Small floating arm trebuchet plans

Here are some stats, note that these apply to the original #2 treb. The treb built for this guide is slight different, partly due to design and partly due to the materials that I had to work with, e.g. the wheels:-

height to top of arm when at rest	9 inches
height to top of counter weight guides	7 1/4 inches
height to wheel tracks	4 1/8 inches
drop of counter weight	3 7/8 inches
counter weight	about 9 1/2 ounces or 270 grams
projectile	4/5 inch or 20mm wooden bead, about 1/9 ounce or 3 grams
ratio of counter weight to projectile weight	about 90 to 1
upper arm/lower arm length	4 9/16 inch (115mm) / 2 3/32 inch (53mm)
wheels	5/8 inch diameter, 9/32 inch wide plus flange 1/16 inch
base dimensions	6 inches long, 1 7/8 inches to outside of counter weight guides
sling length	4 1/2 inches

Disclaimer

The measurements below are not going to be absolute, and are presented here as a guide. In a lot of cases, the actual dimensions of the piece are best determined by measuring and cutting to fit as and when the piece is needed. Lollipop sticks vary quite a bit in dimension and quality, so it's best to buy a bag of them from one supplier and build your treb according the size of stick that you get. My lollipop stick trebs have been based around 1 stick length being the upper end of the arm, with the rest of the treb built to accommodate it. So, the golden rule is measure twice, and cut once.

Much of this treb is made using a hobby knife. Knives are dangerous, and should be treated with respect. If you are going to build a small treb, fingers and knives will be in close proximity for quite a number of hours. Please, please, please look after your fingers. Always observe sensible precautions when using a knife.

Parts

You'll need the following parts:-

a plentiful supply of lollipop sticks,
some 3mm or 1/8th inch diameter dowel,
and some 1.5mm or 1/16th inch thick plywood,
a pair of ball bearings (the inner race should fit the dowel snugly if possible),
some brass copper tube (matching the inner race on the bearings),
a pair of small eye-hooks,
some thin but strong cord or string, 35lb Shanti/Dyneema kite line is ideal,
some plastic sheet or fabric for the sling (a plastic carrier bag is good),
and something heavy for the counter weight ballast (lead sheet or fishing weights are good)

Tools

You'll need the following tools:-

a hobby knife,
an assortment of small drill bits (from 1.5mm to 3mm, or 1/16th to 1/8th inch),
a pin vice,
a rotary tool e.g. Dremel and a selection of cutting/sanding discs,
some clamps (clothes pegs will do in most cases),
a square,
a sharp pencil,
a steel rule,
a cutting surface (so as not to ruin your best coffee table),
sandpaper,
wood glue

Plans

The plans that I've done so far are available, either as a Visio Version 5 drawing, or as a set of graphic images (portable network graphics format, supported by most browsers in these modern times). Either save the Visio file to your computer to print directly from Visio on your PC, or if you don't have Visio then download a Visio viewer or use the links below for graphics versions of the plans. You can search for downloads of Visio viewers at Google

Here are the plans in PNG format, print them to fit an A4 page and they should print close to actual size.

frame parts #1 frame parts #2 frame parts #3 frame parts #4
wheel parts arm parts counter weight carrier parts

The photos

Here are the step by step photos that I took of the treb's construction.

jump to:- frame | wheels | arm | counter weight

The frame

(treb02frame01.jpg 59kb)	1. Square off 4 sticks (frame part #1), and butt two pairs together to make back-to-back L shapes. The upright stick should sit on the work surface, and the horizontal stick should butt up against the upright stick. Sandwich the horizontal sticks, and the bottom of the upright sticks, with 2 off frame part #2, ensuring that you leave about an eighth of an inch between the upright sticks. Repeat to produce a second frame side, pictured left. The upright sticks are the inner legs.
(treb02frame02.jpg 51kb)	2. Use 4 sticks (frame part #3), gluing them to the inside edge of the frame sides to make the outer legs. The outside edge of each outer leg should be 74mm (about 3 inches) from the centreline of the gap between the inner legs. It's not exact, so don't get too hung up about being precise. When the glue is dry, trim the excess stick protruding past the outer legs.
(treb02frame03.jpg 55kb)	3. Cut 4 cross members (frame part #4), each 40mm in length (just over an inch and a half), and use them to glue the two frame sides together. In the picture you'll see some small dowels that were used to hold the frame sides in place while the glue dried, they're not necessary but with care they can be used to keep the frame sides under tension to stop the cross members from shifting.
(treb02frame04.jpg 43kb)	4. Mark a line on the inside of each outer leg, 100mm (about 4 inches) up. Marking the line will be easier now, but we'll be using it later. Cut and glue the plywood base (frame part #5) to the bottom of the frame sides. When the glue is dry cut and glue the end pieces (frame part #6).
(treb02frame05.jpg 32kb)	5. Cut 4 off frame part #7, 63mm long pieces (about 2 and a half inches), and glue them to the inside face of the inner legs.
(treb02frame06.jpg 24kb)	6. Square off 4 sticks (frame part #8), and glue them to the inside face of the inner legs (above the pieces we added in the previous step). Ensure that the 1/8th gap between the inner legs remains perpendicular and consistent.
(treb02frame07.jpg 49kb)	7. The frame so far, with short outer legs and long inner legs.
(treb02frame08.jpg 32kb)	8. Next, cut the buttresses (4 off frame part #9) and glue them in place. You might find it aids keeping them in place by pinning the bottom of the buttress to the end of the frame. Note that the original treb didn't use these, but used a support slung under the wheel tracks. They were fiddly to make and position, and didn't make the frame much more stable. You can see one in the bottom left of this picture of the frame. The buttresses seem to be a much better way of making the frame stable, so the remake of the treb for these instructions went with buttresses.
(treb02frame09.jpg 40kb)	9. Using 4 sticks (frame part #10), reinforce the frame's legs. Note the difference between the front and rear supports (the front uses a whole stick, the rear uses a stick cut to fit).
(treb02frame10.jpg 40kb)	10. When the glue is dry, use a rule and mark the inside face of the inner legs with a line 100mm up the leg. Use the marks you made on the outer legs. Square off 4 sticks (frame part #11), and glue them to the inside faces of the legs. Ensure that you leave the 1/8th gap between the inner legs clear, and align the upper edge of the sticks with the marks you've made. These sticks are the wheel track supports. When the glue is dry, trim the excess stick protruding past the outer legs.
(treb02frame11.jpg 25kb)	11. Cut 1/16th inch plywood into strips, the same length as the wheel track supports. The strips should be 7.5mm wide, or about 5/16th inch. These are the wheel tracks, frame part #12. Glue them on to the upper edge of the wheel track supports, making sure that they stay level while the glue dries.
(treb02frame12.jpg 23kb)	12. When the glue has dried, use another 8 sticks cut to length (frame part #13) and glue them to the existing wheel track support, underneath the wheel tracks. After the glue has dried, trim any of the excess wheel tracks extending past the outer legs. You might like to put a small dowel pin through each end of the wheel track supports.
(treb02frame13.jpg 38kb)	13. Cut 3 track supports (frame part #14), and glue them to the frame. The first should be mounted to the inside face of the stick at the rear end of the frame, the second should be mounted about an inch from the inside face of the front end of the frame and the third should be between the other 2. When the glue has dried, cut the track (frame part #15) from 1/16th plywood, score it along it's length and glue it to the track supports. The pic to the left shows the front end.
(treb02frame14.jpg 43kb)	14. The pic to the left shows the rear end of the frame, with the track flush against the track support.
(treb02frame15.jpg 37kb)	15. Finally, screw two eye-hooks to the rear end of the frame. This is used as part of the trigger.

The wheels

Making the wheels isn't straight-forward. The original treb used a slim pair of wooden wheels for testing, and some machined brass wheels for actual use. The brass wheels really affected the performance, so I decided to make some better wooden wheels for this treb than I did for the original. If you can get some of the right size, then do so. If not, read on...

As with a lot of my tiny treb builds, I was making do with bits and pieces of stuff that I had lying around, which includes a box of bearings. I used some of these bearings in the original treb, and decided to use them on the treb being built for this guide too. I didn't have any more of the flanged bearings that I had used originally, so the wheels would have to be slightly different. The wheels needed a load bearing part to roll on the wheel track, and a flanged part to keep the arm in place as it moved. The flange was easy enough, it's just a disc of plywood larger than the discs used for the load bearing part of the wheel. But the load bearing part of the wheel needed some preparation...

(treb02wheel01.jpg 21kb)

1. Here are the parts needed to make a ring of plywood. At the bottom of the picture is a piece of plywood that has had a hole drilled to fit the outside diameter of the bearing that I'm using. The smaller disc is the same diameter as the hole, so that it fits snugly inside it. The smaller disc has been sanded down so that it is slightly thinner than the rest of the plywood pieces. The larger discs are slightly smaller than the outside diameter of the ring that I want to create, and will sandwich the plywood with the hole in it. The disc at the top of the picture has a screw mounted in it, which is part of the tool set belong to my rotary tool (normally a sanding disc or cut-off disc is fitted to a spigot with the screw).

(treb02wheel02.jpg 30kb)

2. Screw the smaller disc over the screw until it tightens up against the large disc already on the screw.

(treb02wheel03.jpg 28kb)

3. Fit the piece of plywood over the small disc.

(treb02wheel04.jpg 29kb)

4. Screw the other large disc over the whole, until it snugs up against the plywood. Then screw the exposed screw set in to the spigot on the rotary tool, and sand the plywood away by spinning the whole thing around with the rotary tool and sanding it with some coarse sand paper firmly attached to something solid. Take care when doing this, since the square plywood will resist your efforts in making it round.

I went for 16mm diameter wheels, and used a vernier caliper to measure the diameter.

(treb02wheel05.jpg 17kb)

5. After you've got the desired diameter, dismantle the whole thing. You should end up with a ring of plywood, as pictured. Make 4 of them (wheel part #1).

(treb02wheel06.jpg 22kb)

6. When you've made 4 of them, make 2 larger discs (wheel part #2). These will be the flanges for your wheels. I made mine 20mm in diameter. You should then have two sets of the parts pictured, so we can start assembly of the wheels.

(treb02wheel07.jpg 33kb)

7. I enlarged the hole in the large disc so that it was a snug fit for the dowel I'm using for the axles. I used a small piece of that dowel as a guide for assembling the wheel.

(treb02wheel08.jpg 34kb)

8. Press the bearing in to one of the rings (wheel part #1), apply a little glue to the surface of the ring that meets the flange disc (wheel part #2), and push the bearing down over the axle dowel. Don't apply too much glue, otherwise it might get past the shield on the bearing and glue the inner and outer races together.

(treb02wheel09.jpg 37kb)

9. Apply a little glue to the second ring (wheel part #1), and press it over the bearing.

(treb02wheel10.jpg 32kb)

10. When the glue is dry, remove the bearing.

(treb02wheel11.jpg 34kb)

11. Enlarge the hole in wheel part #2 so that it is larger than the diameter of the inner race on the bearing. We'll be using a spacer on the axle of the arm to ensure that the wheel sits at the proper distance from the arm, and the inner race will be snug against the spacer. We don't want the flange of the wheel to be impeding the inner race or the spacer. Replace the bearings, use a little glue or a paper shim to retain the bearings if they have a tendency to work out of the wheel.

The throwing arm

(treb02arm01.jpg 38kb)	1. Cut 2 66mm sticks (arm part #1), leaving a round end on each. Glue them on either side of a whole stick (arm part #2), rounded ends together.
(treb02arm02.jpg 38kb)	2. Square off 2 sticks (arm part #3), and glue them to the sides of the whole stick (arm part #2), with the squared ends butting up to each other.
(treb02arm03.jpg 34kb)	3. Sandwich the arm with 2 whole sticks (arm part #4), overlapping the butt joints and aligning the rounded ends.
(treb02arm04.jpg 37kb)	4. Cut a 12mm (or half inch) length of stick for the tip of the arm (arm part #5), and glue it between the ends of the 2 sticks sandwiched together (arm parts #3).
(treb02arm05.jpg 46kb)	5. Drill holes in the arm for the axles, then glue the dowel for the counter weight and wheel axles. Ensure that the holes are perpendicular to the arm, and parallel with each other. The counter weight axle's hole is 5mm from the thickest end of the arm, and the wheel axle's hole is 58mm from the thickest end of the arm. It's probably best to put in a piece of dowel longer than you need, and cut it to fit when the time comes. I used 1/8th inch dowel, about 90mm in length (3 1/2 inches).
(treb02arm06.jpg 37kb)	6. Drill a small hole in the tip of the arm, slightly angled up when the arm is on a flat surface. Fit a piece of paper clip in to the hole, with about 4 to 5mm showing (3/16 inch). This is the pin over which the free end of the sling will be placed. Just behind the tip of the arm, drill a hole for a small piece of dowel. This will be where the fixed end of the sling is secured. Do not glue the dowel in place, it can be pulled out to remove the sling for tuning purposes.
(treb02arm07.jpg 47kb)	7. Cut some small spacers from brass or copper tubing, and slip them over the wheel axle. The spacers were about 7mm long (just over a quarter inch), though this is definitely one of those pieces were you will have to make them to fit. With the spacers on the arm, put the wheels (with their bearings) on the wheel axle. If the bearings are a little loose on the axle, then you can use a tiny piece of paper between the axle and the bearing's inner race and shim the play out of the wheel.
(treb02arm08.jpg 36kb)	8. Here you can see the wheels on the wheel tracks, and the brass tubing spacers on the wheel axle.
(treb02arm09.jpg 38kb)	9. Here is a side view of the wheels on the track while the arm is in the cocked position.

The counter weight carrier

The counter weight carrier for this treb was designed to hold small pieces of lead sheet, since I had some available. This makes the carriers fairly small and neat. If you intend to use something less heavy than lead you'll need to increase the capacity of the carriers, so remember to make the dowel in the arm longer if you increase the width of the carriers.

(treb02cw01.jpg 34kb)	1. Glue the counter weight carrier's bottom (CW part #2) to one of the sides (CW part #1).
(treb02cw02.jpg 40kb)	2. Another view of the bottom glued to the side.
(treb02cw03.jpg 45kb)	3. Glue the counter weight carrier ends and middle (CW part #3) to the side.
(treb02cw04.jpg 38kb)	4. Glue the other side on (CW part #1). Make 2 in total.
(treb02cw05.jpg 45kb)	5. The counter weight carriers are not permanently fixed to the trebuchet. The lower holes in the carriers are meant to be a snug fit on a piece of dowel, while the upper holes are meant to be a loose fit, and will slide over the counter weight axle on the throwing arm. By making the outer hole loose on one of the carriers the carriers can be reliably press-fitted on to a dowel, and pulled apart again. The carrier on the left has both holes being a snug fit on the dowel, the carrier on the right has only one hole that is snug. It is the right carrier that is pulled off the dowel which allows the counter weight carriers to be removed from the treb.
(treb02cw06.jpg 45kb)	6. Here, the carriers are fitted to the treb. The upper holes are positioned over the counter weight axle on the arm, as the lower holes are pressed on to the lower dowel in the picture.

There's more to come. I'll be completing the treb with a sling and a trigger mechanism.

Here's a detailed guide to making a pouch and sling

If you're interested in seeing more about trebuchets, return to the index for some links.

back to tiny floating arm trebuchet page
return to trebuchet index

This page last updated on June 4th, 2005
Created by hand using 1st Page 2000

(treb02wheel01.jpg 21kb)	1. Here are the parts needed to make a ring of plywood. At the bottom of the picture is a piece of plywood that has had a hole drilled to fit the outside diameter of the bearing that I'm using. The smaller disc is the same diameter as the hole, so that it fits snugly inside it. The smaller disc has been sanded down so that it is slightly thinner than the rest of the plywood pieces. The larger discs are slightly smaller than the outside diameter of the ring that I want to create, and will sandwich the plywood with the hole in it. The disc at the top of the picture has a screw mounted in it, which is part of the tool set belong to my rotary tool (normally a sanding disc or cut-off disc is fitted to a spigot with the screw).
(treb02wheel02.jpg 30kb)	2. Screw the smaller disc over the screw until it tightens up against the large disc already on the screw.
(treb02wheel03.jpg 28kb)	3. Fit the piece of plywood over the small disc.
(treb02wheel04.jpg 29kb)	4. Screw the other large disc over the whole, until it snugs up against the plywood. Then screw the exposed screw set in to the spigot on the rotary tool, and sand the plywood away by spinning the whole thing around with the rotary tool and sanding it with some coarse sand paper firmly attached to something solid. Take care when doing this, since the square plywood will resist your efforts in making it round. I went for 16mm diameter wheels, and used a vernier caliper to measure the diameter.
(treb02wheel05.jpg 17kb)	5. After you've got the desired diameter, dismantle the whole thing. You should end up with a ring of plywood, as pictured. Make 4 of them (wheel part #1).
(treb02wheel06.jpg 22kb)	6. When you've made 4 of them, make 2 larger discs (wheel part #2). These will be the flanges for your wheels. I made mine 20mm in diameter. You should then have two sets of the parts pictured, so we can start assembly of the wheels.
(treb02wheel07.jpg 33kb)	7. I enlarged the hole in the large disc so that it was a snug fit for the dowel I'm using for the axles. I used a small piece of that dowel as a guide for assembling the wheel.
(treb02wheel08.jpg 34kb)	8. Press the bearing in to one of the rings (wheel part #1), apply a little glue to the surface of the ring that meets the flange disc (wheel part #2), and push the bearing down over the axle dowel. Don't apply too much glue, otherwise it might get past the shield on the bearing and glue the inner and outer races together.
(treb02wheel09.jpg 37kb)	9. Apply a little glue to the second ring (wheel part #1), and press it over the bearing.
(treb02wheel10.jpg 32kb)	10. When the glue is dry, remove the bearing.
(treb02wheel11.jpg 34kb)	11. Enlarge the hole in wheel part #2 so that it is larger than the diameter of the inner race on the bearing. We'll be using a spacer on the axle of the arm to ensure that the wheel sits at the proper distance from the arm, and the inner race will be snug against the spacer. We don't want the flange of the wheel to be impeding the inner race or the spacer. Replace the bearings, use a little glue or a paper shim to retain the bearings if they have a tendency to work out of the wheel.