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#1

Knife Sharpening / Re: SG-250 - Oil Better Suited...

Last post by John Hancock Sr - Today at 03:12:24 AM

Still, from a physics standpoint, in a situation where a solid moves in relationship to another, the friction coefficient is the ratio of the tangential force to the normal force. If there is no friction, there is no tangential force applied to the material being cut, so no work being done and obviously no matter being removed.

The work being done can be measured by the material being removed (bonds being broken within the steel) and the generation of heat. You need to maximise the first and minimise the second. We want as little heat as possible and as much material removal as possible for maximum efficiency. This is achieved by ensuring that the relative MOHs hardness of the abrasive is higher than the steel, the abrasive keeps as sharp as possible (friability helps with this) and the surfaces slide past one another as smoothly as possible, which is done by lubrication. The lubricant also assists in removing any heat that may be generated by friction. Water and oil have both been traditionally used on sharpening stones.

#2

Knife Sharpening / Re: SG-250 - Oil Better Suited...

Last post by John Hancock Sr - Today at 03:01:22 AM

Eventually found that my trusted ceramic stones just couldn't handle the harder steels, loads of work, glazing on the stones,

There are different ceramic compounds and they have differing harnesses. It really depends on the MOHs hardness of the ceramic in the stone.

#3

Knife Sharpening / Re: SG-250 - Oil Better Suited...

Last post by John Hancock Sr - Today at 02:57:45 AM

Friction is nothing more than generating heat.

Indeed, the water actually acts as a lubricant as states by Tormek.

#4

Knife Sharpening / Re: SG-250 - Oil Better Suited...

Last post by Sir Amwell - Today at 01:22:16 AM

Maybe you are asking too much of your SG250 stone?
And looking for an alternative like oil to run it through is not going to get more performance out of it?
The SG250 works just fine, through water with occasional regrading,for most everyday day steels, with correct honing gives good results.
But for harder steels?
It just won't cut it.
I spent a long time on bench stones, hand sharpening.
Eventually found that my trusted ceramic stones just couldn't handle the harder steels, loads of work, glazing on the stones, nagura stones, re flattening etc etc.
Oil addition didn't help either.
Diamond or CBN did though.....
Just my two pennorth.....

#5

Knife Sharpening / Re: SG-250 - Oil Better Suited...

Last post by Ken S - Yesterday at 10:50:32 PM

I have seen difficulties which reoccur occasionally, such as frozen EZYlocks and noisy water troughs on T8s. These are frustrating, but can be handled. I don't recall ever reading a previous post recommending oil instead of water with the SG. I am not saying that oil would not work. I am just puzzled that in fifty years of product innovation, Tormek does not recommend using an oillike coolant instead of water. My guess would be that Tormek has good reasons for staying with water. They do recommend ACC as an alternative which offers more rust prevention.

Ken

#6

Knife Sharpening / Re: SG-250 - Oil Better Suited...

Last post by tgbto - Yesterday at 04:35:03 PM

This is patently false and a myth that needs to die. Friction is nothing more than generating heat.

Thank you for your kind, nuanced words.

Still, from a physics standpoint, in a situation where a solid moves in relationship to another, the friction coefficient is the ratio of the tangential force to the normal force. If there is no friction, there is no tangential force applied to the material being cut, so no work being done and obviously no matter being removed.

In the same fashion, a frictionless action of a knife blade on a tomato will press down on the tomato but not cut it. Our job as sharpeners is to make sure friction happens over such a tiny surface that the cells are instantly sheared in two by the focused tangential force without the tomato being deformed by the normal force.

Heat is a byproduct of friction, heat reduces cutting of abrasives, and increases wear ; hence the need to use a coolant in many situations where friction generates a lot of heat, or to reduce friction using a lubricant where the unlubricated action would generate more heat than the tool (drill/belt/wheel) or piece would be able to withstand locally. Cutting fluids act as both.

#7

Knife Sharpening / Re: Small Knife Holder - Sharp...

Last post by Brock O Lee - Yesterday at 04:14:17 PM

I've used the small knife holder to sharpen knives (even large knives) which do not clamp well on the blade, like this Spyderco with a full flat grind and a distal taper. It works well. Just make sure your blade's edge is perfectly in line with the centerline of the jig, otherwise you'll get a wider bevel on one side.

However very long blades might be challenging because your support is not near the midpoint of the blade.

#8

Knife Sharpening / Re: SG-250 - Oil Better Suited...

Last post by Thy Will Be Done - Yesterday at 02:29:37 PM

If you reduce friction you will reduce sharpening efficiency. In another post, you seem to complain that the stone is glazed and needs refreshing... If the stone wears down it means that new abrasives are exposed, therefore maintaining sharpening efficiency. If the stone wears down too quickly, it means it is not hard enough for the material you're trying to sharpen.

This is patently false and a myth that needs to die. Friction is nothing more than generating heat.  Abrasives work by cutting, friction actually reduces cutting of abrasives and increases wear of abrasives.  The stone surface wearing is not at all the same as the stone releasing abrasive grains via bond strength being low enough to do so.  You can easily see this in the fact that if you continue grinding on a piece there will be an increase in the level of polish very quickly and noticeably as the surface gets more and more glazed/loaded.  It will burnish more than cut the steel as this progression happens, burnishing is not desirable.

#9

Knife Sharpening / Re: SG-250 Bond Strength - Too...

Last post by tgbto - Yesterday at 09:05:59 AM

In my experience, the SG is okay for most stainless and carbon steels. Only carbide steels or one type of very scratch-resistant carbon steel require the SB or a diamond wheel. Could it be an issue with applying too much pressure on the wheel ?

#10

Knife Sharpening / Re: SG-250 - Oil Better Suited...

Last post by tgbto - Yesterday at 09:03:24 AM

If you reduce friction you will reduce sharpening efficiency. In another post, you seem to complain that the stone is glazed and needs refreshing... If the stone wears down it means that new abrasives are exposed, therefore maintaining sharpening efficiency. If the stone wears down too quickly, it means it is not hard enough for the material you're trying to sharpen.