Hollow grinding theory

L

lel

Joined
Dec 26, 2006
Messages
139
I never tried hollow grinding myself yet, but being an engineer I decided to do some research and tried to calculate how high the grind line can go with different wheel diameters and stock thickness. I draw the picture, got the formula, entered it to Excel and was amazed by the small numbers I was getting. I thought that something was wrong with my formula so I searched the web. What I found was this calculator based on Rob Frink calculations, which gave me exactly the same numbers I was getting in Excel. Thus if I use 1/8" stock and leave 0.040" for the edge then on 10" wheel the grind line will be just 0.65". Even increasing wheel size does not make a big difference.
So my question is to the makers who practice hollow grinding instead of doing the calculations :jerkit: like I do :D. Do you only use relatively thick stock - 3/16" or even 1/4" when you hollow grind or the edge is not the thinnest part on your knives? Is 1/8" stock non practical at all for hollow grinding unless it's a really small knife?

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Sergiy
 
The blade does not have to sit on one spot on the wheel. You can widen the grind by hitting the top and bottom surfaces of the wheel using a rocking motion. However, you failed to mention the width of the steel used. On a small sized blade .65 with a lil extra room for the edge may not be so bad at all.
 
As murdok pointed out, a 5/8" high grind line on a blade won't be all that bad if the blade is of a smaller size.

Most hollow grinding is on thicker than .125" stock.

As murdok said, the grind can be pushed up higher. Made the first cut near the edge,and walk it up the blade toward the spine in a series of parallel hollows. Even out the curve by eye. It sounds harder than it is.

The tangent of the grind arc should not be exactly at the edge. It can be quite a bit above it ,depending on the blade thickness and style. This will be adjusted in the final edge grinding after HT. The final cross section should be a radius leading to a flat that drops at an angle to the edge.

For a larger hollow grind ,you will need 14" and 16" wheels. The old Sheffield grinders used stone wheels up to 60" to get their hollows in. Some makers make a curved platen that simulates a large wheel.

Stacy
 
I've been making knives for just over 4 years now. The biggest leap on the learning curve for me happened when I stated making the Loveless Style Drop Point Hunter. I learned more than I can begin to tell you guys trying to perfect every aspect of Bad Bobs knife design. IMHO, you can learn almost everything needed to be a good fixed blade, stock removal, knifemaker, by perfecting the skills needed to build that knife alone......... again, IMHO ;) . IMHO ............

I quickly found that to create the deep hollow look he's famous for (on knives similar to that one) requires at least 3/16" stock. Anything less, if you grind to the spine of a knife and carry the hollow tangent to the edge and you'll grind right through to the other side. To work correctly assumes a blade thickness of .187, a width of ~ .920, using a 10" diameter wheel and a finish ground edge thickness of ~ .014.

You can "cheat" using thinner stock, by blending the hollow out beyond the true diameter of your wheel, or just use a large diameter wheel. I also have a 14" dia. wheel which I use quite often. You are not however going to get that deep look that Loveless does so beautifully :thumbup:

I've heard a lot of guys say, "3/16" stock is much to thick for a slicer"
Consider though, the Loveless Style DPH made to his specifications is ground to the spine and has a tapered tang that ends up being ~ .030-.050 at the back of the taper. That being said, the only part of the knife that remains at 3/16" is the area from the back of the guard, forward through the ricasso and a very small ammount of the spine. It results in a knife that, IMHO has a perfect feel. Hands down my favorite hunting knife design, followed very closely by the Herron model # 7.

Loveless Style Drop Point Hunter showing deep hollow grind ..........

SDPH4.jpg


SDPH3.jpg


Herron Model # 7 Style Hunter showing deep hollow grind .............

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Your thinking is spot on lel :thumbup: FWIW, I have a very high end custom skinning knife from another maker that is ground just as you describe. The thickness at the edge being ~ .035 :( and the thinnest part of the grind being about .100-.150 up from the edge. The only reasoning I can come up with for geometry like that is to reduce drag as you get deeper into a cut. I purchased it to skin Beaver (S60V) and since I clean skin them the theory behind the design does little for my skinning technique, in that I use only the fine edge. That knife now sits in my collection. Knives I have made since then work much better for me. A perfect example of "Form Follows Function"

I would like to be very clear about everything I've said being my humble opinion, OMMV ...................... :) What works for me, may not work for you.


:cool:
 
I mostly hollow grind all my blades. Like Stacy said just start the grind at the edge and work it toward the spine. I use any where from a 5" wheel to a 14" wheel. I seen the calcuator but have never used it. I like a smaller wheel because it leaves a sharper ridge especially for double edged blades.

This knife is done on a 5" wheel starting at the edge and flat grinding my way all the way to the top:
gamblersdagger.jpg
 
bladsmth said:
As murdok pointed out, a 5/8" high grind line on a blade won't be all that bad if the blade is of a smaller size.

Most hollow grinding is on thicker than .125" stock.

As murdok said, the grind can be pushed up higher. Made the first cut near the edge,and walk it up the blade toward the spine in a series of parallel hollows. Even out the curve by eye. It sounds harder than it is.

The tangent of the grind arc should not be exactly at the edge. It can be quite a bit above it ,depending on the blade thickness and style. This will be adjusted in the final edge grinding after HT. The final cross section should be a radius leading to a flat that drops at an angle to the edge.

For a larger hollow grind ,you will need 14" and 16" wheels. The old Sheffield grinders used stone wheels up to 60" to get their hollows in. Some makers make a curved platen that simulates a large wheel.

Stacy
Click to expand...

Being kind of new to knifemaking (compared to so many others here) could you please explain the theory behind your statement on a hollow ground knife edge, " The tangent of the grind arc should not be exactly at the edge " ?

Edited to add : Are you talking about, "shouldn't be tangent to" the cutting edge or................ "shouldn't be tangent to" the edge of the grind, which is what I'm talking about.

Farmer Phil said it best "Fascinating. I learn more every day."


And Dayum Bruce :eek: :thumbup: :thumbup: :thumbup: :thumbup: :thumbup: :thumbup: :thumbup: :thumbup:

That knife is BAD ASS !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I'm using all but one of my allotted smileys up there.

I want to be just like you when I grow up !!!!!!!!!!!! ;)
 
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I think Stacey is saying that, if the knife blade is held edge down, the tangent to the hollow that is perfectly vertical should not be at the edge.

I did not know this. I always thought that it should be. This is a very interesting thread.
 
Thank you, guys for very useful information. I see now that hollow grinding is much more complicated than I imagined and the possibilities are not covered by the numbers the calculator shows.
David, I checked your website. I liked your knives and I see that you taper the tangs on many of them. I am trying to make my first tapered tang knife and found that I did something wrong. The tapering I did has a convex shape. I did not do so intentionally, just happened that way, still thinking how will I fit the scales on that tang... I have a couple questions.
Is whole tapered area on your knifes flat? Where does tapering start, right after the guard? Do you use some kind of jig to taper flat and evenly or have some tips how to do it right?
 
T. Hendrickson said:
I think Stacey is saying that, if the knife blade is held edge down, the tangent to the hollow that is perfectly vertical should not be at the edge.
I did not know this. I always thought that it should be. This is a very interesting thread.
Click to expand...

Then I agree with the expert Mr. Hendrickson, and Thank you much for clarifying that ;) :thumbup:
Essentially the short "vertical" you speak of, (one on each side of the blade), or the tangent I speak of, should end up leaving a .010 -.015 land before sharpening (or more,depending on what the knifes intended purpose is).

Wez, thats me (always learning by doing .............. and never giving up ) ;)



:cool:
 
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lel said:
Thank you, guys for very useful information. I see now that hollow grinding is much more complicated than I imagined and the possibilities are not covered by the numbers the calculator shows.
David, I checked your website. I liked your knives and I see that you taper the tangs on many of them. I am trying to make my first tapered tang knife and found that I did something wrong. The tapering I did has a convex shape. I did not do so intentionally, just happened that way, still thinking how will I fit the scales on that tang... I have a couple questions.
Is whole tapered area on your knifes flat? Where does tapering start, right after the guard? Do you use some kind of jig to taper flat and evenly or have some tips how to do it right?
Click to expand...

Thank you very much for the compliment lel.

I taper my tangs up to the back of where the guard slides on (on guarded knives)
On knives that don't have a guard I taper the tang right up to, and sometimes through, the ricasso area.

The easiest way for me to do the taper is to layout 4 parallel lines on the very back edge of the tang. I use a surface plate and height gauge and work from the centerline of the blade as it lays on the surface plate. The 2 inner lines are where the finished taper(or flats) will finish at. The 2 outer lines are where I grind the hollow to, using the same wheel (10" dia.) as I grind my bevels with.
I grind the hollow down the centerline of the width on each side, removing material from near the back edge of the guard, right through the back of the tang, working at an angle of course, until I reach my 2 outer lines on each side of the tang/handle.

Next I switch to my flat platen and using an old wine cork to apply pressure I grind flats until I hit my 2 inner lines that I layed out. I apply pressure accordingly with the cork, to the back of the handle, the middle, or near the guard area until I get the flats positioned correctly.

The hollow grinding done first is not a necessity. It just removes the majority of material, making it 10 times easier, with much less heat when doing the flats, than if I were to start without the hollow grinding. Making any sense ? ;)

Here's a few pics of tapered tangs, from the side and from the top.

The last pic shows me grinding the taper tang flats on my flat platen using the cork.

I hope I helped you rather than confusing you more :o :o

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taperingtang.jpg
 
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Thanks, David. Now I see what I did wrong and at least understand how to do it right. I have also found several tang tapering tutorials. They all suggest to do tapering before grinding the bevels on the blade. Do you also taper before grinding the bevels?
 
I love the look of a hallow grind. here are some of my favorite shots of mine

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i have dont most of my hollows one a 2 inch wheel the deker being a nice guy let me work with his 10 inch and i need one now to do some of my less hollow stuff and i also have 2 plattens that are arced one at 4 foot and the other at 8

the 4 and 8 are for traditional Japanese kitchen knives
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and here are a few shots of the razors they are around 3/16 -1/4 thick spine and edge thickness ie under .007
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DSC05449sm.jpg
 
butcher_block said:
i also have 2 plattens that are arced one at 4 foot and the other at 8
]
Click to expand...

LOVE the blades in your post, Butch... you an' me gonna hafta talk razors soon, it looks like. I've got a batch of 5 I'm working on now...


Any chance you can post some pics of your platens? I'd like to see what they look like...
 
JTknives said:
I love the look of a hallow grind. here are some of my favorite shots of mine
Click to expand...


I don't think you should be able to talk about it until you can spell it right, Jared...:p


...are there knives in your post? I'm having trouble seeing past the curves.;)
 
Matthew Gregory said:
LOVE the blades in your post, Butch... you an' me gonna hafta talk razors soon, it looks like. I've got a batch of 5 I'm working on now...


Any chance you can post some pics of your platens? I'd like to see what they look like...
Click to expand...

i took a pick the other day i ll have ot load it up (it was more proof of concept )

the razors hell drop me a line any time that goes for just abo9ut any one
 
If the tangent is not at, or very near the edge on a hollow gring, doesn't that cancel out the whole theory of the hollow being stronger than the flat....?
 
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