Hollow grind versus chisle grind test. Gentlemen start your engines.

Fred.Rowe

Fred.Rowe

Joined
May 2, 2004
Messages
6,848
My good friend Jeff stopped by last weekend, he had two sets of mower blades for his big zero turn mower that he wanted to sharpen using my 2 X 72 belt grinder along with a bubble jig to keep the angles even.

Both sets of blades were used on his 28 hp zero turn mower and were from the same manufacturer. Putting our heads together, which does not always turn out well, we decided to sharpen one set of 3 blades on the flat platen, the other we decided to try a hollow grind on an 8 inch wheel.

As we sat around the shop having a beer we discussed what the differences might be. I thought the hollow ground blades would tend to cut with less drag as the grass came off the cutting edge. Less bludgeoning and more like slicing. The edges were kept at the same thickness and were not knife blade sharp by any means.

Jeff headed home to mow his 5 acres of grass. He mowed the first half with the chisel ground blades and the second half with the hollow ground blades. He gave me a call the next day and said there was no difference in the quality of the cut; but the big mower did not work any where near as hard to do the same job with the hollow ground blades. He used less gas using the hollow ground blades.

I couldn't decide exactly were to post this but thought since we are talking blades; mower blades are not much different than knife blades.

Fred
 
were the 2 areas the same size? the hollow grind would tend to have less drag cutting but i'm wondering how long the hollow grind edge is going to last compared to the chisel grind.
 
I figured that would be the outcome. (both blades cutting the same) I am sure the chisel ground blades will out last the hollow ground blades by a fair margin. I have sharpened thousands of mower blades, many different ways, including golf course reels and bed knives. All day eight hours a day for years.
 
Fred.Rowe said:
...mower blades are not much different than knife blades.
Click to expand...

I beg to differ... I don't know about you, but I don't have even one horsepower moving my knife blades in a steady continuous sweep, always cutting at the same angle, against soft grass. Even a 90degree edge will cut grass with that kind of speed and torque behind it. Just sayin' :D

I do see how a relieved edge on a mower blade would increase fuel efficiency, but like the other fellows, I also wonder how long that edge will last. Wait till you hit a rock or something with a hollow edge made of soft weak steel!
 
james terrio said:
I beg to differ... I don't know about you, but I don't have even one horsepower moving my knife blades in a steady continuous sweep, always cutting at the same angle, against soft grass. Even a 90degree edge will cut grass with that kind of speed and torque behind it. Just sayin' :D

I do see how a relieved edge on a mower blade would increase fuel efficiency, but like the other fellows, I also wonder how long that edge will last. Wait till you hit a rock or something with a hollow edge made of soft weak steel!
Click to expand...

Actually, I look very closely at the grass after it's been cut. (On golf course greens specifically) We cut them anywhere from 80 to 120 thousandths of an inch, measured with a micrometer. At 90 degrees you will be ripping the grass, leaving it venerable to diseases and pests. But yes, on most grasses 90 degrees will chop it down some :D Sorry I got off on a tangent;)
 
Fair enough, swamp, my biggest concern with cutting grass is that the town board doesn't fine me for letting my lawn turn into a meadow. You golf people are all bonkers... :D
 
james terrio said:
I beg to differ... I don't know about you, but I don't have even one horsepower moving my knife blades in a steady continuous sweep, always cutting at the same angle, against soft grass. Even a 90degree edge will cut grass with that kind of speed and torque behind it. Just sayin'


I do see how a relieved edge on a mower blade would increase fuel efficiency, but like the other fellows, I also wonder how long that edge will last. Wait till you hit a rock or something with a hollow edge made of soft weak steel!
Click to expand...
Sorry to hear you don't have a single horse power under your hood. How do function?:D

I believe the hollow ground edge will break down faster also; just as it would on a knife blade. It was a fun experiment and its always nice sharing a beer with a friend.

Fred
 
I usually convex my mower blades on my slack belt, and take it to a polished edge. The last set I did was shaving hair off my arms... hahah That first cut was pretty sweet though.

Maybe a little bit of overkill (and I'm sure it doesn't stay shaving sharp for long), but I figure tha the convex should still be a little more durable.
 
I'd expect the edge retention would be the same until you hit a rock etc.

In fact, the hollow ground blade may continue to cut well after accumulating more wear. Let me elaborate:

For the last several years I've run a job in some large (60 pound) polyethylene parts. 40,000 pound truck loads at a time. I'll bet we've machined a million pounds of these things by now. The majority of the machining is done at 18,000 RPM fed at 1,800 IPM. In layman's terms,"hauling ass". The plastic is soft, and anything less than a sharp cutter leaves a nasty burr.

The material is surprisingly abrasive. (it is carbon black, 100% regrind)

I experimented with cutter materials and geometry to improve cutter life and feed rates (1,800 IPM is double the original goal). And I ended up with custom cutters ground by Data Flute in a relatively low cobalt carbide with extremely acute geometry and hollow grinds. I can put more than 20 miles on a cutter between regrinds. That's about two tons of chips. The interesting thing (and the way it relates to this discussion) is the very fine geometry continued to cut well after accumulating significantly more wear than more conventionally ground cutters. Cutter change outs during a run reduced by an order of magnitude.

Of course there are no rocks in the work piece...
 
Nathan... I suspect that would be due to the approach angle and relief of the hollow grind, right? The polythylene "in the cut" would not be supported by the flat face of a conventional grind. As the flat grind loses its edge, it would start "paddling" through the medium and jumping, while the concave face would still dig in.

Rick
 
james terrio said:
Fair enough, swamp, my biggest concern with cutting grass is that the town board doesn't fine me for letting my lawn turn into a meadow. You golf people are all bonkers... :D
Click to expand...

Fair enough. When I lived on a golf course they sent us a letter saying there were to be no sat dishes visible from the road. We never removed it. Even after installing one in the back because the other one quit working.:D BTW:They send the fine but don't enforce it :thumbup: that's what happens when you live in "eagle harbor country club" haha!
 
You must log in or register to reply here.
Back
Top