Where a 12 mm guide is used
Twelve millimetres is the size of large furniture dowels, of the holes for threaded rod and coach bolts through timber frames, and of pass-through holes for a cable or a small pipe in a shelf or a cabinet side. These are holes where straightness shows: a dowel hole that leans by two degrees is a millimetre out over the depth of a joint, and the joint does not close. A long guide hole keeps the bit square to the face. The 35 mm offset here places the hole in the middle of 70 mm stock; change it to half the width of yours.
What to measure
Measure the bit across its cutting diameter, and decide which kind of bit you will use, because it matters at this size. A brad-point or dowel bit has a centre spur and cuts cleanly in wood. An ordinary twist bit of 12 mm tends to grab as it breaks in. A flat spade bit does not suit a printed guide at all, since its wide blade scrapes the hole. The guide hole is 12.2 mm for a 12 mm bit. Also measure the bit's working length: the block uses up 25 mm of it before the bit reaches the wood.
Block, fence and material
The block is 25 mm thick, about twice the bit diameter. That ratio is lower than on the small jigs of this site, where the block is three or four diameters, because a thicker block would leave too little bit to drill with; it is still enough bearing length to hold a 12 mm bit square. The fence is 8 mm thick and 15 mm high, for boards of 18 mm and more. Print in PLA for hardness. A plain printed hole of this size wears with every use, as the bit's edges shave it, so for more than a handful of holes set the guide hole to the outside diameter of a steel drill bushing and press one in.
Printing and use
The jig prints fence up, 110 by 70 mm on the bed and 40 mm tall, with no supports, and takes several hours. Use five perimeters and 30 percent infill so the guide hole and the fence are backed by solid plastic. In use, clamp the jig to the work with two clamps, one each side of the hole; a 12 mm bit that catches will spin a jig held only by fingers. Start the drill before the bit touches the wood, feed slowly, and lift the bit out now and then to clear the chips, which otherwise pack inside the guide.
Outline drawing
To scale. Left: top view as printed, fence along the top edge. Right: end view, block 25 mm thick with the fence rising 15 mm above it.
Parameters the generator will use
| Setting | Link parameter | Value |
|---|---|---|
| Model | model | drill_jig |
| Jig length along the edge (X) | length | 110 |
| Jig width including the fence (Y) | width | 70 |
| Block thickness | thickness | 25 |
| Guide hole diameter | hole_diameter | 12.2 |
| Hole centre to workpiece edge | offset | 35 |
| Hole position along the length (0 centres it) | hole_position | 0 |
| Fence thickness | fence_thickness | 8 |
| Fence height | fence_height | 15 |
All lengths are in millimetres. Outer size from these values: 110 x 70 x 40 mm.
Open the generator with these values
This model needs the $7 licence key: one payment, one key, and it opens all five paid models. Get a licence key on Gumroad ($7)
The drill guide jig needs a licence key: without one the generator shows the model locked, lists the values from this link and fills them in as soon as you unlock. The fitted drawer organiser bin is the free model and downloads with no key.