A whole lot of tool in one small package
Far more than just a tool for checking “square,” the combination square is a do-all shop tool. With its sliding ruler locked to the head, you can use it as a depth gauge, a marking gauge, a miter square, and a try square. Loosening the lock in the head releases the rule for use as a straightedge or ruler. In a pinch, you can employ the head as a small level, and a sharp awl tucked inside is always at the ready for marking. Thanks to these attributes, you can turn to a combo square to true up workpieces, lay out joints, and set up shop machines–all with a degree of precision that elevates its status over other tools.
To ensure this level of accuracy, the first order of business is to acquire a square that’s worthy of your work. Not all combination squares share the same quality and ease of use. In this article, we’ll take a look at the parts and nomenclature of combination squares, and I’ll describe what to look for in a good tool. Following that, I’ll discuss the two arenas in which the tool excels: measuring and laying out. Measuring includes performing machine setups, such as squaring bits and blades to tables and fences, as well as checking joints and assemblies for accuracy. Laying out involves making marks on your work, from drawing parallel lines and angles to marking centerlines and pinpointing hardware locations. Read on to find out how to put this extremely versatile tool to good use.
Sizing up a square
If you want precision and predictability in your work, you’ll need an accurately made tool. Referring to Anatomy of a Combination Square (right) keep the following features in mind when shopping for a combination square.
The best squares are made from forged or tempered steel or a combination of both, and should last beyond your lifetime. Look for smoothly-machined contact faces on the head and a ruler with finely etched markings that increase accuracy and allow you to target even 1⁄64" increments (Photo A). A satin chrome finish is preferable to a shiny surface, as it reduces glare under bright light, making reading easier.
The ends of the ruler on a high-end tool are ground square, for precise measuring and layout, and for verifying parallel and square. Cheaper versions are chopped, often leading to rough, out-of-square ends that inhibit accuracy.
Because the ruler on a combination square is graduated differently on all four edges, you’ll often need to reverse it in the head for the measuring needs at hand. So look for a rule that slides out smoothly, and a lock post that rotates easily inside the head so you can flip the ruler over and then reinstall it without struggle. With the locknut tightened, the ruler should feel solid and never slip or creep in the head during use. Last, but not least, a good tool will lock dead square and stay that way at any point along the ruler. This degree of accuracy comes at a cost, though: quality squares are two to five times the price of cheap hardware-store versions.
Anatomy of a Combination Square
Virtually every combination square on the market is anatomically similar, although the quality can vary widely between manufacturers.
Before relying on any square, first check its precision at 90°. If you have a reference square that you know to be accurate, simply nest it against the square in question, inside-to-outside and then outside-to-inside, holding them up to a strong backlight to ensure that they mate exactly. Alternatively, you can check a square using a mechanical pencil or knife, and a panel with a dead-straight edge, as shown in Figure 1. If you’re square is out of whack and you paid more than $40 for it, I recommend returning it.
Measuring
A combo square really shines when it comes to determining dimensions. This includes calculating distances, such as the depth of a mortise or the thickness of a board, as well as measuring relative surfaces, like the angle on the end of a board or the squareness of a drawer or other box construction.
As for simply checking inside and outside corners, a combination square mimics a traditional brass-and-wood try square in every way, except that the sliding ruler on a combo square adds a lot of versatility. And thanks to its tempered and forged-steel construction, a combo square can withstand more knocks and dings than its softer brass counterpart.
The “try” in the tool’s name refers to offering the tool up to the work to test whether adjacent surfaces are flat and square. Perhaps the uncertainty of the word “try” stems from the fact that misreadings are common due to poor technique. The key is to place the head onto the work first, and then slide it firmly along the surface until the rule contacts the adjacent surface. Look for any light (or shadow, depending on your situation) between the work and the rule. If you see even a tiny glimmer, the work is out of square.
To measure inside corners, use the outside of the square. Retract the ruler about 1⁄16" into the head, and then position the square so that the blade and body touch the work (Photo B).
To read outside corners, extend the rule so it’s at least as long as the surface being measured, and position the head on one surface before sliding the rule against the other surface (Photo C). The best scenario is to view the work directly in front of a strong light source, with your eyes level with the ruler. This arrangement lets you see even the tiniest light leaks at the rule, as shown in the opener photo.
Because most miter cuts result in a 90° joint when assembled, it’s prudent to make a final check by holding two miters together and then checking for square on the inside or outside corner. Nevertheless, testing the miter itself is the first step, and the miter square will get you there. To check for 45°, hold the angled face against the work (Photo D). Because the head’s 45° face is relatively small when used for larger work, you can balance the tool by sliding the ruler past center to help counter the weight of the head.
As an adjustable end gauge, a good combo square offers the advantage of having a sliding rule with precision-ground, square ends. You can use the ends in several ways. The best technique is to register the head first, slide the ruler until the end touches the work, and then lock the rule to take a reading. For example, to gauge parallelism of tenon cheeks, hold the head against the stock, and push the rule until the end contacts a cheek. Look for full contact between the rule and the cheek, as the slightest gap tells you the cheek isn’t parallel with the face of the stock (Photo E).
You can measure between nonlinear surfaces–such as the distance from the edge of a router baseplate to the perimeter of a bit–by registering the head against the router base, and extending the rule to touch the bit (Photo F).
Checking the depth of mortises and other stopped recesses is easy. Simply register the head on the work, and then slide the rule until it bottoms out in the mortise (Photo G). Smaller, 6" combo squares, with their 3⁄4"-wide rules, are most convenient for typical woodshop mortises and other common excavations.
You can also use a combo square to check if your boards are the same thickness from side to side and end to end–good info during the machining and dimensioning stage. Lock the ruler so its end is even with one corner of a board, and then check the other three corners using the same setting (Photo H).
Of course, you can also use the ruler on its own to measure distance. On a good square, the marks are accurately milled to the end of the rule, allowing you to butt the end to a surface and read from there (Photo I).
Laying out
Accurate layout is an equally important aspect of the combination square’s talents. In addition to drawing square and 45° lines across your stock (and other angles, as I’ll describe), a combination square lets you lay out dimensions, thanks to its graduated ruler. And using the end of the rule as a guide, you can draw lines at precise distances from the edges of your work.
For making long layouts, manufacturers offer accessory blades in lengths of 18", 24", and 36", all of which will fit their 12" combo square head. These allow the precise layout of dadoes and grooves in cabinetry and other large-scale work (Photo J).
Another very useful accessory is a protractor head, which is handy for projects that involve a lot of angles. Starrett’s protractor head, to name one, allows pivoting the rule to any angle from 0° to 180°. Fitting a long ruler to the head increases the versatility of the tool.
A combo square is commonly used as a substitute for a marking gauge when drawing parallel lines. Just set the ruler to the desired distance, place the head against the work, and draw the square towards you while pressing a pencil against the end of the rule (Photo L).
Use a centerhead to find the axis of round stock, marking two lines approximately perpendicular to each other.
When marking multiple, identical hardware locations, such as those for drawer and door pulls, employ two (or more) squares set to key offset dimensions (Photo M).
By mounting an accessory centerhead on your rule, you can quickly and easily mark center on round or square stock. To find center on a round object, just press the head against the stock, and draw or scribe a line along the inside edge of the rule. Rotate the square approximately 90° and repeat to make a cross mark at dead center (Photo N). As a bonus, the head doubles as a miter square for marking 45° angles on the ends of mitered stiles, rails, and other parts.
About Our Author
Andy Rae is an award-winning furnituremaker whose career spans several decades. He has authored a number of books on woodworking, including Choosing & Using Hand Tools (Sterling Publishing) and The Complete Illustrated Guide to Furniture and Cabinet Construction (Taunton Press).
