Woodworking with Diego de Assis


 
  Shop-made Thread Cutting Tools for Wood 5 of 7  

The Screwbox

Following the dimensions provided in Le Menuisier Ébéniste, I simplified the design somewhat, leaving out some details such as the turned handles and the decoration on the opening for the workpiece.

The bolt I used to make the tap has eight threads per inch. For metal bolts that is a coarse pitch (pitch measures the distance between the peaks of the adjacent threads). For threaded parts that need to move a lot and are in constant use, such as the screw of a bench vise, a very coarse pitch is preferable (fewer threads per inch). And with wooden screws another reason for a coarse pitch is that thicker threads will hold up better.

Hard woods should be used for these parts, as with softer woods the internal threads could wear away. Woods such as Jatobá, Tauari, Peroba-de-campos, Ipę, and Roxinho are excellent for making the screwbox. I used Jatobá (Hymenaea courbaril), which is hard and very versatile.

The screwbox has two parts: a lid or guide plate, which secures the cutter and guides the round stock at the beginning of the cut, and the main body. After the block is cut to dimensions the guide plate is cut away.

The two parts are then put back together and secured with two screws on diagonal corners (placing the screws in line with the grain could split the wood). Then the hole in the lid is bored, with a chip drill, auger, or Forstner bit, marking the desired depth with a bit of masking tape. I used a 1” chip drill.

The first cut should go through the lid, with the tip or lead screw going into the main body to mark the center. The bit is then changed to a smaller size, 7/8” in this example, and the smaller hole is bored through the main body. The difference in the diameter of the holes is exactly the difference between the two bits – 1/8” in this case.

Once the hole has been bored the internal threads are cut in the main body.


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