To assemble boxes, tables, base structures, you can use dowels that are a very easy way to make fast and simple joinery. The dowels are little pieces of wood that you insert between two pieces that you need to assemble. They are glued into holes drilled in the pieces composing the structure.

However, it's very difficult to drill perfectly aligned holes into the two pieces, one millimeter of offset and you can be sure that your pieces will not be aligned as you want them to be. This is where dowel jigs are very useful : they help you to drill perfectly straight AND aligned holes between the two pieces, and their use is really easy. In this instructable, I'll show you how to make the simplest dowel jig of the world, with even two sizes of dowel. Let's work !

Step 1: Supplies

You need :

A piece of wood : 70mm x 50mm, thickness of 14-15mm
A sheet of transparent acrylic : 70mm x 90mm, thickness of 3mm or 5mm
A drill or a column drill (better for perfect straight hole)
Drill heads for wood
- Ø 8mm
- Ø 6mm
- Ø 3mm
A ruler
Bolts and nuts
- Ø 3mm

A knife or a cutter
A square guide (optionnal)
Neocolor 2 Caran d'ache pastel (optionnal, can be replaced by other types of pastel or by paint)
Dowels (for final use)
- Ø 8mm
- Ø 6mm

Step 2: Modeling in Fusion 360 (optional)

I made a 3D model of the dowel jig before all, to be sure that it matchs my expectations. Nothing really interesting to say here, you can find great tutorials for 3D modeling in Fusion 360 on Instructables. I shot some screens for you to get the dimensions of my version of the jig, but you can of course define you own, for the size of the jig, the numbers of holes, the offset between them, etc. Your imagination is the limits :)

Step 3: Cutting

It's a very simple step here. You have to cut two pieces (all the dimensions are in the screenshots of the previous step)

The acrylic sheet
- Mine is a 70mm x 90mm piece
The wood part
- Mine is a 70mm x 50mm piece

That's all !

Step 4: Drilling

Here the tricky step, you have to drill into the piece of wood, on the edge of it. There's 4 holes, 2 for the 8mm dowels, and 2 for the 6mm dowels.

The offset between each hole's centers is 15mm from the edge of both sides (take a look at screenshots of step 1 "Fusion 360"). Draw the lines to guide yourself on the drilling.

Make sure that you drill is completely straight to make perfect holes trough all the piece : 1mm of error and you jig will be useless :'(

Once it's done, drill two another holes (Ø 3mm) on top of the wood part and the acrylic (align them and lock them with a clamp and/or double tape), they will be used for assembling them.

Step 5: Customizing

On the bottom of the acrylic part, mark a line to edge with a cutter or a knife, it has to be perfectly squared on the middle of it.

Once it's done, you can color the line with some color pencil, I use one with a pastel red head, to make the line very visible.

Optional step : paint or color you piece of wood to make it shiny and beautiful, and easy to differentiate with a scrap piece. I use Neocolor 2 pastel for that, beacause it doen't need any time to dry and less messy than paint.

Step 6: Assembling

Now assemble the two pieces with bolts and nuts. You can use screws too, but it's a more definitive solution and with bolts and nuts, you can adjust the offset between the wood part and the acrylic part with spacers (photo 1). It can be useful when your pieces to assemble are thicker than your jig and you want to center the holes.

Step 7: Doweling

Now it's time to use your jig !

Put the two pieces next to the other
Mark a line at the jonction
With you jig, align the red line of the acrylic part on the line, your jig has to be squared and fully pushed again the piece to assemble. Use a clamp to maintain the jig on the piece.
Drill the holes, either the 6mm version or the 8mm (or all of them if you want...)
Repeat all the process for the other piece to assemble.
Now put the dowel inside the holes and... tadaaa!! The pieces are perfectly assembled.

You can use glue in the holes and the edges of the pieces to reinforce your joinery.