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How to mark out a mortise using a mortise gauge?

How to mark out a mortise using a mortise gauge

Shop for Mortise Gauges

Mortise and tenon marked out using a marking gauge before being chiselled and cut into shape A mortise is a cut out of a piece of wood made so that a connecting piece, a tenon, can be inserted. To mark out a mortise you will need a mortise gauge.
wooden block ready to be marked for a mortise or tenon to be cut out, using a marking gauge

Step 1 – Prepare workpiece

If marking a mortise to be joined with a tenon, you will have already marked and cut out the tenon on another workpiece. Position your workpiece for the mortise on a flat surface.

Turning the thumb screw on a mortise gauge to adjust the fence to the right position for the mortise to be marked

Step 2 – Set fence

Decide where on the workpiece the mortise will be cut out. Turn the thumb screw on the fence anti-clockwise to release the fence and move it to the correct measurement then turn the thumb screw clockwise to secure it in place.

Spur adjusting screw moving one moveable screw so the pin measure out the mortise to be removed

Step 3 – Set mortise gauge

Adjust the one marking pin by turning the pin adjusting screw anti-clockwise and pulling it to the required position. If you have your tenon you can use it as a guide for the pins. Turn the screw clockwise to secure it in place.

Mortise marking gauge marking a wooden block with two lines marked out

Step 4 – Mark workpiece

Tilt the gauge in the direction you wish to mark, and drag the tool across the workpiece, either applying pressure to both pins at once or to just one.

Marking both lines at once is more difficult and can lead to the lines being less accurate or one being less visible. It is easier to mark one line first, then go back and mark the other, tilting the gauge slightly to rest each pin separately on the wood.

Width and length lines of a mortise marked out using a mortise marking gauge

Step 5 – Mark connecting lines

To mark a mortise you will need to mark the end lines as well. To do this, readjust the one marking pin so that the pins now line up with the tenon’s width. Then mark the wood, dragging the gauge so that the lines connect to the previous lines made. You should now have four lines marked out making a rectangle.

Chiselling out a mortise in wood, marked out by a mortise marking gauge

Step 6 – Cut out mortise

The mortise can now be removed, most commonly using a chisel.

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