Tuesday, November 16, 2010

Forge your own scythe blade

A question about a previous post (Brubaker scythe blade, Made in USA), followed by the response from Jack Brubaker:

Is there any chance you could provide me with a fuller description of how you went about making the scythe blade itself?  
I have been scouring old books and the internet to no avail for  information and instruction on this aspect of blacksmithing.  I am interested in the details of this as I have no machinery, such as a foot hammer, to pursue this project.  It is my hope that with more details I might be able to reproduce the same effects with simple hand tools. 
-- Carl

I want to write up my experiences with blade forging more completely after I have more practice myself. But, what follows are a few notes that might help. 

It is certainly true that scythes used to be forged without machinery. There is probably somewhere a shop on the fringe of world trade where this is still being practiced, perhaps many. My guess would be that the procedure would be similar to that used when forging with a power hammer, but I may be wrong. Look at the images on the Geyerhammer site to see the steps in forging. 

The only part that really surprised me in watching the smiths at Schrockenfux was the way the blade held the preset curve while being hammered thin. My experience with knife making was that in forging a blade the greater hammering needed to thin the cutting edge to its slight thickness caused the blade as a whole to curve as that part of the blade grew in length more than the thick back of the blade. It is necessary to over compensate for this distortion by curving the blade into a C shape first so it will straighten out as it is thinned.  

The scythe smiths I watched did not have this problem at all. They started with the blank bent to the curve they wanted the final blade to have and kept that curve throughout the forging process.

Having done it once I now see that the difference between a knife and a scythe is that the knife blade has a wedge shaped cross section and a scythe has a thick back but is nearly even in thickness across the rest of the cross section.  I assumed that the thick back would constrain the back from growing in length as the rest of the blade is thinned and grew in length. Two things happen to prevent this from being a problem. First the thick back holds it's heat well because it is thick and just stretches some as the blade is forged. Second the blade is forged with a top die that is canoe shaped hammering on a flat bottom die. (The pointed end of the top die is needed to form the V shaped section between the back of the blade and the diagonal "deer's tongue" that reinforces the connection between the blade and the tang.) 

The long narrow shape of the top die acts as a fuller to move the metal primarily in one direction, to widen the blade as it is thinned. So far this is a lot like knife forging. The difference is that only a small square section is left to form the back while the rest of the blade is hammered very thin. The shape of the blade is watched by the smith and can be controlled by how much hammering is done near the back and how much is done near the edge. If they are balanced the blank will hold its pre-set curve. If the curve goes off it can be brought back with more hammering near one side or the other as if the back was not there.

To hand hammer a scythe blade there could be several approaches. One would be to try to use the cross pein to spread the blade taking care to not hit the back. Start by fullering a groove next to the back to define it's shape and use the cross pein to hammer the rest. This will be a very tedious process requiring many, many heats, unless you are a lot stronger than I am. In the act of taking so many heats on increasingly thinner metal some of the carbon may be burned out of the steel, so the result will be unpredictable.  Heating in a gas furnace rather than a coal fire will probably help limit the accumulated damage to the metal since the sulphur in the coal attacks the steel.

Another process would be to start with a very thin blank that could be cut to shape, and form the back by folding over the back edge once and then bending the doubled metal up at 90 degrees to the blade. There are blades being made this way in large forming dies. It can be done at the anvil with a V block in the hardy hole and working the fold down into the V with a cross pein hammer. Once the fold is formed, hammer it closed on the face of the anvil and form the next fold in the V block. This time just true up the fold at 90 degrees in the V block.

If there are still blades being hand forged somewhere they are most likely being hammered by a team of smiths. The master holding a top fuller and two or more strikers swinging sledges to drive the fuller as the master moves the blank under the blows. This can be a quite powerful process but requires experienced strikers and a lot of team work to really move the metal very much per heat. 

Walter Blumauer from the Geyerhammer Museum told me that in the old days in Austria, a scythe shop without power hammers with about 25 employees (smiths, polishers, finisher, packer, and foreman-owner) would make 25 blades a day. Using water hammers (since the 1500s), each smith could forge 200 to 250 blades a day. When he had met his quota he was done for the day and went home.

The V block seems to me to be the most important tool one would need to have to forge a blade by hand at the anvil, that and a strong arm. It is used to refine the form of the back in place of the double acting power hammer (that strikes both vertically and horizontally with alternating blows) used in the Austrian scythe shops.

One thing worth note is that in the modern practice the entire length of the blade (all but a short end that is hand held by the smith) is forged all over evenly so that the whole blade emerges from the blank at once. Working by hand this will be more problematic since only so much can be done at one heat. If one section of blade is hammered in one heat when the next section is forged there will be a problem of discontinuity where the two forged section merge and there will often be a spot where the blade is less wide where the two sessions of forging merge.

Over lapping hammer blows have a very interesting effect on the way the metal flows (what direction the metal moves under a series of blows). For the sake of example, if a square but flat hammer is used to make a series of heavy penetrating blows, each slightly to the right of the one before it, the metal will move to the right in growth. That is not to say that it won't move out in all directions as is expected from a hammer of this shape, just that there will be more spreading of the metal to the right than the other directions. This is caused by each successive blow landing on the right edge of the depression made by the previous blow. All the force of the hammer in concentrated on the small area where the next blow misses the last blow. 

The effect is the same as if a very narrow hammer face shaped like a straight pein hammer had been used. Scythe smiths use this effect to help control the movement of the metal. They also turn the blank at about 15 degrees to the top die and strike a line of blows down the area near the back or near the edge to both thin the metal and stretch those sides of the blade lengthwise. This can help control the shape of the blade. 

So for maximum widening of the blade, blows will be run in a pattern across the blank. For stretching the edge or back, blows may be struck in series that run up and down the blank along it's length (even though the die is still parallel to the blank). It is also possible to push out the back of the blade if it gets a little to straight (heaven forbid concave) by running a series of blows that start at the middle and move to the back, this will have the effect of pushing out the back. 

Don't worry too much about the exact width of the blade from the back to the edge. Keeping the shape and getting it thin is the main problem. If there is the right amount of metal to start with there should be enough width when the blade is well thinned. If there are excess bulges of metal on the cutting edge they can be trimmed off before final peining of the edge.

I hope to have time to make drawings of these hammering effects and tools that might help later this winter. Austrian blades are forged from 7 to 8 point plain carbon steel. Alloys would interfere with the cold hammering needed to set the tension in the blade in the Austrian method. I continue to use the blade I forged along side a new Austrian blade. I have had more tears in the edge of my blade. I don't know if that is a result of the blade not being hardened and tempered, or is because I used 10 point carbon steel, or because I use that blade for cutting tougher material since it is shorter and heavier. Because I often use it on heavier material I can't judge the blades one against the other effectively, but it remains a useful and effective tool.

-- Jack Brubaker

(Source:  Image "Sensenschmidt-1568" from Jost Amman and Hans Sachs, Frankfurt am Main,