Hi Folks,
Locks are the topic of many informative but also contentious threads on this forum. I believe, that is a good thing and frank, objective discussion of things good and bad about locks is very valuable. Unfortunately, discussions often go down rabbit holes that lead to nothing but the "Cheshire Cat" and the "Mad Hatter". Here is a good lock.
It is the pattern 1756 used on British muskets from 1756 onward with little change until the percussion era. It is a big lock, 6 7/8" long and 1 1/4" wide behind the pan. Its large size makes it easy to see and understand the mechanism. It is also a very good lock to learn lock making because being large, the parts are also bigger and thicker, allowing more margin for error. It is also very simple, no fly detent, no stirrup in the tumbler, no "waterproof pan", no roller frizzen, etc. Yet it was highly successful, helped the British dominate much of the globe, and helped us win our independence. Why? Because it worked and worked well. It is also an excellent showcase for good basic lock design. Let's look inside.
Nothing fancy, just good physics. I built this lock and included a lot of historical details. There is a crown and inspector number and "FG" stamped inside. The "FG" indicate the lock was made by Joseph Farmer and Samuel Galton, both of whom were Quakers. First, note how the mainspring fits well up the foot of the tumbler and the end of the hook tucks right into the instep of the tumbler at full cock. That means it is getting the most mechanical advantage it can by bringing that point of connection on the tumbler as close to the axis of the tumbler as possible. Note the lower leaf of the mainspring is straight when the lock is fully cocked.
Also note the tall tab on the spring for the screw positions the upper leaf of the spring low on the plate. That helps assure a large barrel can be fitted and the spring does not break into the barrel channel. The mainsprings on some modern made commercial locks are poorly designed making it impossible to do that. The fit of the sear to the tumbler is very precise. Look at the fit of the sear spring to the sear. The end of the lower leaf approaches the sear lever at a fairly low angle and the end of the leaf tucks against the back edge of the boss on the sear. This provides great potential for adjusting trigger pull only limited by the fact that the tumbler does not have a fly detent. Consequently, you cannot make a trigger pull much less than 3 lbs. Of course that was no issue for a musket but it would be if you try and turn a Brown Bess into a target shooter. The bar of the sear is located close to the lower edge of the lock plate but it does not change position when at rest, half, or full cock. As a result it is easy to fit a simple trigger such that it doesn't rattle badly at any position of the flint cock. With respect to precision fitting, it is very nice to have extremely close tolerances but not necessary. Flintlocks are not jet engines with blades spinning under high heat at thousands of rpms for hours. The tumbler rotates about 120 degrees max and even over its entire working life, it won't be cycled anything like the fan in your window. Precision machining is nice but not necessary. The greatest flintlocks ever made were made by hand. Even this horrendous India-made Brown Bess lock was made to work reliably without machining.
However, I am NOT excusing poor work, advocating neglecting the skills to produce precise fitting, or questioning the technical superiority of good machine work. Good machining enables high quality work to be done at much lower cost and be repeatable at industrial quantities. Even Paul Bunyan and Blue Babe couldn't keep up with the chain saw. But it is ironic, that I've salvaged many commercially produced locks from poor manufacturing, design, and machine work only using tools held in my impaired hands.
Let's look outside. The pan cover to pan fit is very good, something not hard to do with good assembly technique. Fit the frizzen to the pan, then clamp it in place, drill the thread sized hole through the bridle, frizzen and lock bolster. A drill press is pretty important for that. Then place a very fine shim (1/64" or so) under the back edge of the pan which lifts the frizzen up a tiny bit and drill the clearance hole through the bridle and frizzen. If you don't do that, you may find the front of the pan cover lifted off the pan when you insert the frizzen pivot screw. The shim will prevent that and you will usually only need a few swipes of a file across the top of the pan and bottom of the pan cover to bring the two back in close contact. However, don't fuss with that until you install the feather (frizzen) spring. Get that put in place and make sure the top surface of the spring is flat with the toe of the frizzen, even if you have to bend the spring or file the toe to match.
Doing that will create a nice smooth action of the frizzen on the spring without a side or corner of the toe digging a trench into the spring. There is prevailing rubbish perpetuated online that the feather spring only needs to be strong enough to hold the pan cover shut. The battery (frizzen) must put up enough resistance to the flint to create sparks when everything is covered with greasy fouling and the flint is dull. It does not matter how often you clean your battery and flint while shooting, that basic physics applies. The most common sparking problems I encounter on the shooting line are shooters with weak feather springs and their locks don't spark after 10 rounds. Often the frizzen does not open completely on these locks. Consequently, the owners obsess that their feather springs are still too strong. I take a little piece of masking tape and put it on their feather springs where the curl of the frizzen toe will contact the spring. They fire the lock and are amazed to find a dimple in the tape. The frizzen opened completely but bounced back because the spring was far too weak. Don't kid yourself, you cannot see rebound. Their feather springs need strengthening. In fact, the force required to open the frizzen should be 30-40% of the peak force required to pull the flint cock from rest to full cock. On a Bess lock, I adjust that ratio such that the force for cocking the lock to full is more like a ratio of 25% because the massive flintcock needs a lot of force to get going quickly. Anyway, here is the lock firing. You can see how the micro-second burst of photos shows the great amount of spark directed to the pan, and how they sputter and persist for several moments.
Many flintlock shooters focus on lock speed and sparking but those are not the only critical measures of performance. I recently worked over 2 locks made by the same manufacturer that sparked very well initially and seemed pretty fast but the geometry forced the flint to hit the battery almost straight on and after 3 shots flints were knapped dull. Both locks had no good options for adjusting the length, orientation, and position of the flints because the frizzens were not tall enough and the surfaces curved toward the cock so you could not use a longer flint and close the frizzen at half cock. A shorter flint allowed the top jaw to hit the frizzen before the flint. These locks are just poorly designed. A good flintlock will fire the priming many, many times in a row using the same flint and that criteria is as important as speed and initial sparking. The British expected their pattern 1756 lock used on the Brown Bess to fire the pan 40 times without a misfire using the same flint. The French expected their locks used on the model 1763s (heavy and light 63s) to flash the pan 120 times without a failure and using up no more than 3 flints. Ideally, a good flintlock produces good sparks, is fast, is easy on flints, and flashes the pan on every shot regardless of all but extremely wet conditions.
dave
