Tuesday, February 20, 2018

Tips & Tools with Fred Stutzenberger – Part 15

Tips & Tools #15. Inletting the Lock Internal Parts

In T&T #14, the lock plate was inlet into its panel. Here we will complete the lock inlet by mortising for its internal parts.

A good lock mortise is a measure of good workmanship. When you pull a lock from its mortise and see close tolerances around its moving parts, you know that the craftsman knew what he was doing. So let’s get the innards of your lock nicely inlet so the observer will think you knew what you were doing…even though this was your first experience!


Fig. 1
These are just a few of my little chisels and gouges I use for inletting the internal parts of a lock.

Toward that end, do yourself a favor and take time to make a few simple tools out of scrap materials (stubs of drill rod, broken drill bits, file shanks or battered screwdrivers). You will need some small, specialized chisels with blades down to 1/16” to do a precise inlet. I have gathered together just a few of my treasures-from-trash tools (Fig. 1) to show you what I mean.

Back in the 80s, Warren Fitzpatrick published a neat little template method for cutting an internal lock mortise1. His method consisted of making a paper template that marked the limits of movement of the lock’s internal parts. The template was placed into the plate mortise and pierced with a scribe to show the waste wood to be removed. It was ingenious in its simplicity.


Fig. 2
My lock mortise pattern collection has made lock inletting much easier and more precise.

I used the system as Warren described it, then mechanized his idea in a series of two articles2,3. Since that time, every lock I have inlet has been installed using his modified system. Over the years, I have cut and traced quite a pile of templates to encompass a wide variety of commercially available locks (Fig.2). I have been pleased with the precision the method offered and the time it has saved. I hope you will too.


Fig. 3
The pattern was placed on the plate, the internal parts mounted through it and their range of movement traced on the pattern (the mainspring had already been removed before I remembered to take the picture).

To make the inlet pattern, a sturdy paper such as manila folder or file hanger should be used. The pattern for the Zornes lock featured in T&T #14 was cut from the divider from a green tea carton. It was 0.020” thick, slick, strong paper that made a durable template. All the holes in the plate plus its outer contour were transferred to the template. The template was then placed on the plate and the movable parts were mounted through the holes. The outline of the range of motion of the internal parts was traced on the paper with a bit of a margin (Fig. 3) to allow for pattern shift when in use. The parts were then removed from the lock and the template was punched with a scribe along the tracing. The pattern was then placed into the plate mortise (Fig.4) and reverse punched to transfer the outline to the wood.


Fig. 4
The tracing on the template was punched with a scribe to transfer the outline into the mortise.


Fig. 5
The reference holes have been drilled to establish the third dimension of the mortise.

Any kid could do the next step…just connect the punched dots with a fine felt tip pen. The tracing provides the extent of the mortise in two dimensions, but establishing the third dimension of depth is a bit more complicated. I solved that problem by running a 6-40 tap through each hole in the plate with it clamped in the mortise. This left an indent at every location where a screw would be when the lock was assembled. I used the point from a broken dart to act as a pointer to center each indent for drilling a series of holes at “reference points” of the tracing (Fig. 5). Each hole was drilled to the depth that matched the distance a specific part protrudes from the plate. On this particular plate, the screws holding the bridle, the sear and the sear spring were depth reference points as well as the widest part of the mainspring (at the bend). Once those were drilled, I made use of the milling table to cut clearances to the depth of the tumbler body. Once the ground floor of the moving parts was established, I used the holes for the screw heads to precisely locate the bridle mortise dimensions (Fig. 6).  A 1/8” router bit was used to bring the bridle mortise to depth, followed by a cleanup with the chisels shown earlier and a dental burr in a Dremel Tool.


Fig. 6
Placing the bridle screw heads into their reference holes allows precise location of the bridle. The nose of the sear and the tumbler already have their clearance established.

Now that the bridle/tumbler mortise is formed to depth, inletting the rest of the parts to that depth indicated by the reference holes will be next. The mainspring has its reference hole at the bend. The sear spring screw has its reference hole for locating the sear spring. That long sear arm has its deep hole already drilled.


Fig. 7
I apologize for the blurred photo, but it was the best my little camera could do. The black area on the edge of the sear arm hole is visible and indicates that wood needs to be removed where the arrow is pointing. Very important to have that sear operating free from interference.

Be very careful in regard to the sear arm hole. Your lock and your rifle are only as safe as the sear arm can move freely to do its crucially important job. If the sear arm or the sear spring drag on the wood at any place , you have an unsafe rifle that is a potential killer. Look at Fig. 7 closely. As blurred as it is (taken too closely for my little Olympus camera to accommodate), you can see the arrow indicating the little black flattened section on the front of the sear hole. That is where the rounded right angle of the arm is contacting the wood. That must be removed by cutting a generous opening so that the arm can move freely over the many years you will safely enjoy your rifle. Double check the sear spring arm for adequate clearance also. Don’t neglect checking this!


Fig. 8
The lock mortise is complete except for the removal of a few splinters. Note the recess in the top wall for the cock shoulder to fall. Note that the forward lock bolt hole has not been drilled yet. Better to wait until after the ramrod hole is drilled and its position determined, for there is potential for conflict here.

After inletting all parts aft of the bridle, move on to the mainspring. Unlike the mortises for the rest of the parts, this mortise is of varying depth going deeper as you move forward. Remember at its forward terminus, the mainspring mortise will be very close to where the ramrod hole will be drilled, particularly in a skinny rifle. Do not cut that mortise any deeper than you absolutely have to. In fact, if the bend of the mainspring presses the bottom of the mortise, that’s ideal. Just so the mobile arm of the spring is not impeded in its movement. Let your inletting black be your guide. Be sure to check clearances at all three: full cock, half cock and at rest. At the half cock and rest positions, the cock will press on the side of the mortise. Use your inletting black to mark where wood needs to be notched out there. The shoulder of the cock should come to rest on the top of the bolster without touching wood. In your final shaping, a lot of the wood in that area will be removed anyway so that the notch will be very small. All that is left to do on the mortise now (Fig. 8) is to clean up the little “buggers” that might eventually come loose and get into the works.
Even after pulling the plate in and out repeatedly, the inletting black on the bottom side of the plate mortise (Fig. 9) showed that the fit was still a little too tight. After relieving that, the rest of the mortise was completed. The next job was to drill the hole for the rear lock bolt and establish the position of the sideplate on the panel opposite the lock. That’s a bit more complicated than one might think. We’ll see why in T&T #16.


Fig. 9
This picture was taken before the mortise was finished. Note the inletting black along the wall of the plate mortise. Once this is carefully removed, the lock can be taken in and out with little resistance.


1.Fitzgerald, Warren. A Template for Inletting a Sidelock. Muzzle Blasts, Feb. 1985, p.23.

2.Stutzenberger, Fred. Lazy Man’s Guide to Lock Inletting. Part I. Muzzle Blasts, April, 1997, p.4.

3.Stutzenberger, Fred. Lazy Man’s Guide to Lock Inletting. Part II, Muzzle Blasts, June, 1997, p.40.


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