Mainspring stretch and tempering

[smallpatch]

OK Gentlemen,

I can now testify to the tuna can oil bath method.

I stretched the mainspring about 3/16", heated to red, quenched, put it in a Sucrets can full of oil, and lit it with a torch and let her burn till the oil was gone.  Cleaned her up, installed, works like a charm.

I just don't know why L&R can't just make stronger springs from the beginning.  NOT the first one I've had to do.

Thanks for all your help.

[bob in the woods]

Told Ya !!    

[James Wilson Everett]

Guys,

A whole lot of interesting information here on spring making.  Probably most master gunsmiths kept their "secret" methods in the 18th c. so he would have the advantage of the guy across the road.  But here are some sure facts.

All ferritic metals exhibit identical load/deflection characteristics until the stress reaches their individual yield point.  The load/deflection will always be the slope of the stress-strain curve, better known in engineering as Poission's ratio.  This means that if we have an array of metal parts that are absolutely identical except for their material and/or heat treatment, they will act identically under load.  This is a fact that is well known to mechanical engineers, but little understood by most folks. 

Therefore, if a spring is too strong or too weak, only changing the temper will have absolutely no effect whatever on this.  You have to also alter the shape.  Most often this means changing the distance the relaxed spring tip is beyond the tumbler tip.  Acer Saccharum is correct about the spring thickness, but it also depends upon the above, too.  This is just my way of repeating what he said about spreading the arms.

Perhaps most spring failures are due to an error in the initial hardening step rather than on the tempering step.  When your spring breaks - SNAP! - it could be that the hardening step was done at too high a heat.  Carbon steel should only be heated to the point that a magnet will not attract the hot metal before quenching.  This is the transition temperature and is typically 1550F.  This temperature is tough to gauge by eyeball, but easy with a strong magnet.  Heating to a significantly higher temperature may irreversably damage the metal grain structure.  You cannot see this damage, but it will result in the SNAP!

I use certified 1095 brine quench at 1550F and temper to 810F, it works for me.

Jim Everett

[Bob Roller]

Quenching in brine was an old method of hardening a frizzen.I have never tried it on springs. The method I use is time proven over a long period and I'm comfortable with it.
Cast mainsprings are much improved over the ones made thirty years ago but I still want nothing to do with them.
  The Twigg lock uses external parts from R.E.Davis. They seem to be of good quality and then I can spend all my time on the important internal mechanism which is in reality,the lock itself. I have also made a number of locks on the L&R Small Manton,Ashmore and Durs Egg externals.

Bob Roller

[doug]

     I don't agree with the statement that changing the temper will not affect the strength of a spring.  Some of the first ones that I made, I reheated only to dark blue and they broke before taking a full bend.  A very similar spring reheated to grey (just beyond dark blue) worked well and lasted for many years.  If I heated the same spring too far beyond the transition from dark blue to grey, then it was soft and often would loose its shape when bent

    The lock below is ugly as sin and is very much a work in progress.  The reason I am posting  it however is to make a couple of points.  The first is that it was made to fit in a  hole in a gun and the barrel was extremely swamped with an extremely large breach area.  I found that I had to grind away a lot of the upper leg in order to get the lock to fit up close to the barrel.  I notice that Bob addresses this problem in his Twigg lock by having the end of the upper leg go straight up for a ways until it stops against the inside of the pan.  My point however is that very much to my surprise, a lot of the upper leg can be ground away (side to side) without it bowing upwards which is what I expected to happen.
    The second thing which caught me a bit by surprise is that in my first make of the spring, the lower leg was relatively thick and minimally tapered.  The lower leg would not bend up as high as I wanted and I found that by tapering the lower leg, I was able to get the lower leg to bend over its entire length and able to give the cock more travel.  I had to use more preload to get the cock traveling fast enough.  Even though some modern locks use very strong springs and very short travel on the cock, I think it is a lot easier to give the cock more travel so that it has more time to accelerate
     A final point which I came to realize is that the vertical location of the bend in the mainspring (the middle) has quite an impact on the movement of the tip in relation to the tumbler.  If you can get the bend down near the bottom of the lock plate, the tip of the spring does not move in towards the pivot or shaft of the tumbler.  If the bend is in the middle of the lockplate, and about level with the pivot of the tumbler, as you cock the gun, the tip moves ever closer to the center of the tumbler and it can be challenging to make enough room to fully cock the hammer.

cheers Doug

[LRB]

"Carbon steel should only be heated to the point that a magnet will not attract the hot metal before quenching.  This is the transition temperature and is typically 1550F.  This temperature is tough to gauge by eyeball, but easy with a strong magnet.  Heating to a significantly higher temperature may irreversably damage the metal grain structure.  You cannot see this damage, but it will result in the SNAP!

I use certified 1095 brine quench at 1550F and temper to 810F, it works for me."

  Iron/steel loses magnetic draw at 1414°. Not 1550° 1414° is too low of temp for quenching 1095, and 1550° is unnecessarily high. 1095 with a few minutes of soak at 1475°, give or take a tad is much less likely to produce grain growth. 1550° will cause grain growth.

[MagKarl]

Guys,

A whole lot of interesting information here on spring making.  Probably most master gunsmiths kept their "secret" methods in the 18th c. so he would have the advantage of the guy across the road.  But here are some sure facts.

All ferritic metals exhibit identical load/deflection characteristics until the stress reaches their individual yield point.  The load/deflection will always be the slope of the stress-strain curve, better known in engineering as Poission's ratio.  This means that if we have an array of metal parts that are absolutely identical except for their material and/or heat treatment, they will act identically under load.  This is a fact that is well known to mechanical engineers, but little understood by most folks.  
Jim Everett

No offense, but this is absolutely false.  The facts are: Slope of stress/strain is NOT Poisson's ratio.  It's the elastic or Young's modulus.  It is NOT constant for all metal parts, it's different for every material.  Most steels are very similar though.  Heat treatment and tempering are processes by which we can alter the crystalline structure of steel itself to our advantage, higher yield strength, and less brittle.

We're all here to learn, let's not spoil this discussion of proven practical methods to achieve our goals by spewing BS.

[bgf]

Before there is an argument, I would like to suggest that there are several ways to skin a cat, and that everyone should give the others the benefit of a doubt.  For example, strictly bare non-magnetic heat may not be high enough for adequate hardness with an oil quench, but it likely is if quenching in brine.  Similarly I'm sure Smartdog's combo quench works well in his situation, as I'm sure he did his homework and testing.  The published data for steels is for industrial settings, so less well-equipped facilities like the workshop of longrifle builders might have to work out alternate ways of doing things, much like the builders of originals, and if you change one step, things go differently in the others.  I'd say if it works for you and holds up, keep it consistent.  The reason I'm saying this is because I enjoy hearing how everybody does things and don't want it to stop .  Not only is it educational, but it is interesting simply to see how many different ways people come up with to get to essentially the same goal.

[JCKelly]

I wrote something on heat treating for the muzzle-loader. Don't know how to post a pdf here.  Anyone wants a copy just ask.

I have tried to educate people on the subject of hardness vs strength of springs since my first job about half century ago. I am tired of it.

There was an English born gunsmith, gone now, worked in Pennsylvania. Kit Ravenshear, I believe. His pamphlets on heat treating springs are just exactly suited to what you gentlemen are trying to accomplish. They are also metallurgically correct with no Dark of the Moon lore included.

Get thee a copy.

Or . . . an old recipie I neglected to mention in my pamphlet is for the ideal quench. That is, to quench in the urine of a three-year old goat fed only ferns for three days. I might continue to neglect this one, but at one time people swore by it. 

Get Ravenshear's booklet.

[Bob Roller]

I know little about cast mainsprings but I would think that "as cast"the shape which determines the preloading of the spring may not be enough to give good acceleration at the moment the sear clears the full cock notch.

Bob Roller