Trying to relate my understanding of grinding to the theory

[Richard Hutchings]

Can't wait until my shoulder heals so I can do some woodworking when I wake up. I can't believe I'm following some of this. More popcorn please. Carry on.

[Pat Barry]

When you lean against a wall, or stand on your deck, are you pushing it? How, when you have no forward motion? You don't have to be in motion to exert force. Newton basically said that when you apply force to something (by standing on it, leaning against it, hitting it), there will be equal reaction. Reaction could take a number of forms - object could simply apply equal force back at you (like you leaning on a wall), or accelerate away from you (like you leaning against a cart), or some combination. This is idealized - there is also friction and other issues that you would have to calculate if you were modelling something specific. It is a bit of a mental leap to think that an immobile object is applying a force to you, but if you distrust me here is a set of experiments you could enjoy:

1) Take a nice soft piece of pine (to keep it woodworking-related) and whack yourself on the forehead with it. There will be a bruise on your head, and a dent in pine. Both experienced force.
2) After that heals up, take another piece of pine, lean it against a wall and run into it with your forehead. Again, there will be a bruise on your head and a dent in pine. Pretty much the same outcome, except this time you reversed roles as to what was moving and what was stationary.
3) After that heals up, take yet another piece of pine, and put it and your head into a large vise. Squeeze it slowly and gently, but make sure you stop before you hear cracking. If you hear cracking you went too far - hopefully the board will still be in one piece. Again, your head will have a bruise and pine will have a dent, even though this time you were both stationary.

LOL - this is educational

[george wilson]

You may as well be "pushing" against a stationary rock. The rock doesn't push back. It is just there,and too heavy to move. You HAVE to push against the grinding wheel to get it to cut into the steel. You are getting confused about something.

This whole thing is rather silly.

[bridger berdel]

And most of the coathangers in the world would disappear...

[Marko Milisavljevic]

Imagine the wall was on rails with a giant spring behind it. You lean against it, wall moves until the spring compresses enough and it comes to stop, with you still leaning on it. Is it pushing you now? I think you'd agree it is, yet your body can't tell the difference - it is still feeling the same pressure against it, stationary or sprung wall. Through magic of Newton's third law, even a stationary object that can't initiate the pushing is pushing back when you push on it. It instantly stops pushing when you stop pushing. Like watching yourself in a mirror.

[george wilson]

Actually,I really don't care about this topic enough to waste any more time on it!!

As Jim said,it's one of those that goes nowhere and gets everyone upset.

[Brian Ashton]

supposing you tell me how the wheel is pushing back,when it has no forward movement ? Do you feel a physical sensation of the wheel pushing back against you? The force exerted is straight down,unless you are holding the tool well above the center line of the wheel. Then,from friction,it can push back some,but not a lot. I never have in over 60 years of using a grinder. It really doesn't even kick back like a buffer can(very easily),because the tool doesn't get "tangled in the wheel like it can in the soft fabric of a buffer wheel.

One of the reasons I concluded that newtons 3rd law of motion was at play was by using the example of a motor boat in water. As the boat moves through the water it pushes against the water which intern pushes back causing the boat to rise out of the water. The water is obviously stationary but due to the curve in the hull and speed of the boat it is still pushing back on the moving boat. This I believe is also happening, in a similar way, at the grinding wheel in that as an individual granule of grit on the wheel cuts into and scores the steel it pushes the steel back. The user pushing the steels is pushing at an equal force towards the wheel, thus holding it stationary.

Someone was asking why this is in woodworking... It wasn't explained but it relates to grinding edge tools.

@Jim K. Sorry you think this is a BS thread but one thing is for sure I was putting out the discussion cause I wanted to tap into the wealth of knowledge that exists here and try to develop an understanding of the grinding process. I don't belong to any other forums anywhere so here is where it gets posted. Not sure how this would become a ridiculous debate like sharpening threads simply because this is about understanding the physics behind a process, not a my sharpening method is better than yours pissing contest. If you have ideas I'd certainly like to here them. Ultimately I'm hopping to get a couple nuggets of info that will help to give me a clearer picture on what s taking place or a direction as to where I can do some exploring.

[Brian Ashton]

Hi Brian,



The force it pushes back with doesn't depend on speed of the wheel, in context of Newton's law you quote. It only pushes back as hard as you are pushing it. If it wasn't pushing back as hard, your grinder would experience acceleration in direction of resulting force, ie, it would slide away from you.



I think you would need to study materials science, Newton's laws won't let you meaningfully model interactions you are interested in.

I gave an example earlier of a boat moving through water that I think is similar to what's taking place at the grinding wheel. If the boat rises out of the water relative to the boats speed doesn't newtons law apply to some degree? I.e. the boat pushes against the water, the water pushes back...

Maybe my understanding of material science is off but isn't that the study of developing new materials. Im not sure how the grinding process relates to such a field.

[Brian Ashton]

Brian, what is the purpose of your question? I don't see how it relates to woodworking.

I had to edit this post after realizing this sounds like a snotty remark. I'm such seat of the pants wood worker, I don't use grinding wheels unless I buy a rusty and pitted tool somewhere. After removing the damage, the blade will never see a grinder or belt sander again. I don't even know what angles I sharpen to. My blades cut wood into feathery shavings if that's what I need. No calculations or measurements or theory required. Different strokes for different folks.

At some point as you say you use a grinding wheel for your woodwork tools...

I'm interested in being interested. It's a circular logic that drives me.

[Brian Ashton]

Your question is "What else am I missing?" A big unbounded question, I'd say.

There are lots of general textbooks on this topic that can illuminate some of the principles that you may have not yet discovered.

Take for example this:

http://books.google.com/books?id=FRI...page&q&f=false

Just use the PAGE DOWN key on you keyboard and browse the (abridged) book.

Google Books and the publisher are nice enough to offer you a large portion of the book for personal review. Luckily the first part of such books offer pretty good coverage of known basics. The basic theory and principles you seem to be looking for, that is.

Thx! that's a great reference. That should give me a couple hours of reading to do

[Brian Ashton]

Since we all agree on Newtons law (as proven above by multiple diverse and sometimes inconceivable points), lets figure out what exactly you are trying to figure out Brian? We need more information from you because its not really clear what you want

What I'm trying to gain an understanding of is a method of grinding that I'm sure isn't a new technique but more a forgotten one. This type of grinding relates specifically to putting a bevel on a piece of heat sensitive tool steel or carbon steel by hand. The usual rhetoric everyone espouses is you need to grind at a high speed if you want to remove material quickly... What I've found is the opposite. I've been grinding my tools at about 500 - 600rpms for about 25 years and always knew it was an extremely effective way of not burning a tool... I also had the inclination that it was also faster at removing material. For more than 15 years I never tried to verify it though because I didn't actually care about that. It wasn't till about 8 years ago that I mentioned it on a forum and the supposed experts all burred up and said I was out to lunch... At that time I thought maybe I'm mistaken so I put it to the test to see if what I was thinking was actually taking place. At the time I thought it might be 10% faster, maybe more maybe less I didn't really know. What I found to my total surprise was that I could remove material at about 40% faster without ever getting close to overheating the tip of the tool. I repeated the process a number of times with the same result. When the odd naysayer came over and dared to challenge me on it I showed them, with the same results each time. Couple years ago I mentioned it here with the same reaction from the "experts." Nowadays I don't give a rat ring if anyone believes it or not, my only goal is to understand the process for myself.

My understanding at this point is:

There's an inverse relationship between how much a single piece of grit on a grinding wheel can dig into a steel surface and rip out a strip of material and the speed at which it's moving. I.e. The faster the grain of grit is traveling across the surface of the steel the less it can dig in and therefore the less material it can remove, unless much more force is applied to push the steel into the grinding wheel. This is where I think Newtons 3rd law of motion applies.


But the problem to this is the harder you push the faster the steel heats up... which leads to... The two surfaces rubbing hard together is increasing the heat in the steel which is our main problem in grinding and the primary reason I made a slow speed grinder... This is where I'm wanting to get a better understanding of how to put together a formula that would for the most part explain what's taking place. It's pretty clear now, after thumbing through the two reference documents mentioned that such a task is very complex.

The heat transfer coefficiency of the steel also plays an important part in that it draws the heat building up away. When using a high speed grinder the steel isn't able to draw away the heat fast enough and it over heats relatively quickly as we all know. Whereas a slow speed grinder generates heat at a much slower rate, which allows the steel to deal with it much more efficiently.

So these are three factors I've identified that I think are taking place. How they interact and produce a good result is what I think I happened upon a couple decades ago. The lower rpm of the grinder allows each granule of the grinding wheel to dig much deeper and remove a much larger volume of material with each pass. While at the same time, the heat from the friction builds up much more slowly because the heat generated is related to velocity. To add to this, because the heat buildup is slower the steel is able to draw much of that heat away from the tip. This allows me to dwell much longer on the wheel which intern allows me to remove even more material... Culminating in a much more rapid removal of material compared to the common grinding methodology preached by all the experts.

[Richard Hutchings]

Why didn't you say that in the first place. Now I need to get a slow speed grinder.

[Brian Ashton]

Why didn't you say that in the first place. Now I need to get a slow speed grinder.

I was hoping less was more in this case

If you have a lathe and can find a mandrel that attaches to the outboard side (or you don't mind pulling it off all the time on the inboard side) you have a slow speed grinder.

[Richard Hutchings]

Good idea. Thanks Brian.

[Pat Barry]

It's a circular logic that drives me.

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.