Trebuchet, or counterweight catapult, part II

I will now try to take you through the design of the trebuchet which has been shown in Part I of this post. This description that follows is actually aimed at showing you how in Blender all design is based on very simple starting points. The trebuchet frame is based on a single circle, the hole in which the main axis will eventually be placed, with a diameter of one Blender unit. Around this circle I based an octagon with the same center, but with a diameter of three.

The first circle

The first circle

The octagon is an arbitrary decision. I could have taken a septagon or a nonagon, but decided to go with eight vertices. It looks like this when appended to the circle above:

The octagon and the circle

The octagon and the circle

The next thing is to add faced between the circle and the octagon. To do this, you can use the Bridge option in the LoopTools. If you don’t see this add-on when you go into edit mode, you need to enable it in the File – User Preferences – Addons. Trust me, without LoopTools life is sooo much more cumberbatch cumbersome. Just select both the octagon and the circle, and select LoopTools – Bridge.

I also rotated the hexagon 22.5 degrees. This way one of the faces is flat along the x-y axes. I then extruded the edge down 11 units. Then I rotated it 45 degrees and did it again to get another

Faces added, bottom edge extruded to eight units

Faces added, bottom edge extruded to 11 units

These extrusions will form the legs of the frame.

The next phase is to rotate the design back 22.5 degrees along the X axis to get it vertical again.

Rotated back 22.5 degrees to resume vertical pose

Rotated back 22.5 degrees to resume vertical pose

At this point I went to vertex editing and selected the four vertices at the extreme end of the legs. When you issue the command Scale – Z – 0, Blender will align all of them on the Z axis, thus giving you a flat foot for the legs. Then it is time to extrude the form to give it thickness. I extruded to a thickness of 0.5 first:

Extruded along the X axis

Extruded along the X axis

Had I extruded all the way to a thickness of 1 now, I would have had to use a loop ring to split the mesh longitudinally, then close the axis bore at the middle. It is easier to extrude twice. Now I will select the circle in the top and with the F (Face) command, create a face. This will close the axis bore hole at the middle.

Axis hole blocked

Axis hole blocked

Then all you have to do is to select all the faces on this side of the mesh, including the newly-created block, and then extrude again in the X axis direction.

The final form of the frame

The final form of the frame

When we check this from the other side we can see that the axis hole, one unit in diameter, is now half a unit deep. The rest is solid. For the printing, this is a good thing, because MiniFactory doesn’t have to create any support structures when printing the axle hole.

Axle hole

Axle hole

Now that we have established the crucial part, ie. the vertical frame, all that remains is mirroring this and then creating the parts in the base of the frame. The first thing to do is to extrude the bottom faces of the legs down along the Z axis for one unit.This will create a handy set of faces that you can see selected there in the next image.

Foot extrusion

Foot extrusion

These vertical faces can be extruded along the X axis for first 2 units and then 1 more unit. The reason for this becomes apparent soon.

Twice-extruded vertical faces

Twice-extruded vertical faces

Now you don’t have to go fumbling for a loop cut, when you want to create a bar between the legs. This will form the trough in between the legs along which the payload will be dragged at launch. All you do is delete the faces that are facing each other in the legs, and then select the edge loop and use LoopTools – Bridge once more:

Bridged legs

Bridged legs

Note by the way that the mesh is very clean. There are no extra vertices, edges, or faces anywhere. That is always a cool thing when designing – along with the frequently-repeated “Remove Doubles” command when you have selected the entire mesh. That will make sure you have not inadvertently duplicated vertices, as they always will give you trouble later. Actually, I select the mesh many times during my work and remove double vertices just to be sure, even if I don’t seem to have any.

Back to the show. Now we want to lower the inner edge of the bridged legs. because we will soon mirror the design. Select the edges on the top of the bridged connector and press it down:

Lowered edge of connecting bridge

Lowered edge of connecting bridge

Now we’re ready to mirror the whole frame across the Y axis. First, with the edge still selected, hit Shift+S and then Cursor to selected. This brings the wonderful 3D cursor to the selected item. After that it is easy to install an Empty element into that exact space to use it as the mirroring tool.

3D cursor on edge, Empty added at cursor

3D cursor on edge, Empty added at cursor

The reason you should always use snapping to get the 3D cursor placed exactly is that you simply cannot get anything placed visually so that you can use mirroring tools and other such items with ease.

Now with the Empty in place, it is easy to use the Mirror modifier. All the modifiers are on the Modifiers tab in the properties panel to the right – the one with the wrench. Select Y axis, and use the Empty as the mirror object:

Mirrored frame

Mirrored frame

When you now click Apply, Blender will combine the mirrored object into the original, and merge the vertices, taking care of possible duplicate vertices at the same time.

Without the Apply, the next phase is not possible. We need to extrude the lowest point of the bridge connecting the two towers, so that we get a launching rail for the payload. To do this, we just select the two faces pointing this way, and extrude until the rail is sufficient.

Extruding the launch rail

Extruding the launch rail

 

This essentially concludes the design phase for the solid part. In Part III I will explain the moving parts and show you the print process as well. Stay tuned once more!

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About heikkihietala

Heikki Hietala has worked at the crossroads of IT and language since 1986. He studied at the University of Jyväskylä, Finland. With an M.A. in English Philology and minor degrees in Communication and Information Technology, he has seen action at Microsoft, McKinsey & Company, Lionbridge, Bates Advertising and since September 2003, HAAGA-HELIA University of Applied Sciences. His interests of late have been user interface design and usability, 3D Design using Blender, and information technology for the small and medium enterprises. In his spare time he writes fiction in English. His novel, "Tulagi Hotel", was published in 2010 and a short story collection, "Filtered Light and Other Stories", in 2012. Tulagi Hotel is now available in Kindle and in paperback (also at Akateeminen Kirjakauppa), published by Fingerpress UK. "Hotelli Tulagi" on saatavana myös suomeksi kirjakaupoista kautta maan.
This entry was posted in 3D printing, Blender, Haaga-Helia, Minifactory. Bookmark the permalink.

One Response to Trebuchet, or counterweight catapult, part II

  1. Pingback: Trebuchet, part III – moving parts | Heikki's Blog

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