We used a revolutionary technology in Metal 3D Printing to replicate a crank assembly from a motorcycle that’s over 100 years old.
We were approached by a client who owned a 1914 FN Motorcycle, a very rare, old Belgian motorcycle. Spare parts for this motorcycle have long disappeared from existence with the company no longer running and with the bike being so rare in the first place, spares are impossible to find.
This is where we believed, 3D Printing could be the answer. Reverse engineering is a great strength with 3D printing, especially combined with its cost-effectiveness at low quantities. Any spare part from old machinery can, in theory, be replicated using 3D Printing.
It was very important for the client that the part be as authentic to the original as possible. This wrote off potential ideas to manufacture in plastic and electroplate the part, or potentially use our composite materials for their strength and weight benefits. The part had to be made in metal.
This led us to using Metal 3D Printing. This is still a new technology but there are already a few different methods to produce a metal print including laser sintering and binder jetting. The method we chose however is named “Bound Metal Deposition”. This method is very similar to the common FDM method in plastic printing, except using metal instead.
Metal rods are heated, and then molten metal is laid down onto the print bed in layers. This continues until the part is finished, from where it needs to head to a furnace. The parts print at an enlarged scale so the furnace can shrink that part back down to size and compact the particles up for added strength. An example print can be found in the above image.
The chainwheel and the brake arm were to remain mostly the same, but the crank mechanism in the centre had to change to incorporate the wires that operated the ratchet. This ratchet lifts the arm up and allows the centre piece to spin the other way freely, allowing the bike to reverse.
We took measurements from the existing chainwheel and brake arm so we knew what area we had to work to. From there it was a case of designing the two ratchet arms. The brake lever (main horizontal ratchet) had to allow enough room to revolve around the centre, whilst holding a spring pin to push against the other ratchet. The second ratchet, that rises above the brake lever, is shorter but must also crank outwards 30mm.
This is a tight space and needs enough room to accommodate the cables and the revolving action.
When the design was finalised, it was then a case of printing a prototype. It is always advised to print a plastic prototype before any manufacturing. This is an in-expensive way of ensuring the geometry is correct and provides a great visual aid in ensuring you are happy with the design.
After we had confirmed that the geometry of the design was correct, it was then time to go production, via metal 3D printing.
Throughout production there were some challenges. This part really pushes what metal 3D print can do as there are thin wall sections, large parts, complex geometries, and tight tolerances needing to be met.
The first obstacle was the chainwheel itself – it was too large for the printer. This meant we had to split the part in half, for it to then be welded back together after the printing stage.
Then we printed the main brake arm itself. This was a particularly difficult part due to the length of the unsupported arm. This area was prone to warping and fracturing. To get around this, we oriented the part as flat as we could and placed large supporting structures underneath.
The ratchet arms were simple to print but needed a slight re-design to thicken the walls. Thin walls can cause warping, so this was thickened and the hole sizes were reduced.
After all the printing, all the holes across the several parts needed enlarging. We had to drill and ream out these holes to match the precision required. We produced some bespoke drilling jigs using our composite printer for this. This made drilling far easier as some of the parts were difficult to clamp and orient to the right plane.
After all the adjustments were made, it was simply a case of filing and machine skimming the parts to improve the surface finish. A few coats of paint finished the job to match it with the original.
This was a demanding project that presented a lot of challenges. It was great to finish the project and the client was really pleased with the results. We really wanted to push what the technology could achieve and showcase some of the incredible things Metal 3D printing can do.
The main challenge for this project was in the design work. This is a complex moving piece and all the tolerances were critical. 3D Printing gave us a great base to work with to finish off the part to the precision required for the job.
We’ve no doubt that in the coming years that Metal AM will continue to develop and grow, and in the future will be able to produce precision engineered parts with no finishing required. Great to be a part in using this exciting technology in its infancy.
