Saturday, September 1, 2012

Introduction to Blacksmithing Dies

In the type of work I do I use dies quite a bit.  Mostly they are to aid production of pieces which require self-similarity and increase speed of production.  I still remember the first time I studied the dies in Don Streeter’s book, “Professional Smithing”.

Broadly the topic divides into die making and die using so there are two products involved, the die itself and the objects the die will form.

Most of my opinions have been formed from practice and personal experience.  I haven’t had tool and die training but I have looked for references in blacksmithing books and didn’t find a lot.   Most of what I found on the internet related to industrial dies.  So, I thought it might be useful to report some of what I have found works for me after a lot of experimentation.  I’ll cut to the chase and describe my current practices and perhaps, later, retrace some things which were tried and discarded and maybe some plans for future experiments.

Dies can generally be divided into open or closed types.  Open dies have one swage block with a forming cavity into which a billet is driven to form “half-a-thing”. Half and acorn, half a ball, half a whatever.  The formed object has an interesting face side and a plain flat back side.

Closed dies have two swage cavities which act together to capture a billet and form an object interesting all around.  Closed dies can be further subdivided into those which form highly symmetrical objects such as a ball, an acorn or even a single peanut seed which can rotate uniformly on their longitudinal axis.  But, when in the hull, the peanut has bilateral symmetry or is asymmetric but it does not have longitudinal rotational symmetry. Mostly, I make and use closed dies. There is not much to making an open die after learning how to make closed ones so I’ll concentrate on making close dies.

The thought process begins at the end of the forming process - the formed object.  The simplest example is a ball of a specific dimension.  First, forge that object - the master positive.  If the die use is short-run both the master and even the dies can be made of mild steel, particularly if water quenched.  For long-run use working toward tool steel dies is preferable.  Ordinarily, I forge the first positive out of mild steel, as it is easier to clean up and polish. 

However, when making the first negatives, I usually pick a tool steel for the die blocks.  I have a lot of 5160 and other high carbon scrap and that is my first choice.  To a lesser extend I use S7 or H13.  I cut the die block stock and forge it to a thickness which will accommodate a bit more than half the depth of the master object and perhaps a 1/4” wider and longer and wider than the master.

I make a hairpin spring from about 20” of 3/16” x 1.5” or 2” hot rolled mild steel.  The two blocks are welded so that there is enough space to slip in the positive form.  The block end of the assembly is then heated in the coal forge so that bot are at an even and high heat.  My assistant positions the block end on the hydraulic forging press and I insert the positive and press the blocks together.  For small objects a single heat will do but for larger objects multiple heats will be needed.

After the die is cool I insert a wedge tool I made to open the spring enough to work on the die faces.  Mostly this involves relieving the sharp edges of the form perimeter with a die grinder.  When that work is finished the die is mounted on a saddle for either the press or the power hammer.

Finally, I experiment with billet stock to find what best works to fully fill the die cavity without leaving a lot of excess extruded flash which require cleanup.  I make sure I keep a good tool steel positive in case I need to make another die in the future.

When doing the initial production runs I find that working the objects “to black” and carefully removing scale helps planish and polish the die faces improving the detail of the formed object.  It’s one thing that does seem to improve with use - worked slick.

I’ll get into more details in further posts.

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