Cutting plasticsedge geometry and microbevel

C

ColoradoDave

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Dec 28, 2002
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From time-to-time, I've thought it would be interesting to do some informal study of factors affecting cutting performance on different plastics—after all, plastics are pretty much ubiquitous in our modern world, yet I've seen little mention on the part of knife manufacturers or knife users of what might be desirable in a knife that will be used to cut these materials. My interest was further whetted recently by discussions on these forums where people have mentioned that some of the toughest cutting they have to deal with on a day-in, day-out basis are plastic "blister" and "clamshell" packages, with are typically made from PVC or PET.

It had also occurred to me that plastic might be a good medium for measuring sharpness and edge retention when doing knife performance testing, due primarily to its consistency compared to other materials. So, scouring around the workshop and garage the other day, I found a roll of .080" nylon line—the kind used for lawn/garden string trimmers—and recalling that it took a fair amount of force to cut or snip the stuff, decided to see if something simple and workable couldn't be devised.

As it turns out, the nylon line is indeed tough enough to cut that meaningful, repeatable results can be had with even such a crude instrument as a bathroom scale. And in testing this method, I learned some interesting things, a few of which I wasn't even considering at the time.

Before going into specifics, some general results: A freshly sharpened edge with a primary bevel of ~24° included, approximately .020" thick behind the edge and with a light 30° microbevel, was able to push cut the line with about 11-12 pounds of force; the same blade, after significant use and dulling, required 25+ pounds to cut the line. A thinner, hollow-ground blade, again freshly sharpened with ~24° primary and 30° secondary bevels, but only .010" thick behind the edge, required only 8-8.5 pounds of force. New double edged razor blades were found to sever the line with only about 3-4 pounds of force, while new Stanley utility knife blades, rather surprisingly, required from 14 to 16 pounds to cut the nylon line.

From the above, two things seem pretty clear: first, the nylon line appears to be, as was hoped, a sufficiently sensitive and consistent medium for the purpose of testing relative sharpness and edge retention; and, second, even relatively small aspects of edge geometry, back to about .080" or the thickness of the line, can be expected to noticeably affect results, presumably due to the toughness of the line in terms of its relative resistance to compression or stretching.

The most interesting thing observed is the pronounced effect that the angle of an applied microbevel has on the force required to push cut the nylon line. As indicated in [thread=299465]this earlier thread[/thread], I, like others, haven't ordinarily been able to detect much difference in cutting performance when using knives with microbevels of varying acuteness. I still believe that this may well be the case when working with materials that compress or stretch more easily, but it seems that when cutting nylon, and probably other tough plastics as well, this could be a factor to consider.

Following is a detailed description of the tests used to examine the effect of the microbevel, and results of those tests:

Flat ground 1095 steel blade, sharpened at ~24° included (12° per side) using a medium India stone, with thickness behind the primary edge bevel ~.020". Finish was restored between tests #1 through #6 to assure consistency of the microbevel. Microbevel was applied using a fine ceramic V-rod sharpener. Stropping, when performed as noted, was done using CrO on leather.

Actual readings were recorded to the nearest .5 pound, although this likely exceeds the accuracy of the measuring equipment being used. Average force required to push cut the line is the unrounded mathematical mean of 5 test cuts, and despite being reported to one decimal place, below, should not be taken to reflect greater accuracy than the underlying test data.

Test #1: 30° microbevel applied, 6 passes per side
Average force: 11.6 lbs.

Test #2: 34° microbevel applied, 6 passes per side
Average force: 12.2 lbs.

Test #3: 40° microbevel applied, 6 passes per side
Average force: 13.1 lbs.

Test #4: 44° microbevel applied, 6 passes per side
Average force: 14.2 lbs.

Test #5: no microbevel; 24° single bevel edge applied with medium India
Average force: 13.5 lbs.

Test #6: 30° microbevel applied, 1 pass per side
Average force: 12.4 lbs.

Test #7: 30° microbevel applied, 15 passes per side
Average force: 11.5 lbs.

Test #8: 30° microbevel applied, 15 passes per side, stropped
Average force: 11.5 lbs. (very consistent)

Test #9: edge polished and partly convexed on 600 grit paper, 30° microbevel, stropped
Average force: 11 lbs. (very consistent)

I'll leave you all to drawn your own conclusions, and look forward to any comments. I have several other thoughts, and don't know where this all may lead, if anywhere, but I do intend to start using nylon line to measure relative edge retention ... unless someone points out some reason why this idea is misguided.

Dave
 
Thank you Dave! This is great. I think it is particularly interesting to see the increase in performance when you add a 15-degree microbevel that nominally makes the edge contour more obtuse (see tests 5 and 8). I think this is not only a function of overriding imperfections in the 24-degree edge, it is also a function of a finer grit on your ceramic rod. It would be interesting to see what would happen if you polished your 24-degree edge at 12-degrees per side using ceramic rods or you skipped the rods and simply stropped your 24-degree edge.

I'm interested in talking about the underlying phenomena involved. First lets discuss what we know about nylon fiber from our basic experience. Consider monofiliment nylon fishing line and leaders. These are large scale nylon fibers that are familiar. Nylon line will stretch under tension and partially return to its original length (it has stretches very well elastically and plastically). This naturally makes nylon very resistant to breaking even if the line is nicked. Many materials will start to tear or break when incised by a sharp edge. Nylon does not do that. You have to overstress every stinking bond you break in nylon with your edge, not by wedging apart the material. If you have a flat or rough edge you will tend to mash the nylon rather than parting it. Nylon will tell you a lot about how thin and smooth your edge is.

Nylon is pretty slippery stuff so it will not drag too hard on the blade through simple friction. On the other hand it is stiff enough that it is almost impossible to slice through a solid block of nylon. This is because the material behind and outside of the line of your cut will wedge your blade. You won't be able to wedge through the material without using a hammer. You don't see this in nylon polyfiliment rope. As you press your blade into the rope the fibers will first bend rather than split. Your edge will be in a slight indentation with a little bit of lateral pressure, but very minor. The sharper your edge the less the indention and the lower the lateral pressure (wedging). The net result is that you get a slight bit of extra advantage from a sharp edge and a polished blade when you cut nylon cord. This might even be part of the source of your half-pound improvement when you stropped your 30-degree micro bevel.

I think you have found a sensitive measurement that focuses particularly well on the fineness and smoothness of the final edge of the blade. For example you showed how well it differentiates between razor blades (the double edge blade and the utility knife blade).

If you wanted a test that compared the blade as a whole (showed up more blade drag issues) I would suggest cutting garden hose. If you picked an all PVC variety (low friction) the wedging issues of the blade contour would be the most conspicuous factors. If you picked a rubber-based hose (high friction) the actual blade finish would start to come into play. If you used a fiber-reinforced hose you could look at edge durability when faced by hard glass or nylon fibers.

Anyway, thanks again for a thought provoking and informative test.
 
Thanks, Jeff. A little ways into this experiment, I realized there was a *lot* going on; seemed like I had ~3/32" long plastic nubs all over the place before I was done. And there's a lot more still, I'm sure, to this business of cutting plastics, which it sounds like you've given a great deal of consideration.

Something I'll try to get to soon is testing a fully polished single bevel edge, as you suggest. I may also do some testing of more acute microbevels—similar to what we do with planes and chisels, applying a microbevel only a degree or so greater than the primary bevel, which gives nearly all the performance but saves a great deal of work.

I think you have found a sensitive measurement that focuses particularly well on the fineness and smoothness of the final edge of the blade.
Click to expand...
Exactly my thoughts as well. What I visualize happening is that adding the microbevel is producing a smoother, more refined edge, while at the same time making the edge less acute, and effectively producing a very small "shoulder" just behind the edge. With a 24° primary bevel, the decrease in intial edge angle plus the little shoulder created when you add a 40°-44° microbevel becomes enough to offset the benefit of smoothing/ refining the very edge. I have to admit that this surprised me, particularly since I use only a very, very light touch when applying the microbevel with a V-rod sharpener (in fact I have a hard time imagining the many users who seem to use Sharpmakers for anything other than just that final, finishing touch.) But the nylon line is certainly sensitive to it.

What little bit of work I did with stropped edges was interesting as well. For shaving arm hair, the difference between the finish achieved with the ceramic rods alone versus the added step of stropping of course may seem huge, but the main thing I noticed when cutting the nylon line was just that the stropped edge was much more consistent. And I guess this just demonstrates the obvious: that while you perhaps can't improve much on the finish produced by a fine ceramic rod when it comes to cutting a tough .080" diameter fiber, it's a whole different matter when you're trying to smoothly cut a bunch of tough .002"-.003" diameter fibers all at once along the length of a sharpened edge, where consistency becomes paramount.

Something I didn't mention, but that I think is a whole 'nother issue with cutting plastics, is controllability of the cut when working freehand. I've noticed cutting plastic bottles, for example, that beyond just sharpness and raw ability to cut the material, some blades have much better control than others. My experience here has generally been that convex edges provide better control than flat beveled ones; but why this is so, or how you'd go about trying to study/quantify this, I don't know.

I agree, the garden hose cutting experiments would be very interesting ... though it could get somewhat expensive if that became your standard medium for testing edge retention! I am anticipating a move sometime soon, however, hoping to downsize my homeowner's responsibilities in the process, so don't be surprised if I sacrifice a hose or two for the cause.

Again, thanks for the thoughtful comments.

Dave
 
Following up on Jeff's suggestion, I went ahead and put a polished (fine ceramic, lightly stropped) single bevel, 24° included edge on a ~1.5" section of the same test knife. The resulting edge required only 9 pounds of force to push cut the line.

From there I decided it was time to get more of a sense of how this test medium would work for the original purpose I had in mind, which was a reasonably sensitive test of edge retention.

Using thin cardboard stock, 30" was cut with the section of knife just sharpened. This resulted in the force required to cut the nylon line being increased to an average of 10.4 pounds. After cutting another 30" of cardboard, force increased to 11.7 pounds. Another 60" of cutting increased the force required to an average of 12.3 pounds, and an additional 60" after that brought the average reading up to 12.7 pounds.

(Note that I was actually surprised to find that the inexpensive Old Hickory paring knife being used would still "sort of" push cut newspaper ... which mostly leads me to believe that Sierra Nevada Brewing uses a much better grade of cardboard for its 6-packs than does my usual source for cardboard, New Belgium Brewing.) But anyway....

Finally, being uninclined to maintain any knife with a polished, single bevel edge, I decided to reestablish the edge by applying a very acute microbevel, only ~1° greater than the primary bevel of 12° per side. I expected this to be a bit of an awkward process in itself, just because of the inherent error of using a V-rod type sharpener, compounded by the fact of having to shim it on one side, and repeatedly switching the sharpener left-to-right to work the different sides of the blade. Additionally, I anticipated that working at such a close angle to the primary bevel could require far more swipes or passes to establish the secondary bevel than when applying a more obtuse microbevel.

As it turned out, the acute microbevel applied quite easily, with only ten light passes per side producing a crisp, sharp edge. Following a light stropping, the new edge performed as well as the original, 24° polished edge, with an average for five cuts on the nylon line of 8.9 pounds.

That's all I've got for now; my apologies if I've bored anyone with my long-windedness on what might seem like a pretty obscure pursuit. Suggestions and comments welcome.

Dave
 
The biggest problem with using the force to measure edge retention is that the total force is the combination of the force required to actually cut the material (sharpness dependent) and that required to simply smash the material out of the way. In any case where the material is rigid and of any size, the displacement force is many times greater. The effect this has on a measure of edge retention can be best seen with some math.

Let take a blade which requires 10 lbs to make a cut, and we know that one lbs of this is due to sharpness and the rest is due to wedging (you can make estimates on this based on behavior if you extrapolate the finish behavior and geometrical factors).

Now take that blade and blunt it down to 50 %, the total force increases only to 11 lbs, at the point where a difference can be seen, but it takes an average of a few cuts to notice it, and even then its not going to be statisticall significant unless you are on a fine scale (0.1 lbs).

That being said there is a lot which come out from the results :

1) the extent of the material effects which geometrical aspects influence cutting ability, try this with a thicker and thinner piece of similar cord and the results will be of a similar pattern, but different in magnitude, specifically as the cord get smaller the micro bevel influence will grow

2) there is the influence of angle vs finish as Jeff noted, leading for example to highly polished edge being able to give a high level of cutting ability *at the same angle* as more coarse edges, and thus being able to be ground at thicker ones to maintain the same level of cutting ability and thus have a higher level of durability for impact work

(there is a complementary analogue to coarse edges for slicing)

You might also want to record the standard deviations to give some idea of the significance in the values, however repeated measurement like the increasing edge angles allow this on their own. A simple regression for example would show the increase is significant.

Nice work.

-Cliff
 
Cliff—you confirm what I've been thinking, that the amount of force expended in wedging is the biggest drawback to using nylon line to study edge retention, and effectively rules it out as a means by which to compare sharpness between blades. I expect to still give it a try, at least informally, while testing some new knives I'm expecting soon, and may include some thinner nylon monofilament lines in the mix as well.

One thing which still intrigues me is the effect of finish quality on that portion of the bevel(s) not at the very edge, and have a fairly simple test in mind which may shed some light on the extent to which finish influences force being expended in wedging. When I began this testing, my immediate impression was that bevel finish might not be as significant as I had expected; but again, the large force being used in wedging alone was likely obscuring any ability to observe casually such effects.

If nothing else, this has turned out to be a productive exercise in weighing the various factors influencing cutting performance, and has forced me to sit back and consider what tremendous compromises we're forced to make when choosing and sharpening a knife that's going to be used for general utility work, cutting a variety of materials under widely varied circumstances. It seems clear to me at this point that a blade optimized for cutting tough plastics more than a few thousandths thick would be rather poorly suited to most other practical cutting chores, whether around the home and business or in the wilds.

As always, Cliff, your comments and insights are much appreciated.

Dave
 
You could greatly reduce wedging effects if you allow the line to bend freely as you cut. Only support the line on one side so that the unsupported end of the line can bend away from the cut.
 
In the line that Jeff noted, there was a large difference in force required when I switched from cutting rope on a board to rope extended over a gap or edge of a board (to remove the influence of the board). With one side free to just fall away the wedging influence was lowered significantly. Cutting it under tension also vastly reduces it, eliminating it almost completely in high tension.

To clarify, as my above post may appear to be too harsh, any attempt at measuring the sharpness in *ANY* kind of quantitative way is *VASTLY* better than simply qualitative descriptions. It presents a great source of information and there is a lot you can learn from looking at various attributes.

In regards to your question about edge polish (primary bevel), this has been studied on woods and found to be of no significance, as friction is far less critical than deformation. However this is with a straight push, with a slice the more coarse finish drags the material and the influence of friction goes way up.

This property as well, like so many others, depends on the media. Some materials are rather soft but sticky (cheese) and can exert little wedging action, but are powerful friction sources, potatoes are another, which will bind to a blade heavily, while carrots and turnips fall away readily (the influence of sharpness varies among all three as well, it is of almost no consequence on cheese for example, but very significant on vegetables like onions).

One thing I would stress very strongly, for any test of edge retention is *repetition*. There is just so much random influence (initial edge, angle of the cut, media variance) etc., that you really need to repeat things several times before a decent conclusion can be reached with reasonable confidence.

-Cliff
 
Jeff, Cliff—thanks for the excellent suggestion. I've actually had a chance to try it already, and will be implementing this in future tests.

Coincidentally, I had just received two new knives, a Spyderco Calypso Jr. and a Becker BK11, which turned out to illustrate nicely what you've said about reducing wedging by allowing one end of the line to be unsupported when cutting. The Calypso Jr. has the usual Spyderco factory edge—quite sharp, decent-looking to the naked eye—and was found to cut the fully supported line with about 9.5 to 10 pounds of force. The Becker, with an ugly, asymmetrical, and significantly more obtuse edge which had nonetheless been buffed to semi-shaving sharpness from the factory, took about 14 pounds to cut the supported line.

With the end of the line unsupported, the Calypso Jr. (and the paring knife from the above tests as well) requires only 5-5.5 pounds to cut the line; the Becker, about 6-6.5 pounds. These results seem pretty self-explanatory, and would certainly appear to demonstrate the point you've both made about reducing wedging, and thus making the test more indicative of actual edge sharpness and acuity. BTW, I was expecting there to be some real problems with trying to cut the line with one end unsupported, and that results would be inconsistent to the extent that the plane of the cut deviated from the vertical plane of the edge of the underlying support. Not so, and even though the cut ends of the nylon line were uneven and often significantly angled, I found little problem in getting consistent results from cut-to-cut.

To clarify, as my above post may appear to be too harsh, any attempt at measuring the sharpness in *ANY* kind of quantitative way is *VASTLY* better than simply qualitative descriptions. It presents a great source of information and there is a lot you can learn from looking at various attributes.
Click to expand...
I didn't find your original post at all harsh, Cliff. It was already apparent that wedging accounted for a great deal of the force required to sever the line, especially so with the geometry found on most knives. This is a learning experience for me, and part of my reason for posting is to receive constructive comments, and even criticism if needed.

Regarding repetition in testing edge retention, yes, I agree whole-heartedly. In fact this relates to my more general interest in the possibility of using plastic(s) as a test medium, since it could offer a degree of consistency which many other materials do not, thus eliminating one variable from the mix. Compared to recycled cardboard, or worse still, dirty carpet, you'd undoubtedly have to cut a large amount of PVC, ABS, or whatever other plastic in order to achieve significant degradation of an edge—but, on the other hand, how much used carpet would you have to cut to rule out the effect of different levels of embedded dirt and grit on the results? A lot, I would think.

Once again, my thanks for the comments and interesting discussion.

Dave
 
Material variability is a huge factor, carpet it a horrible one, as you can hit a bad patch and the blade goes blunt in one pass. I never felt comfortable with the edge retention work I did on that media unless I averaged five or so trials, and in general really only felt decently solid when they approached ten. Its a nice media for extreme edge holding, but has its drawbacks.

One otherthing to be careful of is contamination, I recently gathered up a nice selection of clean cardboard, sharpened two blades to identical angles, did all the cutting, etc., and found the results to be really unexpected, one blade when blunter much faster. Then noticed I had been put the cardboard across some sharpen grit when I was cutting it, and quite possibly cut into that. Subsequent trials didn't confirm the initial behavior, and were more inline with the expected performance.

The wood working and friction work was done by Dr. Normal Frantz as part of his PhD thesis, he was studing machine driven blades and looking at chip formation, though a lot of the work can be applied to hand held blades. Lee goes into it in some detail in his book on sharpening.

-Cliff
 
A bit off topic, but related to material variability ... I hate to cut any kind of engineered lumber or similar materials with a decent tool, due to all the sand, metal, and other stuff that gets incorporated into these materials at the factory. Worst of all is particle board/underlayment, which I refuse to cut with a power saw. I'm sure everyone's noticed the little sparks that fly when you cut particle board with a circular saw, due to all the sand; I actually saw a sheet with a "Crescent"-type adjustable wrench cast right into it, and another with one of those cast iron drain covers like you find in basement floors. Imagine hitting one of those with a power saw ... ! Recycled cardboard, I'm sure, is made with no greater care, though at least the inclusions are necessarily small.

Seems my reading list is growing (haven't even obtained a copy of Bryson's book yet) along with all the projects and tests I have in mind. This is becoming a very compelling interest and passtime.

Dave
 
Well now you have me thinking of thin, uniform, plastics as a test medium. A while back I suggested to Cliff the possibility of using ribbon instead of the thread that he uses for edge testing. Now I'm thinking that audio recording tape from audio cassettes might serve the purpose well. It is pretty tough and very thin. If you stuck to one brand of blank tape the results would be fairly reproduceable. I would make a loop in the tape secured with a clothes pin (I would wrap some rubber bands around the clothes pin to make it clamp more securely). I would hang the loop over my upward-oriented edge and hang weights on the clothes pin to measure the cutting force. To test a wider section of the edge or to increase the weight range you could use wider recording tape (such as VHS tape).
 
Sounds like an excellent idea, Jeff. Can't imagine you're going to find anything much more consistent than recording tape.

Wonder what your wife's going to think when you ask for a laboratory-grade recording scale for Christmas? <g>

Dave
 
I tried the ribbon, mainly as using thread is problematic because of blunting being so random, you can get a set of number like :

80,140,60,200,150,110,175

along the edge, as some of the spots where the edge cracks away badly score very low. It then become difficult to say which knife had the better edge retention, it forces a lot of measurements.

The problem with ribbons was that the force required was *too* high, even with moderate blunting the force went into pounds readily. Recording tape is a decent idea though.

I would caution though not to abandon the spot testing completely though because the properties of edges which break apart in such a highly random manner are different than those with equal averages but which are smoother in bahavior (ie, 140,150,160,145, etc.), specifically the ones with high variance tend to be more aggressive on a slice due to the highly fractured edges forming serration like effects.

-Cliff
 
I meant to do this awhile ago and forgot, anyway taking the first four points :

30 11.6
34 12.2
40 13.1
44 14.2

You can do a linear regression and get :

slope = 0.18 (2)
intercept = 6.1 (2.2)
r = 0.99

p values are < 0.01, for b and r, ~0.11 for b, t value for (5%) ~ 3.2

Thus calculated values are 0.18 * angle + intercept, which gives :

30 11.6 11.51
34 12.2 12.23
40 13.1 13.32
44 14.2 14.04

Note the high quality of agreement.

With the strongly defined intercept, you could argue that with zero angle (thus no wedging force) the minimal force would be ~6 lbs, which in fact agrees well with the unsupported cutting results.

I actually meant to do this several years ago and thus separate the effect of wedging vs sharpness in terms of total force but never got around to doing cutting at multiple edge angles, thanks for providing the raw data Dave.

-Cliff
 
Cliff&#8212;I'm the one who needs to be thanking you for the follow-up, and turning this informal little experiment into a very informative exercise. (30+ years ago I was a rather decent math student, but today my aging mind was glad to have the numbers provided so that I could simply plot them on a graph in order to visualize. Surprised my bathroom scale-generated data were so close, though once I got the method down, outliers were rare.)

It's interesting how going through a relatively simple exercise like this translates into useful, practical understanding, and in particular has served to underscore the points often made in your reviews and posts about the complex nature of forces involved in cutting various materials. Recently I was cutting some thin cardboard containers. When I ran into a section where two thicknesses were glued together, it struck me that the force required to cut the two bonded layers was exponentially greater than for one layer. In the past, I wouldn't have been thinking in terms of how the greatly increased stiffness of the material was causing a great deal more binding, and that depending upon the geometry and thickness of the blade behind the edge, that effect can be much greater than would be simply cutting two (unbonded) layers of cardboard at once. It really gives a whole new appreciation for the simple box cutter, and why such a blade performs relatively well even after it has become seriously dulled.

Also, something I wanted to mention on the subject of "standardized" test materials ... I don't recall reading anything about the kind of thread you use for testing sharpness (I assume there are good reasons it was chosen) but whenever I think of thread/string, I think of something where the fibrous construction could make it inconsistent when cutting. One possible exception might be fly fishing line backing, which I think is typically made from finely braided Dacron, and is unusually smooth and free of loose/irregular strands. I recently looked at some under magnification, and did some trial cutting on it, and it seems very consistent. Just FYI, in case you're interested.

Again, many thanks.

Dave
 
What I found really interesting was how well the intercept agreed with the theory, it was a pretty far back extrapolation. You can go into a lot of detail with even simple data, science does not require computers and fancy machines, just a willingness to be creative and inquisitive.

On the thread I use a light baisting thread, because it is very thin and thus induces little wedging and thus measure sharpness with as little an influence on geometry as possible (the thicker the code the more you measure geometry and not edge quality).

When I bought it several years ago (I just asked a seamstress to pick out the lightest thread), and bought a bunch of rolls. I then separated a bunch of strands off of each roll and took a test blade and measured the sharpness repeadily with each strand.

I marked small sections of blade and measured the sharpness in each with each strand to check for consistency. What I found was there was no difference among the strands of thread and that I was limited by the precision of the measurement (+/- 5 g).

I then took them into work and used a much more precise force probe to measure the force. I found this wasn't useful because the variability in sharpness along an edge is greater by more than an order of magnitude than the precison of the probe, so you would get data like :

100.1, 110.2,90.3, 115.2, 105.4, etc.

and thus the decimals were useless data. Even on the very best blades I have never seen an edge vary by less than 10 grams along its length, it usually takes ~10 measurements to generate an average with a precision < 10 g, thus I just use a cheap spring scale to measure the force.

Cutting hemp can be problematic though as its really inconsitent, I have seen one roll give 24 lbs then another 38 lbs on the same blade (recut the first one, its 24 lbs still) as one is a lot harder than the other, this is why I usually keep a number of rolls and just average over the lot of them so the average is decently stable.

[for those that are concernes, yes on the thread I examined if the effect of multiple thread cuts was blunting the edge by rerunning with the same cord of thread, there was no correlation between the first and second round, p value >> 0.05]

-Cliff
 
This is coming back to me now&#8212;revisited your "Blade testing methods" page, which makes it clear.

Been sort of swamped with other things as of late, but with some clearing of the decks in sight hope to work with this some more. Will have to see if I can find a suitable little spring scale to round out the equipment here.

Dave
 
They are pretty cheap and quite informative. You can also check sharpness by drawing the blade under a piece of cord which has a specific weight suspended from it, so the cord is under a known amount of tension (just hold it on the other end with your off hand).

I was using 1/4" poly for this for awhile, it was very consistent, but was so wide, that it made readings imprecise. I switched to 48 lbs hemp as its only a couple of mm wide, but after doing a lot of random sampling on it, found it to be far to inconsistent as its a natural fibre and thus can get 2-3 times as thick in places, this just means you have to take more readings and watch the averages for systematic deviations.

Maybe 1/8" poly would be optimal, or look at a ribbon as Jeff suggested. I think I'll try that when I run out of the hemp.

-Cliff
 
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