Sometimes sharpest is NOT the best option.

S

Squashfan

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I have been working on a cheap gas powered weedeater. Trying to polish a turd is what I feel like I am doing but anyway. It had one of those little blades on the grass guard to cut the trimmer line to the correct length. I got out the belt sander and established some bevels/raised a small burr. And I do mean small. Then I took a extra fine diamond file and knocked the burr off. A quick strop on white compound and I bet the blade would have popped hairs. I didn't check though, it should have been sharp enough. I bolted it back on and fired up the weedeater. I let it run at full throttle a couple of seconds to get the line up to speed. I checked it and I could see the start of a cut. But no cut off line. I went back and forth a couple times but I couldn't get a cut line. Frustrated, I grabbed a fine file and took the apex right off that blade. I fired up the weedeater one more time and... IT WORKED! Freshly cut line from a dull edge. After I thought about it, I seem to remember a story about a paper cutting machine that required a squared off apex. Anyway, it was an interesting experience and a testament to matching your edge to the job at hand!
 
Interesting thought.

Today I was feather sticking and seems my duller knives do a better job.
 
Spent some time with my father in law this weekend, he is always willing to talk some about cutting solutions. On many of the sheer cutters he worked with, the edge was taken to a very high level of sharp and then precisely dulled to where it would cut with max efficiency.

Touring a facility that made scalpels, he noted one side was left with grind pattern, the back was bright polished leaving an edge "like a mini steak knife". This was done on purpose.

The helical cutters I was tasked to recondition came barely sharp. I tuned a handful up where handling them with a 90° cutting angle you had to be careful not to cut yourself. They wouldn't complete a single pass.

No single perfect edge for every job.
 
I leaned the same thing when I oversharpened my lawnmower blade a few years ago... There seems to be an obsession about the sharpest edge but that is not always what you want!
Thanks for sharing the weed eater story
 
It'd be interesting to hear what would happen if the blade were still very sharp with a fully crisp apex, but at a much lower grit finish than the EF DMT + white strop finish (closed to polished/mirrored, I assume). I'd bet the extra 'grab' or bite of the toothy edge (applied by the file) on the trimmer line is what makes most of the difference, as opposed to just creating a blunter apex. Sort of like how a serrated edge will grab & hold a rope during a one-handed cut, so the rope doesn't just slide off the edge.


David
 
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David, that is something I have wondered about too. It probably would work either way. I think I have seen a blade sharpened like you describe. It worked just fine.
 
Many applications that use a blunted edge do so for durability. When I worked in a machine shop that serviced the huge steel mills here in NW Indiana we used to rebuild lots of worn out equipment, and I remember the huge steel cutting shears had to be ground to just-so geometry or the blade would wear out in a very short time. The smaller machines cut steel from 1/16" to over an inch thick, the larger ones up to 4"! None used an edge brought to a narrow apex. IIRC they called for a .015" wide flat on the smaller machine and a .062" wide flat on the larger. If the flat was left too narrow, the edge would be quickly damaged and the deformation would go back way past where the original width of the flat would have been. This same type of edge is used on most mower blades (different angles though). I finish mine with a .015" flat.
 
Maybe it has something to do with the bending action of the line. A somewhat duller edge might give the line that little bit more time it needs to flex. Making the line bend more causing the line to whip around. Then be pulled through the cutting surface. Where a sharp edge would immediately start to cut causing no whip. Then the line would not go far enough past the cutting surface to make a deep enough cut to trim the line. The line would kind of drag across the sharp edge causing the line to look frayed.

Just my hypothesis. I have no way to prove it.
 
I have found sharpness is relevant to the task at hand, just as angles are. I don't want shaving razor sharp on my whittling knives or my hatchets as they will fold over on them selves and dull much quicker. Each item has an ideal balance between angle and sharpness is what I've experienced and usually just touching up the tools is all you need to do, a quick trip to a grinder or file will usually do it on almost everything that isn't an actual knife. the best scissors I've ever owned when taken apart would not cut melted butter but how tightly they bound together when used would trim hair and paper without any issue or snag ever. So yes I agree sometimes sharpest is not the best option, unless it's a filet knife, razor, scalpel or like type item then sharpest is the only option.
 
Wolfspring, if your whittling knives are folding over when carving you have a burr. I went through that when I first started sharpening my own carving knives. A master carver set me straight on what is sharp and not. After the straightening, my chisel will almost whittle hair.

Chris, I saw the way the line looked and you might be on to something. Oh well, it works.
 
I talked to a co-worker who used to be on the cutting crew for a landscape company for years. He took care of mower maintenance. He told me that he put a 1/8" flat on the blade edge and took the burr off. Said that the blade would cut cleaner and longer and wouldn't develop bad nicks if they accidentally dinged a rock or something. I tried it and it works well , and no tip browning with clean cuts.
 
I think the effect is more noticeable in metal working tools and powered equipment. When you start dealing with high amounts of horse-power and force it's translated into extreme amount of pressure. The "cutting edge" is typically just a gross wedge, and wedges rely on high amounts of pressure to multiply the forces concentrated at the tip and force their way through material. When we talk about knives, we're talking about exerting low amounts of pressure and force on the tool which are exponentially magnified at the apex because it is so keen--the more keen the apex, the higher the pressures generated at the very apex, the easier it will wedge through material. However we all know that there's also the need for the wedge's structure to be able to withstand the pressures and forces at this point. When you consider the forces exerted through machinery of almost any kind then if you have an apex that is relatively as keen as one on a knife blade, it's going to degrade very quickly under the pressure/forces. Speed, material, etc. are very important factors and in machining there's actually charts to check what speed to run drill presses at depending on the diameter of the drill bit and type of steel, etc. Too fast or too slow and you'll dull the drill bit out faster because these charts are developed to find specifically the most efficient conditions for the drill bit to cut.

In my opinion any time you begin to incorporate speed and velocity into a cutting instrument having a very keen apex can not only be unnecessary but counter-productive because some material just won't stand up to the pressures, even some steel will be quickly overcome even cutting soft material if the forces become too much for the structure of the wedge to maintain shape. If you think about how some chopping handtools can manifest this just with simple human swinging forces, you can comprehend how the problem is compounded once you start adding in the horsepower of powered equipment.
 
How about "toothy" edges on our knives?

Sharpened a friend's mystery knife for him this week. Damascus blade. Mirror edge left it dull. Finished with a medium stone and it was then "sharp."
 
Maybe, with the weed whacker line, you are breaking it instead of cutting it.
 
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