Hollow Grind vs. Flat Grind Chopper Blades

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Verdaccio

Verdaccio

MGF, Master Grumble Framer
Joined
Jan 22, 2007
Posts
757
Loc
Berthoud, Colorado
Okay I have just discovered that the company sharpening my chopper blades is flat grinding them. I gave them the instructions for hollow grind when I first contracted them and the first time they came back, they were indeed hollow ground.

The company seems to be flat/chisel grinding them now. I want to be pissed...but...

They cut just great! Blades hold sharpness for up to 3 weeks, cut really clean. I am not noticing a difference in performance - if anything they cut better than when the blades were brand spankin' new.

So...aside from having a talk with them about hollow vs. flat/chisel, I thought I would open that contentious topic up here and see if folks have any insights.

Chopper design is like 100 years old, right? So that leads to some questions...

  1. What is the purpose to hollow grind?
  2. Was it simply the "in" thing to hollow grind back then and nobody has thought to change it?
  3. Is there an advantage in hollow grinding over flat/chisel grinding in how the blade cuts, lasts long-term, # of cuts, etc.?
  4. Am I damaging my blades if I continue to have them flat ground by this company?
 
I think we framers have sounded off on this 100 times before. Its well documented in the archives. I would be interested in what your sharpening guys say about the topic.

My experiance was the same as yours. Any measurable difference was overcome by price. My sharpening guy showed me exactly how to flat and how to hollow grind the blades. Hollow grinding is much more expensive becuse it wears his disk (if thats what its called) out much quicker because it only uses the edge of the disk. Flat grinding them allows him to use a much wider area of the disk and therefore is much cheaper. After doing it both way I was unable to tell any difference except my checking account. I don't think you are doing any damage at all.
 
Our sharpener convinced me to try the flat grind and I have been using it since..the blades actually cut better and last longer now than they did with the hollow grind.
 
Two points I didn't notice in the archives...

1. A single switch from one type of grind to the other removes as much metal as several grinds of the same type.

2. My pathetic Lion Miter knock-off blades gave me a lot more useful cuts between sharpenings when flat ground. Crummy blades should be flat ground only, then tossed in the trash.
 
I tend to be obsessive about sharpening blades (I Spyderco our kitchen knives about every two weeks – it’s good therapy).

I noticed that Quality Saw and Knife returned my Morsø blades chisel ground rather than hollow grounds a few sharpenings ago. Like you, I wanted to be upset, but, like you, noticed that the cuts were jes’ fine.

Theoretically, a hollow ground edge should allow for a cleaner cut. Hollow ground will be a bit sharper since the blade is thinner at the edge. However, because of the thinness of the edge, it is more likely to “roll over” after a comparatively short use and need more frequent honing. Chopping especially hard woods is more likely to hasten the need for sharpening.

A chisel ground edge should hold up longer and need less frequent sharpening.

Chisel ground works for me.
 
The company I use for sharpening use the chisel-ground method for yrs. without problem. But the last time I had them sharpened, for some reason, it seems about 1/8" or more of the blade was ground off. The set of blades was about 3 yrs old and we get them sharpened every 2-3 months. Now they don't cut all the way through any moulding unless we turn the "step'' upside down for the last cut. Kind of a pain. Bought brand new blades, they cut great, but I'm leery of sending them to the sharpeners ... What's the usual "lifetime" of chopper blades?
 
You can adjust it to cut deeper if your blades are shallow. However if the taper starts before where the metal head holds the blade, wood will hang up in there. I have one set that is doing that now. I would panic but I think I have a new ,to me, set or two on the way.

If those blades were new and you have only sharpened them 15ish times, it sounds like they are being a little agressive with the sharpening.
 
Jay H said:
YHowever if the taper starts before where the metal head holds the blade, wood will hang up in there.
Click to expand...

That tends to happen right at the apex of the knife joint because the casting is rounded over there. We gained several sharpenings by grinding that back so it came to more of a point.
 
Hey David. Thats an excelent idea. I may try that next time I change the blades. It is really thick right there at the bottom. I don't guess it needs to be. Does that make chopping big profiles easier, even with tall/new blades? I have a tendacy to take to big chops. It seems to cut down to that head then really bind up. I can see where if that were pointed, it would cut better.
 
I don't think it will help on taller blades, as the material will already be pushed away (although maybe not as much with flat ground blades as hollow ground :-P).
 
TomDrama said:
… Now they don't cut all the way through any moulding unless we turn the "step'' upside down for the last cut.

What's the usual "lifetime" of chopper blades?
Click to expand...

I have two sets and they’re still going stong after 22 years.

Tom,

If your blades don’t go all the way down, they can be adjusted for a greater “throw”.

Morso(back).jpg


Loosen the top nut a few turns where the vertical rod joins the yoke. Then turn the bottom nut counterclockwise a few turns. This will pull down the blades.

You’ll have to loosen/tighten a few times until the blades just barely enter the “V” on the table, but once you find the optimal “throw”, the blades will be okay for a few more years (or until some sharpener gets grind happy with the blades again).
 
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