Mainspring tension?

[Scota4570]

I built this lock from a casting set.  The spring is as supplied.  It is a marginal sparker.  The mainspring tension seems a little low.  As measured with a loop on the cock screw, with a fish scale, I get 8# peak between full down and half cock.  Full down is more like 5# to make it just begin to rotate.  Pulling was done at a right angle to the direction of rotation. 

Everything is extremely smooth and nothing binds or drags.  The tolerances between rotating surfaces are very tight.  The relationships of the various parts are pretty much optimal.  The full cock notch is full travel allowed by the bridle. 

Based on the picture, is the position of the spring at rest showing too little pre load?   Should I go ahead and open it up a bit and re-heat treat?  IF it looks about right should I just re-heat treat? 

I draw springs in a lead  bath at 750 based on a thermometer.  I am almost certain that the spring was drawn using the eyeball method. 

[D. Taylor Sapergia]

I like a mainspring to have a preload of about 1/4" below the edge of the lockplate,which is about what yours has.  So I don't think increasing the spring's lower arc is necessary, though it would certainly increase its force, and also its fragility.
How have you heat treated the frizzen?

[Jim Kibler]

To my eye, I think it needs more preload.  Also the spring size and cross section looks a little wimpy.  I prefer strong springs, however.

[Scota4570]

I believe the spring was made from a wax made for an original Baker.  It would be a few percent smaller due to shrinkage? It is significantly thicker than a Kibler SMR spring.

The frizzen was case hardened by the supplier of the casting set.  On the first of these locks I tried to make the frizzen better with more case hardening and made it crack.  I ended up replacing the frizzen.  The first lock was assembled for me.  The lock I assembled works better than that one.   I think a military lock from a proven rifle should work very well.  To that end I troubleshoot one thing at a time. 

My question is more about the expected force needed to cock it.  How is that quantified?  I am pretty sure it would work better if the mainspring spring were stronger.  If I were to re-heat treat that may help.  IF broke it I loose nothing because I can make a new spring to replace the cast sketchy spring anyway. 

I have read several threads and writeups on lock tuning.  I have not found anything about mainspring tension guidelines. 

I just checked one of Jim's SMR locks.  I get 10# at 2" leverage.   If I convert that to 2.25" of leverage I get 11.25# of force to match the Baker lock to the SMR lock.   This lock is 30% weaker than  the Kibler lock.  But if 8-pounds of force to cock the thing is acceptable I'll just move on.   

[Bob Roller]

The mainspring needs more arc beginning at the bend.
Bob Roller

[Scota4570]

I messed with it.  I did add more arc, it looked wrong to me to.  That  increased the preload.  I think my heat treat is more springy.  The force needed to bring it off the resting position is now 14#.   It does spark better. 

For whatever reason as quenched from red orange a file still bit into it.  After the second try getting the same result I went ahead and did the lead bath draw. 

[D. Taylor Sapergia]

Scota:  I built the lock set that came with the Baker set that I bought from TRS, and upon examining my pictures on file, I find that I had to re-arc and re-heat treat my mainspring as well.
As supplied, filed and finished, I found that the lower arm of the mainspring had a disagreeable arc where the arm should have been straight, so I re-arc'd it and then re-heattreated it.  I heated the spring red/orange in a soft flame of an oxy/act torch and some fire bricks, and quenched it in motor oil at room temperature.  Then I re-polished it, and drew the temper on a 1/4" thick copper plate secured in my machinist's vise.  I drilled a divot for the tit on the upper arm, and heated the plate from below with the oxy/act torch moving the flame continuously to bring the copper up to temp slowly.  I had a drop of motor oil on the plate's centre to tell me when the temp was getting close...when it started to smoke, I knew I had to take the heat further away to approach temper slowly, and hold it there while the spring baked.
My effort worked out well, and the finished spring provided sparks like a cutting torch (almost).  Although the rifle has not been used hard, it has been fired quite a bit, and the spring is still working fine.  Again, here are some images...
The first is a before and after picture.  I cannot say how many pounds are required to cock the hammer, but I can say that the lock is balanced, and creates good sparks with no bounce back, and superlative ignition.

[Jim Kibler]

I messed with it.  I did add more arc, it looked wrong to me to.  That  increased the preload.  I think my heat treat is more springy.  The force needed to bring it off the resting position is now 14#.   It does spark better. 

For whatever reason as quenched from red orange a file still bit into it.  After the second try getting the same result I went ahead and did the lead bath draw.

Hardness or strength to which the spring is hardened to doesn't affect the modulus of elasticity (springiness) of steel.  It only increases or decreases the yield stress (basically the load per area that is required such that the spring won't return to it's original dimensions).  So, as long as there is no permanent deformation (change in springs dimensions) during service, the hardness has little effect on "springiness".

Glad your lock is working better.  Not sure what the frizzen material is, but it seems to me I heard of them using 5160 for frizzens at least at one point.  Higher carbon materials such as 1095 are better.

[Scota4570]

I'll have to see how much I like the completed rifle to decide how far I go down the rabbit hole.

I could make a frizzen out of 1095.  I also have a "spare"  frizzen that cracked on the face from my repeated attempts to case harden it.  I could add a piece 1095 to the face of the frizzen. 

 

[Bob Roller]

I think I once read something about special frizzen faces that were added to a soft frizzen for use in very cold climates.
Anyone else know about this??? SOMEONE HAS BEEN POSTING USING MY NAME THAT I HAVE AN EAR INFECTION AND
CAN NOT TAKE PHONE CALLS> THIS IS NOT TRUE AND I HAVE HAD NO CALLS CONCERING LOCKS OR TRIGGERS IN A
LONG TIME.
Bob Roller

[Dphariss]

 A properly made lock should have what is called a “heavy first lifting” on a percussion lock this helps keep the hammer on the nipple at firing. In a flintlock this means as the frizzen is scraped the geometry of the tumbler increases the force applied to the work so the cock is less likely to slow down. Also as the end of the mainspring gets closer to the tumbler shaft as the lock is cocked the reduced (leverage) force at full cock reduces pressure on the sear nose so it can move easier. The good British locks did not have weak springs. Flint or percussion.

[smart dog]

Hi Scot,
Before messing too much more with the shape of the spring try quenching it in brine not oil.  I believe TRS springs are medium carbon steel and in my experience, they may not harden well when quenched in any oil.  I believe TRS actually recommends water quench but after discussions with Wick Ellerbe, I use brine that is warmed to about 120 degrees. 

dave 

[Dphariss]

I did a TRS Manton rifle lock some years back and finally just made a new mainspring from 1095. I think the entire lock was 4140 or some such frizzen was also faced with 1095 to get spark.