Help me understand high speed grinding

[michael langman]

I can clarify this topic as little more,

A grinding wheel is designed to cut steel. The wheel is made up of the grit, and the bonding agent. Different wheels are designed to cut different types of steel. Soft steel and hard steel. Generally the harder the steel, the softer the wheel, and the softer the steel, the harder the wheel, works best.
As the wheel turns at a high rpm, it contacts the steel under a high pressure. Each single grit in the wheel is sharp, and it removes a tiny amount of steel with each grit.
As the grit gets dull from cutting the steel, the pressure increases between the wheel and the steel, and the dull grit is released from the wheel, because the bond is weaker then the pressure applied, and it breaks free.
So the wheel is self sharpening, if the correct wheel is being used for the application.
Grinding wheels are designated by the size of the grit by a number, say, 46 grit, 60 grit, 120 grit. The higher the number the smaller the grit size.
A letter is designated for the hardness or softness of the bonding agent. the letters run from A to Z

So there are many types of wheels to choose from for the many different types of metal to be ground. Also a coarse grit wheel will act softer then a fine grit wheel, so experience and how the wheel is used help a lot in correct grinding.

This refers mostly to surface grinding in a machine shop, but the correct wheels on a pedestal grinder also is important.

In the Navy Machine shop, we were taught to identify types of steel by using a pedestal grinder. The color of the spark. and the length of the spark is different for different steels. For example, high carbon steel has an orange spark that long with many sparkles per inch. If you watch paper burn, which is high in carbon the flame is bright orange, as is the carbon in high carbon tool steel.

Cold rolled steel has a whiter spark and less sparkles on the grinding wheel.

Ypu can lose the temper in steel by grinding incorrectly. If you use a 46 grit wheel that is fairly soft, say no higher the G in bond hardness, and hold the plane iron or chisel, so that the wheel is spinning toward the handle end of the chisel, or the end opposite the cutting edge of the plane iron, the heat will be directed away from the tip of the tool.
Light pressure should be used and moving the tool across the wheel at all times will produce a burn free grind. Correct grinding is a slow process.
Dressing the wheel frequently and quenching the tool in water helps a lot. Because you are not using the high pressures used on a surface grinder then the wheel can get dull more frequently, thus wheel dressing helps.
Slow RPM grinders help those not familiar with grinding wheel choice, and proper grinding technique. A 3600 rpm grinder can grind tools all day long, without softening the tool, but a 1750 rpm wheel can be used, with a finer grit wheel.
The first thing to do to a chisel or plane iron if it is chipped, or very dull, is to grind a flat perpendicular to the point of the tool sharp edge, of about 1/32" to 1/16", preferably with a 60 grit wheel or higher. Very lightly as not to distemper it.
Then proceed to remove the material across the intended angle of the tool as explained above.
The tool should never get too hot to be held in the hand at the point of the grind. About 120 degrees is very hot to the hand.
I hope this was helpful in explaining the answer to your question.

[Warren Mickley]

I think you completely missed the original question, Michael. When the grinding is producing sparks from the interaction of stone and steel, how hot does the surface of the steel get? and if this temperature exceeds the temperature that would affect the hardness, how deep into the steel does this effect go. The problem is that you can't stick your finger in there while grinding to see if the temperature is "hot to the hand".

[Kees Heiden]

It certainly is possible to damage the steel with high speed grinding. Michael explains how to prevent that from happening. Soft wheel, coarse grit, a light touch, dressing and quenching. I thought his post had good additional information allthough it didn't directly answer the original question, which was allready answered in earlier posts.

[Patrick Chase]

I think you completely missed the original question, Michael. When the grinding is producing sparks from the interaction of stone and steel, how hot does the surface of the steel get? and if this temperature exceeds the temperature that would affect the hardness, how deep into the steel does this effect go. The problem is that you can't stick your finger in there while grinding to see if the temperature is "hot to the hand".

Warren is right - that is indeed the real question of this thread. I think he and I have different opinions as to the answer (and I've already stated mine), but that question really is the crux of the matter.

[george wilson]

A minute fragment of steel,ripped loose by the grinding wheel,is so small that it does get very hot and can even go to white hot. It just doesn't have the mass to absorb the friction of being ripped loose,whereas the main body of the tool is millions of times more massive. If you do continue to grind without frequent quenching,especially when the tool starts becoming ground thinner and thinner,it too will begin to have a much reduced mass. Then,it cannot long stand the friction of the grinding wheel. It will soon begin to turn yellow,then brown,dark brown and blue. And these colors can change nearly instantly when your edge has been ground thin. There is nothing wrong with grinding if you are skillful enough to understand these things and quench more and more frequently as your tool is ground thinner.
'
As has been mentioned.do not grind to a final sharp,ragged edge. Leave a little "line of light" along the edge that can be as little as 1/64" wide. This is accomplished by grinding as little as possible in the beginning to produce a flat edge on your tool before grinding. This is particularly used when resharpening gouges,and is how you can grind the final edge straight across,like it should be in most cases.

By the way: I have never been SUCKERED into buying a "slow speed-1750 rpm bench grinder". It is not necessary. Just learn how to grind in the first place.

[Mel Fulks]

George, 1725 is MEDIUM speed ...the ones at Colonial Williamsburg are slow speed!!

[Patrick Chase]

George, 1725 is MEDIUM speed ...the ones at Colonial Williamsburg are slow speed!!

That's why he put "slow speed" in quotes :-)

[george wilson]

Durn Toot'in those wheels in Williamsburg are SLOW. I hated those things. My 2 LEAST FAVORITE 18th. C. ways of doing things with authentic tools was DRILLING HOLES,and GRINDING TOOLS. I think we did not have a good source of sandstone these days,anyway. It was much too fine and hard. In those days,it was existing technology,and I'm sure they knew where to get the best cutting sandstone.

I have an unused,original Civil War grind stone wheel in my shop. I never have gone to the trouble to use it. But,I'll bet it would cut better than the stones that the purchasing dept. got custom made for us to use.

ABOUT DRILLING HOLES: If you have any of those infernal old "center bits",the ones with the in curving sides,they were never made to drill straight holes with. They were made to drill a SHALLOW HOLE just deep enough to start a SPOON bit into. Then,you could drill a straight hole(If you didn't lean crooked on your brace!).

[David farmer]

A minute fragment of steel,ripped loose by the grinding wheel,is so small that it does get very hot and can even go to white hot. It just doesn't have the mass to absorb the friction of being ripped loose,whereas the main body of the tool is millions of times more massive.

I feel the original question was answered.
The image I had in my mind was a microscopic photo taken at the instant an abrasive particle was plowing through the steel. While the chip is still connected it would be glowing hot but at the remaining connection point there must be a very abrupt heat gradient from glowing to cool enough not to change the temper.

[Kees Heiden]

Durn Toot'in those wheels in Williamsburg are SLOW. I hated those things.

One of the reasons why Sheffield got so big was the availability of water power in the area. All that water running down from the Peak district allowed them to setup numerous water grinding mills. They were not stupid back then. In the joiners or carpenters shops they had to rely on handpower, poor guys

I am a lazy sod really appreciating the high speed grinder

[Kees Heiden]

I tried to understand some scientific articles about the temperature gradient in grinding operations, but it ain't easy! It is also doubtfull how usefull these articles are because they deal mostly with grinding as a shaping tool where they remove a lot of material in as short a period as possible. That's different from our grinding to get tools sharpened. I couldn't find scholarly papers about that process, maybe it isn't so important for the industry? We work with less grinding depth (decreasing the temperature), but the objects we grind tend to be very thin (razor thin at the end) which is a negative factor

What I learned from this casual literature research. In conventional grinding the total amount of energy input is going for some 60 - 85% into the workpiece. The grinding particles make chips, but they also mow and plow into the surface, heating up the workpiece considerably. The energy to make a chip is distributed equally over the chip (which is flying away) and the surface of the workpiece. The chip is very small as explained above, so heats up a lot, resulting in a glowing spark. But the surface tends to heat up a lot too. Easilly going over 100 degrees Celsius.

Now it is anyones guess how deep this surface temperature radiates into the workpiece (the tool we want to sharpen). The surface temperature and the temperature gradient depend on a lot of factors.

- The grindstone. The type of grit, its grainsize, the hardness. The well known bleu Norton 3X wheels have a very open and loose structure, making them perfect for grinding toolsteel.
- The conductivity of the grinding grit. The new CBN stones have very good heat conductivity. They manage energy transfer rates of less then 10% instead of the 60 - 85% of conventional AlOx stones.
- The bulk of the workpiece, thicker is better, but we want to grind an edge, which is unfortunate ;-) This is why you shouldn't grind all the way to a sharp edge, leaving some work for the flat bench stones.
- The hardness of the steel, harder heats up more. Also more carbides (chrome, vanadium etc carbides) makes grinding harder and leads to more heat.
- Cooling with fluid. The well known wet grinders use water or some kind of cutting fluid to remove heat. Our quenching at the dry grinder is of course a very poor cousin, we tend to be just a little too late, cooling after the tool got too hot allready. So, like explained in previous posts, you need to quench more often as your edge gets thinner as a preventive meassure.
- Cutting depth. Leaning the workpiece hard into the grindstone is a sure way to overheating. keep a very light touch.
- The speed of the tool moving along the grindstone. This is your hand moving the tool left to right, back and forth. You don't want to keep the tool stationary. The grindstone heats up your tool where it touches the metal. This is also why a crowned wheel leads to less heat. The grinding grit only touches the bevel surface over a short width, while the rest of the bevel's surface not touching the stone has time to cool down again.

So, I don't know if I answered anyone's question, but it gives you an idea of what happens and what influences the energy transfer into the tool you want to sharpen. It is very easy to overheat an edge on a high speed dry grinder, so it takes some skill to prevent damage. Because the high speed dry grinder is the quickest and cheapest way to remove a relatively large amount of hardened tool steel, it is a usefull skill to learn.

[David farmer]

Great stuff Kees. Thanks for sharing what you learned!

I couple decades ago, I realized there was more to grinding than just grinding.
I was annoyed at how slow shaping a chisel was where I worked. I grabbed a roll of adhesive backed sandpaper (3M 219U) and put a 24" length of the coarsest stuff we had onto a granite surface plate. I stuck the chisel into an Eclipse jig and thought, " I'm going to get this done now!" A few fast and athletic trips back and forth and I went to check my progress. Woops! I touched the chisel near the edge and promptly burned the cr@p out of my fingers. Since then, I've wondered exactly how a grinding wheel can remove so much material without generating that kind of heat.
I have a better understanding now, but It seems there must be some minute layer of damage. Apparently just not thick enough to be relevant if any further honing takes place.
I do find it useful to have a mental image of whats really happening, even if it's going on outside the range my senses.

[michael langman]

My apologies for getting off of the topic. I read the title of the topic. Help Me Understand High Speed Grinding, and so it goes.

Now back to the real topic at hand.

Heat treating a piece of steel is not an instantaneous process. Parts being heat treated have to be held at the critical temperature for a certain amount of time.
Sharpening planer blades on a surface grinder is nothing like standing in front of a pedestal grinder and sharpening a planer blade. So the scientific studies are not out there for us Neanderthals.

The part being ground on a surface grinder is never on the same point of contact for more then a fraction of a second. AS the wheel is spinning hundreds of pieces of grit are removing minute particles of steel by cutting them off. Grinding wheels are cutting if they are coarse enough.
The depth of cut is not deep enough to transfer too much heat to the part being ground. Therefore the part cannot lose it's temper if all things are being done correctly. The big flash of red someone sees between the grinding wheel and the part they are pushing against the wheel is the metal being removed from the mass of steel they are grinding. The part itself is not the same color. If they push hard and hold the steel there it will distemper.
When I was grinding in the shop which I did most of my life because grinding correctly was a longer process then milling steel or doing other things it was customary for me to dress the wheel on the grinder and then I would run my finger over the periphery of the wheel while the grinder was running. Other people thought I was nuts, but I was removing the higher pieces of grit from the wheel that would have ruined the finish on the piece I was grinding. I didn't burn my finger, and those pieces of grit stood out like a sore thumb, and were easily removed.
Most professional grinders know that you always finished a part especially if it was a cutter by taking .0001 of an inch per pass for the last half thousandth of an inch. And the tempering range of high speed steel is higher then tool steel, but still high enough to not cause distemper when grinding.
If you are so concerned as to ruining your PMV planer blade, then send it back to the place you bought it, and let them sharpen it.

[John K Jordan]

Now it will be easier to spring for that new CBN wheel.

The CBN wheel is a big improvement. I resisted switching to CBN for years but eventually replaced most of my grinding wheels as the price came down. I even put a 10" 600 grit wheel on my Tormek for dry grinding with other 8" CBN wheels on the "1/2 speed" bench grinders. (Most of my grinding is shaping and sharpening lathe tools.)

I used to think CBN grinding wheels were a relatively new advancement in grinding but I recently found this reference from almost 30 years ago. It may help to explain the heat advantage:

Heat-producing grinding force and grinding power can he reduced by using sharp wheels, soft wheel grades, and frequent dressing (King and Hahn, 1986). Another method of controlling temperature is to employ Cubic Boron Nitride (CBN) wheels. The lower maximum temperature produced by CBN wheels, as compared to conventional alumina wheels, is said to be due to the good thermal conductivity of CBN, i.e. the CBN itself removes a large fraction of the heat from the grinding zone (Neailley, 1988).
(from http://www.abrasiveengineering.com/therm.htm)

BTW, I bought all mine from Ken Rizza at Woodturners Wonders. He sells wheels machined from aluminum. It's probably not enough of a difference to matter but aluminum does have a higher thermal conductivity than the steel used in most other wheels.

JKJ