View Full Version : Plunge Line/Grind height
08-23-2010, 08:01 AM
So I'm underway with my first fixed blade attempt using hand tools! Thanks to Tracy and Lora for getting the supplies out to me so quickly.
Am planning to flat grind the blade, but am not sure if I should go FFG, or to what amounts to a high Flat Sabre grind. And also not sure if I should add a swedge.
I've seen makers here do both - what are everyone's thoughts/reasons for grinding as high/low as they do? Thanks!
08-23-2010, 08:23 AM
IMO, it all depends on the steel type and the design/intended use of the blade. A full flat grind will give you a super fine edge, but it sometimes lacks in strength. My solution to this is to do a flat grind in order to reduce weight, and then utilize a convex edge for added strength. Ideally, plunge cuts should terminate between 2/3 of the blade's width from edge to spine, to terminating just short of the spine. Just make sure you do not break the spine with the plunge cuts.
I mentioned steel types....some steels, namely the higher alloy steels will not hold up well with a super fine cross section at the edge. These steels tend to be somewhat brittle in thinner cross sections. That's one of the reasons that you often see some of the stainless/high alloy steels with hollow grinds that exhibit short plunge cuts, with overly large edge bevels....simply put, it's a compensating measure because if the edge was thinner, it would be too brittle.
I might be getting too technical there....suffice to say that you want the thinnest finished edge on a blade that you can, and still ensure that there is enough "meat" at the edge to support hard use. Thinner edges equal less cutting resistance, making the knife easier to cut with. In reality it's a balancing act to achieve a thin edge, reducing cutting resistance, while at the same time leaving enough material to ensure adequate strength to hold up, without chipping.
I could go on and one, but I think you get the idea. Practice and experience are the best teachers....I know that might sound like a cop-out, but it's true.
08-25-2010, 07:54 AM
Thanks for that Ed.
I've gotten the blank designed and cut out, and have the profile almost smoothed out. Worked with a drill and hacksaw to get the blank, so it was slowish going. Satisfying nonetheless!
I'm working with S90V, which I know, is a very ambitious first try. I've worked with metal before in other non-knife capacity, so hopefully there'll be some carryover
As I understand, S90V has good wear resistance, but toughness may be an issue. I'm using .172" stock. I've seen thinner blades full flat ground before, but still not sure what I'll do. Have a few more days to mull it over while draw filing ;P
I must admit that I don't have a particular use in mind for this blade. It's an amalgam of different things which I liked in several knives. Still can't decide if I should make it a dedicated 'slicer' or an 'all rounder'. leaning more towards the latter tho. In that case, I guess a Flat grind with a relatively low-ending plunge line might be best?
08-25-2010, 12:06 PM
I let an individual talk me into making him a pair of my EBKs out of S90V....ONCE! NEVER, NEVER, NEVER again! I would suggest getting the blade as close to finished as humanly possible before heat treating....you can hand sand on S90V all day long and get nowhere. When I used it, I got so frustrated with finishing that I went back to a 220 grit belt, then to a scotchbrite belt, and quit. Having worked with high carbon/alloy steels for most of my career, I found S90 to be very brittle by comparison too. Depending on the grind your applying, that fact must be take into consideration.....what I had to do was leave the edge much thicker before final sharpening than I normally would in order to prevent chipping. (approx .020-.030)
08-25-2010, 01:01 PM
I suspect S90V would be better for a dedicated slicer than an "all-rounder", as Ed explained.
Another thing to think about when planning bevels and grind-lines, is the overall angle of the finished blade, compared to the thickness of the stock. For instance, the bevels on a Ka-Bar are fairly keen but the bevel only goes about halfway up the blade (roughly 1/8" stock). If you copied the same profile and angles, but used 1/4" stock, the bevel/plunge would go up much further on the blade.
I can see this in my head but I'm not sure if I explained it in a way that makes sense... :o
I think planning the bevels for the cutting performance you want, and choosing stock thick enough for the strength you want, is more important than any set formula about where the grind line ends up.
08-26-2010, 08:42 AM
Looks like I'm heading towards a flat saber grind, rather than a FFG. Maybe a small swedge at the top.
I should have probably clarified that by 'all-rounder' I actually meant 'urban' all-rounder. Camping chores probably won't come up on a regular basis.
We'll see how it goes I guess. Also have some 3V carbon steel to work on, which might be better for an outdoors blade. :)
One at a time...
Thanks guys for the input!
08-26-2010, 11:47 AM
3V has a great reputation for holding an edge a long, long time and also being very, very tough. Naturally it also has a reputation for being an extreme bear to sharpen. I think it would be great for an outdoors blade, but you better bring a diamond hone along in case it does get dulled a little. This is only based on reading the manu's spec's and other makers' experience with it, so take it with a grain of salt. I have a couple bars of it but haven't gotten around to using any yet.
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