SVM-45 optimal blade thickness?

[Lape]

Hello
As many of you have been noticing, the SVM-45 jig gives different angels once a knife depending on thickness. The jigg is constructed for one thickness of the blade, if you want the exact angle on both sides.

What is that exact thickness?
What is Tormeks rekomendations to compensate for this problem?
Is there any plans to make a pivoting jigg?

[Jan]

Based on my measurements and considerations it is the thickness of 2.5 mm.

If the blade is thicker than 3 mm (1/8") or thinner than 2 mm the side asymmetry starts to be visible (for people who are aware of it). For thin blades you can compensate it using suitable shims, while for thick knifes only Wootz's solution can resolve the problem.
https://forum.tormek.com/index.php?topic=2962.msg16395#msg16395

Jan

P.S.: Based on my recent geometrical modelling incorrect knife mounting in the knife jig can result in bevel asymmetry much larger than the one caused by excessive blade thickness.

[Ken S]

Welcome to the forum, Lape.

Your observation about the bevel angle variation with blade thickness is correct. Jan's observation about greater variation being possible with how the blade is placed in the jig is also correct.

How much variation should we expect or tolerate? In the world of metal machining, things are never "exact". The closest we come to exactness is "within tolerance". Tolerances vary depending on many factors. One would expect tighter tolerances with a Rolex than with a Timex. Even an object which reads right on with a precision tool like a micrometer is limited by the precision of the tool.

Knives could be sharpened with much more precise bevel angles than the standard Tormek jigs produce. However, how much demand would there be for a jig which cost several hundred dollars? Or, for a sharpener charging fifty dollars at a farmers market?

I think Wootz' modification of the Tormek knife jig is a real improvement. I can see where a collector with a knife which needed to be resharpened would be willing to pay a premium for very exacting sharpening. If Tormek made a self centering very precise knife jig, I would be willing to purchase one. So would a few forum members and professional sharpeners. Tormek makes a superb drill bit jig, the DBS-22, in that class. They also sell the 4000 grit SJ 200 and 250 grinding wheels in that cost range. I suspect the forty dollar knife jig outsells the two hundred forty dollar drill bit jig more than substantially.

I first began using a Tormek in 2009. Since then, much of the Tormek line has continued evolving. The T7 had numerous improvements, the most notable being the stainless steel EZYlock shaft. The T3 made a giant leap and became the T4, much improved. The T7 evolved into the T8, with many substantial improvements. Almost all of the jigs and accessories have been redesigned and improved.

The standard knife jig was redesigned with zinc machining and a new locking mechanism. I would not be surprised to see Tormek produce an upgraded, self centering version someday. I certainly have no inside information about this; it just seems a logical step forward. Recently the Tormek engineering staff has had a very full plate. We have seen the new T8; a much improved SE-77 straight edge jig; more minor redesigns of several jigs now made of machined zinc; plus a new website. That is quite a workload for a very small company.

Looking back at the entire history of the Tormek, innovation and change has been continual. Knife sharpening is a primary market, especially in Europe. Tormek has always been the premier leading company in wet grinding innovations. The redesigned gouge jig is an outstanding example of this. The older model was the state of the art. In my forum review of the new jig, I expressed the opinion that I felt many turners long satisfied with the older jig would want to switch to the new jig. That opinion has bern echoed by forum turners. I can foresee a day when I will have the same opinion about a new redesigned Tormek knife jig. I look forward to that day. In the meantime, the present Tormek knife jigs offer very adequate accuracy for general sharpening.

For very fussy work, grinding one bevel and then reversing the blade before final grinding of the second bevel should improve consistency.

Ken

[cbwx34]

As an alternative to grinding part of the jig (Wootz's solution) for thicker knives... could you not also shim the blade so it is canted slightly in the jig?  (Basically alluded to in Jan's "incorrect knife mounting" statement).

[Jan]

Recently I have seen a belt sander sharpener with a knife jig which may be self-centring the blade.

Jan

[cbwx34]

Most of the guided clamp systems have come up with a self centering clamp of some sort.  (In fact, I have one that may be adaptable to the Tormek, see picture).

Just to add a bit to my earlier post, if I had a knife that was, 3.5mm thick... where the Wootz solution is to use a jig he ground .5mm from... could I not shim the base of the blade .5mm (on the base level side) ... which would essentially tilt the blade down?  I know that part of this would depend on how wide the blade is (spine to edge), but for most knives, it should be close enough to negate the difference you're trying to correct for.  (May actually be a thinner shim, since the angle is really what is being compensated for).

May not be ideal in a commercial environment, where speed is a factor, but for the home user an alternative solution.

[Jan]

Cbwx34, your procedure may require juggler skill. 

It would be probably easier to compensate the side asymmetry by suitable turning of the knife jig adjustable stop at each upside down flipping of the jig.

One turn of the adjustable stop typically corresponds to 1.5° change in the bevel angle.

Jan

[cbwx34]

Ha... you may be right.

So, is there a recommended angle change (using the Adjustable Stop) based on blade width?

[Jan]

Cbwx34, full bevel angles symmetry for thick blades requires some effort. I do not do it often.

I have no general recommendation concerning the angle change of the Adjustable stop because it also depends on blade protrusion, stone radius and bevel angle.

To estimate the suitable angle change of the adjustable stop use the Tormek Angle master. Set the angle for the first side, flip the knife jig and estimate the angle change necessary to get the same angle for the other side also.

Jan

[cbwx34]

Ok.  Thanks.  I took a 3.8mm thick blade and tried to measure the angle difference on it (with the Angle Master and a digital gauge)... came in between 1.4-2 deg.  So guess 1 rotation would adjust for it.

Just to satisfy my curiosity (throwing a wheel in the mix has always thrown me off a bit)... the 1.5° change you mentioned earlier would also be affected by stone radius, correct?  (I know it's probably not much... just seeing if I got it right in my head).

Thanks again!

[Jan]

Cbwx34 you are welcome! You are correct, the change is small.

For 125 mm wheel radius, shortening of the blade protrusion in the Knife jig by 1 rotation of the Adjustable stop, results in bevel angle increase by circa 1.5°.

For 100 mm wheel radius, shortening of the blade protrusion in the Knife jig by 1 rotation of the Adjustable stop, results in bevel angle increase by circa 1.8°.

In both cases I have assumed bevel angle 15° and kenjig protrusion 139 mm.

Jan

[Ken S]

Very useful information. If desired, we could even subdivide the rotation, essentially making the threaded end stop a micrometer. We would certainly not need twenty five or fifty divisions, however, a half, quarter or eighth turn might be useful.

Interesting thought path.

Ken

[Lape]

Thanks to all of you for very good answers. I have tried the shims. Works great.

[Lape]

Hello again.

According to my recent measurements the jigs center is aligned with blades exactly 2 millimeter thick.
Hmm?

[Jan]

Lape, originally I have thought also that the Knife Jig works symmetrically for 2 mm thick blades, but was aware that the accuracy of this figure is limited.

Later I have specified this thickness to 2.5 mm based on logical arguments. 2.5 mm is the thickness of the steel guide bar (1) of the Small Knife Holder. This thickness guarantees that the Small Knife Holder inserted in the Knife Jig works symmetrically and the same is true for the knife blade of the same thickness in the Knife Jig.

Jan