Keystone Dowel Jig

Preamble

This is my attempt at designing the best doweling jig that can possibly be made entirely out of plastic. 

The major problem with plastic jigs is that they don't generally last very long—usually about the duration of a single project—before they're worn out and need to be tossed. This design greatly reduces that issue by using snap-in dies/keystones for the actual drill holes. Once the hole gets too loose to be useable any longer, just print a new keystone!

At this time, I've only made jigs to fit the most common American lumber and dowel/bit sizes, since that's what I use. If there's sufficient demand, I'll happily upload a metric version as well. :)

I played around with the idea of a single, adjustable jig base that would work for any size lumber, but abandoned that for a couple reasons. One is that a doweling jig should be fairly precise, which is difficult to accomplish in any adjustable tool. The second is that printing and assembling the jig from multiple parts significantly increases the margin for error (especially when using a budget printer like mine). My single-piece jigs all fit beautifully on their intended lumber. My multi-piece attempts all had a good deal of slop. They were probably fine for pilot holes, but not for dowels.

The only significant downside with this design is that each jig will only work with a specific lumber size. This doesn't seem like that big a deal since in my experience, a typical project rarely utilizes multiple thicknesses of lumber in any meaningful way, at least not when it comes to doweling. 

The print orientation of the jigs also requires some supports as well as a shorter layer height due to the sideways-facing threads. This increases the print time, but not by a lot. It's well worth it for the ensured dead-on accuracy of printing the bases as one piece.

Anyway, let's get to it.

Jig Bases

The jig bases will each fit over one of the following standard (American) thicknesses of lumber.

• 1½" (i.e. 2×4, etc.)
• 1"
• 3/4"
• 1/2" (small keystones only; see below)

Each base features two slots for keystones, one on the edge of the board and one on the top face. The slots are perfectly aligned with the center of the board and with each other. Properly measured, this should ensure perfect dowel placement every time!

The top and bottom of the bases also feature threaded holes for the included thumbscrew, so the jig can be tightened in place. These may not be necessary unless your boards are slightly undersized or have been planed. If you use them, do so gingerly. Overtightening will flex the jig and skew the angles of the keystones.

Additionally, each of the jig bases also comes in a variant which can be used when drilling all the way through the face of a board. The standard versions are intended for drilling halfway only.

Keystones

The keystones come in a variety of (American) bit diameters. I chose the ones most commonly used for doweling as well as for simple pilot holes. If I missed any obvious sizes, let me know and I'll add them!

The keystones come in two types, small and large. The small keystones are compatible with any size base, but only go up to a 5/16" bit diameter. The large keystones go up to a 1/2" bit diameter, but are not compatible with the 1/2" base.

The large keystones are a tad easier to work with, so I'd recommend sticking with those unless you need to go all the way down to 1/2" lumber. The small keystones are also a good choice, however, if you never anticipate needing to drill anything larger than 5/16". Naturally, the small keystones result in less plastic being wasted once they're worn out. The choice is yours.

I have also included a removal tool for each size of keystone. The small ones, especially, can be difficult to get out by hand. With the tool, though, you shouldn't have much trouble removing keystones even if you have big, meaty fingers. 

Printing Instructions

• Leave all objects in their current orientations.
• All are intended for a 0.40mm nozzle.
• PETG (or harder) is recommended. 
  No PLA, unless you want these to last only a handful of uses.

JIG BASES

• Supports required: YES, but only for the keystone slots, not the threads. 
  Snug support style is recommended.
• Perimeters: 2–3
• Infill: 25%
• Layer height: 0.10–0.12mm, so the threads come out clean and without curled overhangs.
  You may also consider a variable layer height. Start at 0.15–0.20mm, then reduce the height on the layers with threads.

KEYSTONES

• Supports required: NO
• Perimeters: 3
• Infill: 25%
• Layer height: 0.15–0.20mm

THUMBSCREW

• Supports required: NO
• Perimeters: 2
• Infill: 15%
• Layer height: 0.12–0.15mm

REMOVAL TOOLS

• Supports required: NO
• Infill: 50%, for added rigidity. 
  For the small tool, consider adding 100% density modifiers on the narrow parts.
• Perimeters: 2
• Layer height: 0.15–0.20mm

Good luck!