Installing a Half-Mortise Door Lock

During the 18th century locks were commonly used on casework to secure spices, important papers, jewelry and other valuables. In fact, nearly every cabinet door and drawer, as well as lids on chests, desks, and other casework were fitted with a lock to keep the contents private and secure.

Times have changed and most people today are not concerned with keeping everything in the house under lock and key. However, doors on fine casework usually still require a lock; the key serves as a handle and the lock bolt works as a catch (Photo1). Also, a lock adds curiosity and mystique to a door (especially if you cannot remember where you hid the key).  

Choose a Lock

                      Open the pages of a furniture hardware catalog and you’ll see a multitude of lock choices. However, you can quickly narrow down the choices to three broad categories:  half-mortise, full-mortise, and surface mount locks.  I’ll focus this article on installing half-mortise locks because they are the type most commonly found on 18th century furniture (Photo 2).

Among half-mortise locks you’ll see a number of design variations and each are for specific applications. For example, the lock for a slant-front desk is quite large and has an angled plate to correspond with the angled rabbet on the lid of the desk (Photo 3).

Chest locks come supplied with a keeper which fastens to the underside of the lid; hooks on the keeper engage with the lock to secure the lid (Photo 4).

Locks for doors and drawers often have two keyholes positioned ninety degrees apart which makes this type of lock interchangeable; it can be mounted vertically for a door or horizontally for a drawer.

Keep in mind that when purchasing a lock for a door you’ll need to select the correct “hand” configuration. In other words, you’ll need a right hand or left hand lock depending upon the direction that you intend for the door to swing. Unfortunately this designation is not universal between suppliers and so you’ll need to inquire when purchasing the lock.

Once you determine the type of lock that you’ll need you’ll want to determine the best size. Essentially a larger lock will position the keyhole and escutcheon further away from the door edge. Of course you’ll want to make certain that the lock will fit within the dimensions of the door stile.
As you select a lock keep in mind that locks are typically used with an escutcheon; the escutcheon is a decorative plate or liner which protects the wood surrounding the hole from wear and provides the finishing touch to what would otherwise be a dull keyhole in a beautiful piece of furniture. Escutcheons are often made of brass although you can make an escutcheon from contrasting wood or an old ivory piano key.

When using a manufactured escutcheon I always make a selection when I place the order for the lock. This ensures that the lock and escutcheon are properly sized so that they will work together. Also, during the 18th century, escutcheon styles continuously changed in keeping with the furniture styles. If you desire that your cabinet appears authentic to the period then you’ll want to peruse the hardware catalogs and choose an escutcheon that blends with the style of the furniture that you’re constructing.
 

Position the Lock

Once you’ve selected an appropriate lock you’re ready to begin. The first step is to carefully fit the door and install the hinges. Once the door is swinging smoothly on its hinges the next step is to determine the exact location for the lock. The distance of the lock from the door edge is determined by the distance from the edge of the lock to the pin on which the key pivots. To locate the height of the lock I first stick the escutcheon to the door stile with double-sided tape to have a look at the location. (Photo 5).
 

From an aesthetic point of view, I prefer to position the escutcheon above or below the centerline of the door. This concept can also add to the utility of the lock. For example, if the lock is being fitted to an upper door on large casework such as a secretary desk, it may be easier to reach the lock if it is positioned slightly below center. In contrast, on the lower door of the corner cabinet illustrated here, I positioned the lock slightly above the centerline of the door for the same reason.

Also, I make sure to locate the lock so that the lock bolt will not interfere with interior shelves. Once I’m satisfied with the placement of the lock, I mark crosshairs which indicate the location of the pin on which the key pivots.

The horizontal line of the crosshairs indicates the height of the keyhole (Photo 6). To accurately locate the position of the keyhole from the edge of the door, I measure the distance from the edge of the lock to the center of the pin (Photo 7) and mark the distance on the door (Photo 8).
 

Making the Cuts

With the initial layout complete the first step is to drill a hole through the door stile. I select a drill size which closely matches the diameter of the keyhole in the escutcheon. (Photo 9). Next, I turn the door over and layout the mortise for the lock mechanism on the inside face of the stile.

I carefully measure the distance from the pin to the edge of the mechanism (Photo 10) and from the edge to the plate to the mechanism (Photo 11).

Next, I transfer the measurements to the door (Photo 12).

With the layout complete I’m ready to cut the mortise for the mechanism (Photo 13).

But first I’ll scribe a layout line for the shallow recess to receive the back edge of the lock plate; the reference surface for the marking gauge is lost once the mortise for the lock mechanism is cut. I use the lock to set the marking gauge (Photo 14) and scribe a mark on the door stile (Photo 15).

To cut the lock mortise, I use a laminate trimmer. The laminate trimmer is really just a small one-handed router and it is the perfect size for cutting recesses for hinges and locks. Unlike the deep, narrow mortises for joinery, lock mortises are shallow and wide. A laminate trimmer equipped with a straight bit makes the job fast and easy. To determine the cutting depth of the router, I position the lock on the router base and set the depth slightly more than the combined thickness of the lock mechanism plus the plate (Photo 16).

The small amount of additional depth prevents the lock mechanism from bottoming out in the mortise. Then I carefully rout the mortise up to the layout lines (Photo 17). For your personal safety it’s important to clamp the door securely to a solid workbench and wear eye protection.

Now I place the lock in the mortise and use a knife to mark the layout lines of the shallow recess to accept the back plate (Photo 18).

Afterwards, I carve the recess by paring across the grain with a sharp chisel (Photo 19).