Wheels, how do I get strong removable ones? I'd rather not purchase them because I bet they'd be expensive. A trip to the scrapyard is probably in order. Other ideas?
(4) Arm ratios!? (This is the ratio of the throwing arms long end to it's short end; the ends are split by the axle's position). What's the advantage of a large ratio (5.5:1) vs a small one (2.5:1)?
Step 8: Further Order of Operations
Here's the plan for the next week or so (I'd like to hear thoughts):
(0) get wood for the base ends and make 'em
(1/2) put the small additions onto the main support as per the plans
(1) drill axle holes in main support; fix the possible alignment problem by assembling both of the main supports, putting the axle through them, leveling/squaring everything, and then drilling the bolt holes for the top piece's connection to the rest of the main support
(2) continue refining model