How to kill your edge fast....

numberthree

numberthree

Joined
Dec 14, 2000
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I'm in the midst of building a recording studio and today was the day I put in the first layer of insulation. I had to cut several of the pieces of fiberglass insulation to make them fit various nooks and crannies, and tried a couple of knives in the process since they were on hand: a CS recon tanto (Carbon V) and an Erikkson Mora 2000 (Sandvik stainless). A couple of short cuts ( < 12")through the seemingly soft insulation rendered both edges completely dull. I was amazed to say the least! I guess glass and steel don't mix ...(duhhh)
 
I have a Cold Steel Bushman that my father borrows to use to cut insulation with! It works quite well. Then again, my belt sander does a quck re-sharpening job too :)
 
Maybe its the way you are cutting or what you expect the blade to do as compared to a utility-style razor blade. I recently insulated a whole small room, ceiling and walls. Required some 48+ individual insulation bats, all of which had to be cut one way or another. Tried a couple of knives including a Dozier K1, my 4" Taz utility (0.1" thick D2), and a 3.5" upswept skinner (1/8" stock). I also used a heavy metal yardstick to press down (compress) the insulation as I cut.

All three knives cut a dozen bats before requiring a resharpening. True, I had to be careful near the end not to tear the paper backing by sawing a little through the paper prior to going back and slicing the insulation, but that didn't happen until near the end of each knife's usefulness...

What did you cut on? I laid each of my bats on a piece of plywood so I wasn't cutting on a concrete floor...
 
A couple of months ago I dropped some Insulation in the attic. I made sure to compress and cut on top of wood. After a handful of cuts my 710 needed a tune up. I was surprised and checked to see if I pulled my cuts over a nail or something. Nope.
Cheers,
Gord
 
Never really thought about it, but I guess that makes sense, as it is fiberglass. I do remember finishing a basement over 10 years ago and using a utility knife to cut insulation, and using a whole pack of blades.


Erik
 
I used a M-16-03Z to cut a few bats (2 or 3) lengthwise at work and it didn't dull the blade much. It would still shave hair afterwards.
 
I assume you were cutting a paper
backing. I'm a printer and cut paper
and cardboard all day, opening packages
and cartons. This seems to dull a knife
very fast...

-Rebus
 
The fast dulling effect of the fiberglass may indicate that I'm not copletely getting rid of the burr on my edges too...
 
There's nothing like a burr to make a seemingly sharp knife go dull in no time flat.

A burrless edge will not feel very sharp, and it won't shave as well as you'd expect (after all, knives have a far worse geometry for shaving than a straight razor, so they SHOULD shave more poorly).

However, if you place your thumb lightly on top of the edge, and push down as though trying to cut your skin (please, show extreme caution if you try this), you will sense the extreme sharpness of the edge. The blade will seem hungry to enter your flesh.

I found this to be the case after using my new Edge Pro for a while. I sharpened my wife's kitchen knives again, where before I was using a Sharpmaker (and I *know* I was leaving micro-burrs on the knives, they were dulling much too quickly). Her comment afterwards was that they didn't feel as sharp! And they didn't! But boy has she noticed the difference in actual use.

Which has made me realize: These aren't shaving blades, people. Whoever taught us that it should bounce hairs off our arms, really just made us favor edges with burrs. Excellent edges show under usage, not the type of trivial tests that knife knuts seem to love. At least, that's been my experience so far.
 
JohnW :

Whoever taught us that it should bounce hairs off our arms, really just made us favor edges with burrs.
Click to expand...

While, burred edges can shave well, not all edges that shave well are burred. As for the usefullness of shaving sharpness, shaving hair is just light precision cutting, a blade that can't shave cleanly can't do a lot of other light push cutting well either, and will in general be out cut by a blade that can shave well at the same grit finish. How much performance is gained is dependent on how much of the performance is influenced by sharpness, so the difference will be greatest the lighter the cutting task.

That being said, burred edges are probably the most common problem in sharpening, especially in the production arena as almost every time I handle a NIB production knife that isn't sharp, all you have to do is give it a light honing to remove the burr and it is near optimal. This is just a case of being sloppy in the sharpening. An edge is formed with a belt and a couple of rough passes are taken with a buffer to clean the edge, if this isn't done right it will leave a burr or round the edge over.

To see the structure of burrs check out Lee's book on sharpening where he shows magnified pictures of sharpened edges at various grit finishes as well as various stages of polish. You can see exactly what a burr is, how it is formed and how to remove it. Once this has been done the cutting performance is still very high, and yes the blade will shave. You can do more reading on this if you browse the wood working groups for information on the "Scary Sharp" method of sharpening. This is basically using sandpaper on glass, in a large number of grits. Again care is taken here to remove the burr, and the edge that results will shave very well.

As for fibreglass insulation, I worked with that lovely material for a few summers quite a few years back and it will dull blades very rapidly. However you do see a lot of edge roll as the material is so hard, on those snap off blades all that it requires to get them back to full sharpness again is a ceramic rod. It is faster to sharpen them with a couple of strokes than to replace the blade.

-Cliff
 
Thanks for clarifying that for me, Cliff. Never cease to learn new things.

I guess what I meant was: because a burr will shave well, people tend to stop sharpening as soon as the blade shaves, not realizes that there's more to be done.
 
Try using your blade on some tar paper...:( The edge on my Spyderco Delica SE didn't last too long. Won't ever do that again....:mad: :mad: :mad:
 
Hello folks,


Yes, fiberglass insulation dulls a steel knife fast, even talonite goes down pretty fast (ok not as fast as steel, but fast enough to need resharpening before I had the ceiling done). Then I found something odd... my buck 110 buckcote.. I tried that one, and the self-sharpening theory of the one-side TI-NI ceramic coating works really well.. You do need to get used to the assymetric edge. But, I can say that for insulation, the buck 110 buckcote keeps cutting approximately 3 x longer then talonite.

Side note.. on cutting meat and skinning, talonite outcuts the buckcote easy by a factor 2.. I cannot explain it, guess buckcote was made to cut insulation :D

Oh.. it also depends greatly on how you are cutting.. I tried two paring knives (excactly the same) in D2, one lasted several meters, one didn't even make one meter.

greetz and take care, bart.
 
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