Making the wooden screw

Making the spindle screw and nut are perhaps the most demanding tasks associated with building a wooden press. The thread’s dimensions are dictated by two factors, the arc through which the spindle can be rotated by the bar before being constrained by the presses cheeks and the clearance needed for the bed to pass below the platen. Both were determined experimentally. The maximum rotation of the spindle was just over one third of a revolution  and the clearance needed between platen and bed was about 21mm. This implied that the thread lead, the distance that the spindle would travel through the nut in one revolution was about 64mm. The spindle for the press was made from a cylinder of beech wood (900 x 150 x 150mm) onto which was drawn the.thread outlines.

thread-drawings-2.jpg
Left: one start thread.   Right: two start thread.

I decided at this stage to use a two start thread. This was not essential but it did have the advantage of allowing a thicker thread core as well as reducing the quantity of wood to be removed from the thread and nut. To set out the thread on the spindle, two strips of paper 32mm wide and about two meters in length were prepared. These were then fastened in a spiral around the wooden blank with their edges abutting and with their starting points 180 degrees apart. Next the edges of the paper strips and the two ruled lines running down their centres were pricked through about every 50mm to mark the wood underneath. Once done the papers were removed and the pin pricks on the wood joined producing a number of interlocking spirals. The spirals produced by the paper edges mark the position of the bottom of the V-cut threads. The two parallel spirals 4mm apart on each strip mark the top surface of the threads (which don’t come to sharp points) and indicate where the cut downwards on both sides must start.

 

 

figure 19
Chiselling out the thread

After marking out the thread, the depth of the V-section cut has to be determined by fitting a hand saw with a depth stop then sawing downwards to the thread depth. Next the sloping sides of the threads are carved out using a chisel. This is done by carefully carving out a small portion at the start of one of the threads. Once this is correct it is used to help control the cutting of the spiral by keeping most of the chisel’s flat surface pressed against it whilst allowing only about a third of the chisel’s edge to cut into new wood.

Next – Making the wooden nut