S&W 1st. Mod. DA mainspring

[Exmilcop]

I have an antique with a brutally strong mainspring. My sense is that the spring is new old stock or a well made replacement, but it's far too strong for it's purpose. I'd estimate the trigger pull in double action at around 20 lbs.! Trying to thumb cock in single action is also very tough. I'd bet trying to find a donor pistol or a good used mainspring would be problematic, but what are the odds a very good metallurgist would be able to draw the temper and then re-temper with lessened tension?

 

[Protocall_Design]

The proper amount of temper in spring steel is not very adjustable. The spring needs to be thinned down to what works right. The amount of metal is the determining factor. You may be able to thin that one with a file or belt sander, keeping it cool as you go, or have a new one made.

 

[Exmilcop]

The proper amount of temper in spring steel is not very adjustable. The spring needs to be thinned down to what works right. The amount of metal is the determining factor. You may be able to thin that one with a file or belt sander, keeping it cool as you go, or have a new one made.

I'm pretty tool handy, mainly woodworking, but metallurgy is outside my wheelhouse though I have some knowledge. The problem with thinning the spring, as I understand it, is that the material removal must be even or I run the risk of creating a stress riser that could lead to total failure. Then it's a case of trying to find a replacement and lots of luck with that one.

 

[Protocall_Design]

You can get annealed flat spring stock from Brownell's and try your hand at making your own. It's a good skill to have. Usually, making the spring is the easier part. Heat treating and tempering are the more finicky part. It is doable in a home shop with just a propane torch, once you get it figured out. I would figure on it taking a few tries to get it right.

 

[mmaher94087]

Post #2 is spot on. Since you want to replace the spring anyway, why not try your hand at thinning the one you have? First off, I recommend pulling the stocks off and watching the action of the spring as it moves while cocking the hammer. If the spring is too long, it will feel tough to cock or too strong. That is because the hammer is not only trying to bend the spring in an arc but also compress (shorten) the spring causing a very, very slight 'U' bend in the middle of the spring. The spring only needs to be a 1/16" too long to manifest this problem. If the DA spring was replaced with a NM#3 SA spring it will cause this problem as the springs are different even though the DA and SA revolvers are called the Model 3.

Should you thin your spring then first heat it evenly to red hot and let it cool at room temperature. This anneals (removes the springiness and softens of) the metal. Thin the annealed spring on the inside of the arch as evenly and smoothly as possible. Once thinned, draw-file (file the long length of) the area to remove any cross-spring file marks. Sand the filed-on side of the spring with 180 grit or finer Emory cloth to smooth. Nicks, gouges, pits and scratches will promote stress risers.

Once the part is thinned, heat it evenly to cherry red and drop it in a bucket of water or oil. This hardens the steel but also makes it brittle. Lightly sand to make the part shiny for the next step.

Next, draw the temper to make it a spring. I use a flat steel sheet about 5"x10" that has a right-angle bend about 3/4" from the 5" side so that it can be clamped in a vise and overhang the water or oil bucket. Place a small pile of washed and dried sand on the plate and nestle (still visible) the part in it. Washed sand removes any salt that could contaminate the steel and cause problems. The sand around the part spreads the heat uniformly. Heat the sandpile from under the plate with a Propane, Mapp or Oxy-Acetylene torch until the part turns blue. Scrape the part into the bucket to cool and stop the tempering process. Test the spring. If it works; good, if it breaks; you have a template for a new attempt.

My next lesson will be self-brain surgery. Sign up now.

 

[2152hq]

Springs are soft enough to be cut with sharp files,,,SHARP files.
You don't have to anneal (soften) the spring first and then re-heat treat it again to do a slim down job on an over weight flat or V spring.

You can file in any direction you care to to first remove the material from the spring. BUT to finish up you must file length wise on the spring,,not across the piece.

Thin the spring with a very gradual taper from bottom to the top end where it engages the hammer. Often called a Buggy Whip shape in older books on the subject. (But that terminology may be lost on many today.)

Along with tapering the top and bottom 'flats' of the spring,,you can taper the outside edges as well.
This will also make the spring lighter but don't overdo it and save this part of the operation for doing after the above tapering of the flat edges has been done.

Check to OAL as stated above to make sure the spring isn't too long and being kinked when the hammer is cocked.

Then all the file marks must be polished off. Start with 180grit, then go up through the grits till you reach at least 600grit.
Make sure all the previous grit marks are removed. If not, those heavier marks are what the spring-meisters call stress risers and can cause failure when the spring is compressed.

This super polish job goes for the edges of the spring as well.

..You can use power equip to do alot of this work, but unless you are familiar with the job I would not recommend it.
The Dremel is one of the first things people reach for and though it can be useful for some of this work,,it can ruin a lot as well.
A belt grinder is the best power equip to use and with proper care you can do the entire job on one if the size is right.
The numerous grit belts needed makes the cost for doing one job high, but if you use the machine and belts for general work it's there all the time anyway.

It doesn't take all that long to do by hand with files, polishing grit cloth and bench vise.
Plus you don't have to be concerned with re-heat treating the spring when done.

 

[steelslaver]

Thin out the spring. Only option.

The elastic modulus of steel can NOT be changed. Heat treatment can only change the elastic limit. You want a piece of steel to flex easier or harder you have only 1 option. Change its geometry. That is it. Even changing its alloy ingredients will only result in tiny tiny changes

Today's lesson elastic modulus vs elastic limit

Fact:
If you take 3 bars of steel. All 1" wide, 1/8" thick and 12" long.
#1 piece is 1095 heat treated exactly like making a file
#2 piece is 5160 heat treated exactly like making a leaf spring
#3 piece is regular old mild steel

Clamp one end of each down exactly the same and set up a dial indicator on the end of each. Then, hang a 5# weight on each. Believe it or not each will move the exact same amount. Adding another 5# will still result in all the indicators reading the same. This is because for almost all intents and purposes all steels have the same elastic modulus. 200GPa

This will continue until they begin to reach their elastic limits, which can be changed by both heat treatment and alloy.

The 1095 treated as you would a file has a very short and abrupt elastic limit and will fail catastrophically. Short meaning before it flexes very far it will fail in an abrupt and catastrophic manner by suddenly snapping. Reached its elastic limit

The 5160 treated like a spring will have a much longer elastic limit and will flex much farther before it fails, first by permanently deforming (bending) then at some farther point breaking.

The mild steel will reach its elastic limit and become permanently deformed

1095 is commonly used to make files, it is also used to make high quality cable. The cable bends around sieves repeatedly because it is heat treated to a spring temper and each strand has a small cross section.

 

[steelslaver]

I should have added this

If I took identical sized pieces of 1095 and then left one piece in the totally annealed soft state, then took the other 2 pieces to critical (1500f) and quenched them, then left one of those 2 pieces in its fully hard state then spring tempered the other at 800f and then set the 3 pieces up the same as in the first test, I would get the same results.

All 3 would deflect the same amount under a given load as the original pieces. The fully hard piece would fail quickly and abruptly as in the first test, the spring tempered piece would fail similarly to the spring tempered piece of 5160 in test one, but its elastic limit failure point would probably occur at a somewhat different point, the deed soft piece would bend.

Alloy will NOT change the amount of deflection under a given load (elastic modulus). It, like the heat treatment can change the elastic limit (failure point)

Alloy changes many things, but not the elastic modulus. A piece of mild steel taken to 1500 and quenched will not noticeably harden, a piece of 5160 will harden to almost 60 RC while a piece of 1095 will harden to about 65RC. A lot of that is the carbon content. Mild steel. .18&, 5160 .60% and 1095 .95%. D2 will get harder than either. It has 1.5% carbon and as iron can only absorb about .84% carbon the excess carbon in the D2 bonds with the vanadium in it and forms vanadium carbides which are extremely hard. This gives it extreme wear resistance, it is hard to sharpen and hard to dull. One of its uses is punch press dies. But, those carbides set up in the steels grain boundaries and although it has almost identical elastic modulus as 1095 or 5160, it has an extremely short elastic limit and will snap with almost no deflection Nobody in their right mind would make D2 into a spring.

 

[Exmilcop]

To all you gentlemen who so kindly shared your knowledge of steel, thank you. I must share a real "head-scratcher" with you all. I took the spring out to see if I could re-configure it a bit to lessen the tension. I noted that it's shaped to mate to the cut-out in the butt, and upon reflection, decided to just reassemble it and ponder on it some more. After I put 'er back together, PRESTO! The trigger pull and single action are far less stiff than they were and it's performing within normal specs. I never changed or altered a dang thing. Go figure!

 

[Alk8944]

Don't even think of trying to re-heat treat the existing spring, you will destroy it! The proper way to reduce the strength of a flat spring is to narrow it side-to-side, not try to thin it. It can also be bent slightly, but this is a delicate process and can result in the spring being broken, so I won't tell you how it is done and warn against trying this!

In my life as a gunsmith I have made numerous flat and V-springs for muzzle-loading locks and set triggers. One of the handiest things for making springs is a thermostatically controlled lead pot used for bullet casting. you need a lead thermometer to go along with it to regulate the temperature of the lead bath used to draw the temper of the spring.

If you choose to modify the existing spring this can most easily be done by grinding the sides of the spring on a belt sander. Be sure to grind along the length of the spring and then polish the sides with fine aloxite cloth or paper to be sure there are no nicks or scratches that would cause the spring to break.

Understand I am not recommending that you try this! I do recommend you get used to the spring as-is and live with it! I cannot believe the spring is as powerful as you claim, my #3 certainly is not! Without digging my gun out of the safe to check my recollection is the #3 has a strain screw like all later S&Ws. I haven't seen the gun for probably 20 years! If so, back the strain screw off about 1/2 turn, and if this lightens the hammer pull enough then simply shorten the strain screw slightly until it is. Remember though that S&W had a reason for the tension the spring was designed to have!


Oops, You said .38 DA. I was thinking about a different post when I mentioned the #3! The .38 DA has a strain screw too, try backing it off instead of screwing with the spring! Numrich Has used strain screws if you end up needing one.

 

[2152hq]

Why is thinning a flat spring the wrong way to reduce it's strength?

I agree that narrowing it can be done and get results as well.
It's an easier job to narrow the spring by it's edges and keep things under control than it is to grind or file the necessary lengthwise non-uniform taper of the spring down properly to reduce strength.

Both dimensions should balance. Why simply narrow an unnecessarily thick heavy spring that is too heavy for the job.
File (or grind) the entire profile so it becomes the useful smooth power source that only a flat spring can give you.

You can re-heat-treat a spring. If done correctly it won't ruin it.
However, it also won't change the power of it if it was done correctly to begin with.
If you try to lessen the power by making it a 'softer' spring by drawing back a bit further, you only make it respond less as a spring and it takes a set.
Draw back less, and spring is too hard and will likely break.

Harden and draw back where it should be and you have the same spring you started out with.

If you anneal the spring and change the bend in the spring or if a V spring you change the distance between the arms, or arch the arms, Then when hardened and properly drawn back you will have a different feeling spring in the mechanism.
But that is only due to geometry. You changed the shape of the spring.
Just like narrowing and/or tapering the spring as above.

I too use a lead pot with thermometer to draw back/temper the springs I make. Works very well. I've made many over the years for a great many guns. Mostly SxS shotguns and DR's.

I find I would rather make a spring than use most any of the cast replacement springs available for some older guns.
They harden and draw back OK.
But the quality of the castings leaves so much to be desired that the work involved in most of them to get them ready for the gun isn't worth it a lot of the time.

The older gun trades in England and around Europe had tradesman that just made the springs for the guns that others were making.
A specialty like so many others.