Construction of the L’Obsidienne 
road roller (4)
Page created on 7/03/2025; updated on 09/03/2025.
Making new parts (continuing)
Front wheel frame
So I’m going to replace this frame with a brass one, and here is the diagram.
Ideally, I’d like to use a profile with the right cross-section, but I’ve only got 1 mm and 1.5 mm squares. I tried milling the 1.5 mm section to a width of 1 mm, but holding it for machining is difficult, and bending it 90° isn’t easy either. So I came up with the idea of superimposing two 0.5 mm sheets. Here is the sequence of operations.
- Cut two strips of brass approximately 2 to 3 mm wide by 70 mm long (bearing in mind that the expected average perimeter is 59 mm).
- Bending against an aluminium template with the inside dimensions of the frame, minus 0.5 mm, to take into account — roughly — the spring effect. A support part locks the slats during bending. This part can be moved to either side of the template. Bending is done by hand, by pushing with a steel plate (model coach ballast!).
Click on the image for a closer look at the assembly.
- Once the complete turn has been made and squareness checked, the strips are cut to the right length, overlapping each other, and placed in an aluminium soldering template. I included a layer of aluminium foil to avoid the misadventure I had of a partial soldering on the aluminium: if aluminium foil is soldered on, it will be easy to remove; but that did not happen.
- Soldering the assembly using a torch with flux and tin-silver solder. I used the torch because of the template’s high heat dissipation.
Click on the image for a closer look.
- Excess solder is removed using a file, rotary brush, and fibreglass.
- Milling on each side and grinding to the required thickness of 1.5 mm.
- Milling of two 2.5 mm wide grooves as per drawing.
- Drilling of:
- 2 ⌀ 1 holes for the wheel axle;
- 2 ⌀ 0.6 holes for the articulation;
- 4 ⌀ 0.4 holes for attaching the scrapers.
All this was not without mistakes and rework! Here is the resulting part.
The scrapers are mounted on pins made from sections of ⌀ 0.4 nickel silver rod and soldered. The pins are then shortened and filed to simulate bolt heads. See the results at the end of the page.
Front suspension arch
You may have seen this coming. Yes: the arch support that connects the body of the machine to the front wheel frame is also pretty ugly! But rebuilding it is more complicated, because of the varying thicknesses from top to bottom.
I don’t have precise data, but with the help of several photos, I’ve estimated the maximum width at 2.1 mm (compared with 2.6 at present), and the minimum at 1.4 (compared with 2). One solution would be to machine a solid brass part. I’d need sheet metal thicker than 2.1 mm, which I don’t have; or two 1 mm sheets, machined at an angle to obtain a thickness of 1.4 mm, and soldered together. In both cases, I’d have to saw the arch into a circular shape and then adjust it, which is not my forte! I could still fall back on this solution if the one below fails.
So I finally tried, as with the frame, to stack thin metal sheets, in decreasing numbers from top to bottom. Here is the diagram.
- sheet metal, thickness 0.5; length 28
- sheet metal, thickness 0.5; length 17
- sheet metal, thickness 0.2, length 8.5
- sheet metal, thickness 0.5, length 21.2
- sheet thickness 0.2; length 23.3
The empty spaces will be filled by solder. The manufacturing steps are as follows.
- Prepare brass strips approximately 3 mm wide and of the length shown on the drawing (the outer strips can be cut slightly longer for a better final fit).
- Drilling the strips exactly in the middle, ⌀ 0.8.
- Approximate bending of the strips.
- Stacking of parts slid onto a ⌀ 0.8 mm nickel-silver rod, then light soldering in the middle. The aim is to hold them together, while avoiding stiffening the whole.
- To hold the components tightly together, a 2 mm-thick aluminium template is made, consisting of two complementary parts, one female at the outer radius of the arch (8.2 mm), one male at the inner radius (7 mm). Here too, a layer of aluminium foil has been inserted between the two parts of the template.
- Soldering the assembly, using a torch, with flux and tin-lead solder..
Click on the image for a closer look at the assembly.
- Still in the template, first parallel milling of the faces to achieve the required width of 2.1 mm, successively on one face and the other.
- A second inclined milling cut was made on each side, with a 0.3 mm shim under the ends of for first side and a double 0.6 mm shim under the ends for the second. Here is the result:
- Levelling (9.6 mm) of the ends: this is done by grinding with a diamond disc. To ensure symmetry, an epoxy support is drilled with a ⌀ 0.8 hole, and the part is planted in it by its axis.
Click on the image for a closer look at the assembly.
The part is held in place by HDF soft jaws.
Click on the image for a closer look. You can see that the epoxy strip ensures that the axis of the part is perpendicular, although the latter is barely visible.
- Rounding the ends with a file.
- ⌀ 0.6 hole drilling, 0.8 mm from end.
- End milling. This ⌀ 0.8 × 1.6 milling, forming a clevis, allows the part to overlap the wheel frame and articulate with it, as in reality. Machining is done very carefully, by 1/10 mm passes because a 0.8 milling cutter is fragile. I broke one anyway…
Click on the image for a closer look of the part.
The arch and frame are mounted and burnished. Here is a comparison with the original parts.
Click on the image to see details of the (functional) joints.