Long Knife Jig asymmetry problem

[wootz]

Recently started using the Long Knife Jig SVM-140, and noticed that bevel on one side gets always wider than on the other.
Trying to understand what's happening, having turned it over to the second side checked the angle again with the AngleMaster, and was surprised to see the angle is not the same.
The problem is with both kitchen knives and thicker hunting ones.
Now to obtain the same bevel on both sides, I have to adjust the angle every time I flip over the jig.

The only explanation I have is the way they manufacture the jig does not center the clamp right.
Have never had this issue with the regular Knife Jig SVM-45.

Has anyone had similar problem? My jig is of newer design (without locking screw on the stop).
Am thinking of grinding inner surface of one of the clamps to compensate for the asymmetry, but decided to check with you guys first.

[Ken S]

Welcome to the forum, Wootz.

The Tormek factory in Sweden is presently closed for the annual holiday time. Tormek will be back on the fourth. Tech support should have an answer for you. (support.tormek.se is the email address) Stig often checks the forum. He will be back on the fourth.

I suggest you do not grind down your jig until we hear from Sweden.

Ken

[wootz]

Thank you, Ken

[Ken S]

Let us know what you find out.

Ken

[wootz]

I found out that... the stone was out of true. 
The stone was slightly conical, causing the change in angle on flipping over the blade.
Truing the stone has eliminated that.

[Ken S]

Excellent, Wootz! New knowledge is always welcome. Thanks for sharing and keep posting.

Ken

[Elden]

Welcome to the forum, Vadim.

    In a previous thread, there was some discussion concerning uneven bevel angles resulting from the design of the knife jigs. One side of the jig is fixed while the other side is a movable jaw. 
   Theoretically therefore, only one certain knife thickness will achieve an equal bevel angle on each side when ground. On thinner knives, a shim could be used to center the knife in the jig. Thicker knives will have a greater difference in the side to side bevel angles. It was pointed out, that the knife could be turned over in the jig in order to achieve the same bevel angles.
   I suppose that the field of invention is open to a jig that would center the knife to its centerline.

[Stickan]

Hi,
On regular chef knifes or blades where the back is no more than 3-4 mm its extremely little difference in the edge-line. The knife jigs are designed to fit the most common knife models. We have several knife-sharpening business companies who uses our jigs and even several knife resellers and brands who recommends us for sharpening their knifes.
On more powerful knifes, like a hunting knife with a wider back you can see the difference more. So than you need to mark both sides.
A thing that almost never comes up in a discussion like this is than many knifes don't have the  the edge centred in the blade. It´s pretty easy to see, before you start sharpening a knife, hold the knife with the tip upwards and check if the edge-line is the same on both sides, you will be surprised how many knifes that are off-centred, and many times it´s the more expensive hand made knifes.

The jigs are designed to sharpen the most common sizes, some knifes you need to find and learn the technique so you get a "fabric" edge line.

Best,
Stig.

[wootz]

kb0rvo, your post is my gut feeling explained in plain and precise terms.
With the next hunting knife I'll definitely try marking position of the clamp on the blade, and turning over the blade in the jig rather than the jig itself. And if it delivers better result, for collectible knives I can put up with the inconvenience.

Such a clever and responsive forum, pure pleasure to look in.

[Stickan]

Wootz,
Please check if the edge is actually in the middle of the blade aslo!

Best ,
Stig

[wootz]

Wootz,
Please check if the edge is actually in the middle of the blade aslo!

Best ,
Stig

Sure, it factors as well.
Presuming the bevel mirrors as you turn over the knife jig was wrong for all the above reasons.
Now, with thicker knives, as I turn over the jig, I mark the bevel and use the jig adjustable stop to equal the grinding angle with the other side, normally the needed adjustment is within half a turn.

Thanks again, guys, for your hints.

[wootz]

I realized the problem with adjustments every time as the knife is flipped over, is that when I have to alternate sides, it is impossible to maintain strictly the same angle, and because of that I cannot get sharp edge.

I ended up by buying a second knife jig, and filing away 1 mm from the static clamp.
Now I have a dedicated jig for knives 3mm and over thick.
Not that I recommend this to everyone, but if you sharpen many thick knives, this helps to minimize the bevel asymmetry.

[Ken S]

Well done, Wootz!

I agree, Elden, this field is very open for invention. Here is an idea which occured to me this morning:

Imagine a machine shop (or woodshop) V block which holds a round work piece in place to be accurately measured, cross drilled or sawn square. Now, imagine that V block being replaced with a simple home shop made  wooden cradle designed to hold a Tormek knife jig parallel to the flat surface (like the saw or drill press table).

With the knife clamped in the jig, and the jig setting in the cradle, slide a piece of wood with a parallel line scribed next to the edge of the knife. Ideally, the sharp edge of the knife should be the same height as the scribed line. Flip the knife and jig over and check the other side with the scribed line. If the knife is symmetrical, both sides should be the same height compared with the scribed line.

The height of the scribed line could be shimmed with paper or cardboard for different knife thicknesses. Shims could also be used to reposition the knife to make it symmetrical in the jig.

While this might seem like overkill, the extra care would be appreciated with collectible knives and fine kitchen cutlery.

The "table" could be a flat piece of MDF, wood or (my choice) baltic birch plywood. The cradle could be loose or secured to the table. It could be sized to be portable for mobile sharpeners.

Ken

[Jimmy R Jørgensen]

[Jan]

I realized the problem with adjustments every time as the knife is flipped over, is that when I have to alternate sides, it is impossible to maintain strictly the same angle, and because of that I cannot get sharp edge.

I ended up by buying a second knife jig, and filing away 1 mm from the static clamp.
Now I have a dedicated jig for knives 3mm and over thick.
Not that I recommend this to everyone, but if you sharpen many thick knives, this helps to minimize the bevel asymmetry.

Thank you, Wootz, for sharing with us your courageous solution. 

Important is, whether it really eliminates the bevel asymmetry. I am wondering, if the asymmetry is fully eliminated or rather reduced.

Recently, for some geometrical considerations, I prepared a sketch of the knife jig. I concluded that the knife jig is working fully symmetrically for knives which blades are not thicker than about 2 mm.

So, to modify the knife jig for 3 mm thick blades, for maintaining symmetry with respect to flipping, I would remove 0.5 mm from each clamp.

To modify the jig for 4 mm thick blades, I would remove 1 mm from each clamp.

Jan





P.S.: Ken, in the sketch above you can see settings for a kenjig: projection length 139 mm, support-stone distance s = 80 mm, which for stone radius 125 mm should guarantee the bevel edge 15 degrees.

As s' = 79 mm you can see my small correction of s, which reflects the way how we measure the support-stone distance with a kenjig.