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+--- Thread: Salutations to Stone Sharpeners! (/showthread.php?tid=217)
RE: Salutations to Stone Sharpeners! - me2 - 12-22-2017
The issue of heat during sharpening is a surprisingly controversial one. The issue is not whether the body of the blade gets too hot, but whether the edge gets too hot. Most people who power sharpen hold or regularly touch the blade bare handed, and know enough to look for color creeping up the blade. However, if the color formed on just the edge, in a strip only 1/32" wide, would we see it if we didn't think to look for it?
I have 4 knives that led me to stop using my power sharpening equipment. One is in 1055 steel, another is something like 1095, one was A2, and another is S7. All 4 were sharpened on my HF 1x30 belt sander with 220 to 320 grit belts, and power stropped with a leather belt using compound on the same sander. All 4 had edges that would dent during use afterward. Hand sharpening stopped the issues. Additionally, the 1095ish blade was sharpened on a water cooled wheel followed by the Sharpmaker. Then it was sharpened on the HF and power stropped. It was used on the same work after both, and was damaged significantly by edge denting following the HF sharpening. Changing speeds or adding coolant would probably solve the problems, but that required a step up in complexity. Going back to stones was much less complicated. All 4 of these blades had denting issues. If you suspect your current sharpening procedure is causing this sort of damage, I would recommend the bamboo skewer test. Chop a bamboo skewer into pieces. If the edge dents, you've got an issue. If not, then any softening that did occur is secondary other factors and may not be noticed in use.
Something else to think about. The colors seen during grinding/sharpening are oxides that form on the steel surface. Anything that affects how these form affects the color seen when sharpening. For a 1095 blade, a blue color indicates a fairly narrow temperature range. What is the temperature range for turning a stainless steel blue? I don't know, but I can only imagine it is considerably higher. Here is another question that Jan referred to above. If the blade at a point 1/2" away from the edge gets warm to touch (100 to 120 F), how hot did the very edge get, as that is the point of entry of all the heat available to raise the temperature of the whole blade.
One final thought on the grinding heat topic. A fair amount of research has been done into how much heat is generated while grinding. The most reliable study I've read shows a temperature of somewhere near 1000 F generated during grinding. Some other first hand conversations indicate the temperature can (not is or will be, but can) be much higher. I know GE regularly does tests to see if grinding their parts has resulted in excess grain growth in nickel alloys meant for high temperature applications.
Is overheating possible? Yes.
Is it always detectable before use? No.
Do we know how much softening has occurred? Typically no.
Would I knowingly pay someone to sharpen one of my blades used for fine cutting with high speed and/or uncooled power equipment? No.
Note this question had several qualifiers; knowingly, fine cutting, high speed, etc. If you want to use an angle grinder to sharpen a blade used to cut shingles, I don't see a problem other than needing to buy a new knife really soon.
In the end, is a sharp blade that's a little softer better than a dull blade? Depends on the user, but I'd say yes.
RE: Salutations to Stone Sharpeners! - grepper - 12-22-2017
This is an interesting thread.
Those little HF 1x30’s run fast and quickly heat a blade. I wonder if you would have seen the same problems using the slower Kally 1SM with a 42” belt?
I know that when using a very light touch I get almost no perceptible heating with the Kally. That’s not to say that the apex of the edge does not get hot. I don’t know.
It makes sense that the very thin apex could get hot. I wish I had some way to measure it so I knew for sure.
RE: Salutations to Stone Sharpeners! - Jan - 12-22-2017
Mr. Mark, thanks for your explanation, appreciated. The temperature 470°F was chosen only as a suitable number for my heat transfer example.
Mr. Me2, you are correct, steel composition affects the heat temper colour. In stainless steel, the chromium content is the most important factor influencing oxidation. The higher chromium content, the more heat resistant the steel. This results in delayed development of temper colour.
Other factor affecting heat temper colour is time. Standardised colour charts assume heating for approx. 1 hour. When the steel is heated for longer time than the heat temper colour falsely indicates that the steel was exposed to higher temperature.
Jan
RE: Salutations to Stone Sharpeners! - scott.livesey - 12-22-2017
if your blade changes color, you are getting it too hot. if you are making sparks, you are getting it too hot. with a very fine edge, heating an area 0.1mm square enough to make a spark, you now have a spot of soft steel. if you read the forums, you hear stories of my xxx knife didn't cut well till the third or fourth major sharpening(all the overheated areas were ground away). I sharpen with stones after starting the edge with a wet belt sander. main reason is resharpening. how is the customer going to resharpen a belt ground edge?
on heating the edge, the best cure is prevention. wet grind at low speed, finish by hand.
I still don't get the fascination with Hitachi cutlery steel. I mean white is 1095 or W1. blue is similar to O7 or 1.2519. my biggest gripe is Hitachi refuses to export the steel to the states. yes I know I might get a little piece from Dictum or another European company and end up paying CPM-90V prices for 1095 in a fancy wrapper.
RE: Salutations to Stone Sharpeners! - Ken S - 12-22-2017
This is an interesting topic. Grepper asked how the difference if a 1x42 Kally is used. I would ask one step further, what if the same Kally was used, except powered by the Penn State Industries variable speed motor running at just over one third the RPM of the Kally's Baldor motor?
The surface feet per minute (SFM) is determined by multiplying the circumference of the wheel by the RPM of the motor. The factory Kally has the advantage over the Viel due to its four inch diameter drive pulley. The factory issue Viel comes with a six inch diameter drive pulley. When equipped with the Viel four inch pulley, this advantage is neutralized. When used with a variable speed motor run at a fraction of the RPM, the sfm is reduced much further. The result is not as cool as a Tormek, however, I can hold a piece of metal in my fingers much longer than with a full speed grinder.
I am curious about your thoughts.
Ken
RE: Salutations to Stone Sharpeners! - scott.livesey - 12-22-2017
i have a 2x72 with an 800 rpm motor and a 3" drive wheel and a VFD. I do most finishing at 300-500 sfm.
RE: Salutations to Stone Sharpeners! - Rupert Lucius - 12-22-2017
(12-22-2017, 08:05 PM)scott.livesey Wrote: i have a 2x72 with an 800 rpm motor and a 3" drive wheel and a VFD. I do most finishing at 300-500 sfm.
In my opinion you have machine set up for sharpening - again my opinion, nothing better, nothing.
Rupert
RE: Salutations to Stone Sharpeners! - scott.livesey - 12-22-2017
the issue is resharpening and touching up. with a belt sander you usually end up with a convex edge that will work well but can't be fixed on a stone. i sharpen with the sander till i can cleanly cut copy paper then finish on stones
RE: Salutations to Stone Sharpeners! - Rupert Lucius - 12-22-2017
(12-22-2017, 08:31 PM)scott.livesey Wrote: the issue is resharpening and touching up. with a belt sander you usually end up with a convex edge that will work well but can't be fixed on a stone. i sharpen with the sander till i can cleanly cut copy paper then finish on stones
Is your edge convex if you work ON the platen?
Touching up do you go direct to the Atoma 140? (per Mark R.)
Maybe Mark will come by and get me straightened out or lined out. Again you have a great machine for sure in ref to FPM (belt travel).
Rupert
RE: Salutations to Stone Sharpeners! - scott.livesey - 12-23-2017
for touching up, I use a Jewelstik 1200 grit bench hone. for repair, I start with Norton Crystolon or India coarse/fine, then a Smith's diamond fine, then the Jewelstik. for some knives, I stop after Norton India fine.