How do you measure "thickness behind the edge"?

Discuss Spyderco's products and history.
carpdiem
Member
Posts: 182
Joined: Sun May 20, 2007 2:30 pm

How do you measure "thickness behind the edge"?

#1

Post by carpdiem »

I've seen this referenced a bunch, but don't understand it.

Where do you measure?

If you measure right behind the edge, can't you get arbitrarily close to zero?
User avatar
Evil D
Member
Posts: 27147
Joined: Sat Jun 26, 2010 9:48 pm
Location: Northern KY

Re: How do you measure "thickness behind the edge"?

#2

Post by Evil D »

I've always assumed it's measured at the top shoulder of the edge bevel. This way you're measuring the thickness of the blade and not the bevel or edge.
User avatar
The Deacon
Member
Posts: 25717
Joined: Fri Sep 24, 2004 10:33 am
Location: Upstate SC, USA
Contact:

Re: How do you measure "thickness behind the edge"?

#3

Post by The Deacon »

Evil D wrote:I've always assumed it's measured at the top shoulder of the edge bevel. This way you're measuring the thickness of the blade and not the bevel or edge.
That's always been my assumption as well.

I'm also of the opinion that, unless you have access to extremely sophisticated measuring equipment, it's prone to major error since both minor errors in positioning a measuring device at that exact point and minor variations in the bevel width can impact the results.
Paul
My Personal Website ---- Beginners Guide to Spyderco Collecting ---- Spydiewiki
Deplorable :p
WTC # 1458 - 1504 - 1508 - Never Forget, Never Forgive!
User avatar
Evil D
Member
Posts: 27147
Joined: Sat Jun 26, 2010 9:48 pm
Location: Northern KY

Re: How do you measure "thickness behind the edge"?

#4

Post by Evil D »

The Deacon wrote:
Evil D wrote:I've always assumed it's measured at the top shoulder of the edge bevel. This way you're measuring the thickness of the blade and not the bevel or edge.
That's always been my assumption as well.

I'm also of the opinion that, unless you have access to extremely sophisticated measuring equipment, it's prone to major error since both minor errors in positioning a measuring device at that exact point and minor variations in the bevel width can impact the results.

Yeah, and this is exaggerated by whatever sharpening technique is used and how accurately you create the bevel. Best case scenario would be to take several measurements from all along the blade and get an average. Even then when you consider things like distal tapers there's no telling what numbers you'll get unless those grinds are done so accurately that they only effect spine thickness but I doubt it.
User avatar
bearfacedkiller
Member
Posts: 11411
Joined: Sat Jan 04, 2014 1:22 pm
Location: hiding in the woods...

Re: How do you measure "thickness behind the edge"?

#5

Post by bearfacedkiller »

I have seen makers list the thickness behind the edge, thickness 1/4 behind the edge and at the spine. I have always believe that thickness behind the edge is exactly as D described it. It is where the primary grind and bevel meet.
-Darby
sal wrote:Knife afi's are pretty far out, steel junky's more so, but "edge junky's" are just nuts. :p
SpyderEdgeForever wrote: Also, do you think a kangaroo would eat a bowl of spagetti with sauce if someone offered it to them?
MacLaren
Member
Posts: 12573
Joined: Wed Apr 16, 2014 12:59 pm
Location: High in the Blue Ridge of NC

Re: How do you measure "thickness behind the edge"?

#6

Post by MacLaren »

Yeah, Ive got a Bradley Guardian done in FFG, that the maker said is .010 behind th edge. ....but I just dont see it....
carpdiem
Member
Posts: 182
Joined: Sun May 20, 2007 2:30 pm

Re: How do you measure "thickness behind the edge"?

#7

Post by carpdiem »

If you measure at the point where the primary grind and the edge bevel meet, then you have another major problem: the thinner the edge grind, the "thicker" the knife.

For example, take a full flat ground knife with a single edge bevel. Full flat so that the primary grind thickens as you go towards the spine. Now, a 40 degree inclusive edge bevel will meet the primary grind at some point, and have some thickness there. If you compared that knife to an otherwise identical one with a 30 degree edge bevel, the 30 degree edge bevel would meet the primary grind closer to the spine of the knife, thus resulting in a thicker measurement for what should be an obviously sharper knife.

What.
User avatar
HarleyXJGuy
Member
Posts: 1395
Joined: Fri Aug 29, 2014 8:50 pm
Location: Michigan

Re: How do you measure "thickness behind the edge"?

#8

Post by HarleyXJGuy »

Carefully?
On my radar: 110V Military, Police 4 and some sweet Rex 45 Military action.

Newest Spydies: S90v Ti Military, Pacific Salt and a special Kiwi.
User avatar
Evil D
Member
Posts: 27147
Joined: Sat Jun 26, 2010 9:48 pm
Location: Northern KY

Re: How do you measure "thickness behind the edge"?

#9

Post by Evil D »

carpdiem wrote:If you measure at the point where the primary grind and the edge bevel meet, then you have another major problem: the thinner the edge grind, the "thicker" the knife.

For example, take a full flat ground knife with a single edge bevel. Full flat so that the primary grind thickens as you go towards the spine. Now, a 40 degree inclusive edge bevel will meet the primary grind at some point, and have some thickness there. If you compared that knife to an otherwise identical one with a 30 degree edge bevel, the 30 degree edge bevel would meet the primary grind closer to the spine of the knife, thus resulting in a thicker measurement for what should be an obviously sharper knife.

What.

This is true, but the same thing is true if you reverse the numbers and make the edge 60 degrees inclusive and you end up with a tiny little bevel, the edge behind the bevel will be thinner. Still though, aside from comparing two of the same knives that are sharpened in these very different ways, this is still a pretty reliable way to measure a blade and have a mental image of how it's ground. It isn't perfect but it gives you an idea of what the grind is like. It works much better over a large range of different model knives, for example with Spyderco they shoot for 30 inclusive on all their knives, so you can easily compare a Tuff to a Caly 3 and know by looking at the thickness behind the edge that the Tuff is a much thicker blade, even if you had never seen either knife, because you know they're both sharpened to 30 inclusive.

However, if we measure behind the bevel, then how do you measure a scandi or zero grind?
carpdiem
Member
Posts: 182
Joined: Sun May 20, 2007 2:30 pm

Re: How do you measure "thickness behind the edge"?

#10

Post by carpdiem »

So then it's only useful for comparing knives with the same edge geometry?

I have no idea how you'd measure for a scandi or zero grind, which makes it all the more confusing to me when I see discussions that treat width behind the edge as some sort of overall sharpness measure.
User avatar
paladin
Member
Posts: 1932
Joined: Sun Feb 27, 2011 5:51 pm
Location: Hotel Carlton-San Francisco

Re: How do you measure "thickness behind the edge"?

#11

Post by paladin »

you measure the thickness behind the edge with the sliceamommeter...very scientifical
zhyla
Member
Posts: 2190
Joined: Tue Apr 20, 2010 2:12 pm

Re: How do you measure "thickness behind the edge"?

#12

Post by zhyla »

carpdiem wrote:So then it's only useful for comparing knives with the same edge geometry?
No, it's still relevant. It's essentially a measure of how thin the blade was before the secondary bevel was added. When you grind primary bevels onto a knife you typically don't take it down to a sharp edge, you leave a little bit of a flat on the edge. I usually take it down to .005" before adding the secondary bevel. If I instead left .02" on that flat it would be a much thicker wedge to push thru something, regardless of the secondary bevel shape.
User avatar
The Deacon
Member
Posts: 25717
Joined: Fri Sep 24, 2004 10:33 am
Location: Upstate SC, USA
Contact:

Re: How do you measure "thickness behind the edge"?

#13

Post by The Deacon »

I'd say "thickness behind the edge" is one of a number of measurement that are relatively meaningless out of context but, when taken together, can give you a decent idea of a knife's suitability for a specific purpose.
Paul
My Personal Website ---- Beginners Guide to Spyderco Collecting ---- Spydiewiki
Deplorable :p
WTC # 1458 - 1504 - 1508 - Never Forget, Never Forgive!
User avatar
MCM
Member
Posts: 3008
Joined: Sun Jul 27, 2008 8:04 am
Location: Left Field......

Re: How do you measure "thickness behind the edge"?

#14

Post by MCM »

I associate low numbers with slicing.
Agree its not the only measurement that counts, but what really drives it home are Jim Ankersons test results.
It kind of proves what I have always thought all along. Just my 2.
And to answer the question, I use a Dial Caliper. Close enough for my needs. (Read as / easy to see)
:spyder: :eek: :spyder: :eek: :spyder: :eek: :spyder:
More S90v & CF please.......
User avatar
The Deacon
Member
Posts: 25717
Joined: Fri Sep 24, 2004 10:33 am
Location: Upstate SC, USA
Contact:

Re: How do you measure "thickness behind the edge"?

#15

Post by The Deacon »

MCM wrote:I associate low numbers with slicing.
Agree its not the only measurement that counts, but what really drives it home are Jim Ankersons test results.
It kind of proves what I have always thought all along. Just my 2.
And to answer the question, I use a Dial Caliper. Close enough for my needs. (Read as / easy to see)
But slicing what? Thick blades with deep hollow grinds, like a straight razor's, tend to be thin behind the edge. They're great for lopping off hairs but would be a poor choice for slicing cheese or vegetables. At the other extreme, a plane iron is anything but thin behind the edge, and yet it can shave paper thin slices off a block of hardwood.
Paul
My Personal Website ---- Beginners Guide to Spyderco Collecting ---- Spydiewiki
Deplorable :p
WTC # 1458 - 1504 - 1508 - Never Forget, Never Forgive!
User avatar
Ankerson
Member
Posts: 6917
Joined: Sat Jul 24, 2010 1:23 pm
Location: Raleigh, NC

Re: How do you measure "thickness behind the edge"?

#16

Post by Ankerson »

Cut stuff Kinda like this. :)

.006" behind the edge, 15 DPS, 400 grit SIC.



https://youtu.be/K_R1yxULRuQ

https://youtu.be/o1QSk-fcHbg
User avatar
Ankerson
Member
Posts: 6917
Joined: Sat Jul 24, 2010 1:23 pm
Location: Raleigh, NC

Re: How do you measure "thickness behind the edge"?

#17

Post by Ankerson »

Just a few quick measurements that I took with my Micrometer to put things into perspective. :)

20 LB Printer paper - .0035"

Phone Book Paper - .002"

Double edged razor blade - .003"
User avatar
MCM
Member
Posts: 3008
Joined: Sun Jul 27, 2008 8:04 am
Location: Left Field......

Re: How do you measure "thickness behind the edge"?

#18

Post by MCM »

Sharpening Plane Irons
https://youtu.be/VOvqbrdNZBU" onclick="window.open(this.href);return false;

I guess I never thought much about planes as they slice thin material...........
:spyder: :eek: :spyder: :eek: :spyder: :eek: :spyder:
More S90v & CF please.......
Cliff Stamp
Member
Posts: 3852
Joined: Sat Dec 31, 2005 2:23 pm
Location: Earth
Contact:

Re: How do you measure "thickness behind the edge"?

#19

Post by Cliff Stamp »

It is just the transition point where the edge bevel meets the blade bevel and it is only one measurement of the blade geometry, it doesn't describe the entire geometry. For some cutting it isn't even the critical aspect.

As the force on a knife decreases away from the apex and falls off rapidly to zero when looking at the knife you work backwards from the apex :

-sharpness of the apex
-angle of the apex
-thickness of the apex
-angle of the edge
-thickness of the edge
-angle of the blade bevel
-thickness of the blade bevel

On some knives there is only one bevel so it is simplified, however most knives tend to be multi-bevel grinds. In general, as cutting gets deeper the material "sees" more of the knife and so the measurements back from the apex are important. If cutting is very shallow then the measurements closer to the apex are more important.

For example which one of these cuts wood better (in slicing) :

-Typical Mora, 1/16" thick, one bevel ~11 dps, no apex bevel

or

-Ka-Bar, 5/32" thick, sabre grind/9 dps, edge bevel 0.045"/17 dps, apex bevel, 0.001"/20 dps

The answer is that it depends. If you do light carving so the cutting is shallow then the Mora outperforms the Ka-bar by ~2:1 because the main influence is the edge angle and the angle ration is ~2:1. However if you ramp up the force then the wood sees much more of the knife and the performance starts to equalize and at very high force (~50+ lbs) they are not significantly different.

If you have done a little high school physics then do a work integral where the force at a given height is proportional to the square of the displacement and it falls off with a simple linear function with a bound of zero at the top contact point. If you play around with these equations in excel it can do the integrals for you (just do a point approximation/sum) and you can look at how the cutting ability is influenced by moving around the angles/thicknesses.

Just be aware that these two approximations (force is proportional to the displacement, falls off linear to zero at the top contact point) vary from one material to another. Not all materials are well approximated by them but it works well for any material which tends to split/open as you cut it.
carpdiem
Member
Posts: 182
Joined: Sun May 20, 2007 2:30 pm

Re: How do you measure "thickness behind the edge"?

#20

Post by carpdiem »

Cliff Stamp wrote:... Just be aware that these two approximations (force is proportional to the displacement, falls off linear to zero at the top contact point) vary from one material to another. Not all materials are well approximated by them but it works well for any material which tends to split/open as you cut it.
Thanks for the very detailed explanation. As it turns out, I'm actually a physicist by background, so this approach and level of detail is greatly appreciated and makes perfect sense.

One more question, though, Cliff: do you have any good sources or papers on the cutting force approximation you described? I'd love to get even further into the details.
Post Reply