Long Knife Jig asymmetry problem

[carvingcat]

After a bit of advice from Tormek Support I've managed to get my jig sorted!

Did they tell you straighten it out? So you were no longer afraid you might break it?

That's correct Herman, Tormek said if it broke they would send me a replacement jig so I had the confidence to go ahead and bend it straight.

[Ken S]

I call that excellent customer support.

Ken

[wootz]

For a look at another sharpener's solution to this problem, see
http://amktactical.com/epages/3c926a50-9aba-43a5-9571-098ee03f1288.sf/en_US/?ObjectPath=/Shops/3c926a50-9aba-43a5-9571-098ee03f1288/Products/24
Watch the video from about 1:30.

Thanks for that, Steve. Have you guys watched it?
Recently I got a few knives which to bevel properly I couldn't do without shims, so I watched that video, and applied their approach to SVM-45.
Shim is placed on the bottom jaw, i.e. between the static part of the clamp and the blade.
Maths for Tormek SVM-45 are as follows.

Spacing from the centreline of the handle to the bottom of the knife clamp (the static part of the clamp) is 1.5mm.
Ideal for 3mm blades, but well acceptable to 2.5-3.5mm thick as well.

For knives outside this range, shims can be used. I use blades from a Feeler Gauge with 1cm wide blades as shims.
Measure thickness of the knife at the clamping spot, and divide by 2 - you get spacing to the centreline of the knife.
Difference between 1.5mm and halved knife thickness is filled with a shim.

Example 1
Knife thickness 1.7mm, divided by 2 = 0.85mm.
1.5mm - 0.85mm = 0.65mm shim.

Obviously, for knives thicker than 3mm you should not put shims in the standard SVM-45 - for them I use a second jig with 1mm filed off the static clamp (described earlier in this topic).
Spacing from the centreline of the handle to the bottom of this jig is 2.5mm.

Example 2
Knife thickness 4.6mm, divided by 2 = 2.3mm.
2.5mm - 2.3mm = 0.2mm shim.

These calculations have been proved practically.

Hope this will be useful to you where a perfectly symmetrical bevel is required.

[Jan]

Wootz, thank you for sharing the practical examples. 

I have only a minor comment. Previously I have expressed my opinion that the knife jig works fully symmetrically for knives which blades are about 2 mm thick. (Reply #31 of this topic.)

Recently I have specified this value to 2.5 mm. It is the thickness of the steel guide bar (1) of the Small Knife Holder.



This thickness guarantees that the Small Knife Holder works symmetrically and the same is true for the knife blade of the same thickness in the Knife Jig.

Jan

[wootz]

Brilliant thinking, Jan!
Thank you for the correct figure. I happily stay corrected - the exact clamp centreline is 1.25mm.

[Jan]

You are welcome, Wootz! 
I am also happy that it was possible to determine this important figure in this elegant way.

Jan

[Elden]

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. 

Jan,
   Out of curiosity, is the above mentioned bevel angle a measurement of the edge angle or of the chord? The drawing appears to picture the chord, however, I know that is not what you were illustrating.
   Does the KenJig set up for the edge angle or the chord angle? That is really what I am wondering about.

[Jan]

Elden, you are correct, at the scale my drawing is shown, it is not recognisable what kind of line segment is delimiting the bevel angle.

Let me illustrate it on a chisel type bevel. Chord has both endpoints A and B on the grindstone and against chord we measure the hollow. The chord angle, not shown in the drawing, is 28.1^o.



Tangent line touches the grindstone at A and never enters the grindstone's interior. Tangent to the grindstone is square to the radius.

The line which delimits the edge angle is a tangent line segment. Please note that in the  example shown here the edge angle is 25^o, while the heel angle is 31.2^o.

Also Kenjig sets up for the edge angle and not the chord angle. The chord angle corresponds to the mid angle, i.e. the edge angle at the midpoint of the tool, where the hollow size is maximal.

Jan

[Elden]

   Thank you, Jan! That satisfies my curiosity. It really did not matter,but was a thought born out of previous discussions. Due to thickness of most knives, I suppose there would be less difference between the chord and tangents angles than that of a chisel. Thick knives might be an exception to that statement.
Edit: Because the taper ground into the knife, there really shouldn't be a great difference between thick and thin knives.

[Ken S]

Good point, Elden.

The longest bevel I remember was the mortising chisel Jan posted a while back. A bevel that long would have considerable hollow.

Ken

[Jan]

Yes, Ken. 
For my ancient 3/4" mortise chisel the calculated hollow was some 1 mm and the Elden's chord angle was by some 7^o larger than the edge angle!  For aesthetical reasons the chisel was ground flat.

Jan

[Ken S]

Jan,

I am pleased that you can appreciate both the logical beauty of mathematics and the aesthetic beauty of a chisel bevel.

Don't change.

Ken

[Jan]

Lovely and aptly said, Ken! Appreciated. 
Everything has its beauty and our task is to see it.
(Modified according to Confucius.)

Jan

[Ken S]

A question from Texaspro inspired an idea. It is borrowed from sharpening skew chisels. With skew chisels, the amount of skew by setting the skew amount to a perpendicular line drawn on the grinding wheel. To draw this line, lower the support bar until it touches the grinding wheel. (Be sure the grinding wheel has been trued.) Using a fine point Sharpie marker, draw a line across the grinding wheel, using the support bar as a straightedge.

Set the jig to the desired bevel angle. With the knife jig with the knife mounted onto the support bar with the sharp edge touching the line. Without moving the grinding wheel, flip the jig over. If the knife is set symmetrically, both sides of the blade should touch the line.

This may be easier to see if the the support bar is raised to hold the knife at a right angle to the grinding wheel.

Ken

[cbwx34]

A question from Texaspro inspired an idea. It is borrowed from sharpening skew chisels. With skew chisels, the amount of skew by setting the skew amount to a perpendicular line drawn on the grinding wheel. To draw this line, lower the support bar until it touches the grinding wheel. (Be sure the grinding wheel has been trued.) Using a fine point Sharpie marker, draw a line across the grinding wheel, using the support bar as a straightedge.

Set the jig to the desired bevel angle. With the knife jig with the knife mounted onto the support bar with the sharp edge touching the line. Without moving the grinding wheel, flip the jig over. If the knife is set symmetrically, both sides of the blade should touch the line.

This may be easier to see if the the support bar is raised to hold the knife at a right angle to the grinding wheel.

Ken

I (of course) went and tried this on a thick (3.75mm) hunting knife... and it worked... you could easily see the difference.  (Part of me thinks this has come up before?... or maybe a "deja vu" moment)...

Anyway... give it a try!