I'm used to sharpening my blades to a fine edge but the ones on my new Grasshopper, though ground to the right angle, are really dull.
Are blades supposed to be dull so the cutting edge lasts longer, or are new owners supposed to sharpen them before using? Are they shipped dull for safety or to deter rust, or did someone forget the final step?
Pics Join Date: Feb 2003 Location: NW Oregon Posts: 5550
re: Mower Blades: How Sharp?
My guess is that sharp blades would be much more prone to cracking. A fine edge can be dinged a lot easier than a dull one and cracks start from there.
[QUOTE=auerbach;169090] Are blades supposed to be dull so the cutting edge lasts longer, or are new owners supposed to sharpen them before using?
Are they shipped dull for safety or to deter rust, or did someone forget the final step? [/QUOTE]
Yes, blades are intentionally dull. The proper way to sharpen a blade is to, well, not sharpen them.
The science of it is, as you surmised, if the blades are sharpened such that there is ~1/8" blunt edge on the business end they will actually cut better, longer, than if they are sharpened to a knife edge.
If they are sharpened to a knife edge they very quickly get worn too blunt because of several factors. Among them, the edge is too thin to withstand the many tiny impacts caused by the millions of tiny pebbles found in topsoil. The other problem is that grinding them to a fine point causes the metal to lose it's temper as a result of the grinding.
From a safety point of view, the fine edge degrading will cause a lot of very fine steel particles to be fired out of the mower.
For sure, the duller you keep the blades the less they'll need sharpening and the more they'll resist damage. And yes, these ones have almost a 1/8" flat spot instead of a sharp edge.
But every blade-sharpening site I found stresses the need for sharpness to cut the grass cleanly. Are they all wrong?
Or could it be that the installed blades were designed for the vacuum system that I also got, where they aren't so much for grass-cutting as for chopping edge-damaging debris that then gets vacuumed into the hopper?
[QUOTE=auerbach;169122] For sure, the duller you keep the blades the less they'll need sharpening and the more they'll resist damage. And yes, these ones have almost a 1/8" flat spot instead of a sharp edge.
But every blade-sharpening site I found stresses the need for sharpness to cut the grass cleanly.
Are they all wrong? [/QUOTE]
No, it's just the word "sharp" in this context means ground to a 45° bevel on the top side only, and with a 1/8" blunt face edge.
This is a lawn mower after all, you're not going to shave with it, just cut grass.
I've seen blades ground to a razor edge lots, and on a situation like a golf course quality piece of turf, it will probably work, but on your lawn it sure won't.
The blades will stay "sharp" a lot longer ground as described above, and give you a better, less damaging cut longer, than they will if you grind them to a fine edge.
The answer I just got back from the blade people is that the blades come coated (mainly for safety), which makes the cutting area look dull, and that using them will remove the coating and reveal them to be very sharp. Which is what happened.
I was with my Dad when he bought our first power mower, a "Cooper Cyclo Mower" sometime in the fifties and still remember the salesman telling Dad that you never need to sharpen the blade. Dad did sharpen it anyway, never is a long time.
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