A dowel plate is a simple tool, consisting of a thick piece of iron with holes of different sizes. The edges of the holes are sharp and will cut dowels of a specific diameter, when pieces of wood are hammered through it. It is difficult to find a medieval reference to this tool, however a possible presentation of a dowel plate is found on a woodcut (detail shown left) in Van der Dyngen Erfyndung by Polydor Vergil from 1537 (Augsburg, Steiner, fol. VC r - also found as figure 219 in Van Tyghem, 1966). In principle, dowels could (more laboriously) be produced by hand using a chisel.
Dowels were used ubiquitous for all kinds of medieval woodwork: wooden frames of houses and sheds were secured by large dowels, mortise and tenon joints of furniture pieces by smaller ones. Wood for dowels need to have a straight grain, otherwise they would break during hammering. Dowels need to be slightly larger than the hole, which was drilled by an auger or brace.
The dowel plate in our medieval toolbox was also made by Dennis Riley.
These two images are from the miniature 'Building the ark of Noah' in the Bedford Hours. On the left a man can be seen drilling a hole for a dowel with an auger, whereas on the right a man is seated on the roof , hammering a dowel in a previous drilled hole.