Machine Design

A simpler tape measure provides two tools in one

Edited by Leslie Gordon

Smaller, lighter, and less costly than the traditional 16 to 35-ft-long tape measures for outside jobs such as framing buildings, the Indoor Tape Measure from R. Clark DuBois is for use around the house or shop. Only 3 × 2.5 × 1.4 in., the outer case houses a concave steel tape that is actually two measuring tools in one — it can work either as a measuring tape or as a 7-ft long ruler. The tape is graduated on each side and, like all measuring tapes, has a hook on the end.

The outer case can be so small because — unlike other tape measures — the design works without the need of a lock and rewind spring. The case is oak so the device looks good sitting in a bookcase or kitchen drawer.

To take short measurements with the Indoor Tape Measure, just pull the tape out a foot or so, and it stays there. Because the concave tape wants to be straight, energy is stored when you push the tape back into the case, where it rolls into a coil.

To take long measurements, such as for a drape or piece of plywood, pull the tape out about 5 ft. The stored energy makes the tape self-extend, freeing it from the case. The tool is now a separate, 7-ft-long, light, stiff, straight, and accurate measuring rule. Without a heavy case on one end, the rule is easy to maneuver.

The major problem in designing a tape that pulls free from its case was how to push the tape back in. Without a rewind spring pulling on the inside end of the coil, friction lets only short lengths of a few feet be pushed into the coil inside the case. The trick was expanding the coil as more tape is pushed in. Small, internal injection-molded plastic components let the coil do this. DuBois’ recent patent, 7,370,432, covers the concept.

As a free rule used concave side down, the numbers on the tape are easy to read and the graduations touch the work. Thus, users need not worry about parallax errors. Users can read the tape to accuracies of less than 1/16 in., helpful for those liking to, for instance, build furniture as a hobby.

To get such accurate measurements with a conventional tape, users must push down its edge to the surface so the graduations rest on the work. This sounds easy to do, but it takes action with a hand that might have other things to do. Generally one hand holds the tape while the other holds a pencil and paper. Often, a hand is also needed to hold the work or a light. The Inside Tape Measure solves the problem of needing three hands.

The rectangular case is also functional: In a shop, it can be used as a square. In addition, with the tape pulled out several feet and the case standing on its end, the tape extends vertically — handy for pinning a skirt or checking the lengths of drapes. On its side, the extended tape is on its edge, which puts the graduations near the work surface. Contact R. Clark DuBois at [email protected] or (508) 209-3267.

© 2011 Penton Media, Inc.

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