Trying to relate my understanding of grinding to the theory

[george wilson]

Just grind your tool!!!!!

[Brian Ashton]

Brian, I do think that this "circular logic', by definition, will lead you no where and leave you frustrated. Its best to reorient your logic in a linear fashion that takes you toward a solution.

Hey, you're the one who wanted more details...

[Brian Ashton]

Just grind your tool!!!!!

There's a t shirt in there somewhere. Stay calm and grind your tool. Or how bout, Shut up and grind your tool

[Bob Woodburn]

Brian, below (indented) is a brief verbal sketch of the relevant characteristics of a grinding wheel quoted from : http://en.wikipedia.org/wiki/Grinding_wheel

Given a wheel specification defining all five characteristics below, manufacturers generally recommend a range of speed and feed and depth of cut (the latter 2 comprising what is being called here force or pressure). Speed is typically stated in wheel circumference feet per minute.

I assume the manufacturer's recommendation is an attempt to optimize the desired metal removal rate, final finish, heat effects, wheel wear, frequency of dressings needed, and so on and so forth for industrial processes.

A woodworker forming bevel edges that are meant to have durability may focus on heat effects and require absolute minimal pressures in the hope of minimizing plastic deformation and burrs at the edge. Fast metal removal at an edge may not be a goal.

I hope you can see that it is meaningless when generalizations are made about wheel speed alone, without stating the five characteristics of the wheel at issue and stating which grinding results one is trying to optimize and which are irrelevant.

If you want the best opinion about why your wheel works the way it does at low speed, you should talk with the manufacturer. I'd be interested to know what their experts have to say.

"There are five characteristics of a cutting wheel: material, grain size, wheel grade, grain spacing, and bond type. They will be indicated by codes on the wheel's label.
Abrasive Grain, the actual abrasive, is selected according to the hardness of the material being cut.

• Aluminum Oxide (A)
• Silicon Carbide (S)
• Ceramic (C)
• Diamond (D, MD, SD)
• Cubic Boron Nitride (B)

Grinding wheels with diamond or Cubic Boron Nitride (CBN) grains are called superabrasives. Grinding wheels with Aluminum Oxide (corundum), Silicon Carbide or Ceramic grains are called conventional abrasives.
Grain size, from 8 (coarsest) 1200 (finest), determines the physical size of the abrasive grains in the wheel. A larger grain will cut freely, allowing fast cutting but poor surface finish. Ultra-fine grain sizes are for precision finish work.
Wheel grade, from A (soft) to Z (hard), determines how tightly the bond holds the abrasive. Grade affects almost all considerations of grinding, such as wheel speed, coolant flow, maximum and minimum feed rates, and grinding depth.
Grain spacing, or structure, from 1 (densest) to 16 (least dense). Density is the ratio of bond and abrasive to air space. A less-dense wheel will cut freely, and has a large effect on surface finish. It is also able to take a deeper or wider cut with less coolant, as the chip clearance on the wheel is greater.
Wheel bond, how the wheel holds the abrasives, affects finish, coolant, and minimum/maximum wheel speed.

• Vitrified (V)
• Resinoid (B)
• Silicate (S)
• Shellac (E)
• Rubber (R)
• Metal (M)
• Oxychloride (O)"

[Pat Barry]

It might be an interesting science project to study grinding force versus wheel type versus RPM versus blade hardness versus grinding angle, etc but I do think this would require quite a lot of expensive instrumentation and well designed experiments to gather useful data. At the end of the day you will still have tradeoffs to consider and no clear cut single best method. I do not think and ground shaking discoveries are to come of this however and I therefore kinda like George's pragmatic approach to "Just grind your tool!!! " those might be great words to live by and recall for future discussions

[Brian Ashton]

It might be an interesting science project to study grinding force versus wheel type versus RPM versus blade hardness versus grinding angle, etc but I do think this would require quite a lot of expensive instrumentation and well designed experiments to gather useful data. At the end of the day you will still have tradeoffs to consider and no clear cut single best method. I do not think and ground shaking discoveries are to come of this however and I therefore kinda like George's pragmatic approach to "just grind your too " those might be great words to live by and recall for future discussions

Not entirely sure why you think I'm trying to break new ground. I certainly have no desire to do any rigorous study that's for sure. I'm also not trying to find or convey that I have found the holy grail of grinding. This is my method, I have no interest in spreading the gospel just want to know why it works the way it does. At present I've gone as far as my limited knowledge will take me so I was looking for some other perspectives and or guidance as to where to look further. I already said it's probably a process that's been forgotten and I just happened to stumble on it again. When it comes to putting a bevel on a piece of steel by hand I doubt there is anything new to discover that hasn't been learned over the past 400 or so years. All I'm doing is something that I like to do - learn. Learning is one of those spices of life. Maybe you think I'm obsessing over this or something but I can assure you I maybe think about it once every couple years. It's like I have all these pots of interesting ideas that are simmering away on the back burner. Every once in a while I pull the lid off one and give it a bit of a stir... At this moment I'm simply trying to adding a bit of flavour and seeing what comes of it. When I've satisfied my curiosity I'll put it back on the back burner and maybe revisit it again in a couple years, maybe sooner, maybe never.

Anyways I've had a few good posts, thx guys, that I can ferret out and ruminate over. Watch this space in about 2 years.

[Richard Hutchings]

Thanks to you Brian, I've now given some thought to something that I never would have thunk to think about. I'm going to try to flush it out but I have to say it was interesting.

[Pat Barry]

Not entirely sure why you think I'm trying to break new ground. I certainly have no desire to do any rigorous study that's for sure. I'm also not trying to find or convey that I have found the holy grail of grinding. This is my method, I have no interest in spreading the gospel just want to know why it works the way it does. At present I've gone as far as my limited knowledge will take me so I was looking for some other perspectives and or guidance as to where to look further. I already said it's probably a process that's been forgotten and I just happened to stumble on it again. When it comes to putting a bevel on a piece of steel by hand I doubt there is anything new to discover that hasn't been learned over the past 400 or so years. All I'm doing is something that I like to do - learn. Learning is one of those spices of life. Maybe you think I'm obsessing over this or something but I can assure you I maybe think about it once every couple years. It's like I have all these pots of interesting ideas that are simmering away on the back burner. Every once in a while I pull the lid off one and give it a bit of a stir... At this moment I'm simply trying to adding a bit of flavour and seeing what comes of it. When I've satisfied my curiosity I'll put it back on the back burner and maybe revisit it again in a couple years, maybe sooner, maybe never.

Anyways I've had a few good posts, thx guys, that I can ferret out and ruminate over. Watch this space in about 2 years.

Its been and interesting discussion certainly. I think if you boil it down to making some of the variables in the process fixed and focus on the relationship of speed vs pressure and time and then measure the result in terms of material removed you will find that the formula will be something like speed * time * pressure is proportional to material removal. Therefore, in order to compensate for the faster grinding wheel speed and get the same result on your slower grinding wheel you willl need to increase either time, pressure, or both. Its purely that simple. Temperature of the tool in the grinding conditions is most driven by frictional heating, material thermal conductivity, etc and that in turn is driven by pressure and exposure to the frictional force and therefore that includes both grinding wheel speed and time of pressure application. Again, all of this is well understood by mechanical engineers. The fact that you feel that you can obtain a cooler tool with large amounts of material removal at low speed indicates to me that you probably press very hard for a short period of time. Using this same sort of pressure on the high speed tool will lead to higher tool temperatures unless you compensate by significantly reducing time of pressure application. All the other variables involved (type of grinding medium, coarseness of the medium, wet or dry grinding, tool steel hardness and tool steel material, grinding angle, etc, etc, etc) are also in play in the real world making the process extremely complicated. This is where the school of hard knocks provides the best guidance. If you get acceptable results using the low speed grinding wheel, then great, that's all we are striving for.

[Eric Schubert]

Here's a quick rundown of my understanding of the original questions posed, Brian:

While the wheel isn't actively pushing back against the blade being ground, physics dictates that in order to remain stationary an object must push back with the same amount of force that you are exerting upon it. So your horizontal force against the blade presses it to the wheel. That force is transferred to the wheel, which is transferred to the bearings or wheel mount, which transfers to whatever the grinder is mounted to, and then to the floor/wall. So, imagine the table or bench pushing the grinder back against you pushing the blade to the wheel.

Imagine sliding a box along the ground.

The force of friction of the box sliding along a surface has a relationship to the normal (perpendicular, gravitational) force of the object against the surface and the coefficients of friction of the surfaces in contact. The higher the force of gravity (heavier the object) the more force you use to push and slide the box. The same is true with the blade on the wheel. The higher your force pressing the blade against the wheel, the higher the force of friction being generated that the wheel must overcome to keep spinning at the same speed. This force of friction is tangential to the wheel.

If you are sliding the box at a constant speed, your force equals that of the frictional force. And since your blade does not move, the force you use to push tangentially to the wheel plus any help from the tool rest will equal that of the opposite tangential force of friction.

Here's an illustration:

Forces.png

Flat illustration:
F(n) = normal force (perpendicular)
F(o) = you pushing to slide the box along the surface
F(f) = frictional force working against you sliding the box (varies with F(n) and surface roughness)

Wheel illustration:
F(b) = your force pressing the blade against the wheel
F(n) = the normal component of F(b) on the wheel
F(t) = the tangential component of F(b) on the wheel
F(f) = frictional force on the wheel/blade

Since your force is at an angle as you create the bevel on the blade, break your force down into the normal and tangential components. Only the normal component generates frictional forces. The tangential component is probably a combination of your F(t) pressing against the frictional force and the tool rest holding the blade in place. These forces combine to offset the frictional forces and keep the blade stationary. If these forces do not balance, the blade moves (probably flying across the room somewhere...).

F(f) = F(n) x Coefficient of Sliding Friction

So, let's say you're pushing with 10lbs of force and your coefficient is 0.2, then your frictional force is 2lbs.

The coefficient varies with surface roughness of the two faces in contact with each other. You can often look these values up as a combination of whatever materials you're using and how rough the surfaces are. For instance, the coefficient of friction for dry wood on metal is roughly 0.2. But again, this varies as the texture changes on the surfaces of either the wood or the metal.

Make sure to use the sliding friction coefficients, not the static coefficients. It is harder to get an object started sliding than to keep it sliding. Static coefficients are to calculate the initial force required to get an object moving from a stop, hence the larger coefficient values.

Hopefully this at least helps. If you need further clarification, let me know.

[Eric Schubert]

By the way, I have to add...

Theoretically, the frictional force is purely a function of the normal force and the coefficient of friction. Speed of the box on the flat surface should make no difference whatsoever.

However, in reality, this can change quite a bit with a texture that isn't consistent or that changes as it wears. Or even a very rough texture can dig into the edge or side of an object and cause extra frictional forces to be generated. So, take my post as pure theory that may very with actual, real-world conditions.

Also, since a wheel is round, its opposing force is not necessarily purely tangential. Some of it is normal, as well. (To be normal/perpendicular, you'd have to push directly toward the center of the wheel. Doing so makes grinding very difficult, so we hold the tool we're grinding to the side where the wheel is spinning away from us for safety.) This complicates things.

Now, the heat generated is another matter entirely. This is due to the work being done by the frictional force as it opposes your own forces. Since you're grinding and removing material, heat is also generated as you break the bonds of the molecules of steel or iron from one another. (Just imagine very tiny frictional forces causing more work to be done here, for the sake of simplicity.)

Let's think about the sliding box again. You put work into sliding the box a particular distance. The farther the distance, the more work being done. An equal and opposite amount of work is then done by the frictional force to oppose you. All of this work done by friction goes toward generating heat.

Work = Force x Distance

Force to push the box = 10lbs
Distance = 10ft

Work = 100 lb*ft

How to take that work and calculate change in temperature in a tool is difficult. Some work generates heat in the wheel, some generates heat in the steel, and a small amount is even dissipated into the air around both. You'd need to know the thermal properties for each of them to make that calculation happen.

[george wilson]

And,that and $5.00 will get you a cup of coffee at Starbucks!!!

Just grind the TOOL!!!!!

[Noah Wagener]

" "Isaac Newton, the man who made the brilliant observation that something unimpeded will fall to the ground but never figured out why. His disciples are zeroing in on when." "

Winton, are you suggesting a Shapton 120 could serve as a grinder? Could you compare its speed to the 1,000 and its hardness?

[Tom McMahon]

Unless I missed it, somewhere in this thread has the square area of contact been mentioned. When I was in high school metal shop I was taught to use a wheel with a radiused face to limit the area of contact. Joel from Tools for Working Wood wrote an article about it. https://toolsforworkingwood.com/store/blog/48

[Richard Hutchings]

I am with you George. They're having fun on another level and I still believe it has nothing to do with woodworking. Yet, I still come back to read.

And,that and $5.00 will get you a cup of coffee at Starbucks!!!

Just grind the TOOL!!!!!