hollow vs. flat vs. convex

S

SkaerE

Joined
Sep 9, 2001
Messages
428
ok, i know what they all LOOK like, but im trying to see the benefits of each.

hollow is generally thinner? like a skinning knife? more fragile.

convex is a good tough blade, thicker than the others, not as sharp of an edge, kinda a chopping blade?

flat is a good compromise between the two?

so do i have this down pretty well? anything i missed?

which do you do the most often? how come?
 
I would think the full V is more fragile (if that is the way it should be stated) than hollow. It seems too that the hollow is not thinner than a full V of the same spine width. The hollow (concave) curves outward from the edge while the flat runs straight away toward the spine. I did not do well in geometry class. Am I wrong?

Roger
 
Originally posted by rlinger
I would think the full V is more fragile (if that is the way it should be stated) than hollow.
Click to expand...

Uhm...why? :confused:

There's less material behind a hollow ground blade's edge than behind a flat ground one's. That means less durability, doesn't it?
 
Tim,

Perhaps you're right. I can only go by what I think a curve is compared to a straight line. I see the curve immediately running outward away from the edge and a line running at an angle away from the edge. Isn't the center of the wheel at the edge on a hollow grind? If true does that mean that the rest of the wheel is running outward and away? Am I wrong? I don't know. It just seems to me that a full V flat grind of the same spine thickness can not be thicker at any part away from the edge than a hollow grind. I would be wrong for sure if the center of the wheel is held above the edge, but that would make the edge thicker than the area just above the edge. - ???

Roger
 
Roger,

take a sheet of paper and draw a blade viewed from the tip. Draw it with a flat grind at first. Then draw a concave circle over it. You will notice that the latter leaves less space (which would equal material in a blade) in it.
 
The way you describe it is true. But I do not see a circle in a hollow grind. I see an arc with its deepest part at the edge. I see a straight line intersecting with an arc but not exceeding the arc.

Roger
 
If the straight line doesn't exceed the curved one, the hollow ground blade's spine has to be thicker, which then would be apples and oranges.
 
It is correct, regardless of wheel diameter the spine of the hollow grind can not be as thin as that of the flat V so long as edge thickness of both blades and all other factors are equal. This assumes the edge of blade is held no lower than center of wheel while grinding. The arc is curving more rapidly outward from the edge than the straight line is moving outward from the edge. I don't know.

Roger
 
I think this image will show up. All other things being equal, hollow will have less material left in the blade, then flat, then convex. Pic show flat vs hollow. Hope it shows up.

Steve

attachment.php
 
I make different knives for different purposes. The small knives that are designed for fine cutting are always hollow ground. Knives designed for chopping are either convex or flat ground depending on their length. Longer knives develope more momentum and need to be ground similar to an axe for strength at the cutting edge. Some may not agree, but this is only one mans oppinion.
 
Here is a representation of what I did on a cable damascus bowie.

The specs:

Blade Thickness: 0.30 in
Blade Width: 1.50 in
Hollow Grind: 8 inch contact wheel.

I ground the blade about .75 inches up from the edge and left the edge at about 1/16 inch thich. Here is a diagram of my hollow grind vs. a flat grind:

edge1.jpg


The largest difference between the to grind lines ends up being only 0.0183 inches. Does that amount make a difference in the performance of the blade? I don't know (well it does make a difference, but would it be negligable?). Why do I do hollow grinding? Because that is all I know how to do at this point.

Of course this all depends on the diameter of your contact wheel too.

Hope this helps
 
As Laredo's drawing shows the difference is very subtle. One could also factor in the strength of a triangle vs the strength of an arch. I like hollow grinds for their appearance, but I suppose it is arguable that the very edge is more fragile than the other two grinds. It's thinner cross section allows it to be readily re-sharpened/re-aligned to razor sharp, which is one reason it was used on straight razors.
 
so a convex grind goes completely to the blade's edge while the other 2 have a secondary edge?

is that correct?
 
It looks like that in the drawing, yes. Any of the three grinds however, could be zero ground or have secondary bevels.
 
Loredo7mm,

Your drawing, while impressive, displays what I believe is a proper hollow grind but it does not - I think- display a 'full V flat' grind.
Is it true that a full V flat grind goes all the way to the spine?

It does however, appear that I will have to conceed on your particular drawing. I believe and hope that in my previous posts on this subject I specified 'full V' in comparing the two grinds.

Roger
 
Convex edge not as sharp...What are you kidding me!
Try this.
Using D2 steel in 3/16 or 5/32nds stock.
Start out with a Flat grind to remove half of the bulk.
Then switch to a convex all the way to the edge. Take to a 600 grit or so finish. Put a final edge on it with a 15 micron 3m belt then run the edge so it barely contacts a buffing wheel to knock off the burr then tell me if it's sharp.
If the HT is done right this will be one of the sharpest edges you will see.
 
I in no way claim to be an expert here, but I believe the variations in ideas of flat grind here, at least by some, have it divided into flat grind and saber grind, or so I've seen it explained here before. In these, flat grind was all the way to spine, and saber grind was only partially up blade(so all of my blades, except for one, were actually saber ground, not flat)

Again, just repeating what I"ve read in threads here before
 
I think you're partially correct etp, however it would be possible to do any of the grinds in the sabre style. Benchmade balisongs of the weehawk style are done in both hollow and flat grinds, and both are a sabre grind of sorts. Regarding sharpness, I believe Robert is describing a Moran edge (correct me here if I'm wrong) and they are indeed very sharp. Any of the grinds in question can be very sharp.
 
Boy is this a fun thread to read..

A hollow grind when done properly will have a very thin edge were a convex will have a thicker edge,and a flat grind all the way "V"ed from spine to edge will also have a weak edge but a little stronger than a hollow...The Moran edge as it is called is inded a very strong and usful edge and will get razor sharp and stay that way.Buck had this type edge years ago but people coulden't sharpen them after awhile themselves with out a way of re setting the convex edge.
The moran edge is actually a thicker flat grind at the edge then a convex edge cut in instead of a thin small jumped on edge that is usually hollowed on a wheel.
Now Jerry hossum does a hollow grind on his battle Bowies,but he leaves a tear drop on the edge so it has a moran edge thus it is thicker and not as fragile.It kind of looks like a big fullered out groove in the blade.Now a cutlass grind is not just a half ground blade,it is actually more like a dagger only the back side of the blade isn't all the way to a cutting edge,I show how to do this grind on a Bowie in my tutorials,This was the grind preffered bu Hubber in the old days,It gave less drag when cutting through a object as the center of the blade was the thickest part and not the spine.Also a convex grind is supposed to bee thinner at the back edge than the center of the blade,the whole side of the blade is a convex.
Really in my opinion they all have there place on certain knives,But most people that forge blades will either have a flat grind or a convex grind as that is what they forged in and so why mess up your forging.Now you can also do a hollow like they did in the 1800 on the old classic Bowies,Only thing is you need a 5-6 foot wheel to do this,The blade does have a slight hollow but looks almost flat.
Basically it will depend on how thin you make the section just above the cuting edge as to how weak the edge will be.And if you are just jumping on a edge on a thick blade edge so it has allot of drag when cutting.
I have heard these arguments before,and the best thing I ever heard was if you forge don't hollow grind just keep the flats you put in or convex the whole blade if you foged it that way,leave the hollow grinds for stock removal work as you didn't fight to get the bevels anyway.
Hope this makes sense.
Also this is just my 2 cents worth on the subject.
Bruce
 
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