How to temper a mainspring?

[B-Davis]

I am looking to lighten the trigger pull on a Burnside carbine I have. The current trigger pull is a good 11 lbs.
I would like to follow the advice of Dave France and take some metal out of the main spring, since the Burnside does not have a sear spring. With this in mind, Lodgewood sells an untempered mainspring.
I would like to use this spring, but am unsure as to the tempering process. I know the spring has to be heated up, and dropped into oil, but then what?
What is the most basic technique for a non- technical person to attempt? I have heard of dropping the spring into a pot of melted lead as another potential process. Does this work?
So, what would be the most basic, but successful method to try? I would like to try this at least once to see if I can do this.
Another method that I have heard of to lighten the Burnside trigger pull is to use a Spencer or Sharps mainspring, has anyone ever done this modification? I would be interested in this too!
thank you for any info!
Bryan

[Pat in Virginia]

One simple method I have seen my gunsmith use to temper a spring he made for me is to hold it over a can of oil with a magnet. He then uses a torch to heat up the spring moving the torch so the temperature is uniform on the spring. When the proper temperature for tempering is reached the spring simply falls from the magnet into the can of oil. Apparently, at the right temperature for tempering, the heated spring looses it magnetic attractability (the electrons come into alignment or something esoteric like that) and the magnet therefore releases it.

The spring is then properly tempered and there is nothing else to do in that regard. You want to do your work (grinding, etc. - heat generating activities) on the spring before you temper it. My spring was indeed properly tempered by this method.


Pat in Virginia

[B-Davis]

Interesting! Now, another question, since I am still very new at this. After a spring is tempered, does it then have to be reheated and annealed. Or does this process defeat the purpose of tempering?

[Mike Stein]

I'd look at the sear and tumbler engagement before touching the spring. Sounds like you got a dangle to the angle that needs adjustment.

[R. McAuley 3014V]

Pat,

This may be the test method your gunsmith had derived his from:

Popular Mechanics Magazine, Vol. 18, No. 4 (October 1912), “Shop Notes,” entitled “Magnetic Test for Heat in Hardening Steel”, pp602-03.

http://books.google.com/books?id=A94DAAAAMBAJ&lpg=PA5&pg=PA5#v=onepage&q&f=false

[Ron/The Old Reb]

You need to heat the spring till the spring is a uniform yellowish orange color. This is hard to do with a torch. The sear spring can be done with a torch but the main spring is hard to get a uniform color with a torch. I use a small charcoal grill like a hibachi and a hair drier. When the spring is uniform in color all over drop it in water not oil. Then you have to temper it. Do this by heating it till starts to turn a a blue color then set it on a piece of wood and let it cool don't put it on metal to cool. You want it to cool slowly. You can also temper it by putting it in molten lead at 850 degrees. But I can never get the blue color this way, but it works. Be sure you have it well polished before you harden it.
Have fun.

[Maillemaker]

The process terms are getting twisted a bit.

The entire process is called heat treating. Iron requires a certain amount of carbon in it in order to be hardenable through heat treating. Obviously things like springs are made from carbon steel so that they can be heat treated.

If you heat steel up to its critical temperature and let it cool slowly, this is called annealing, and will soften it.

If you heat steel up to its critical temperature and let it cool quickly, this is called quenching and it will result in a hard, but brittle item.

To retain the hardness while eliminating the brittlness, you temper the item after it has been quenched. This can be done a few ways.

One way is to remove it from the quench before it has completely cooled. This is called a slack quench.

A more controlled way is to re-heat the item up to a certain temperature. Tempering temperatures for carbon steels vary from 400-800 degrees, depending on the alloy and the desired hardness.

If you re-heat in air, oxides will form on the outer layer of the object. The color of the oxide layers tells you the temperature reached.

If you reheat in a vat of molten metal, like lead, no oxides can form as no oxygen can reach the object.

Here is a good article on the subject:
http://en.wikipedia.org/wiki/Heat_treating

Here is a downloadable Heat Treating Handbook that is very, very useful:
http://www.secowarwick.com/en/infocenter/heat-treating-data-book/

It will tell you what oxide colors correspond to what temperatures. It will tell you what hardness you will get for any alloy for any tempering temperature.

The problem with the main spring is you do not know precisely what alloy it is made from. Also, I don't know what hardness to shoot for for a main spring. So you will have to guess on the tempering temperature.

Basically you will heat it to bright orange heat (the magnet trick is goo), and quench it in something like transmission fluid. This makes it hard, but brittle. Then temper it by re-heating it to around 600 degrees. This keeps it hard, but tough.

Too hard, and your main spring breaks. Too soft and it's not a very good spring. It's better to go soft than hard - you can always re-heat treat.

Steve

[spadegrip]

The process terms are getting twisted a bit.

The entire process is called heat treating. Iron requires a certain amount of carbon in it in order to be hardenable through heat treating. Obviously things like springs are made from carbon steel so that they can be heat treated.

If you heat steel up to its critical temperature and let it cool slowly, this is called annealing, and will soften it.

If you heat steel up to its critical temperature and let it cool quickly, this is called quenching and it will result in a hard, but brittle item.

To retain the hardness while eliminating the brittlness, you temper the item after it has been quenched. This can be done a few ways.

One way is to remove it from the quench before it has completely cooled. This is called a slack quench.

A more controlled way is to re-heat the item up to a certain temperature. Tempering temperatures for carbon steels vary from 400-800 degrees, depending on the alloy and the desired hardness.

If you re-heat in air, oxides will form on the outer layer of the object. The color of the oxide layers tells you the temperature reached.

If you reheat in a vat of molten metal, like lead, no oxides can form as no oxygen can reach the object.

Here is a good article on the subject:
http://en.wikipedia.org/wiki/Heat_treating

Here is a downloadable Heat Treating Handbook that is very, very useful:
http://www.secowarwick.com/en/infocenter/heat-treating-data-book/

It will tell you what oxide colors correspond to what temperatures. It will tell you what hardness you will get for any alloy for any tempering temperature.

The problem with the main spring is you do not know precisely what alloy it is made from. Also, I don't know what hardness to shoot for for a main spring. So you will have to guess on the tempering temperature.

Basically you will heat it to bright orange heat (the magnet trick is goo), and quench it in something like transmission fluid. This makes it hard, but brittle. Then temper it by re-heating it to around 600 degrees. This keeps it hard, but tough.

Too hard, and your main spring breaks. Too soft and it's not a very good spring. It's better to go soft than hard - you can always re-heat treat.

Steve

I would look into what Mike Stein wrote. Look at the SEAR SPRING and tumbler, not the main spring.

[B-Davis]

Hence the problem! The main spring IS the sear spring! I would rather experiment with a $30 part than a $100-$150 part.:D Remember, the Burnside is a back action lock.
That is why I want to buy a $30 mainspring.
I am looking at following Maillemakers advice.

[Eggman]

I would look at stoning (with small Arkansas stone) the full cock notch. You can also look at thinning the end of the mainspring -- which is the sear spring. I would not go the replacement mainspring route. What you're contemplating is a high skill process that our gunsmith boys trained at, usually at some length. That said, if you stay on the track you've indicated you may want to practice on some cheaper pieces first.

[Mike Stein]

You can try a lighter spring and still end up with a heavy trigger if the geometry of sear to tumbler engagement is wrong. Though this link shows the 1861 Springfield lock, the discussion of sear to tumbler is universal enough for you to see the geometry. Geometry will be an issue even when your mainspring doubles as a sear spring. Spring tension will effect hammer speed and can effect ignition (not go off reliably) or be effected by ignition (hammer blowback).
http://www.nwtskirmisher.com/useful-locks.shtml

Get a good lens and take a look. A couple of light passes with stone may be all you need. Do know that you need to go slow and test often. Metal does not grow.

Good luck with whatever path you choose.

Mike

[Pat in Virginia]

Richard,

I think my gunsmith learned the magnet trick when he was going to gunsmith'ing school in Colorado a few decades ago. I'm amazed that you knew where to find that article.

For those of you who may be interested, the pertinent part of the article from the Oct 1912 issue of popular mechanics, Magnetic Test for Heat in Hardening Steel, Richard mentions reads as follows:

"A very simple method by which anyone who wants to harden a few tools may obtain as good results as a professional mechanic, though, perhaps it will take him a little more time, is by use of a magnet. The temperature at which steel should be quenched to secure the maximum hardness is just above the point where the carbon in the steel changes from the free to the combined state. Curiously enough at this same point steel becomes non-magnetic, and by taking advantage of this fact one can harden steel almost perfectly. ..."

It reminds me that "There is nothing new under the sun." There is no telling how far back that knowledge has existed.


Pat

[spadegrip]

Hence the problem! The main spring IS the sear spring! I would rather experiment with a $30 part than a $100-$150 part.:D Remember, the Burnside is a back action lock.
That is why I want to buy a $30 mainspring.
I am looking at following Maillemakers advice.

Sorry, I never looked at the lock internals on my Burnside. My bad.

[Chris Sweeney]

You said that the Lodgewood spring was "untempered." Is it annealed or hardened?
Hardening and tempering are not the same thing. The magnet trick works at about 1400+ degrees f. (or more, depending on the steel). It's cherry-red at this point.
Spring steel is pretty high in carbon, so if you quench it cool, it will be very hard, but also very brittle. To temper a spring you want to heat it back up to about 500 - 600 degrees, then quench again.

It's near impossible to file on a fully hardened piece of spring steel, and not easy on a semi-hard tempered one. If you are going to be removing a lot of metal you may want to anneal it by heating it up to the non-magnetic state, and let it cool VERY slowly. Then when you're done filing, heat it up to cherry again, quench, then temper.

I bought a Zouave replacement springs once that was untempered but hardened. Thought it was ready to use. Second time I cocked the hammer, it snapped at the V. Now I know to draw a file across the surface to check hardness first

[B-Davis]

Thank you for all the suggestions and ideas. I actually took a different route. I went to a gun show today, I lucked out. A gentleman sold me two original sharps mainsprings for $40. I already modified one, and have trigger pull down to five pounds. I now can save the original spring and have a spare!
Sincerely,
Bryan Davis