Combination Square

Starrett's clamp uses a counterbored nut and helical spring both to stop the bar shifting and to exclude dust and debris. A layout tool lives in a workshop full of chips.

Scribe a line across a board with a try square, flip the square over and scribe again from the same point. Two lines mean the square is not square — and the error is half the gap.

Prepare a straight steel rule about 300 mm long with parallel edges. Everything the tool claims depends on this bar being straight and its edges parallel.

Cut a slot along the blade's centre line for the clamp to run in. Off-centre and the head will sit skewed at every position.

Mark millimetre divisions along one edge, starting from the very end of the bar. The zero must be the physical end or every depth reading is offset.

Make a stock with one face square to the blade slot and one at exactly 45°. Check both against a known reference before going further.

Make a hook that engages the blade groove, drawn up by a nut against a spring. The spring keeps the hook loaded so the blade cannot creep between settings.

Loosen, slide the blade its full length, lock, and try to move it by hand. Any movement under hand pressure is movement under a mallet too.

Scribe, flip and scribe again as in step 2. Adjust the head face until the two lines coincide. This is the acceptance test for the whole tool.

Turn to the 45° face and mark a mitre. Verify by marking two mitres against each other — together they must form a right angle.

Sit the head on a workpiece, drop the blade into a recess and lock. Read the depth directly off the graduations — one setting, no transfer, no second tool.

Set a fixed projection, hold the head against the edge and run a pencil at the blade end while sliding along. You have scribed a line parallel to the edge.

Set a small level into the head with a raised arch over it. Starrett guards it because an exposed glass vial in a toolbox is a broken vial.

List what the one object now does: square, mitre, rule, depth gauge, marking gauge, level. Note that they cannot disagree, because they share one edge and one scale.

The patent. US 215,024, "Improvement in try-squares", filed 26 February 1879 and granted 6 May 1879 to Laroy S. Starrett. ⚠ Secondary sources frequently give 26 February as the grant date; it is the filing date. The grant is 6 May.

Sliding the stock is a smaller change than it sounds and a bigger one than it looks. A try square's head is fixed at one point on the blade, so the tool has exactly one function and one reference length. Letting the head clamp anywhere turns the same two pieces into a family of tools, because what a layout tool is depends entirely on where its reference face sits relative to its scale. That is why step 14 is the payoff: nothing was added except a slot and a clamp, and six tools came out.

The dust clause is the mark of someone who used tools. Starrett was a farmer and inventor before he was a toolmaker — his earlier patent was for a meat chopper — and the patent's insistence that the clamp exclude dust and debris is not the language of a designer working from drawings. A layout tool spends its life in sawdust and swarf, and a clamp mechanism that packs with chips loses its setting silently, which is the worst way for a measuring tool to fail. Guarding the spirit vial under an arch is the same instinct.

Why the flip test matters more than the tool. Steps 2 and 9 teach the single most useful habit in layout work: a square can only be checked against itself. Scribe, flip, scribe again — if the lines diverge, the error is half the divergence, and no amount of care in cutting will rescue work laid out with a square that is 0.5° off. This is also the reason a combination square is the tool a machinist checks first and replaces without hesitation when dropped.

What it became. Starrett founded the L. S. Starrett Company in Athol on the strength of this and subsequent tools, and the combination square is still made to essentially this design, with interchangeable heads — protractor and centre-finder heads slide onto the same blade, extending the same idea further. The tool in a modern workshop drawer is a direct descendant, and a maker who understands why the head slides understands most of what precision layout is about.