Anyone know if you can change a flat grind to a hollow grind?

C

CrowsCutters

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Oct 5, 2021
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Been trying to find an answer but haven't had any luck at all. Any help on this would be appreciated. I've got a bugout that I'm customizing heavily and I wanna put a hollow grind on it. However I'm not sure how to go about it or if it can even be done.
 
With the right tools I am sure it can be done. Doing so DIY though is going to be very difficult especially to maintain something close to symmetry from one side to the other. I know there are some folks on the forums here that used to do regrinds that you might look into paying for it to get done by someone that knows what they are doing.
 
It absolutely can be, but the degree to which you're able to do it will be inherently limited by the starting geometry. Full-height hollow grinds can be some wicked cutters if made nice and gradual.
 
Aren't the Bugouts an .090" blade?
That doesn't seem to give you a whole lot to work with...
 
As far as I understand, the chief reason for a hollow grind is to make the blade stock behind the cutting edge thin. That way, as you sharpen the blade a lot, and go further and further into the blade stock, you still have a relatively thin profile. In theory, this eliminates the need to do a lot of thinning (reprofiling) on the blade as it ages and you sharpen it a lot.

For a brand new blade that is not hollow, I would think you could achieve a similar result by thinning the blade stock a good bit. I would simply drop the edge angle fairly low (maybe 13 degrees to start) and end up with a somewhat thin blade behind the edge AND a very very sharp thin edge.

Hollow grinds are something a factory can do pretty easily. But a DIY consumer? Hollow grinding is not easy and requires (as far as I know) pretty specialized equipment.

Brian.
 
The bugout is 0.090", roughly 3/32".
Even leaving an edge thickness of only 0.015" and using a 14" wheel (uncommonly large), the grind height would only be 0.74".
It would be best to design a reblade unless you want to significantly change the profile.
The hollow grind calculator on DC knives lets you play with the parameters.

Online Hollow Knife Grind Calculator - D. Comeau

Determine the height of a hollow or concave grind based on the contact wheel diameter and the steel thickness. For makers of razors and knives.
dcknives.com
 
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Not much point in hollow grinding an already thin blade.
What are you trying to accomplish? If you're after lower resistance and a keener edge, just reground (maybe convex!) at a lower angle.
Convex edges are, btw, not blunter, but rather sharper, in the sense that they remove the fatness of the bevel behind the edge (which adds resistance during a cut in less flexible materials), which isn't doing any cutting anyway. Just focus on the apex angle first, and remove the unnecessary bulk behind the edge.
 
I've done a few FFG-to-FHG conversions and they work great if done right.
 
Luke Dupont said:
Not much point in hollow grinding an already thin blade.
What are you trying to accomplish? If you're after lower resistance and a keener edge, just reground (maybe convex!) at a lower angle.
Convex edges are, btw, not blunter, but rather sharper, in the sense that they remove the fatness of the bevel behind the edge (which adds resistance during a cut in less flexible materials), which isn't doing any cutting anyway. Just focus on the apex angle first, and remove the unnecessary bulk behind the edge.
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Same thought here. It's much, much simpler to thin & convex the shoulders of the flat-grind's v-bevel and get nearly all of the same benefit you'd get from the hollow grind. Namely, that benefit would be much less drag in cutting thick or tough materials. And taken a step further, you could also polish the thinned convex behind the edge, which makes cutting even slicker. And as time goes on, you can continue to thin out the steel behind the edge, simply by laying the blade a little bit lower in angle each time you sharpen it up.

Very thin & high hollow grinds are great cutters when done by experienced hands. But there's a lot more risk in attempting it yourself, for reasons already mentioned.
 
This thread led to the checking of my assumptions, and they held up well. I have 3 factory hollow ground blades, a Case Trapperlock, A Boker Merlin, and a Benchmade 480 Shoki. They are the best non-kitchen slicers I own, with two 0.11 blade stock, and the 480 at 0.09. Yes, both Mini and standard Bugout's are solid slicers, but I wish I could jig them up to hollow regrind them even (agree with FortyTwo Blades, heat can be managed). If I can get a MagnaCut reblade for 940 grail, it will be 0.09 hollow grind just like the 480! I think Osborne's geometry would still be useful with 0.90 hollow ground given cassette frames strength.
Wonder how much extra, a hollow grinding option would be on Benchmade's Custom build site?
 
M.C. Cognet Douk-Douks are a full-height hollow grind. One of the only factory knives I know of that have them in knives larger than small gentleman's folders.
 
bgentry said:
the chief reason for a hollow grind is to make the blade stock behind the cutting edge thin.
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as far as i understand, the chief reason for a hollow grind is because it's the cheapest and fastest way of any grind .
economics 101 .
 
kreisler said:
as far as i understand, the chief reason for a hollow grind is because it's the cheapest and fastest way of any grind .
economics 101 .
Click to expand...
Interesting. I havent heard that what would make it cheapest and easiest? I always figured doing a saber grind was easiest. Less area to grind, less material to grind, and the primary grind is flat which is far easier to do on modern tools. Hollow ground blades would seem to be harder to get even from one side to the other and not getting it too thin.
 
all my oldest cheapest kitchen knives are hollow ground. all u need is a 5cm wide grinding wheel and then you're able to grind blades of 500cm length!

ah never mind.
i was just the home expert speaking. the one with the internet connection and a web browser 🤣
 
If you use two opposing grinding wheels, you can just run bar stock between them - boom! instant two-sided hollow grind. It allows automation.
 
For that reason it can be an economy grind, but economy hollow grinds should not be confused with well-done ones, much like economy low saber grinds shouldn't be confused with high or full-flat grinds.
 
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