Direction of the Grind

[Ed Tipton]

Almost all of my blades use a flat grind with a convex grind on the edge. I always leave grind marks in my blades since I usually stop at 240 or sometimes 400 grit. I have used both longitudinal or lengthwise grinding on some of my blades and horizontal or crosswise grinding on others. Since I do grind using a platen, I find that the longitudinal grind is much easier and faster to do, since all I do is hold the blade against the belt using a magnet, but on most of the knives I see, they tend to be cross ground. Does it matter? Is this something that is strictly left up to the discretion of the maker, or is there a reason why guys are cross grinding so often. I see no reason as to why it should matter, and, if not, I will probably start grinding all of my blades longitudinally. Of course, I do grind horizontally when putting on the final edge.

[Ed Caffrey]

It really doesn't matter, as long as the grinds come out looking neat and professional. The only down side that I can see to longitudinal grinding is plunge cuts.....most buyers expect that feature, and about the only way to achieve it is via to grind horizontal.

It's really all about what works the best for each individual.

[Ed Fowler]

The direction of your final grinding and buffing can make a significant difference in the lateral strength of the blade. This is an old lesson learned by metal workers and the Battelle Memorial Institute in research they did for the US Army Air Command way back in 1940. Vertical grinding that leaves scratches on your blade leaves stress raisers that will become very obvious should you decide to test a knife or two to destruction.

[jdware]

Ed, (Fowler, that is) can you elaborate on your last comment a little more? Are stress risers only a factor in the heat treating or are post heat treat stresses also prone to happen at the stress riser? What grit finish is acceptable going into heat treating? What grit finish is needed to avoid stress risers? How does the direction of the finish effect the lateral strength (are you saying that a blade with the finish running the length of the blade would be less prone to break across the width of the blade than one with the finish running cross ways?
thanks
J

[Ed Fowler]

I never let a blade lay on my bench with 36 grit scratches on the surface, I always make a few passes with a 220 grit belt before I shut down for the day. I sand my blades to a 220 grit before hardening. This seems to be the limit, you can sand finer (belt grinder) but 220 grit seems to be fine enough. When I leave 36 grit scratches on a before hardening, they will be much deeper than the bottom of the scratch and show up in the etched blade. I find blades with heavy scratches in the edge before hardening will chip in accord to the scratches and I have to grind more steel off of the edge to develop a tough edge.

An acute stress raiser can cause a fracture during heat treat.

Take a file and scratch a vertical line on the side of the blade, put it in a vice and bend it, it will break at that scratch.

There are stress raisers by design, many are more serious, but if you are making a well thought our knife for use (aka function) you should know how to avoid them. Any Metallurgical text should cover them.

Yes I am saying that blades with vertical scratches will not be as strong as blades with horizontal scratches no matter how "professional" they look. Final sanding and buffing is an easy way to add lateral strength. Try it your self on some of your test blades or pick up a few wall mart cheap knives and test them. My old high school physics text book had a few pages about why metals fail, the first machinist to take me under his wing (mine blacksmith back very early 1900's) would lecture me about stress raisers while he repaired metal parts that failed through design or heat treat faults.

A friend of ours was killed when a shaft broke in the mine he was working in. My friend was invited to examine the shaft, testified in court and the jury rewarded the company and our friends wife well.

This is not rocket science, just a natural consequence of regularly testing blades to destruction, then you will know for yourself. If you have more questions please don't hesitate to ask.

A classic text on the subject is "The Prevention of the Failure of Metals Under Repeated Stress" by the Battelle Memorial Institute. Written in the 1940's it is the best discussion of stress raisers I have found.

[jdware]

Thanks Ed. I think I understand pre heat treat .... at least a 220 grit finish. But post heat treat ..... is anything short of a full polish inadequate to get full strength. A 400 grit vertical belt finish is a weaker blade than a 400 grit lengthwise finish? - and both are less strong than a full polish?
J

[Ed Fowler]

A 400 grit vertical finish will not be as strong and tough when compared to a 400 grit lengthwise finish. In the experiments conducted by the Battelle memorial the final buff added additional strength. They were testing parts to failure as lives depended on the strength and toughness of aircraft parts and the differences were significant. As knife makers we can take advantage of their knowledge.

I would suggest that you try different finishes and see what difference these variables make in your knives, we have found significant differences. You never know the potential strength of one of your knives until you test one to destruction.

Vertical strength is a very significant attribute of a knife that may be needed in extreme situations. A lack of vertical strength is the most significant cause of blade failure. Stress raisers come in many forms, your name etched , stamped or engraved on the side of your blade can be a significant variable, this is why the ricasso is the best place to put your mark on the blade. The notch, commonly known as the choil is another significant stress raiser as are serrated edges when it comes to lateral strength. A round hole is more significant than an oblong hole where the length is 3 times as long as the height of the hole.

When you look at a Scagel folder blade you will never see a round hole in a blade his folders all have oblong holes. Nothing new, just a lot of little things forgotten or ignored and when blades fail the cause is attributed to "abuse" rather than design faults.

As an example I was demonstrating grinds to a student over a year ago, I was set for hip surgery and left a hardened and tempered blade laying on my bench with 36 grit vertical scratches for about 7 months. A man stopped by my shop last summer wanting me to explain "temper" he was on his way to teach knife making! I decided to demonstrate the edge flex to explain temper, I sharpened the blade and when I tested for edge flex, the edge chipped out due to the stress raisers that developed in the steel beneath the bottom of the scratches over time. Normally I never let a blade lay on my bench with heavy vertical scratches on it.

This is why testing our knives is one of the most significant aspects of making knives.

Good questions!! Thanks

[jdware]

Thanks again Ed. A lot of good information and a lot to think about. Will definitely study this further.
J

[Ed Fowler]

You are most welcome!

One of my first introductions to this phenom was when 36 grit scratches kept re-appearing after I had sanded them out when I etched my blades. It took me a lot of thought to figure where they were coming from.

Steel is much like glass, it keeps changing over time, the change is very slow but it happens. One of my goals is to reduce the future rate of change through my methods. I believe we as knife makers can do it!

[DAN VAN]

I sand my blades to 400 grit before heat treat to get rid of stress risers and so I have alot less polishing time later on a hardened blade. Also a better finish helps close up the pores in the steel to keep the blade from rusting as easily. Dan