A mortise gauge is an indispensable woodworking tool used to accurately mark parallel lines for various joinery, most notably mortise and tenon joints. Its primary function is to scribe two parallel lines simultaneously, ensuring consistent width for your mortises and tenons.
Understanding the Mortise Gauge
A mortise gauge, often confused with a basic marking gauge, features two movable pins (or spurs) rather than just one. This design allows for precise, repeatable marking of parallel lines, crucial for the fit and strength of woodworking joints.
Key Components:
- Head/Fence: The primary reference surface that slides along the edge of your workpiece.
- Stem/Beam: The main body of the gauge, graduated for measurements.
- Adjustable Pins/Spurs: Two sharp, movable pins that scribe the lines. One pin is typically fixed, while the other can be adjusted.
- Locking Screws/Wedges: Mechanisms to secure the head and the movable pin in place once adjusted.
Setting Up Your Mortise Gauge
Accurate setup is paramount for achieving precise joinery. Follow these steps to prepare your mortise gauge:
- Determine Mortise Width: Measure the exact thickness of the tenon you intend to create, or the desired width of your mortise. This measurement will dictate the distance between the two pins.
- Adjust the Pins:
- Loosen the locking mechanism for the movable pin.
- Carefully adjust the movable pin until the distance between the inside edges of both pins matches your desired mortise width. You can use a ruler, calipers, or directly reference the tenon.
- Pro Tip: For some marking tasks, such as highlighting holes for joinery or single-line marking, you may need to make a slight adjustment to get the single spur to function as a marking gauge.
- Set the Offset (Depth):
- Loosen the locking screw for the head/fence.
- Slide the head along the stem until the pins are positioned at the desired distance from the edge of the fence. This distance will be the offset of your mortise from the workpiece edge.
- Securely tighten the locking screw for the head.
- Check Pin Sharpness: Ensure both pins are sharp. Dull pins will tear wood fibers rather than scribe clean lines.
- Test Mark (Optional but Recommended): Before marking your final piece, make a test mark on a scrap piece of wood to verify the settings and pin sharpness.
Marking with a Mortise Gauge
Once set up, using the mortise gauge is a straightforward process requiring a steady hand and consistent pressure.
- Position the Gauge: Place the head/fence firmly against the edge of your workpiece. This edge must be straight and true, as it acts as your reference.
- Apply Pressure:
- Apply firm, consistent pressure to keep the fence tight against the workpiece edge.
- Apply gentle downward pressure on the pins to ensure they scribe into the wood.
- Scribe the Lines:
- With steady hands, push or pull the mortise gauge along the length of your workpiece. Many woodworkers prefer to pull the gauge towards them for better control, especially on longer marks.
- Make multiple light passes rather than one deep pass. This helps to achieve cleaner, more accurate lines and reduces the risk of the pins deflecting.
- Side-by-side highlight the holes with a pencil so they are more visible after scribing, particularly for small or intricate marks, can improve clarity for subsequent cutting or boring operations.
Practical Tips for Mortise Gauge Use
- Grain Direction: When marking across the grain, the pins can sometimes follow the grain lines. Make very light passes initially to guide the pins straight.
- Workpiece Stability: Secure your workpiece in a vise or with clamps to prevent movement during marking.
- Eye Level: Position yourself so you can view the pins and their relation to the wood surface at eye level, ensuring accuracy.
- Cleanliness: Keep your gauge clean and free of sawdust, which can interfere with smooth movement and accuracy.
- Reference Face: Always work from the same designated "reference face" and "reference edge" of your workpiece for all marking to ensure squareness and consistency.
Mortise Gauge vs. Marking Gauge
While both are marking tools, their distinct designs serve different purposes:
| Feature | Mortise Gauge | Marking Gauge |
|---|---|---|
| Pins | Two adjustable pins | One fixed or adjustable pin |
| Purpose | Scribing parallel lines (e.g., mortises, tenons) | Scribing a single line (e.g., cut lines, rebates) |
| Versatility | Can often be adjusted for single-line marking | Limited to single-line marking |
By mastering the use of a mortise gauge, you can achieve professional-quality joinery with precise, repeatable results.
