A simple jig for grooving short dowels
Carefully fitted dowels will trap both air and excess glue when inserted. Unless you provide an escape route, that air or adhesive may prevent you from bringing together the joined pieces. If you force them together, the built-up hydraulic pressure may cause the dowel to pop out of its hole when you're not looking.
For dowels 11⁄2 " to 2" long, build a jig out of scrapwood as thick as the dowel length. Near one end, drill a hole the same diameter as the dowel and then crosscut the scrap so two-thirds of the hole remains as shown right. Insert the dowel in the jig and cut a 1⁄16 " saw kerf 1⁄16 " deep with a bandsaw or scrollsaw. One groove lets air and glue escape.
—George Mitro, Jr.., El Paso, Texas