AmericanLongRifles Forums
General discussion => Gun Building => Topic started by: Jim Kibler on February 13, 2012, 09:49:32 PM
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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
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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
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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.
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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?
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Is this a test? ;D
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
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Just trying to involve other people rather that me blabbing away as usual :) Doesn't seem to be working too well though.
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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
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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.
(https://i12.photobucket.com/albums/a246/Tom45-70/Breech%20Designs/Triggers/NotchesandSears.jpg)
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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
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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.
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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
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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.
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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.
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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?
(https://americanlongrifles.org/forum/proxy.php?request=http%3A%2F%2Fi12.photobucket.com%2Falbums%2Fa246%2FTom45-70%2FBreech%2520Designs%2FTriggers%2FSearandNotchD.jpg&hash=51ed277ca1380d646848267070f84ea71677ea48)
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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.
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Andy, I read your post and I agree completely. Thanks.
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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
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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.)
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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?
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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.
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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
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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
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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
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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
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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.
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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
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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
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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
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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.
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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.
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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!
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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
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"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
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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.
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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
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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