lock set-up / trigger function

[Jim Kibler]

Guys,

As mentioned in the thread pertaining to trigger setup, it might be good to start a topic on how lock design, set-up, tuning etc. effects trigger function.  Specifically the goal of a lock in relation to trigger function is to minimize travel and pull weight.  So does anyone have ideas relating to characteristics of a lock that impact either trigger pull weight or trigger travel?

Thanks,
Jim

[Jim Kibler]

Guess I will start out with a few characteristics.

Travel:
Full cock notch depth -  Obvioiusly the more shallow the full cock notch depth is, the less sear travel is required to trip the lock.  Care must be taken though.  As the notch depth becomes more shallow, it must be shaped with increasing care.  There is of course a practical limit to a notch depth. 

Fit between sear hole and screw - If this fit is not very close, the slop must be taken up first with trigger movement before the sear tip begins to move when pulling the trigger.

Weight:
Mainspring strength-  The stronger the mainspring, the more force is excerted on the tumbler and sear tip.  In my view heavy mainspring strength is important and desireable for other lock characteristics, so I would not consider adjusting spring strength to tune trigger weight.  It is worth understand it's impact however.

Mainspring tip location at full cock-  The closer the mainspring tip rides relative to the axel of the tumbler, the less torque is imparted on the tumbler.  This will decrease the force between the sear nose and tumbler and decrease the force necessary to trip the lock.


There are quite a number of other variables that remain.  Lets hear your thoughts.

Jim

[Acer Saccharum]

Mentioned by Dphariss is the safety consideration of the lock holding full cock under duress.

The lock should hold full cock WITHOUT the sear spring. The spring is meant to push the sear into its notch, it's not meant to be the force what holds the gun at full cock.

My reasoning:
If your sear spring should break(it happens) while the gun is at full cock, the gun won't go off accidentally. Once broken the gun won't re-cock. THAT's the time to find out that the sear spring is broken, not when you're bringing the gun up to your shoulder.

[Jim Kibler]

Mentioned by Dphariss is the safety consideration of the lock holding full cock under duress.

The lock should hold full cock WITHOUT the sear spring. The spring is meant to push the sear into its notch, it's not meant to be the force what holds the gun at full cock.

My reasoning:
If your sear spring should break(it happens) while the gun is at full cock, the gun won't go off accidentally. Once broken the gun won't re-cock. THAT's the time to find out that the sear spring is broken, not when you're bringing the gun up to your shoulder.

So what does this say in terms of a desirable full cock notch shape for safety and trigger pull considerations?

[Acer Saccharum]

Is this a test? 

Jim, by your post, I suspect you have your own thoughts on this.

Here goes:

To me it says that the full cock notch face must be on a true radiant line from the center of the tumbler. If the notch is angle is one way or the other from this line, it will either pop the sear nose out without the sear spring pressure to hold it in, OR ..
if the notch is angled the other way, the sear will be wedged toward the center of the tumbler. The sear nose, in order to come out of full cock, will have to force its way out of the notch, causing the hammer to rotate backwards. Depending how much back angle there is on the fullcock notch, the trigger pull can be increased dramatically, because the mainspring has to be compressed by the action of pulling the trigger.

Maybe somebody can make that read logically.

Tom

[Jim Kibler]

Just trying to involve other people rather that me blabbing away as usual   Doesn't seem to be working too well though.

[AndyThomas]

To me it says that the full cock notch face must be on a true radiant line from the center of the tumbler.

Tom's got the right idea, but the wrong angle. The full cock notch face should be perpendicular to the line running from the center of the sear pivot screw to the face. As he said, the cock (or hammer on cap guns) should not move either forward or backward as the sear moves across the tumbler face.

I've repaired two locks which would fire by pushing on the back of the cock. A dangerous situation! I'm sure they didn't come this way from the factory. This needs to be understood before messing around with a lock.

This can be checked by holding the cock (without a flint, please) with one  thumb, while slowly moving the sear with the other. Again the cock should not move either forward or backward, until it trips at the edge of the notch.

As Jim pointed out, a loose pivot screw / sear can cause a lot of problems. I repaired an antique pistol lock, which had this problem, by modifying a Siler sear to take up the slack. Makes me wonder how many old guns don't have some Siler parts in them.

Andy

[Acer Saccharum]

Here's what I'm trying to say, in picture format.

A will pop right out of cock without a sear spring to hold it in.

B will require lots of effort to trip the sear.

C is neutral as far as angles. It's what I try for when I tune my locks.

[Acer Saccharum]

Just trying to involve other people rather that me blabbing away as usual   Doesn't seem to be working too well though.

Seems to have drawn me in....blah, blah, blah.....Hahahahaha.

Tom

[Pete G.]

Do not reduce he full cock notch below the radius of the of the half cock, especially on a lock without a fly. If the halfcock notch bangs into the nose of the sear every cycle it will wear the sear to the point of not holding. A lock with a detent helps, but still can place enough stress on the sear nose to cause some damage.

[Jim Kibler]

What's interesting is that Andy's response and Tom's are slightly different.  If a line drawn from the tumbler center through the tumbler notch is perpendicular to a line drawn from the sear pivot point to the face of the tumbler at the notch, they end up being the same.  This is shown by the dotted lines in Acer's drawing "C".  It is quite common that this angle is a little greater than 90 degrees, however.  With this being the case, if the notch is simply cut at a line drawn from the tumbler center, the sear rotation will be such that the tumbler will have to rotate slightly in order for the sear to trip.  Does this make sense?

Now, in practice, the notch is so small that this slight difference usually isn't significant, but it's good to understand the principle behind things.  So, although you often hear the response Tom gave as the desired full cock notch angle, from a purely technical standpoint, I like the one Andy gave.  Although in practice they often quite close to each other.

Jim

[Dave Faletti]

I don't recall if its considered positive or negative rake but there needs to be enough of an angle on the engagement surfaces that the sear is pushed into place with enough force to at least overcome the friction.  If someone starts pulling on the trigger than releases, it needs to reset and not just stay where its at.

[Jim Kibler]

Do not reduce he full cock notch below the radius of the of the half cock, especially on a lock without a fly. If the halfcock notch bangs into the nose of the sear every cycle it will wear the sear to the point of not holding. A lock with a detent helps, but still can place enough stress on the sear nose to cause some damage.

With a tumbler setup for a one position sear having the full cock notch below the radius of the half cock notch is basically unavoidable if you are going to have a reasonable full cock notch depth.  The fly does insure the sear rides up over the half cock notch.  The fly is of course especially important with set triggers.  In the case of a single trigger with no fly, the sear must rotate sufficiently to ride up over the half cock notch when the trigger is pulled and the sear breaks free of the tumbler notch.

Incidently I just looked at a Chambers round face virginia lock and the full cock notch is indeed at a smaller radius than the half cock notch.  It of course does contain a fly.

[Acer Saccharum]

Even tho' this notch face is not on a radial line with the tumbler axel, there is no camming by the sear when the trigger is pulled. Am I understanding this correctly?

[Acer Saccharum]

AH, and in regard to clearing the half cock notch, if the setup is like A, in the earlier illustration, the angle of the notch will push the sear nose out of the notch, and cause the sear nose to NOT clear the half notch.

When the trigger is pulled, you want the finger to be the force that's holding the sear away from the half notch. B and C ad D will all clear the notch OK. B will have such a hard pull, the trigger will slam out of the notch when it finally goes.

[Acer Saccharum]

Andy, I read your post and I agree completely. Thanks.

[Jim Kibler]

I don't recall if its considered positive or negative rake but there needs to be enough of an angle on the engagement surfaces that the sear is pushed into place with enough force to at least overcome the friction.  If someone starts pulling on the trigger than releases, it needs to reset and not just stay where its at.

I've never woried about this, Dave.  I figure if you are squeezing the trigger it really doesn't matter.  I try to avoid any rake that would make pull harder, but still want the setup to be stable.

Jim

[Acer Saccharum]

A way to tell if the notch angle is right, and a point Andy touched on, is to clamp the lock solid, put a dial indicator high up on the cock, and watch the needle as the sear moves out of the full cock notch.

(I'd do this without the mainspring in, just pushing the cock forward with a finger.)

[Jim Kibler]

A few more things to add to the list...

Trigger pull weight related:
Smooth polished surface on sear nose and full cock notch - (previoiusly mentioned by Dan)

sear spring strength - Spring needs to be strong enough to snap sear into half and full cock notches effectively and reliably, but does not need to be excessively strong.  An excessively strong spring will add to trigger pull.

Are we missing anything else?

[Jim Kibler]

Here are a couple more that just came to mind. 

Sear ratio -  ratio of distance from sear pivot point to trigger contact over distance from sear tip to pivot point.  Not something that can be adjusted on an existing lock, but certainly has an impact on both force to trip the sear and distance it must travel.

Distance full cock notch from tumbler axis - As distance from full cock notch to tumbler center increases, force between notch and sear nose decreases and therefore reduces trigger pull.  Again not something that can be adjusted on an existing lock, but good to understand.

[Dphariss]

I don't recall if its considered positive or negative rake but there needs to be enough of an angle on the engagement surfaces that the sear is pushed into place with enough force to at least overcome the friction.  If someone starts pulling on the trigger than releases, it needs to reset and not just stay where its at.

Won't reset.
No firearm I know of will bolt gun sure won't
If the sear is captured to this extent the pull is going to be pretty heavy.
The difference in an angle that will hold the sear and one that will allow it to be forces out by the mainspring pressure is not all the great to the naked eye.
But resetting would require a notch much like "B".
I used to figure it as a radius off the sear screw. The sear nose at this radius, its the radius it follows as it moves and the notch at a radius that lightly captures the sear so it will not pop out if the sear spring is removed.
Tom's "C" drawing is correct so long as the sear stays in with no spring and full mainspring pressure.
Some firearms will have a "hook" on the sear notch that holds the sear in. But when it wears away the lockwork can become dangerous.

Dan

[Dphariss]

Do not reduce he full cock notch below the radius of the of the half cock, especially on a lock without a fly. If the halfcock notch bangs into the nose of the sear every cycle it will wear the sear to the point of not holding. A lock with a detent helps, but still can place enough stress on the sear nose to cause some damage.

Best to shim/pin or install a screw in the tumbler to limit sear engagement in a deep notch, cutting down the FC notch must the carefully done in the proper manner and if its really deep a shim of some sort needs to be employed.

Dan

[Dphariss]

I've repaired two locks which would fire by pushing on the back of the cock. A dangerous situation! I'm sure they didn't come this way from the factory. <snip>

Andy

I would not assume a lock I bought or a complete firearm could not be "pushed" off. There are those that rely on the sear spring. If they have a "hook" on the full cock if it is stoned or worn away often it will then be possible to "push" the lock off full cock.
Some are softer than they should be and use will then cause them to get "light".
I just did a talk at our Guild meeting on an American made lock, new, that had a fly that was too short and it would prevent the sear entering the 1/2 cock. Sear would hang up the fly and the edge of the 1/2 cock and would push off easily. I did a rebarrel and convert to flint on the gun for a friend.
The fly was not quite shaped right either or it would not have hung in this manner.
Shot with set triggers it worked normally, though I think long term it would damage the fly and/or sear nose. But in cocking and uncocking etc it was possible to set it in the false 1/2 cock. VERY dangerous.
ALL locks need to have a careful examination. The notch engagement and the fly function especially.
Dan

[westbj2]

I agree with Tom's "C" drawing.
Proper lock function beyond the angular geometry requires these considerations as well.
1) Tumbler shank being ROUND with a clean smooth surface and mating with a ROUND (reamed) hole in the plate at 90 degrees to its surface.  The fit tolerance should be .001" to .002".  Notches across the tumbler edge must be perpendicular to the plate and absolutely uniform in depth.  Depth of the full cock notch need only be as deep as the face of the sear nose thickness.  Tumbler support nub opposite the shank must be ROUND and on the same C/L as the shank.
2) Sear pivot hole must be ROUND (reamed) and parallel with a line across the face of the sear nose.  Sear screw fit relationship to the mating hole should be within .001".  Sear screw body should have just the right amout of thread to bottom out on the plate when the bridal position is correct.  Too long a screw thread will result in the bridal binding the sear when properly tightened. 
Most importantly, all of these factors (and more) must work in unison.  If only one is off or incorrect there is an effect on the entire operation.
I just looked at the relationships of trigger and sear on some high grade British work now on the bench.  I this case, the trigger is pivoted about 1/2" ahead of and about 3/16" above the sear pivot location.  Contact point of the trigger/sear is a near vertical lift  Pressure point of the trigger radius is about 3/8" ahead of the sear/trigger contact point and about 3/4" behind the trigger pivot.
Jim
 

[Acer Saccharum]

I agree on the hole tolerances on the rotating surfaces of tumbler/plate/bridle. Also sear to sear screw fit. Every bearing surface hardened and polished. The squareness of the holes and components is often overlooked, but will cause binding and slow lock time.

Mainspring must not rub on plate.

I also like to eliminate any contact of tumbler and sear with the plate by relieving everything on the part except a small bearing ring around the hole of the sear and the shank of the tumbler. Is this called 'ghosting'?

Tip of mainspring and cam of tumbler must be fitted to each other for even bearing surfaces, and those surfaces highly polished.

Casehardened plate and parts speeds up a lock tremendously.

No wonder a well made custom lock is very expensive.