Knocked Up Joint
From DT Online
A Knocked Up Joint or Double Seam is commonly used to fix an end to a cannister or food can. The canning industry would normally use a Seaming Machine (see A Dash of Science for further detail) but they were traditionally hand made by Tinsmiths. If made properly a Knocked Up Joint should be water-tight without being soldered but, of course, they can be soldered as well if required.
Making the Joint
- To make the base, mark out a circle with its radius equal to the cylinder radius plus twice the width of the Seam.
- Form a rim round its outside with a depth equal to the width of the Seam (do this by working around the circumference little by little with Flat Nosed Pliers and truing up with a Boxwood or Rawhide Mallet on a Mandrel Stake).
- The length of the cannister needs to be extended by the width of the Seam so that a flange equal to this can be formed around one end (as before, do this by working around the end little by little with Flat Nosed Pliers and truing up with a Boxwood or Rawhide Mallet on a Bench Block, Tinman's Anvil or similar flat surface).
- Place the flanged cannister inside the rim of the base and flatten the rim down on to it using a Boxwood or Rawhide Mallet down on to a Bench Block or similar surface.
- The joint formed at this stage is known as a Paned Down Seam
- To complete the Knocked Up Joint or Double Seam, the cannister is held on to a Mandrel Stake and folded over again as shown.