Convex profile knife

[van]

Good morning,
before writing I did some research on our site but found nothing about it.
My question is: on Tormek T7 or T8 is it possible to sharpen a knife that has a construction with a convex profile? Without altering the structure of the knife during the sharpening phase. By convex profile I mean that from the edge to the bevel the sides are forged in a convex manner. (Normally the knives are flat or concave profile)
Thank you

[Hannsi1957]

thats simple to answer NO WAY

[jvh]

My question is: on Tormek T7 or T8 is it possible to sharpen a knife that has a construction with a convex profile? Without altering the structure of the knife during the sharpening phase. By convex profile I mean that from the edge to the bevel the sides are forged in a convex manner. (Normally the knives are flat or concave profile)

Hello,

IMHO it's not possible to make a blade with convex grind (you need free belt for that) but it's possible to sharpen knives with the convex grind.

Pictures below illustrate what happen on the convex blade when sharpening.

First picture shows ideal setting with angle 17,24°, stone doesn't affect convex grind at all, only apex is grinded.  This is the ideal case, but probably not reachable. But if you set higher angle than ideal, only apex is grinded and convex grind stays untouched.

Second picture shows incorrectly set angle to 12,61° and grind 6 mm long. You can see that this affects convex grind to the depth 0,125 mm. This case is just for illustration, setting angle 12,61° and discovering a 6 mm long grind means that you ignore all control. But is grinded depth 0,125 mm too much?

Third picture shows incorrectly set angle to 16,86° and grind 1 mm long. You can see that this affects convex grind to the depth 0,004 mm. This situation could be from real life. You can set apex angle with marker method and check edge when grinding, 1 mm long grind is clearly visible. Grinded depht 0,004 mm is negligible and moreover you can restore convex apex with honing.


jvh

[van]

Thanks jvh,
always available with clear and exhaustive technical explanations.

[cbwx34]

My question is: on Tormek T7 or T8 is it possible to sharpen a knife that has a construction with a convex profile? Without altering the structure of the knife during the sharpening phase. By convex profile I mean that from the edge to the bevel the sides are forged in a convex manner. (Normally the knives are flat or concave profile)

Hello,

IMHO it's not possible to make a blade with convex grind (you need free belt for that) but it's possible to sharpen knives with the convex grind.

Pictures below illustrate what happen on the convex blade when sharpening.

First picture shows ideal setting with angle 17,24°, stone doesn't affect convex grind at all, only apex is grinded.  This is the ideal case, but probably not reachable. But if you set higher angle than ideal, only apex is grinded and convex grind stays untouched.

Second picture shows incorrectly set angle to 12,61° and grind 6 mm long. You can see that this affects convex grind to the depth 0,125 mm. This case is just for illustration, setting angle 12,61° and discovering a 6 mm long grind means that you ignore all control. But is grinded depth 0,125 mm too much?

Third picture shows incorrectly set angle to 16,86° and grind 1 mm long. You can see that this affects convex grind to the depth 0,004 mm. This situation could be from real life. You can set apex angle with marker method and check edge when grinding, 1 mm long grind is clearly visible. Grinded depht 0,004 mm is negligible and moreover you can restore convex apex with honing.


jvh

Good explanation, but why is the first example "not reachable"?  (Maybe I'm not awake yet).

Just to add a bit of info, here's a picture I "stole" from another forum a while back, showing the difference between grinds and bevel widths, angles, etc....



... to achieve the same angle, the bevel width must change.

thats simple to answer NO WAY

It's not "no way" (as demonstrated above)... just have to realize what's going on.

You can also, for example sharpen at several angles, as explained in Steve's "Multi Bevel Method", then blend it in on a strop.

And if you're good enough, freehand, even on a wheel, usually results in a convex edge.

[jvh]

Good explanation, but why is the first example "not reachable"?  (Maybe I'm not awake yet).

Hello,

I thought that you need ideal conditions on all sides to get such result. It means exact grinding angle, knife guidance, trued stone, precise material removal and also a perfect profile of the knife. I mean that is hard to met all these conditions without any deviations, therefore I wrote "probably not reachable".

jvh

[van]

Hello,

I saw the 2nd video made by Tormek, in the final part of the video we talked about the convexity related to the sharpening of an ax. Perhaps that method could also be applied to a convex forge knife.

[cbwx34]

Good explanation, but why is the first example "not reachable"?  (Maybe I'm not awake yet).

Hello,

I thought that you need ideal conditions on all sides to get such result. It means exact grinding angle, knife guidance, trued stone, precise material removal and also a perfect profile of the knife. I mean that is hard to met all these conditions without any deviations, therefore I wrote "probably not reachable".

jvh

Thanks.

Hello,

I saw the 2nd video made by Tormek, in the final part of the video we talked about the convexity related to the sharpening of an ax. Perhaps that method could also be applied to a convex forge knife.

I went and watched it... the first method he shows is what I referred to earlier, (the Multi Bevel Method)... you can do that and then "blend in" the bevels on the leather wheel.  The 2nd (freehand)... it's pretty hard not to make it convex.

I guess it depends on what you're goal is... I find that most convex knives (usually hunting/camping knives) that are billed as being very durable, are that way because they're so thick at the edge... usually in the high 20°s per side (or more).  Thinning will make them cut much better than they do with the convex edge, IMO.  (Like I said, depends on use and knife type).

[Hannsi1957]

i dont understand the discusion... the question was

is it possibler to grind convex on Tormek

simple answer NO

[cbwx34]

i dont understand the discusion... the question was

is it possibler to grind convex on Tormek

simple answer NO

Think beyond the simple.