Guild of Metalsmiths
Metalsmith - V 21.2 :
by Randy Hengl, Guild Member and member of the Metalsmith's Editorial Board
In the process of performing restoration work on 17th and 18th century firearms, one of the most difficult tasks for me has been the fabrication of springs. The sickening sound of a snap as you place one under pressure for the first time, or the stupid feeling that you develop as the "spring" collapses and lays flat like a tin can run over by a Mack truck, led me to seek the wisdom of more learned people than myself.
My first mentor's suggestion that I place one half of the profit that would be derived from the job in the bottom of my forge prior to starting. This 'offering to the gods' was not my idea of a solution.
The second person I sought council with suggested that I contact the original manufacturer. In this case I found that my communications lines to the spirit world were down.
In desperation I began to search for what other sources of knowledge were available. I found that a lot has been written but: .
My conclusion is that most of the people who have written about spring making have spent their time writing about spring making and have not made any.
The principle underlying the manufacture of a spring is the elastic property of the steel from which it is made. You have to have the right steel! Spring steel is a plain carbon steel with a carbon content of around .7%. Steel below .2% carbon will not react to heat treatment and above 1% will be impossible to control in a small shop environment. The American Iron and Steel Institute specification number 1070 works well. Some sources recommend 1095 for springs but this product is a lot fussier about its heat treatment in hardening and tempering than the home shop worker needs to deal with. General Motors use 5160 in their leaf and coil springs this is a chromium steel with .6% carbon and works in heavy applications but is tough to forge when working on a reduced scale. The process of making a spring consists of: Forging, Filing, Polishing, Heat-treatment in the form of hardening and tempering, and a final polishing prior to fitting. The craftsmen I attempt to duplicate did not have access to the finely rolled and sized stock that we have today. My mother may have raised a fool, but that was my sister, every hammer blow avoided is one saved for the future, and Brownell's in Montezuma, Iowa, or Dixie Gunworks of Union City Tennessee sell pre-annealed 1070 spring stock in 12" lengths in thickness from 1/32 to 3/8" and widths to 1" for a dollar amount that is less than the wear and tear on my body than it takes to rough shape it to form.
From flat stock, forging may be done with almost any heat source but I have found that the "soft' heat of a charcoal fired forge seems to produce a better spring. What ever the heat source you use, it will need to produce a heat between 1450 and 1550 degrees Fahrenheit. I suggest that you allow the stock to preheat slowly before moving it into direct flame. When the stock has reached a even cherry-red color remove it from the heat and forge it to final with and thickness. DO NOT WORK THE METAL BELOW A CHERRY RED HEAT. (If reproducing a broken spring use calipers set to the various thickness will assist sizing during this first stage.) Allow the stock to slowly cool in lime or ashes out of air flow prior to any, required final filing to shape. After filing the inside side of the future spring that will be inaccessible after bending to shape MUST be polished dead smooth. Any file marks left in the spring will produce stress points which will develop cracks during use.
In forging to final shape you will find it helpful to mark the stock with chalk or a 'silver pencil' in the places where bends are to be made. Placing the curves, or bends, in a spring so that compression will be distributed evenly is the secret to proper function. Bend the spring a little less than you feel is necessary, so that at the point of least compression the spring will not be too weak and fails to function. Return the stock to the heat source and gradually heat it to a cherry red glow. Move slowly and work only at full red heat to prevent the molecules of steel from becoming injured which will produce fractures in use. Uneven heating is the cause of most of the defects in a forged spring. File marks and slight scratches that allow stress fractured are the second cause of failure.
Hardening and Tempering
If steel is heated to a high temperature, bright or cherry red, and then quenched in oil or water, the atoms of iron and carbon re-arrange themselves in a formation which give the metal a very hard and brittle property. This process is known as hardening. If this metal is heated again to a lower temperature, dull red, and allowed to cool slowly it will revert to its soft state. Hardened steel if only partially re-heated will lose its brittleness and springiness will appear as the heat increases. This process is known as tempering. A pretty simple explanation of heat treating but all you need to know to turn pieces of stock into a serviceable spring. Selecting the right heats will require some experimentation. Every metal treating, blacksmithing, gunsmithing book and even in what I have written here speaks of heats in colors. Ever notice how many colors mother nature uses in painting cherries, and we all speak of cherry red. Cherry red, in medium sunlight, is approximately 1500 degrees. On an overcast day, in a dim shop, 1500 degrees will appear brighter and in bright sunlight will seem duller. A hint: What we are looking for is the 'critical heat", when you reach this heat the magnetic attraction of the metal will disappear. When all else fails the use of one of the temperature testing products such as Tempilaq or Tempilstick comes in handy especially for some of us that are color blind to most shades of red. When your stock reaches this heat quench it in OIL, moving it around so that it cools evenly. Your spring is now very hard and brittle.
Temperinga.) Lead melts at about this temperature and you may submerge the spring in molten lead until it reaches the point that the lead no longer sticks to it. Submerge the part fully, and remember that steel floats in lead. b.) Self-cleaning ovens 700 degrees in their cleaning cycle and you can clean your electric oven and temper the spring at the same time, (My wife hates this trick). c.) Fiercely burning motor oil burns at about this temperature. if you fill a shallow tin with oil to cover the spring and heat it with a torch until it sputters and burns with a roaring flame until the oil burns away you will reach 'draw' or temper.
The next step is to reduce this hardness by tempering. The ideal temperature for tempering steel with .6 to .7 % carbon is 700 degrees f. This may be accomplished in several ways.
You may have read that springs should be tempered until they are blue. If you polish a piece of steel and heat it slowly until various colors appear. The blues begin to appear at about 580 degrees, at this temper you are in danger of having your spring still brittle and it may break.
After the above has been done the spring is ready for final polish. before any polish is done examine the spring with a magnifying glass to detect any flaws.
Cracks of a circular form in the corners indicate uneven heating. Start again!
Cracks of a vertical nature or dark fissures indicate the steel has been burned. Start again!
Dimples, show that there are hard and soft places and indicate that the heating and cooling has been uneven. Start again! When the spring is perfect in ever respect polish it to mirror finish and place it under test**.
** At THIS point you may wish to make that offering to the gods!
Guild of Metaismiths Volume 21 No.2 June, 1997
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