Methods to change from scandi (or with microbevel) to convex? How to make it thin.

[DavidL41]

I've seen a few things here and there talking about how people mess up convex grinds, making them too thick behind the edge. From what I hear it is a myth that convex's are bad slicers (the opposite because it can get thinner and the convex taper adds the bulk) .

1.What is the proper idea of a convex and is it thinner behind the edge than a scandi when done to the same primary angle?

2.To my question, are there any ways that you prefer to take a scandi or a knife with a seconary and primary edge to a convex? Do you start your taper half way down the bevel so it doesn't end up too thick behind the edge?


3.How would I establish a really thin convex thats bevel is 2 inches wide? The convex I had in mind looks like this. It seems the angle on the convex is between 5 and 10?

 

[DavidHoback]

Ok, you said quite a few things there. Not sure if this really answers you but, if your trying to change a blade that is already ground, heat treated & edged, than I would recommend using the slack on a belt grinder. Go slow, and keep it cool. Your angle will be almost completely dependent on the grind in place already.

 

[FortyTwoBlades]

Ok, you said quite a few things there. Not sure if this really answers you but, if your trying to change a blade that is already ground, heat treated & edged, than I would recommend using the slack on a belt grinder. Go slow, and keep it cool. Your angle will be almost completely dependent on the grind in place already.

When grinding a convex primary you'd want to put multiple facet grinds in at different angles and then just use the slack belt to blend them together. Try using the slack belt the whole way and it'll be prone to putting the "wrong" sort of convex on.

 

[HeavyHanded]

First off you have to determine the surface area of the convex - how far up the primary grind it will go. If you take a convex and a Scandi that have the same spine thickness and you start the grind at the same distance from the spine (leave a flat section with a shoulder extending from the spine), the Scandi will be more acute and have less metal behind the edge.

To convert it to something more acute you need to run the grind further up the edge toward the spine.

To make it really thin you have to think more along the lines of making a flat grind that has just a bit of curve - it is not quite flat. As with a flat grind, the spine thickness to width is the constraint for how acute the edge can possibly be. The flat grind will have even less steel behind the edge, but will also finish with a much more acute terminal angle.

Where you put the bulk of the meat will determine the minimum possible angle at the edge, and this is something you will want to control, so per 42 Blades you should shape the edge with a plan, do not allow a conformable surface or slack belt to determine the shape of the arc.

The Scandi below was done on a wet wheel with a jig that allowed 1.5° shift from shoulder to edge, terminating at 12° (IIRC). The convex beneath it was done on a belt grinder with a jig and goes from 0° at the spine to 12° at the edge per side, finish stropping will get it up to the 26 or so needed for better edge stability. As with every grind, making it thicker at the edge is never the problem.

 

[DavidHoback]

When grinding a convex primary you'd want to put multiple facet grinds in at different angles and then just use the slack belt to blend them together. Try using the slack belt the whole way and it'll be prone to putting the "wrong" sort of convex on.

Try reading what I wrote BEFORE quoting me there sport! I wasn't talking about a flat bar of steel.

READ: "trying to change a blade that is ALREADY GROUND, HEAT TREATED & EDGED" Key word there being ALREADY! Of course I make a flat grind first on my platen, before anything has been done. In order to remove enough material behind the edge. (I don't make multiple though). But the blade in question sound to me, to already have a finished grind. And the OP is looking to change it to a convex.

Putting the "wrong" sort of convexing on a blade can happen anytime care is not taken

 

[FortyTwoBlades]

Try reading what I wrote BEFORE quoting me there sport! I wasn't talking about a flat bar of steel.

READ: "trying to change a blade that is ALREADY GROUND, HEAT TREATED & EDGED" Key word there being ALREADY! Of course I make a flat grind first on my platen, before anything has been done. In order to remove enough material behind the edge. (I don't make multiple though). But the blade in question sound to me, to already have a finished grind. And the OP is looking to change it to a convex.

Putting the "wrong" sort of convexing on a blade can happen anytime care is not taken

Easy there, killer. If looking to convert a scandi to a convex without altering the edge angle it's still going to require pre-faceting the blade at least a little before hitting the slack belt or else you're going to be there quite a while if using the very light pressure that would be required not to thicken the edge. Grinding on a slack belt takes way longer than on a platen because of the reduced pressure, and between that and the comparatively low degree of accuracy possible on the slack belt it still makes sense to use the facet-grinding method. Scandis are technically as close to a flat bar as you can really get, as it's just an edge bevel of a certain angle imposed on a flat blank with no other relief grinds and what the OP is talking about is essentially adding a blended/seamless relief grind.

 

[HeavyHanded]

Just a guess, but 42 might have been adding to this line:

... Your angle will be almost completely dependent on the grind in place already.

The OP needs to know both of these factors, esp since he's alluding to what could easily be a fairly involved regrind, and especially if contemplating doing it by hand.

Edit to add: beat me to it....

 

[DavidL41]

When grinding a convex primary you'd want to put multiple facet grinds in at different angles and then just use the slack belt to blend them together. Try using the slack belt the whole way and it'll be prone to putting the "wrong" sort of convex on.

If I was to get a convex like the video above. a very low angle and thin grind would I put a secondary bevel and lay the slack belt where the secondary bevel shoulder and primary shoulder touch the belt?

 

[DavidHoback]

Easy there, killer. If looking to convert a scandi to a convex without altering the edge angle it's still going to require pre-faceting the blade at least a little before hitting the slack belt or else you're going to be there quite a while if using the very light pressure that would be required not to thicken the edge. Grinding on a slack belt takes way longer than on a platen because of the reduced pressure, and between that and the comparatively low degree of accuracy possible on the slack belt it still makes sense to use the facet-grinding method. Scandis are technically as close to a flat bar as you can really get, as it's just an edge bevel of a certain angle imposed on a flat blank with no other relief grinds and what the OP is talking about is essentially adding a blended/seamless relief grind.

LOL! This is why I hate the interweb! You can't see voice inflection in print. I should have posted smiley face I guess. I was laughing while typing that reply. I thought the "sport" comment would show more joking aspect than it did. My bad.

KILLER! ME?!? HA! Not likely! Can only use one arm, see out of one eye, don't move around too quick and hurt all the time! So, haven't killed in years.

I do understand what you are saying. The OP asked a question and nobody answered him after several hours. So I gave him my thoughts as to how "I" would do what I thought he was asking. I know I don't own a big fancy shop, and under my name you don't see anything here. But a google search would show I speak from DOING, not from what I read others do.

 

[FortyTwoBlades]

Yeah I was more of expanding on your post to clarify that it'd be a good idea for someone who doesn't already know what they're doing to take the facet grinding approach, as it's generally the best and safest way to go about it. If you already know the ropes with a slack belt and the amount of work being done is light you can get away with not doing it, but comparatively that's something for folks who already have some experience under their belts.

 

[DavidHoback]

I hear ya. And your right. I guess I only ever learned it this way. And I sure did mess convex a blade or two incorrectly on a slack belt. Although I quickly went to a very short length of slack. Having the slack run very short, like on a rotary platen, does a much better job. Can also use foam between the platen & belt. Also works fairly well.