Intro
Story
Manufacturing
Materials List
Updates

Introduction
In this section, an overview of the steps we took to make the surdo will be given. I have chosen to make clarifying sketches instead of "official" drawings, as this would take too much time. Hopefully, everything is clear to the reader, if not, don't hesitate to contact me.

Remarks
All dimensions are in millimetres. The sketches are not to scale! As far as I know, all dimensions are correct, but typing mistakes do happen, so do check critical dimensions before any machining.

Shell
For the shell (cilinder), 1 mm thick aluminium sheet was used. Figure 1 shows the required minimum dimensions. You will need about 200 mm more (100 mm on either side) to prevent a straight leading and trailing edges. This excess material can be cut off afterwards. If this is not possible, take the minimum dimensions and manually bend the edges before connecting.

Before rolling, the edges need to be folded and filled to create a rounded edge to create a large enough radius to prevent the skin from wearing too hard. This was done by laying a wire of 3 mm diameter in the folds along the sheet (see also figure 1).

Fig. 1: Aluminium sheet dimensions. Click for a larger version

To ensure a smooth joint, a kind of step should be made by folding a corner at one end of the sheet, see side view in figure 1 and figure 2. It depends on the equipment you have to your disposal, I made it before rolling the cilinder as I did not have the means to do it afterwards. Make sure to make the step in the same direction as to where the other edges were folded.

Fig. 2: Connection principle. Click for a larger version

After folding, cut the shaded corners seen in figure 3a.

Fig. 3a: Sheet with folded edges. Click for a larger version

Fig. 3b: Section A-A. Click for a larger version

Now roll the folded sheet, make sure the folded edges are on the inside of the cilinder. When the ends meet, it should be possible to connect both ends by simply sliding them over each other and then riveting them together. See also figures 4a and b. Make sure that the size of the cilinder is correct by fitting a skin. Also avoid leaving a gap between the two ends, as the skin could pull both ends further together.

Fig. 4a: Connection Click for a larger version

Use clamps to hold both ends together while drilling and rivetting, if possible extra around the hole you are drilling.

Fig. 4b: Section E-E. Click for a larger version

The rim
For the rims, two strips of regular steel of with minimum dimensions of 15x3x1626 was used. Add 200 mm for straight edge correction if possible.
Roll both strips as with the shell. After rolling, weld both ends together. Check the diameter before welding by comparing it to a skin, if possible.

After welding, mark out 8 points at equal distances around the circumference of the ring, drill holes and cut M12 thread. Insert an M12 bolt in each hole and tighten it. In figure 5a is schematically shown what you will end up with.

Fig. 5a: Overview rim. Click for a larger version

Now mark out the position on the bolts for the holes through which the tension rods are to be inserted. Also mark the distance to cut off the head of the bolt. The dimensions are given in figure 5b. At the dotted line marked with "D" the bolt should be cut off. The trailing threaded end at the inside of the ring should also be cut off.

Fig. 5b: Detail rim before cutting bolt. Click for a larger version

The result of cutting and drilling is given in figure 5c. In this figure it is also shown how to make the slots where the tensioning rods are inserted to prevent twisting. Only one rim needs to have these slots, by the way.

Fig. 5c: Detail rim after cutting bolt, showing slot detail. Click for a larger version

The rims are now finished. I chose not to secure the insertion points (yet) to see how that works. You may want to fasten them using e.g. "Loc-Tite ^TM" or by welding.

Tension rod
The tensioning rods can be made from 6 mm diameter steel rod. Cut thread on one end and then flatten the other end. Use gas welding or cutting equipment to heat one end and then hammer it on an anvil. In figure 6, the basic shape is given. The sizes given for the flat end are approximations.

Fig. 6: Tension rod. Click for a larger version

After ensuring the rods fit, round the sharp edges of the flat end to prevent injury.

Use regular M6 nuts to fasten the rods.