What a combination square is
A combination square pairs a steel rule, called the blade, with a sliding head, often called the stock. The head can be locked anywhere along the blade, which is where the “combination” comes from. A single tool serves as a try square, miter square, depth gauge, height gauge, marking guide and straightedge.
Typical blades are around 12 inches long, though other lengths exist. The head usually has two working faces: one at 90 degrees to the blade, and one at 45 degrees. Many heads also include a small spirit level and a scriber stored in the head. Features vary by maker, so check what yours has.
Checking the tool first
A combination square is only as useful as it is accurate. Test the 90 degree face the same way you would test any square. Hold the head against a straight board edge, draw a line along the blade, flip the head to the other side of the line and draw again. If the lines coincide, the square is true. If they split, the tool is out, and the gap is double the actual error. Check that the blade is straight and that the head sits snug on it without wobbling. Dirt in the slot or a bent blade are common causes of a problem.
Checking for square
To check whether a corner is square:
- Seat the head firmly against one face or edge.
- Slide the blade down until it touches the adjacent face.
- Look for light between the blade and the surface. No gap means the corner is square; a wedge of light means it is not.
Check at several points along the edge, and look at both faces of a board. For dados, rabbets and shoulders, set the blade into the groove so you can see whether the walls are square to the face. Light can leak even where the angle is good if the surface is rough, so run a fingernail or eye along the joint.
Marking lines
For a 90 degree line across a board, hold the head against a straight edge and draw along the blade. For a line parallel to an edge, set the head to the distance you want by sliding the blade, lock it, then slide the head along the edge with a pencil held at the end of the blade. This works as a rough marking gauge. For precision work, use a sharp point or the scriber, but a dedicated marking gauge is steadier.
45 degree lines
The 45 degree face gives you a quick miter line at the corner of a board. Seat the head against a straight edge and mark along the blade. Two 45 degree lines meeting at a corner form a 90 degree corner when two boards are joined, the basis of miter joints in frames and trim. These joints are known for opening up if the angles are not exact, which is why precise marking and cutting matter.
Measuring depth and height
Because the blade slides through the head, the tool measures how far something sticks up or down:
- Depth. Rest the head across the top of a groove, mortise or hole, and slide the blade down until it touches the bottom. Lock it and read the depth against the rule, or set the depth first and test other grooves against it.
- Height. Stand the tool on a flat surface, with the blade extending upward, to check the height of a part or to set the height of a saw blade or router bit. This is a quick way to compare settings.
- Setting up repeatedly. Set the blade to the desired measurement once, lock it, and use it as a reference for several setups. A saw blade or router bit can be adjusted until its tooth just meets the blade.
When setting a blade or bit with the tool, unplug the machine first and keep fingers clear of the cutter.
Other uses
- As a straightedge. The blade alone, removed from the head if designed to do so, can serve as a short straightedge or ruler.
- Marking the center. Some sets include a center head for finding the center of a round workpiece, but that is an accessory, not part of every square.
- Using the level vial. A small vial can give a rough check of whether something is level or plumb. It is not a replacement for a proper level.
- Setting up tools. Check the fence of a saw or router table for square to the table, and verify a blade’s angle.
Care and storage
- Keep the blade clean. Wipe dust, pitch and oil from the surfaces, because the markings should stay readable and the head must slide freely.
- Protect the edges. A nicked head will not sit flush. Do not use the square as a hammer or pry bar.
- Prevent rust. Dry the tool, and wipe it with a light coat of protective oil or wax if it sits unused. Store it where it will not be knocked about.
- Check periodically. Re-test after drops or heavy use.
- Replace if damaged. Out-of-square tools should not be used for precision work.
Limits of the tool
A 12 inch blade is fine for small parts but short for a large panel. For long checks use a framing square or the diagonal method. A combination square’s fixed head angles also do not replace a sliding bevel when you need an unusual angle, and the tool is not made for rough framing with rafters, where a speed square is often faster.
Used with care, it becomes one of the most heavily used tools in a small workshop. This is general information, not professional trade advice.