Cliff Stamp
BANNED
- Joined
- Oct 5, 1998
- Messages
- 17,562
For some time now I have been interested in getting a large bowie class blade with the optimal cutting profile, constrained by the durability requirement that it should be able to handle full power chopping in knotty wood. I am not concerned about actual edge damage but I don't want to induce a ripple in the primary grind, that is the limiting factor. If knots were not a factor then you could basically flat grind a two inch wide blade from quarter inch stock, with just a hint of an edge bevel, blade thickness 0.005" to 0.01" thick behind the bevel. However because knots are much harder and can cause a blade to receive a lateral impact when they break or get pulled out of the wood, the edge must be left much thicker to prevent rippling.
How thin can you go? Well that is dependent on the steel, the type of knots, how hard the blade is being swung, and the control of the follow through. From experience a maker can judge the first two factors, however the last two are obviously dependent on the user so there is no universal answer. I brought up the subject with Phil Wilson some weeks ago, specifically looking at CPM-3V for a blade steel. Why 3V? It is a high alloy steel (high strength, and wear resistance), that could approach 60RC and still not be concerned about lack of impact toughness. Phil felt that an edge thickness of 0.02" behind the bevel was a good point to aim for, and I was more than pleased with that idea. The steel will be hardened to 58/59 RC, full cryo treatment, and the toughness at that RC will easily be in excess of what is required.
This brings me to this blade from Tramontina. It was given to me as a gag gift. You can usually see them in flea markets and such, they are very cheap. It is a big knife, ten inch blade and has a stock thickness of about one eight of an inch. The blade has a full distal taper and is about two inches wide at maximum with a full flat grind. The edge is about 0.020 - 0.025" thick behind the bevel. I was actually interested in working with it for a few reasons. First off I was curious about the raw chopping ability and edge durability. Lastly, I was interested in experimenting with the flexibility in some detail, which I was assuming was very high due to the profile and most likely under hardened steel. I have no idea of the steel type nor the heat treatment, a few web searches did not turn up anything. This is obviously a large unknown.
On to the chopping. As expected due to the thin cross section the blade penetrated quite well. Just eye balling the cuts, they compared well to the depth of the chops with the Wildlife Hatchet from Gransfors Bruks. This is even more impressive when you consider that I was just using a wrist snap with the bowie, but a full shoulder powered drive with the hatchet. So does this mean the bowie chops as well as the hatchet? Well no, as there is more to chopping than just raw penetration. The cross section of the bowie is so small that it doesn't deflect the wood at all. The only way the wood will clear is if the cuts the bowie makes actually intersect. You can't pry the chips out with the bowie either as the blade just deflects too easily. Thus even on a small piece of wood which the hatchet blows apart in about five hits, the bowie needs more than fifteen because of all the multiple notching that is has to perform.
Ok so how about edge durability? Will the combination of steel and geometry how up to impacts on hard knots? I chose a piece of wood with a solid ring of knots and attempted to cut them out. After five chops the edge on the bowie had taken a semi-circular deflection about 3 mm across. This is pretty much what I would expect. I have seen similar deflections on much thicker knives. The problem is that the steel is usually really soft about 45 - 50 RC, and the overall heat treatment is obviously not high so it is really weak and deflects easily. It can handle clear wood, but any knots or small diameter hard wood will cause the edge to deform if any lateral impacts are experienced because of the very high pressures induced by the small contact areas.
As for the bending. I pounded the tip of the bowie (2 inches) into a stump with the poll of the axe to hold it in place. Using just my wrist I bent the blade and let it relax until I saw it take a slight set. This happened at about 55-65 degrees, and the deformation was just in front of the viced point and only to a few degrees, just enough to be seen. The blade was then bent to ninety degrees and allowed to relax. Now the induced bend was easily visible and about 11-13 degrees. The blade was then taken to 135 degrees and again relaxed. The induced set had now increased to 17-19 degrees. Finally the blade was bent to a full 180+ degrees, a bit beyond 180, as the handle was now almost touching the side of the stump. When relaxed the set had now jumped up to 28-32 degrees. As well the blade had also taken a bend in front of the handle, it was slight, less than ten degrees. The blade was returned very close to straight but just bending it the opposite way to ninety degrees a few times.
In short; one eight inch flat ground blades are not that useful for chopping if multiple cuts are required, really thin edges on low quality heat treated steels will deform on hard woods, you can bend the hell out of long blades with distal tapers on thin stock and having the steel fairly soft doesn't hurt either, it is also really easy to bend, just wrist torque was required. No real surprises anywhere.
-Cliff
How thin can you go? Well that is dependent on the steel, the type of knots, how hard the blade is being swung, and the control of the follow through. From experience a maker can judge the first two factors, however the last two are obviously dependent on the user so there is no universal answer. I brought up the subject with Phil Wilson some weeks ago, specifically looking at CPM-3V for a blade steel. Why 3V? It is a high alloy steel (high strength, and wear resistance), that could approach 60RC and still not be concerned about lack of impact toughness. Phil felt that an edge thickness of 0.02" behind the bevel was a good point to aim for, and I was more than pleased with that idea. The steel will be hardened to 58/59 RC, full cryo treatment, and the toughness at that RC will easily be in excess of what is required.
This brings me to this blade from Tramontina. It was given to me as a gag gift. You can usually see them in flea markets and such, they are very cheap. It is a big knife, ten inch blade and has a stock thickness of about one eight of an inch. The blade has a full distal taper and is about two inches wide at maximum with a full flat grind. The edge is about 0.020 - 0.025" thick behind the bevel. I was actually interested in working with it for a few reasons. First off I was curious about the raw chopping ability and edge durability. Lastly, I was interested in experimenting with the flexibility in some detail, which I was assuming was very high due to the profile and most likely under hardened steel. I have no idea of the steel type nor the heat treatment, a few web searches did not turn up anything. This is obviously a large unknown.
On to the chopping. As expected due to the thin cross section the blade penetrated quite well. Just eye balling the cuts, they compared well to the depth of the chops with the Wildlife Hatchet from Gransfors Bruks. This is even more impressive when you consider that I was just using a wrist snap with the bowie, but a full shoulder powered drive with the hatchet. So does this mean the bowie chops as well as the hatchet? Well no, as there is more to chopping than just raw penetration. The cross section of the bowie is so small that it doesn't deflect the wood at all. The only way the wood will clear is if the cuts the bowie makes actually intersect. You can't pry the chips out with the bowie either as the blade just deflects too easily. Thus even on a small piece of wood which the hatchet blows apart in about five hits, the bowie needs more than fifteen because of all the multiple notching that is has to perform.
Ok so how about edge durability? Will the combination of steel and geometry how up to impacts on hard knots? I chose a piece of wood with a solid ring of knots and attempted to cut them out. After five chops the edge on the bowie had taken a semi-circular deflection about 3 mm across. This is pretty much what I would expect. I have seen similar deflections on much thicker knives. The problem is that the steel is usually really soft about 45 - 50 RC, and the overall heat treatment is obviously not high so it is really weak and deflects easily. It can handle clear wood, but any knots or small diameter hard wood will cause the edge to deform if any lateral impacts are experienced because of the very high pressures induced by the small contact areas.
As for the bending. I pounded the tip of the bowie (2 inches) into a stump with the poll of the axe to hold it in place. Using just my wrist I bent the blade and let it relax until I saw it take a slight set. This happened at about 55-65 degrees, and the deformation was just in front of the viced point and only to a few degrees, just enough to be seen. The blade was then bent to ninety degrees and allowed to relax. Now the induced bend was easily visible and about 11-13 degrees. The blade was then taken to 135 degrees and again relaxed. The induced set had now increased to 17-19 degrees. Finally the blade was bent to a full 180+ degrees, a bit beyond 180, as the handle was now almost touching the side of the stump. When relaxed the set had now jumped up to 28-32 degrees. As well the blade had also taken a bend in front of the handle, it was slight, less than ten degrees. The blade was returned very close to straight but just bending it the opposite way to ninety degrees a few times.
In short; one eight inch flat ground blades are not that useful for chopping if multiple cuts are required, really thin edges on low quality heat treated steels will deform on hard woods, you can bend the hell out of long blades with distal tapers on thin stock and having the steel fairly soft doesn't hurt either, it is also really easy to bend, just wrist torque was required. No real surprises anywhere.
-Cliff
