The Adjustable Try Square: An Eccentric Pin That Trues the Tongue

"In the drawings herein referred to, a is the tongue, b is the beam, c is the flattened pin, d is the eccentric pin, e e are the screws which hold the beam together". "By turning the eccentric pin the tongue will swing or vibrate in proportion to the throw of the eccentric. As the tongue swings, the oblong hole h, allows it to slide on the flattened pin, and the flattened pin being made to fill the hole the narrowest way, will always turn to favor the angle of the tongue."

**Beam.** Saw 15 cm of hardwood 30 × 15 mm from hardwood board and saw a slot 3 mm wide and 5 cm deep into one end for the tongue. **Tongue.** Cut 20 cm of 25 × 3 mm mild steel flat bar and file one long edge dead straight. Near one end drill two 5 mm holes 3 mm apart and file between them into an oblong hole; 30 mm further along drill a 10 mm hole. **Pins.** File two flats on the shank of an M5 bolt so it fills the oblong hole across its narrow way, and bolt it through the beam's cheeks and the oblong hole. For the eccentric, cut a disc 3 mm thick from 10 mm aluminium rod, drill it 5.5 mm half a millimetre off centre, file a screwdriver slot across its face, and fit it in the 10 mm hole on a second M5 bolt through the beam. Snug both nuts. **Test.** Hold the beam on a board's straight edge and draw along the tongue. Flip the square and draw again. Measure the gap between the lines at 20 cm with the caliper and compare with the notebook. Turn the eccentric until the two lines coincide, then tighten both nuts. Check it against a bought try square.

**Joel Whitney** of Winchester, Middlesex County, Massachusetts, received US 17,991, dated 11 August 1857. His beam was a wooden centre piece between two metal plates held by screws, or all metal in two parts; the eccentric's journals were fitted to holes in the plates and slotted for a screwdriver, and radial lines on the plate could guide the tongue to other angles. **Honest limits.** A hardwood beam with a sawn slot, filed bolts and an aluminium eccentric stand in for his plated beam and journalled pins. The notebook treats the eccentric's swing as a small angle; its lengths and errors are examples. Your flip-test gaps are the result.