Setting The Marking Gauge

Take a mortice gauge (or combination gauge) and set the pins to the exact width of the ½” (12mm) chisel that you will be using later to chop your mortise hole. The reason why you must set the pins to the chisel, instead of just measuring, is that the width of chisels vary noticeably according to the manufacturer. Therefore, make sure you’re taking your width right from the ends of the pins, as anything different would only create problems.

You must now set the stock of the gauge so that both pins will be centred, i.e. equidistant from the outsides. Rather than calculating and measuring this distance (which usually implies micro adjusting later), take an estimate, tighten the nut and mark the two points on your tenon piece. Then register the gauge against the opposite face and mark the two points again to see how far off you are. Whatever the distance is between the first and second points, half that distance and slide the beam accordingly. Check again, and repeat if necessary.

Once the pin points align from both sides, you’re ready to mark both the thickness of the tenon and the width of the mortice hole. Double check that the pins haven’t moved and still correspond to the width of your chisel.

Watch Paul centring a gauge taken from the Setting Up a Gauge Guide.