When did the "push cut" become the "Gold Standard" of sharpness?

[marcangel]

I've noticed ALOT of discussion about "push cutting" newspaper, toilet paper and everything under the sun as the de facto test of blade sharpness lately. To be perfectly honest, I just don't understand what this is all about, and have found my "user" knives to be considered pretty freakin' sharp by almost anyones standards, but few are great "push cutters". I took it upon myself to reprofile my ZDP-189 Leek (I got one of the first batch) with diamond through ultrafine sharpmaker rods and it's certainly shaving sharp and cuts toilet tissue effortlessly with a pull cut, but not a push cut. Personally, I don't think the blade geometry of the Leek is well suited for push cutting, and unless I'm using a big ass Chef knife, I don't "push cut" anything so who really cares about this?

Is push cutting a function of sharpness or edge geometry or both? Is it some silly mojo test cooked up by someone with too much time on their hands? Is it a reliable test for only certain applications? Someone, please enlighten me.

 

[Cliff Stamp]

Push cutting is generally more sensitive, it takes a much higher sharpness to push cut newsprint than to slice it cleanly for example. Even very aggressive slicing edges from x-coarse DMT or 90 grit AO can still push cut newsprint when properly sharpened. For these types of sharpness measurements the blade geometry is fairly insensitive because the paper is only checking a tiny part of the blade.

The benefit is highe rinitial cutting ability and greater edge retention. I have done cardboard trials for example with fairly "low end" blades where they were still clearly slicing newsprint after 50 meters of 1/8" ridged stock cut through the ridges on a small fraction of the blade, usually 3-4 cm. Now if the blade was just that sharp when it started then obviously it would be much blunter at the end.

To be clear, increasing the sharpness of a blade at a given finish so it can push cut is very different from increasing the grit. The former increases the slicing aggression simultaneous with push cutting, the second tends to decrease it and the associated edge retention.

-Cliff

 

[stevekt]

What is a push cut?

 

[Jeff Clark]

"Push cutting" is to force an edge through a material without any slicing action. Shaving is a form of push cutting. Pushing a chisel through wood is push cutting. Detail whittling is mostly push cutting while just whitling a point on a stick is slicing combined with pushing.

Looking at a blade's push cutting sharpness is sort of simple and pure. It measures how thin and straight that you got the edge. To some degree it measures your final honing angle. It removes a lot of variables from your measurements. It isn't a measure of general purpose cutting ability. For most things I do with a knife I have some longitudinal slicing action combined with my cutting.

 

[gud4u]

It isn't a measure of general purpose cutting ability. For most things I do with a knife I have some longitudinal slicing action combined with my cutting.

I think this is true for most cutting applications.

For testing edge quality, I prefer a very slow full-edge draw-slice from the heel to the tip of the blade. From that slice, you can determine:
- Relative sharpness of all areas of the blade.
- Any edge defects from any area of the blade.

Push-cutting is fine, if you're interested in determining sharpness of a specific point or small zone somewhere along the edge - but reveals nothing about the uniformity of sharpness over the full edge.

I also like a slow draw-slice entering into the surface of the newsprint to test how well the point of the blade is dressed. Tip-sharpness is often overlooked, whether new-in-box or user-sharpened.

 

[Jeff Clark]

There are a lot of ways to cheat at slicing performance. If I am in a big hurry I sometimes sort of strop an edge along a diamond-coated rod at a rather high angle. I produce an edge that is not very durable, but will slice well for a while.

 

[Cliff Stamp]

Push-cutting is fine, if you're interested in determining sharpness of a specific point or small zone somewhere along the edge - but reveals nothing about the uniformity of sharpness over the full edge.

Push cut something with full blade width contact.

-Cliff

 

[marcangel]

Push cut something with full blade width contact.

-Cliff

I was hoping someone would say that! That is where I think "push cutting" becomes problematic, and why I used the example of the Leek, since it is a modified whancliffe. It is next to impossible to test the entire length of the blade simultaneously with a push cut for anything except a true Wharncliffe. Even that gets to be difficult, unless you hold the knife by the back of the blade, trying to push cut flat things like newspaper or tissue. This technique might work with larger fixed blade knives to judge sharpness, but for a folder with a blade less than 3.5" it becomes much more difficult proposition. Since most folders have blades < 3.5 inches, and even Chris Reeve states "if you need a blade more than 4" for a task, you should be using a fixed blade." I'm still not convinced that push cutting is really a valid test for many folders.

 

[Jeff Clark]

I don't think anybody only does a push cut test. Typically I look at an edge for dull spots, feel the edge for sharpness and uniformity, slice some newspaper to feel for dull spots, do a little shaving with different sections of the blade, and then do a push cut test. The push cut test is the final test of whether the edge is sharp as I think. I wouldn't even try and do a full blade push cut test since I virtually never do that kind of cutting.

 

[STR]

For carving wood like say when I'm making a face on a walking stick push cutting is a good thing. It comes in handy to have a nice highly polished edge for a precise push cut when carving in tight areas like when you are making eye sockets where you cannot move the blade in a slice motion for very far at all or it will mark somewhere other than just the spot you want to take out wood from.

I have a four blade Case Congress 6488 I bought new in 1975 that I've carried off and on for 30 plus years and I've always kept the two sheeps foot blades highly polished and the other two spear point types with a much courser edge for great slicing in softer materials like rag cutting and stuff like that. The highly polished edges are sharp as all get out but they don't do as well for slicing rags, or even rubber hoses and other such things like the courser ones do. Like Jeff said, for daily tasks a courser edge fits the bill and it does push cut fine in most mediums so you get benefits from both types that way.

I've found on many of my knives that I'm quite happy with the edge I get with the Edge Pro using the 220 grit stone. I often stop with that one on a lot of my EDC knives because I've not seen any real everyday benefit from going on to the 320 and then the 600 followed by the 1200 followed by polish tapes. Except on my wood working knives I general use that 220 grit for everything.

STR

 

[Dog of War]

Push cutting makes for a simple test that somewhat isolates factors of edge geometry and consistency important to most cutting. Disregarding edge finish and looking only at geometry, the edge that push cuts better should be expected to slice better. Remember slicing is a combination of forces, in part the same downward force as push cut, plus the force of drawing the blade. So slicing is a more complex action, making it more difficult IMO to use effectively as a casual test for most of us.

If pure push cutting performance isn't important to you, Phil Wilson has recently made some excellent posts testing edge retention cutting rope with a slicing cut. I don't think of his method or the simple push cutting tests I've been running lately to be any kind of "gold standard" but either one gives quantified relative results that mean a lot more than just subjective opinion of sharpness.

 

[oldrifleman]

Ok, really dumb question. When you are testing you edge using a push cut on say copy paper, are you holding the edge stright up from the edge of the paper or at an angle? Steven

 

[Cliff Stamp]

It is next to impossible to test the entire length of the blade simultaneously with a push cut for anything except a true Wharncliffe.

Generally, most push cutting tends to focus on a small section of edge, a fraction of an inch or so. There is of course nothing stopping multiple checks made along the edge to verify it is all equally sharp.

Disregarding edge finish and looking only at geometry, the edge that push cuts better should be expected to slice better.

Indeed, and stay that way for longer.

-Cliff

 

[rebeltf]

Is it some silly mojo test cooked up by someone with too much time on their hands?

Often get that idea after reading certain posts....

 

[hardheart]

I'm still not convinced that push cutting is really a valid test for many folders.

Valid test of what? You do a push cut test to see how well your knife push cuts. I also don't understand the blade length comment, you can do less slicing with a shorter blade. Razors, utility blades, and whittlers are used to push cut more often than slice.