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Hi,
How are sparks not a sign of burning?
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Chilling blade before using a powered belt?
- Thread starter Pendexter
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How are sparks not a sign of burning?
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Iron, Chromium, and a handful of other alloys used in some steels are pyrophoric. Quickly knock a small enough piece off and it will combust (spark) all on its own. So the bit that came off is burning, but that doesn't mean the chunk it got knocked off from is any hotter than ambient. When you use flint and steel, its the steel not the flint that makes the sparks. The steel might not even get warm unless you really get on it.
The whole thing gets a little tricky when it comes to feed rates etc because those are also dependent on amount of pressure applied and how sharp the individual abrasive is. So while sparks can be a sign of overheating they are no guarantee. I can throw spark using a fresh 60 grit Norton Blaze belt but the workpiece won't even be warm.
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Generally more carbon, more sparks.
I run half speed motors and a German steel will not throw a single spark but old carbon steels look like a fireworks show. Using waterproof abrasives and running them saturated with water buys you a bit more contact time and reduces dust.
You can always make another pass so move the blade quick across the grinder.
I run half speed motors and a German steel will not throw a single spark but old carbon steels look like a fireworks show. Using waterproof abrasives and running them saturated with water buys you a bit more contact time and reduces dust.
You can always make another pass so move the blade quick across the grinder.
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This ^.
I also am of the opinion belt grinders make a poor choice for fine finishing of a blade. Once you get above 400-600 the likelihood of heat buildup with short duration contact increases considerably. They are fantastic for resetting bevels.
I also am of the opinion belt grinders make a poor choice for fine finishing of a blade. Once you get above 400-600 the likelihood of heat buildup with short duration contact increases considerably. They are fantastic for resetting bevels.
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Depends on the speed and the belts. Fine belts on my 1hp 2x72 can quite quickly overeat a thin blade unless I'm using a cork polishing belt (those things are magic with heat mitigation, I swear) but the same grit on my 1/3hp 1x42, which runs much slower, are unlikely to burn steel even if I try.
Also, the sparking when steel is getting ground is sort of complicated. Most of the heat happens after the bit of steel comes away from the parent material as it reacts with oxygen in the air. Similar to how an oxy/acetylene torch can continue to cut steel even if the flame is extinguished if conditions are correct (care is taken to maintain travel speed at the correct rate, gas jet volume and pressure is right for the steel thickness, etc.).
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Hi
eKretz and HeavyHanded,
What temperature is the steel dust before it begins to burn by itself in air (pyrophoric)?
What temperature is the steel sparks after it begins to burn?
How far are the burning sparks away from the metal/abrasive contact point?
Hi
The workpiece doesnt do the cutting, the last 1 micron does the cutting.
I linked it above, but here it is again, the video shows
1) no sparks
2) blade in hot blue flaming fire
3) still holding with bare skin after 45 seconds in blue flame
How many millimeters do you think got burned?
*hint at least 10mm was in the flame, the edge apex (sharp part) is the last 1/1000 mm (1 micron).
That torch thing is not the same as what happens when sparks come off steel when it's abraded - that's not a very good analogous example. I can't tell you what temperature the steel particles are at the instant they come away from the parent steel (not burning at this point, but quite hot still) but as far as the burning/oxidizing particles that are the sparks I've seen different studies referenced that judged those to be in the range of 500°C - 1000°C as they burn - but this happens after they come away from the parent material. Most of the heat is carried away with the detached particles when using a sharp abrasive, this percentage of heat carried away changes depending on the shape (which influences sharpness/dullness factor), size and type of abrasive.
I can also say that when I use a belt on tool steel, whether it's a knife or otherwise, I hold much closer to the point where the work is being done, not 1¼" or more away. On a knife specifically, my fingers are usually directly opposite the belt contact point when finish grinding. It is immediately apparent if the blade is getting warm. I am not going to try to say one way or the other whether belt grinding WILL alter the temper at the extreme apex in every case, but I am perfectly willing to believe that it CAN, especially under the right conditions. Further study is necessary to discover whether this is preventable or not, with many variables contributing to the possible outcomes.
I can also say that when I use a belt on tool steel, whether it's a knife or otherwise, I hold much closer to the point where the work is being done, not 1¼" or more away. On a knife specifically, my fingers are usually directly opposite the belt contact point when finish grinding. It is immediately apparent if the blade is getting warm. I am not going to try to say one way or the other whether belt grinding WILL alter the temper at the extreme apex in every case, but I am perfectly willing to believe that it CAN, especially under the right conditions. Further study is necessary to discover whether this is preventable or not, with many variables contributing to the possible outcomes.
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This ^
In the case of powered grinding you're dealing with a lot of variables, beginning with kinetic friction. Then tensile strength, abrasive footprint, shape of the abrasive. The amount of heat generated might be in the hundreds of degrees or it might be next to nothing as per the flint and steel comparison.
When I do a lot of work with an axe or hatchet the bit gets warm to the touch, as does the edge when I'm cutting cardboard - warmer than most knife steel when I'm using a belt grinder. Are folks going to suggest that chopping wood with an axe or cutting strips of cardboard ruins the temper of your knife?
Starting with the assumption that there is a lot of heat generated and then coming up with ways to explain why you can't readily detect it ignores the fact there might well be nothing to detect. I'd also say that retention testing unless done with a lot of subjects is not going to be much help either - one individual might just not do a good job with belt grinder as they do with whatever other methods they're using.
I'm not in a good position to say one way or the other, as I don't much like how my edges perform right off the belt and normally finish them by hand - possibly removing the micron of damaged edge. I haven't noticed any difference that can be detected offhand.
Another factor to consider is the surface area in contact with the belt. It's a LOT easier to overheat a piece of steel when there's a lot of contact. Dragging an abrasive through a large distance and creating a long trough (relatively - we're speaking microscopically, of course) imparts a ton more heat than just zipping it through a short one.
There's a similar analogy to the abrasive sparking situation for those familiar with machining - if there are any of you guys lurking about. Abrasives work basically like microminiature machine tools, producing microscopic chips and tool marks, etc.
Those with plenty of experience will have noticed how sometimes when roughing off a lot of steel on a machine tool without using flood coolant that sometimes the parent material of the work (NOT the chips, those are almost universally hot - they're analogous to the sparks in this theoretical analogue) can stay almost room temperature. Other times it can be so hot that water will sizzle if you dribble some on.
There are several factors here also, but some of the relevant ones are, again, the size, shape, sharpness and material of the cutting tool. The speed of the cutting tool and the angle at which it's presented to the work, etc. Also whether it's making point contact or broad contact, on the lathe whether the chip is thick or thin (high feed rate or low, deep cut or shallow) for milling tools whether the chips are thin on tool entry and thick on exit or vice versa (conventional or climb milling)...so on and so forth.
There's almost always a lot more going on with most any process than meets the eye. It's tough to get a handle on what exactly is going on in aa very deep way without quite some level of expertise/experience with the process and study of the principles behind it.
There's a similar analogy to the abrasive sparking situation for those familiar with machining - if there are any of you guys lurking about. Abrasives work basically like microminiature machine tools, producing microscopic chips and tool marks, etc.
Those with plenty of experience will have noticed how sometimes when roughing off a lot of steel on a machine tool without using flood coolant that sometimes the parent material of the work (NOT the chips, those are almost universally hot - they're analogous to the sparks in this theoretical analogue) can stay almost room temperature. Other times it can be so hot that water will sizzle if you dribble some on.
There are several factors here also, but some of the relevant ones are, again, the size, shape, sharpness and material of the cutting tool. The speed of the cutting tool and the angle at which it's presented to the work, etc. Also whether it's making point contact or broad contact, on the lathe whether the chip is thick or thin (high feed rate or low, deep cut or shallow) for milling tools whether the chips are thin on tool entry and thick on exit or vice versa (conventional or climb milling)...so on and so forth.
There's almost always a lot more going on with most any process than meets the eye. It's tough to get a handle on what exactly is going on in aa very deep way without quite some level of expertise/experience with the process and study of the principles behind it.
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Hi,
How is that important?
The video is offered to show that yes you can touch the blade while the apex is getting burned and not feel it.
1cm of blade was in flame, thats 10,000 microns, where as the edge apex is the 1st micron.
I don't see one can touch the 1micron apex without grinding of fingers
or detect and react before it is overheated
Takes less than 4/1000 seconds to overheat and cool off 1micron apex,
where as it takes 10-30 times longer than that for fingertips to react
Hi,
Ok, 500 Celsius sparks,
but
how far are the sparks away from the point of contact?
sparking videos I've seen show sparks at the point of contact
some discoveries less important *cough* more lube
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I'm waiting for research showing the top speed I should be using when cutting cardboard to prevent ruining my temper. In the meantime I'm gonna smear a bit of this on my knife just to be safe:
Wow this thread has gotten me down a deep rapid hole. Much learning and discovery to be had in this forum.
A couple of comments/questions:
1. Here's a really informative thread on this topic of belts and tempering with an experiment with temperature-indicating lacquers. You might see some familiar names in that discussion
https://www.bladeforums.com/threads...g-belt-sanding-initial-results.1357231/page-1
2. Time and temperature: tempering is usually done after quenching, 2 hours x 2 times.
Post #11 in the following thread explains it much better than I could hope to.
https://www.bladeforums.com/threads/temper-too-long.645024/
That got me thinking about the transition from austenite to martensite. If it takes 2 hours at 400 deg. f x 2 times to get most of the austenite to convert to martensite, then how much conversion is actually taking place when the "last micron" is heated?
3. Looking at the images online, martensite appears to be over a micron in size (I could be wrong). Does that mean that the martensite structure is chipped away or sharpened at the edge? I obviously don't really know what martensite is. I'll read more into it when I have time.
4. Is the ratio of austenite to martensite what is causing the change in hardness?
A couple of comments/questions:
1. Here's a really informative thread on this topic of belts and tempering with an experiment with temperature-indicating lacquers. You might see some familiar names in that discussion
https://www.bladeforums.com/threads...g-belt-sanding-initial-results.1357231/page-1
2. Time and temperature: tempering is usually done after quenching, 2 hours x 2 times.
Post #11 in the following thread explains it much better than I could hope to.
https://www.bladeforums.com/threads/temper-too-long.645024/
That got me thinking about the transition from austenite to martensite. If it takes 2 hours at 400 deg. f x 2 times to get most of the austenite to convert to martensite, then how much conversion is actually taking place when the "last micron" is heated?
3. Looking at the images online, martensite appears to be over a micron in size (I could be wrong). Does that mean that the martensite structure is chipped away or sharpened at the edge? I obviously don't really know what martensite is. I'll read more into it when I have time.
4. Is the ratio of austenite to martensite what is causing the change in hardness?