Tanto Blade (Forging) – Crossed Heart Forge

Process of Forging & Shaping a Tanto Blade

A charcoal fire is used to heat the steel for shaping with hand hammers. I often use a heavy hammer for rough shaping and a lighter one for the finishing work. The shaping of a blade is divided into a hot and a cold stage, and each stage further divided into two steps. First a specific type of blank (sunobe) is forged, the shape of which will determine the finished dimensions of the blade. Then the blank is forged into the shape of the blade and allowed to cool slowly. The cold shaping begins with the profile and then moves to the bevels. Once these are finished, the blade is ready for Yaki-ire, clay tempering.

Forging the Blank (Sunobe)

In this stage, the volume of steel is allocated to each area of the blade and tang. This distribution will largely determine things like distal taper, proportions, and style of the blade, though the sunobe looks very little like its final shape at the end of this step. In traditional swordsmithing, a practiced eye can determine the outcome of a student’s test at the sunobe stage, without even waiting for the forging of the final shape.

Antique horse-drawn carriage springs.

The steel has low alloy and high carbon content.

The hammer used for most of the shaping work, made from a piece of axle (You Need A Japanese Swordsmith’s Hammer).

The bar or billet is cleaned and forged out to a rectangular bar (wakashinobe).

The tip begins as a reversed 45 degree angle, the longer side will become the cutting edge and tip.

This photo at a later stage shows why cutting and turning up the tip is important to the flow of the grain along the edge.

The rectangular sunobe tapers away from the mune machi in both directions.

Forging to Shape (Hizukuri)

Forging a sunobe into the shape of a blade is a process of working up and down the steel a section at a time, forming the bevels and establishing the geometry of the knife. As the bevels are narrowed, the outward moving steel tends to curve the blade away from the edge, this must be anticipated and compensated for throughout the process. If it is not addressed early on, there will be no way to correct it later. Keeping the temperature as low as possible and forging almost into the black range each heat is one way to help refine the grain structure of the steel. Enough edge material must be left to have a 2mm thick edge after forging or 1.5mm after filing and before yaki-ire.

Back into the fire.

The mune is forged first before the edge is thinned out.

A charcoal forge heats a small section of the blade at a time and prevents major scale loss.

Forging with water on the anvil during the hizukuri stage creates steam explosions which blow off the fire scale, keeping the steel surface clean and free of impurities.

A reverse sori is forged in anticipation of the effects of yaki-ire, tanto should end up straight.

Rough Shaping (Ara-Shiage)

This stage of cold shaping has two distinct steps, the first to outline the profile, and the second to establish the bevels and sides of the tang. Files are used to profile the blade in a blacksmithing leg vise, adjusting and refining each line slowly, removing to check the overall silhouette often. A sen dai (staple vise) and a sen scraper are used for the rough work on the bevels, filing and drawfiling for the final work. The edge should still be 1.5-2mm thick after all of the rough shaping is finished in order to reduce the risk of warping or cracking during yaki-ire.

Mune (spine)

The mune is forged in during hizukuri but often needs a fair bit of work to define and true up. Using a sen scraper or an old rasp to remove forge scale before filing is a simple way to increase the longevity of the files. At this stage any irregularities in the planes or angles of the spine should be cleaned up and a degree of reverse sori in proportion to they type of hamon intended.

Sighting down the spine and regularly removing the blade from the vise to check the geometry will help prevent uneven angles and wavy lines. The spine should be as complete and accurate as possible before moving on to the munemachi.

Draw filing is a useful technique for refining the surface once the planes have been established.

Mune Machi (spine notch)

The munemachi is partially forged in during hizukuri but must be filed after filing the mune. If the order is reversed the file will mar the edge of the munemachi and cause it to droop, opposite of what it should be doing right at that point. The notch is placed according to the tang and blade length and filed square to the mune rather than “kinked” upwards. Check often to prevent the notch from becoming lower on one side.

Take great care not to slip and run the file over the mune at this point as it will necessitate removing a large amount of material to correct.

Nakago Mune (spine of the tang)

The areas of the spine on either side of the munemachi should align with one another in a fairly straight line. The spine of the tang continues this line towards the tip of the tang.

A steep 45 degree chamfer reflects light and reveals a high spot in the centre and establishes a line to be filed down to. Checking often and sighting down the spine will help prevent any bulges or dips in the flow of this line.

If the motokasane is particularly wide a very slight rounding is sometimes applied across the spine of the tang.

Ha (edge)

Moving back to the tip, the blade is flipped over in the vise and the edge profile cleaned up from tip to base of the tang. Reverse sori must be included here in anticipation for yakiire. Establishing an edge line with a steep 45 degree chamfer gives a guideline to file down towards. If the edge is too narrow, particularly in the area of the hamachi, it must be filed down until it is at least 1mm thick in this area, better 1.5-2mm thick all the way along.

Sight along the edge often and hold up to the light to check the profile.

Ha Machi (edge notch)

The hamachi is not forged in during hizukuri as it is thin enough to file in. Placing the hamachi is done by approximating 90 degrees from the spine next to the munemachi and marking the hamachi at a perpendicular point. Great care should be exercised not to allow the file to bite too deeply and move the hamachi out of line with the munemachi. Read more about the geometry of this area.

The machi should be in line with one another and perpendicular to the mune.

Nakago Ha (edge of the tang)

The line from the hamachi is continued to the end of the tang. Using a kata will help develop the sense of proportion of thickness and taper required to create this part of the tang geometry. In the meantime this stage may need to be revisited after establishing the bevels. It is also helpful to check if the length of the tang is correct before committing to a final profile here.

A steep 45 degree chamfer showing a bulge in the centre that needs more work.

Nakago Jiri (tip of the tang)

The blade is placed point down in the vise and filed to finished shape. If there is an excess of material here beyond a couple of mm, a hacksaw may be used first. The most common form is rounded or slightly rounded, called kurijiri.

The tip of the tang should be rounded in one direction but flat/square in the other.

At this point the profile has been completed but the bevels remain as-forged.

Another example of the water forged surface of the bevels before filing.

Hira Ji (blade bevels)

All of the above profile work should be complete and as accurate as possible before moving on to the bevels. After the bevels have been refined in the sen dai, moving back to the vise will mar the bevel surfaces.

The goal of this stage is to true up the lines along the spine and edge and then flatten the area between them. Filing a steep 45 degree chamfer along the edge establishes a line and then the material is removed down to meet it.

Hirazukuri (single bevel) tanto are mainly flat from the mune to the ha, but should have at least a small degree of convex or haniku (“meat” behind the edge), to provide strength. Because the edge is still 1-2mm thick the bevels should be about as flat as they can be filed at this point to save time after yakiire.

Great care must be taken at the now-exposed hamachi and munemachi as too much pressure on the file or sen and the entire shape will be broken. Thinning the hamachi too much and rounding off the edge line would require the whole edge profile and bevels to be taken down further to realign with it (leaving some ubuha, at least at this stage, is one form of protection from this error). Thinning the vulnerable corners of the mune at the machi would require the rest of the mune to be thinned accordingly and the bevels to be re created to align with it.

The simple sen dai clamps the blade flat between wooden wedges and blocks and does not mar the surface.

Filing at a diagonal angle first to work on specific areas, checking often for dips and waves.

Draw filing to check flatness and continuity. Great care is taken not to apply too much pressure to the exposed corners of the ha or mune at the machi.

Nakago no Hira (tang bevels)

As with the blade bevels, a steep chamfer is filed on all edges to set the target lines. Filing down towards them and checking regularly from multiple angles to keep the geometry as even as possible. Finishing is done with careful draw filing. Care must be taken not to remove too much material around the machi, either where the habaki will sit or on the blade bevels next to them. Finally, all edges are very lightly chamfered by drawfiling to remove burrs without altering the shape of the edges.

Filing down to the lines set by the chamfers.

Drawfiling to check continuity. The blade will be left with this rough surface to help keep the clay on during yakiire.

A comparison between the raw material and the completion of ara-shiage.

The next process is Yaki-Ire, clay tempering.