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Builder Spotlight

July 2019 Issue




Adjustable Jig Posts

An easy way to keep a wood jig in alignment.

In order to keep the airplane flying without big trim tabs hanging in the breeze, you have to keep things straight and precise during construction. For homebuilts that require a jig for building wings, the manufacturer usually provides plans or suggestions for building the jig, and often the specified material is wood because it is readily available, inexpensive, and easy to work with. But this presents a problem if the workshop is located in an area with large swings in relative humidity—such as my hangar on the outskirts of Portland, Oregon, where I’m building a GlaStar.

A hanging plumb bob verifies the jig’s precise position.

The GlaStar wing jig design calls for a 16x2-foot workbench made from 2x4-inch lumber with 4x4-inch vertical wood posts at the ends. Even if the workbench is constructed level and square, and the posts are plumb, it will only stay that way until moisture changes in the wood cause something to come out of alignment. The brackets holding the GlaStar’s wing to the wood post incorporate a plumb bob, making it easy to confirm alignment before any work. If the plumb bob is off its mark, the builder must make adjustments to the jig to bring it back into alignment. Sometimes this can be tricky if the misalignment is in several places.

To minimize the potential for warping, I used steel posts and also constructed a simple mechanism that allows easy initial adjustment of the posts and any future adjustment if the wooden workbench on which the posts rest pulls them out of alignment.

Drill and tap the vertical post for the two adjustment rods or use tee nuts.

Drill 5/16-inch holes in the post, and use a 3/8-16 tap for a 3/8-inch threaded rod from the hardware store.

For a wooden post use two tee nuts.

Each post is held in position by two 3/8-inch threaded rods that pass through holes tapped in the post’s centerline—one at the top of the workbench, and one at the bottom. The two rods are perpendicular to each other. The post is tightly sandwiched between wood members parallel to the threaded rod, which makes the assembly tight enough to hold everything firmly in place, but still allows for movement along the rods. Each end of the rod has a large washer and a pair of jam nuts.

Two nuts and a fender washer for each end of the threaded rod.

Adjusting the angle of a post only takes a turn of the jam nut. In the photo with the arrows, turning the top nut clockwise will lean the post to the right.

Turning the nut clockwise will move the post in the direction of the arrow.

Home improvement stores typically sell a 36-inch threaded rod for about $4. You’ll need eight nuts and four fender washers per post, so the cost is very low.

Aluminum posts can also be tapped for a threaded rod, but a wooden post can be used for the jig, too. You’ll need to drill a hole through the wood post slightly larger than the threaded rod, and use a pair of tee nuts fastened to the post over the ends of the hole.

Be sure to make the hole in the tabletop large enough to allow the vertical post to move in all four directions.

Wood members parallel to the bottom threaded rod hold everything firmly in place, but still allow for movement along the rod.

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