Introduction: Improved Drill Centering Jig
After extended use of the original drill centering jig, we discovered a number of opportunities for improvement and identified some of the weaknesses that developed over long term use. To be fair, the original jig ended up getting a lot more use than was originally expected and held up well all things considered. One of the first things that became apparent was that a stationary jig mounted to the drill press table would be more effective. After drilling hole after hole with a portable jig, it was easy to allow it to tilt slightly which forced the hole off center. Another issue was that the square tubing in the original jig provided a large surface area with which to clamp the board, but it also trapped wood chips from the drill occasionally throwing the hole off center. Eventually, the hole in the guide bar enlarged due to the friction from the drill bit, which was expected, but could be eliminated in a stationary design.
The new jig has now had nearly as much run time as the original and aside from the occasional recalibrating to center, has no major drawbacks or replacement items. The addition of a dust collection hood was a major improvement from a convenience standpoint, but is not necessary. For what it is intended to do, which is consistently bore a specific diameter hole in the center of a board over and over, the jig has performed exceptionally well. As an added bonus, the jig is not limited to one size drill bit.
To ensure the jig does not flex, I first cut two 3/4” steel box tubes and two 1” steel u-channels to a length of 8”. The u-channel will serve as the clamping surface against the wood creating minimal opportunity to trap wood chips while remaining rigid. I also cut two 3 1/2"” lengths of 3/4” square tubing to serve as the parallel rotating members. (fig. 1 & 2)
The next step was to bore three holes in the two 3 1/2"” pieces of steel square tubing. One hole is centered and drilled at a ¼” diameter. (fig. 3)
The other two are located 1” from either end and drilled at a 5/16” diameter. In order to ensure the holes would match, I stacked them and drilled both pieces of box steel at the same time. To ensure alignment, I drilled the center hole first and then ran a ¼” bolt through both pieces before re-clamping to bore the side holes. (fig. 4 & 5)
I then drilled 5’16” holes 1” from either end of the 8” steel tubing. Again, drilling through both pieces, once the first hole was drilled, I dropped in a 5/16” carriage bolt to ensure alignment. (fig 6 & 7)
One side of each of center holes in the 3 1/2” pieces of the square tubing needed to be enlarged enough to accept a ¼” socket. The bolt cannot extend above the surface of the square tubing because the wood will be riding on top of it. Therefore I opened the hole up on one side with a 3/8” drill bit and then widened the hole a little more with a small grinder. (fig. 8, 9, 10 & 11)
On the opposite face of the 3 1/2” piece of steel box tubing, the two 5/16” need to be squared to in order to lock a carriage bolt. I in the absence of a hole broach, I used a small file to square off the holes. (fig 12 & 13)
The u-channel needs to be drilled with some small holes to mount to the perpendicular face of the 8” square tubing. The holes need to be offset so the bottom surface where the clamping structure will rotate is flat. (fig. 14)
Once the holes were drilled in the u-channel, I lined up the u-channel with the box channel and drilled through the box channel using the holes as a template. (fig. 15)
It is time to assemble the clamping unit. Using 1/8” bolts, I attached the u-channel to the 8” box channel. (fig. 16)
I then used 5/8 x 2” carriage bolts to attach the two pair of box channels ensuring that the expanded holes are facing up. (fig. 17)
It is now time to build the mounting base. I used a piece of 5/4 x 8 poplar which was plained to ensure flatness and cut to a length of 18”. I drew a center line and placed the clamping assembly on the wood and drilled the 1/4” mounting holes using the assembly as a template. (fig. 18)
I used a countersink bit on the back of the wood to create a countersink for the bolts so the base would mount to the drill press securely. (fig. 19)
In order to promote the longevity of the jig considering the repetitive use, I cut a light gauge piece of sheet metal to place between the clamping jig and the wood base. With the correct number of wide flange washers on the mounting screws, the heads of the carriage bolts can ride on the sheet metal and stabilize the jig. I also drilled 2 large holes in the base and the sheet metal to accommodate two large bolts mounting the base to the drill press. (fig. 20 & 21)
After utilizing ¼” x 1.25” bugle head bolts to attach the clamping assembly to the base, I installed some plastic thumb blocks to make the clamping easier. I also installed some rests on either side of the base to reduce the tendency of material to tilt out of the jig. (fig. 22)
It is now time to set the depth of the hole and calibrate the jig to center. This requires patience and I suggest re-calibrating the jig every few hundred holes, but it is well worth it. The jig is quick an accurate if you have to repeatedly drill holes dead center in stick lumber.
Rod Gunter is General Manager at Gunter Building Solutions and has over 20 years of experience in the homebuilding and cabinetry industries. Rod has been responsible for building over 200 homes above the $500,000 price point. Rod has trained large groups including all the major home centers on selling skills, construction techniques and sustainable natural wood products. Rod resides with his family in Holly Springs, North Carolina. Gunter Building Solutions owns WoodAirGrille.com, a leading manufacturer of wood return air filter grilles and wood return air vents.