Doweling Jig

I needed a jig to help my drill 8mm holes in the ends of hardwood boards for my Outdoor Table. These holes needed to be accurately spaced, otherwise assembly would be difficult if not impossible. 

I started with a piece of 80x20mm mystery steel bar. (It later turned out to be a really fairly hard tool steel!) The bar was milled on all sides to 70x18x22mm. 

3 holes were drilled symmetrically, 20mm apart. They were positioned to be 8mm from one face, and 10mm from the other - this allows me to dowel boards which are 16 and 20mm thick. These seemed to be the most distances for an 8mm dowel. Spacers can be used to generate other distances if required. Holes were drilled to 6.5mm before being "reamed" with an 8mm end mill.

Using an 8mm drill bit straight away tends to open up the possibility of a crooked hole (where the exit hole is not coincident with the entry). This can happen even with a centre drilled point. I'm also not a fan of drilling a smaller hole first. as this exacerbates the wandering problem due to a smaller drill, and it causes a lot of chatter when a larger drill goes through.

Two M6 holes were tapped on the side to create the final jig. 

I've mounted it to a piece of aluminium stock, which can be clamped to the piece of wood being doweled.

UPDATE: Woodworking tolerances are often seen as mediocre by metalworkers, who can easily work down to +/- 0.05mm. With a cruel sense of irony, my jig is actually too precise! The holes are so close to 8mm that a regular drill struggles to go through, let alone a woodworking drill with it's pointy peripheral tips. I've got to make the holes bigger....