lock set-up / trigger function

[Jim Kibler]

Lots of good information here.  Back to Tom's diagrams and full cock notch shape.  For those who are not buying that the method Andy described and Tom drew in diagram "D" I would ask you think about it a little more carefully.  For a given lock layout, draw a line from the tumbler centerline through the full cock notch.  Then draw a line from the sear pivot to the full cock notch.  If this isn't 90 degrees which is very typical, a simple radial line (drawing "C")  will not be ideal for a full cock notch geomentry. If this angle is greater than 90 degrees, which is not uncommon, the angle of notch created from a simple radial line will tend to trap the sear.  Now as said previously, in practice there is typically not a great deally of difference given the small size of the notch.  It is good to be aware of.  Sometimes it will make a difference.

As to full cock notch depth.  As mentioned previously, if it is being shortened, care must be taken for good notch geometry, otherwise an unstable condtion could result.  I would try to avoid shimming a full cock notch.  First, this is because I like one position sears.  Second I've never found an issue with reducing the full cock notch to a reasonable depth even in locks setup for one position sears.  I have designed and made tumblers on locks I've buillt with a one position sear and a reasonable full cock notch depth and not had any trouble with sear / half cock notch interfearance.

Back to items in a lock that direcly affect trigger pull weight or travel...  About the only other one I can think of is the location where the sear spring contacts the sear.  Again this is generally not something that is adjusted, but it will impact the amount of torque put on the sear and the resultant trigger pull. It is good to make sure the spring is not binding on the area on the sear where the sear goes through and it's good to polish these contact surfaces.

Jim

 

[Dphariss]

At one time I was working on locks that had been "fixed" by making the FC notch 3 times or more too deep. Shimming in one form or the other was the only option, short of welding. Given the alloy 4140 I figured shimming or a hard pin was best.
The deep notch screws up everything.
I have not seen a lock in our context that needed this. But its something people need to understand.

As Jim points out the spring can cause more problems than some might expect.
In some cases the sear spring can actually jam the sear to some extent if its too long
Dan

[Acer Saccharum]

In some cases the sear spring can actually jam the sear to some extent if its too long
Dan

Yes, the tip of the sear spring, if jammed into the corner between the arm and pivot barrel will cause a much higher pull than if is is pushing on the arm alone.

But don't over do the shortening of the sear spring, the farther out on the sear arm the spring pushes, the more effect it has on the trigger pull. The spring tip should be almost up against the barrel of the pivot, but not touching it when the sear is at its most depressed position.

Tom

[fm tim]

The modern gunsmiths refer to the sear notch angle, regardless how measured, as positive, neutral, or negative.

Positive implies that the cock moves slightly back when the trigger is pulled.  the sear in a positive trigger, if released, should return to full engagement.  This is safest, but has an obvious impact on trigger pull. 

A neutral trigger will have no sear movement when the trigger is pulled, and the sear will stay where it was when the trigger is released. 

A negative trigger will actually let the cock move slightly forward before it releases, and, if the trigger is released, the sear be in an unstable situation which wants to force the sear out of engagement except friction holds it.  In this situation, the sear may release and let the cock fall if jared.  This is unsafe in any firearn because dropping it may release the trigger,  It does have the lightest pull however.

[westbj2]

Sear springs.
If you agree that the pinnacle of lockmaking was achieved by the britts in the mid 19th century, then how they did the sear spring becomes relevant.
As noted above, the contact of the lower spring limb is always just behind the barrel of the sear body/arm junction.  Almost without exception the spring itself is characterized by a rather long set of upper and lower limbs which are narrower (width of spring) than much work seen elsewhere.  The lower limb (usually about 10% longer than the upper limb) tapering in thickness and width from the bend forward.  A good deal of preload is seen when the spring is mounted with the sear absent....sometimes approaching 1/4".  When the sear is mounted and the spring in place, I am always amazed by the mechanical advantage of the sear over the spring behavior.  Very little pressure is required on the sear bar to exit the F/C notch.   

[Jim Kibler]

Interesting stuff westbj2.  So what impact does a spring with this configuration have on function?  What you are describing is a spring that has a large working length and a relatively small cross sectional area.  A significant preload is necessary for a spring like this to exert sufficent force on the sear.  The interesting thing about a spring with this configuration in relation to sear function for me is that as the spring is compressed by rotating the sear, the change in force the spring exerts doesn't change rapidly.  Compare this to a spring with either larger cross sectional area or shorter working length.  In this case the force the spring exerts for a given compression will rise more quickly.  So the practical impact of a sear spring as Jim describes is that as the sear is tripped, the force from the sear spring doesn't rise as much as the sear is rotated and the spring is compressed.  At least that's how it seems to me!

[Bob Roller]

Here is a trick I use. After the springs are done and function tested,I harden and temper the sear and the "fly". The tumbler is still quite soft.almost as soft as 12L14. I assemble the lock and fire it several times,maybe 7 or 8 and this develops an instant wear pattern on the full cock notch. When it releases easily.I then  disassemble the lock and harden and temper the tumbler. I use 0-1 for the "fly"and the sear and 1144 for the tumbler and have for decades and have had no reported problems. As has been stated earlier,I don't care about methods and am interested only in results be it in making a complete rifle. I am comfortable with what I do and how I do it.
This method works best with a linked mainspring and could create problems with the ramp and cam type of tumbler.
Bob Roller

[westbj2]

"The interesting thing about a spring with this configuration in relation to sear function for me is that as the spring is compressed by rotating the sear, the change in force the spring exerts doesn't change rapidly"
Jim,
You have described the function well.  Put another way is this.  With the sear spring in place and the sear placed on its pin, the sear body must be positioned so that it is approaching a vertical orientation to clear the end of the spring and slide into place against the plate.  When the sear is rotated against the spring and toward its normal horizontal position, the pressure exerted by the spring is diminished. Kind of the reverse of the hammer pressure let off visa-vi the mainspring, tumbler and link.
Jim

[Acer Saccharum]

Jims, the closer the tip of the sear spring is to the sear barrel the less force is generated out at the lever end of the sear bar.



Bob Roller, I like your idea for fitting the hard sear to the soft tumbler. That's a practical man, for ya.

[Jim Kibler]

Jims, the closer the tip of the sear spring is to the sear barrel the less force is generated out at the lever end of the sear bar.

Yep, but this of couse could be compensated with a weaker spring if the sear spring bearing point were further out on the sear arm.  It's not as simple as this though.  The closer to the sear pivot point the sear spring rides, the less the sear spring is compressed during the movement of the sear in firing the lock.  With any spring, force is proportional to the amount it is deflected or changed in shape.  That is as the spring is compressed it's force also increases.  So if you want the spring force to increase as little as possible when the sear is rotated, it becomes beneficial to locate the contact point close to the pivot point.  At least this is what makes sense to me.

Jim

[Bob Roller]

I have used this "Hard to Soft" fitting method for years. A lot of locks I make are also sent with double set triggers and I test these with the soft fit method by clamping the front of the lock plate in a smooth jaw vise and snapping the hammer against a block of wood or a plastic screw driver handle and when it feels "broken in"I trip the lock 3 or 4 times with the triggers that will be sent with  the lock.In other word,take them out of the box and they are ready to use without a lot of extra work.

Bob Roller