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Installing a Half-Mortise Door Lock


During the 18th century, locks were often used on casework to secure spices, important papers, jewelry and other valuables. The most commonly used type of lock fits into a half-mortise (a mortise that’s open on one side).

I use a combination of power tools and hand tools to create those half-mortises–the kind of work I particularly enjoy. It’s not a difficult task, but after installing dozens of these locks, I’ve realized that it’s important to do each step in a particular order.


Why use locks today?

I made this 18th century corner cupboard the way it would originally have been built–with half-mortise locks, instead of handles or pulls. The key serves as a handle, and the lock’s bolt works as a catch. If you’re building a period piece, you’ll probably be installing similar locks, because they’re often an essential element of a piece’s design and history.

While most folks today don’t need to keep everything in the house under lock and key, locks do have a place in contemporary furniture. A lock adds curiosity and mystique to a door (especially if you cannot remember where you hid the key!). And without a pull or handle, a door can have a much sleeker look.


Buy the right lock

The first step, of course, is to get the correct lock. That’s not as easy as it may seem. Locks for doors and drawers often have two keyholes positioned 90° apart, which makes this type of lock interchangeable; it can be mounted vertically for a door or horizontally for a drawer. Ordering a lock for a door can be tricky, because locks come in right hand and left hand versions. Unfortunately, this designation is not universal among suppliers, so you’ll need to inquire which is which when purchasing the lock.

The next item to address is the lock's size. Size determines the distance between the edge of a drawer or door and the keyhole and escutcheon. The larger the lock, the longer this distance will be. For doors, the width of the stile will determine the maximum size of the lock.

Selecting the right escutcheon is just as important as selecting the right lock. Escutcheons are often made of brass, but you can make one from contrasting wood or an old ivory piano key. During the 18th century, escutcheon styles continuously changed in keeping with furniture styles. If you want your cabinet to look authentic, find some photos of genuine pieces made in the same style and compare their escutcheons with what's available in a catalog or online. I usually order an escutcheon from the same company that supplies the lock to make sure the lock's key will fit the escutcheon.


Position the lock

OK–you’ve bought the lock, and you’re ready to install it. Let me walk you through how to do this on a door (the procedure for installing a drawer lock is very similar). First, carefully fit the door and install the hinges. Next, determine the lock’s exact location by positioning its escutcheon. The distance of the escutcheon’s center from the door’s edge is a given– it’s the distance from the edge of the lock to the pin on which the key pivots.

The best height for the lock isn’t as obvious. From an aesthetic point of view, I prefer to position the escutcheon above or below the door’s centerline–not right on it. If the lock goes on the upper door of a large piece, I’ll probably place the lock slightly below center, so the key is easier to reach. If the lock goes on a lower door, I’ll place it slightly above center, for the same reason. In either case, make sure to locate the lock so that the lock bolt will not interfere with interior shelves.

To determine the lock’s exact height, stick the escutcheon to the door stile with double-faced tape (Photo 1). Step back and take a look at how the escutcheon balances with the other brass hardware and the lines of the cabinet. I usually try moving the escutcheon up or down a bit, just to see how it looks, and wait a day before making a final decision.

Once you’re satisfied with the placement of the lock, mark crosshairs inside the circular opening of the escutcheon to indicate the location of the pin on which the key pivots. Remove the door and draw a horizontal line through the crosshairs (Photo 2). To accurately draw the vertical line, measure the distance from the edge of the lock to the center of the pin (Photo 3) and mark this distance on the door (Photo 4).


Cut the mortise

Next, drill a hole through the stile at the intersection of the crosshairs. Select a drill size which closely matches the diameter of the keyhole in the escutcheon (Photo 5). Flip the door and lay out three lines for the lock’s mortise on the inside face of the stile. First, measure the distance from the pin to the top edge of the mechanism (Photo 6) and mark this on the stile. Measure the mechanism’s width, including the brass edge plate (Photo 7) and mark this on the stile (Photo 8). Lastly, measure the full height of the mechanism and mark its bottom edge on the stile. Shade in the mortise (Photo 9).

Before you cut the mortise, use a marking gauge to lay out the far edge of the recess for the lock’s backplate. Set the gauge directly from the lock (Photo 10), then scribe the line (Photo 11). Why do this now? The reference surface for the gauge will be removed once you cut the mortise.

I use a laminate trimmer with a 1/4" bit to rout the mortise. Place the lock on the router’s base and set the depth-of-cut slightly more than the combined thickness of the lock mechanism and the plate (Photo 12). Clamp the door securely to a solid workbench and rout the mortise up to the layout lines (Photo 13).


Cut two recesses

Place the lock in the mortise, then use a knife to mark around the lock’s backplate (Photo 14). Carve a recess for the plate by paring across the grain with a chisel (Photo 15). You could mark the depth of this recess with a gauge, but I just do it by eye, checking as I go.

Next, position the lock in the mortise and mark around its side (Photo 16). A series of shallow chisel cuts across the grain (Photo 17) make it easier to pare this shallow recess (Photo 18). Elongate the keyhole with a rat-tail file to accommodate the key.

Replace the lock and drill pilot holes for the screws. Remove the lock and run steel screws into the holes (Photo 19). Steel screws are much less likely to break than brass screws, so I always use steel screws first, to tap the holes. Remove the steel screws, replace the lock, and install it with brass screws (Photo 20).


Cut the bolt mortise

Next, cut a mortise in the cabinet’s stile to accept the lock’s bolt. First, extend the bolt and mark its location on the stile, leaving about 1/16" leeway on either side of the bolt (Photo 21). Measure the distance from the face of the door stile to the bolt (Photo 22) and mark this distance on the cabinet (Photo 23). As you cut the mortise, gradually work your way toward the face of the cabinet (Photo 24). The final shaving from the mortise allows the bolt to slide smoothly into place.



This story originally appeared in American Woodworker April/May 2010, issue #147.

April/May 2010, issue #147

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1. Begin by placing the lock’s escutcheon on the door using doublefaced tape. Mark the center of the escutcheon’s circular opening.

2. Remove the door and draw a line through the mark.

3. Measure the distance from the edge of the lock to the pin that receives the key.

4. Mark this distance on the door.

5. Drill a hole through the stile at the intersection of these two lines.

6. Next, lay out a mortise to receive the lock’s mechanism. Measure the distance from the center of the pin to the top of the mechanism.

7. Measure the mechanism’s width, including the brass plate.

8. Transfer both measurements to the inside face of the door.

9. Mark a line for the bottom edge of the mechanism and shade in the mortise.

10. Before cutting the mortise, set a marking gauge to the width of the lock’s back plate.

11. Mark this distance.

12. Adjust a laminate trimmer slightly deeper than the combined depth of the lock’s mechanism and back plate.

13. Rout the mortise up to the lines, freehand.

14. Place the lock in the mortise and scribe both sides of the back plate.

15. Pare to the lines, going across the grain to prevent tearout.

16. Place the lock back into the mortise. Scribe around the three sides of the back plate’s edge.

17. To aid in paring this recess, make a series of shallow cuts using a chisel.

18. Then pare across the grain.

19. Mark the lock’s screw holes, drill pilot holes, and drive steel screws into the holes.

20. Remove the steel screws and install the lock using brass screws.

21. Place the door back on the cabinet and mark the location of the lock’s bolt.

22. Measure the distance from the front of the door’s stile to the bolt.

23. Transfer this measurement to the cabinet’s frame.

24. Cut the bolt’s mortise. Done!