Center Finder for Chair Stretchers
Finding the perfect center to drill mortises for stretchers between two round chair legs can be quite a juggling act and a time consumer if you make a lot of chairs like I do. I came up with a simple solution using two center-finding heads and two grooved metal rules from combination squares. The rules are held together with a homemade clamp box, which allows the rules to slide between the legs and be locked precisely in place.
The top piece of the clamp box has a dado for the rules, a recessed T-nut, and a 1/4-in. brass screw for locking the rules. A piece of hardwood glued at the bottom holds the whole thing together.
As you tighten the knurled screw, it pushes on a copper penny, which acts as a pressure plate to lock the two rules in place.
I cut both rules short (about 9 in.) to give a measuring range of 12 in. to 16 in.—the distance between most chair legs. You can customize the length of the rules to suit your needs.
The device is easy to use. First I locate the height of the stretchers on the legs. Then I align the device with those marks and extend it until both legs of the center finders are rubbing the perimeter of each leg. Finally, I advance the rule tips to touch the legs, lock down the clamp, and mark the center point of the legs. The jig ensures that I drill aligning mortises, and it also helps me determine the length of the stretcher with precision.
Detailof clamp box