initial sharpness and edge profile effect on cutting lifetime

[Cliff Stamp]

I am posting this here instead of the reviews form as it isn't a review of a knife and secondly because many of the threads here on detailed discussions of such issues include posts which dismiss the arguements as if the level of performance was not significantly different. As an example of the effect of edge optomizion consider the following :

This is work done on a no-name "stainless steel" knife :

http://www.bladeforums.com/forums/showthread.php?t=422935

Background info on the graphs and the numbers :

http://www.physics.mun.ca/~sstamp/knives/model.html

The initial edge was very obtuse at about 21 degrees per side, after reprofiling it was reduced to 14 degrees per side, thus the initial angle was 50% more obtuse. This has a profound effect, producing a cutting advantage of more than five, which is how much more material it can cut before blunting to a given state. Joe Talmadge noted similar many on one behavior with his post on optomizing the Axis some years back.

As an illustration to general use; with the initial profile and sharpened to shaving finish, the knife could slice 3/8" hemp on a 3 cm draw with 20.0 (5) lbs. After the edge adjustment it cut the 3/8" hemp again through 3m with 16.5 (5) lbs after cutting 45 m of 1/8" ridged cardboard. So not only does it have a dramatic increase in initial cutting abilility the increase in edge retention makes an extreme effect on the cutting lifetime.

Note further that this profile is not as of yet optomized for cardboard in either angle or grit. This steel also doesn't have the ideal characteristics for such work and thus the geometry limits will be lower than more suitable steels. However, to clearify, this knife, with this "low end" steel, will dramatically outperform many knives in "superior" steels if the edge profile and grit are more optomized.

The above data is only an average (median) of 4 and 3 runs for the initial and adjusted geometry which is why the data is still a bit noisy and the uncertainty in the fit results a bit large. However it is still very clear that the nature as well as the rate of blunting has changed. I am still not happy with the initial sharpness of the knife so I think it can still be improved.

It no longer has the chunky feeling it did initially, but as of yet I have been unable to get it to shave cleanly above the skin or move it out past an inch on push cutting newsprint. It will do most tests of sharpness like fillet a hair, cut letters off of newsprint, shave smoothly on the skin with no draw, etc. . But it won't pass the really high tests like Landes no-tension hair cutting.

The main issue it removing the burr, it is one of the most demanding steels I have seen. On a curious note, and something which I don't understand fully yet, if you strop on plain leather after honing, the edge blunts quickly and stops shaving. A few passes can clean the edge, but going up to 10 passes per side reduces the sharpness quickly. It would seem odd for this to be an abrasive effect, as it continues to shave after about 40 cuts through the cardboard.

The cutting average calculated in the above was an average from 15-40 of the intersect function. Below 15 m the function basically has a veritical asymptote because the stock blade didn't reach that level of sharpness and the adjusted blade takes about that long before it is cuts the same as the stock blade when fresh.

-Cliff

 

[Ben Dover]

I'm not sure what you said, but it sure makes me want to learn a heck of a lot more!:jerkit:

 

[shecky]

I bought a couple of folders from the local $.99 Store, and the infamous $.97 Ozark Trail folder. All suffered from the same problem: obtuse rolled edge. The biggest problem with them is the overall weak construction. One $.99 Store knife was a linerlock with a scary-flimsy lock. The other was identical to the Walmart Ozark Trail, with an added patriotic graphic on the handle. Pivots were a bit loose on all, the lockbacks have some play but turned out to be secure enough for someone who uses a knife like my Mom. The handles are easy enough to destroy with minor abuse. I managed to destroy one by using the knife to pry some bark off a log.

The blades on all were actually not too bad, particularly if one spends some effort to get rid of the burr. All seemed to be the same blade design, with partially serrated pattern. The linerlock blade was marked "440", the others simply "Stainless". The only one still intact is the Walmart one, the others either destroyed or disassembled/parts lost. I honestly don't think the lockbacks were all that bad, even with those awful serrations.

 

[sprokker]

Thanks Cliff, it's nice to see some quantitative results backing up the qualitative experience I think we all have with edge geometry.

 

[Cliff Stamp]

I managed to destroy one by using the knife to pry some bark off a log.

Yes, I could pop this one apart similar, I noticed the same behavior in bark removal awhile back and had to pinch the blade tight around the pivot. The initial edge really is a huge drawback, you have to cut that off, remove the frequently damaged metal and then apply a solid sharpening.

I am still trying to get this one to shave above the skin. I regound it to 9/10 degrees per side last night and with an x-coarse DMT finish I could get it to catch hair above the skin on a slice, which is pretty close and push cut newsprint at 1.5".

I'm not sure what you said ...

Essentially :

With the stock edge angle sharpened to a 600 DMT finish the knife sliced 3/8" hemp with 20.5 (5) lbs. With the reduced edge angle it needed only 11.5 (5) lbs, which shows the much higher level of cutting ability. Now most people know this, now anyway, at one point this was also highly contended.

However what a lot of people argued for a long time, and some still do is that at the lower edge angle while yes you may have higher initial cutting ability the edge retention is lower so you quickly end up with the knife cutting worse than if the edge angle was higher. This isn't true. After cutting near 45 m of 1/8" ridged cardboard the knife still only took 16.5 (5) lbs to slice the hemp and was thus still slicing the rope better than it did with the initial higher angle which was freshly sharpened and unused.

...it's nice to see some quantitative results ...

Yes, the above is still a bit rough in terms of analysis methods though. I figured out last night how to improve greatly the cutting advantage uncertainty which I'll implement on the next data set.

-Cliff

 

[HoB]

Or to say it in one sentence: With a more accute edge/blade profile not only do you increase initial sharpness (right after honing) but you also increase edge retention *significantly*!

The only caveat here is, that blunting was through a soft and abrasive medium (cardboard).

 

[Cliff Stamp]

Yes. It is important to note at some point you will see the performance reverse as the cross section becomes too low. These cross sections are however very low. I will be adding some data of this nature shortly. For the harder and stronger cutlery steels this limit will of course be much higher than it is for this steel, which as Joe has noted on many occasions is the real way to separate which steel is superior.

Yes this media is quite soft, however the same is seen on harder materials. For example, I have adjusted the angles on many chopping blades and have seen the same thing. The performance is often very dramatic with the initial very obtuse edge even denting so it can be seen by eye by after reducing the edge angle and applying a large relief this is prevented and the edge retention much higher.

Specifically for example I recently compared a reground Fiskars to the Wildlife, and the Fiskars stayed with the Wildlife for 1000 chops into various semi-seasoned woods before it start to be difficult to slice newsprint. With the initial edge geometry the axe had trouble slicing newsprint after a quarter of such work.

-Cliff

 

[Cliff Stamp]

I ran another trial with the angle further reduced to nine degrees and refined the sharpening technique significantly. The final runs were using the following :

-form the primary on 200 SiC
-polish up to 0.5 micron
-sharpen at 600 DMT really light

The 600 DMT was a secondary bevel at about 11-12 on a 9-10 degree primary and took 100 feather light passes. This takes about 5-10 passes on a quality steel which doesn't need the primary polished either. The no-name can now push cut newsprint far past an inch and that and the shaving ability could be significantly improved with a higher polish. I was just using the 600 DMT for a slicing edge retention trial.

To polish the primary takes about 100 passes per side on the 200 grit, 200 with an 800, similar with a 1000, and again with a 4000. These are all small stones, 1"x4". This is only about five minutes and doesn't need to be ultra careful. I find waterstones fairly forgiving anyway. At each stage I check the edge under magnifiction to make sure it is even and the micro-serrations minimized.

The reason this is necessary is because you can't use much force with the 600 DMT or it burrs. You can't remove any large scratch or missing edge piece as that would take forever since the force is so low. This allows sharpening from start to finish in about 7-8 minutes. This is still really slow but it was taking me 15-20 minutes a week or so ago.

Using this I was able to increase the initial sharpness to 0.23 (1) cm on the light cord, significantly improved but I think optimal is much higher. I did four runs with the reduced edge angle and on the last two runs I was able to get about half of the edge to cut at 0.010-0.015 cm. This lack of consistency may simply be a limitation of the steel because it simply would not actually get any sharper with time on the stone and I was able to eliminate all traces of a deformation burr.

I think what was happening is that there was a fracture burr problem because even moving up to 1200 DMT just to check didn't actually increase the arm shaving ability. In any case though, with the average sharpness of about a quarter of a cm the knife was starting to be able to shave above the skin on a slice which isn't bad. Here are the results of the new run with the existing data :

The reduced angle maintains a higher sharpness for an extended period of time. However after about 5 m it starts to fall behind the 14 degree edge but is still well above the stock profile. At the end, it actually slices 3/8" hemp with the same force as the stock profile does when freshly sharpened, about 20 lbs showing the extreme influence on cutting ability of a relief grind. To really show the complete picture you could cut a binding material to show the effect on cutting ability alongside the light cord, I have done this in the past, but not in as much detail.

From the above it seems clear that a nine degree edge is too low for this knife for optimal edge retention on cardboard. This is clear when you look at the edge under magnification because it is more rippled. You can tell this without careful measurement because at 15 degrees it retains shaving ability for an extended period of time but at about nine degrees it starts to deform faster and by about 5 meters you can see by eye visible deformation.

It would seem to imply that there is a point between 14 and 9 degrees which would be optimal for edge retention and that would be the suggest best final edge angle. Of course this doesn't mean the entire bevel gets this angle, just the micro-bevel. I would estimate that a close to optimal profile would be full flat so a primary edge of about 5 degrees with a small microbevel at 12-14 degrees. This would also offer a massive increase in cutting ability due to the extensive relief of the 5 degree bevel.

I was planning to illustrate this with another run using a micro-bevel but ran out of cardboard. I may do this later on with another knife with a stronger and finer carbide steel. It would be expected that the minimal angle tolerance would be much higher with the stronger and finer steel as in general I consider 10 degrees to be fairly obtuse and generally run that on the heavier work utility knives (non-metals).

-Cliff