Portsmouth Block Mills

The Portsmouth Block Mills form part of the Portsmouth Dockyard at Portsmouth, Hampshire, England, and were built during the Napoleonic Wars to supply the British Royal Navy with pulley blocks. They started the age of mass-production using all-metal machine tools and are regarded as one of the seminal buildings of the British Industrial Revolution. They are also the site of the first stationary steam engines used by the Admiralty.

The block-making processes using the machines

A pulley-block has four parts: the shell, the sheave, the pin for locating the latter in the shell and a metal bush, or coak, inserted into the sheave to save wear between it and the pin. Blocks can vary in size and in the number of sheaves.

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The process of making the shells

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• Cut slices from the trunk of a tree, and from these slices by means of the circular saws cut rectangular blocks from which the shells were manufactured.
• Bore a hole in the block for the pin, and at right angles to this a hole or holes to receive the morticing chisels,(depending on the number of mortices). The clamp used to hold the block at the same time indented locating points by which the blocks were secured in the later machines, thus ensuring consistent location and measurement in the subsequent processes.
• Mortice the blocks by a self-acting machine. The morticing chisel reciprocated vertically, and at the same time the vice gripping the block was gradually moved each cut. Once the length of the mortice had been cut the machine automatically stopped to allow the block to be replaced with a new one.
• Cut the corners off the block by a circular saw with angled guides.
• Shape the 4 faces of the blocks to a shallow curve. This was done by a machine where a number of blocks were clamped in the periphery of a revolving wheel. The cutter was swept in a curve across the faces of the blocks as they rotated. The radius of the curve was controlled by a former. After each cut the blocks were turned 90 degrees to bring up a new face.
• Each block was then placed in a machine which scored a shallow groove, by means of a revolving cutter, to give a location for the securing ropes.

The process of making the sheaves

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• Cut a slice across a trunk of Lignum Vitae. The machine for this allowed the log to be rotated at the same time as the circular saw operated, ensuring that an equal thickness was maintained. The position of the log for each new cut was controlled by a leadscrew ensuring great accuracy.
• Make a circular disc from this slice by means of a rounding saw, which simultaneously bored out the middle and shaped the outer edge.
• Mill out from each face a profile to take the outer face of the coak
• The coak was inserted into the sheave, and a retaining ring rivetted to keep it in place.
• Broach out the hole in the coak to the size of the requisite pin.
• The finished sheave was faced-off on both sides in a special lathe, and the rope groove was machined on the edge.

The process of making the pins

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• The pin blanks were forged slightly oversize with a square left on one end.
• They were turned to size on the circular part in a special lathe.
• They were given a burnished finish between hardened dies
• One source says they were then tinned to preserve them from rust.

The process of making the metal coaks

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• These were cast in bell-metal and the mould left grease-retaining grooves in the inner bore. One end of the coak had a flange and a loose ring was supplied for the other end, together these parts gave a seating for the rivets which fixed the coak to the sheave.

Assembly process

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• The shells were smoothed by hand with a spoke shave and then the sheave and pin assembled. There were stored in the Block Mills and issued as demanded.