OT ? - Hardness & abrasion questions

  1. If I want to use a piece of plate (float) glass to lap a piece of steel, would it make sense to cover the glass with contact paper, or something similar, prior to applying the lapping compound, in order to prevent abrasion of the glass?

  1. What is hardness? Strength of the molecular attraction? Inherent property of an individual molecule?. What is it? Seems like this is something I should have learned in 6th grade.

  2. Silicon carbide (or flint, garnet, AL2O3, etc.) sandpaper only lasts a short time when sanding relatively soft wood (or metal, for that matter). Aside from any mineral content in the wood, why? Is it only a function of the abrasive paper's bonding material? The abrasive particle's tendency to fracture? Will an infinite amount of soft material always wear down a limited amount of a harder material? Could a heavenly cotton candy machine produce enough coton candy to wear down the Hope Diamond?

I've an inquiring mind, but, contrary to their slogan, the National Enquirer is _not_ answering these pressing questions. At least _they're_ responding to my emails. CNN, MSNBC, et. al., aren't replying at all - I'm not sure if it's some sort of conspiracy, or that they're ashamed to admit that they don't know the answer.

R, Tom Q.

Reply to
Tom Quackenbush
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Hm, glass deforms little so lapping compound will stick in the steel more, no? So the steel would tend to lap the glass? Unless it's hardened. Then it'll be about even. I doubt contact paper would be a good idea...wouldn't that be a bit uneven for your application?

'Far as I know.

Sometimes.

Depends on what you look at... it might be molecules sticking together strongly (diamond is a continuous structure, for example) or merely in each other's presence (graphite and mica are loosely layered). Most plastics (HDPE, etc.) have the same bonds (carbon-carbon) as diamond, but not as many, and broken up by the hydrogen alongside it, so aren't as strong. Plastics are also made of long strings, which probably don't crystallize (making it a formable glass?), but hold it together like fibers in a piece of cardboard.

Crystallization is also another matter. I don't know if abrasion, at least small level (like polishing) affects this any, but at least large scale abrasion and other large-scale stresses (like bending of a bar) tend to concentrate around grain boundaries. An extreme example of this is gray cast iron, which has free graphite flakes in its composition. The base metal (pearlite, etc.) is quite strong, but because the flakes are weak and sharp, they concentrate stress and reduce strength to only 30-60ksi, with nearly no ductility.

Dunno. I suppose it's simply the action of two surfaces rubbing on each other; sooner or later, some atoms are going to be ripped out.

Only if you notice the grit disappearing. Bad paper on tight corners can be amazingly useless...

Unlikely, you aren't applying enough force. Although the very first sharp corners might disappear for this, if the material has cleavage planes. I think most grits have little or none.

AFAIK, yep!

Maybe you should e-mail Dan Rather...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

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Reply to
Tim Williams

I think you're asking several unrelated questions here. You can find discussions of steel hardness and strength, and where they come from (strained chemical bonds, IIRC) in a basic materials-science text.

But there is a new function on Google that might be just what you're looking for. It's called Google Scholar. Click on the "more" above the entry box in Google and you'll see it. I just tried it with ABRASION RESISTANCE HARDNESS and got around 2,500 hits. They're mostly adademic references, including a lot of white papers. They seem to have some of the answers you're looking for.

Abrasive paper usually is limited by dulling: tiny fractures that break off the sharp edges. Brittle materials break and chip for a variety of reasons, of which the hardness of the material they're abrading is only one.

It's quite a piece of technical info you're biting off to answer all of those questions. Have fun.

Ed Huntress

Reply to
Ed Huntress

You're probably right about the contact paper; but I'm only talking about few thous. tolerance. Contact sheet brass or copper would probably be better - I think I've heard of such, but I've never seen any.

OK. I'm pretty sure read something similar, and it would explain the behavior of some cast iron that I _most certainly_ did not abuse.

Even though I mentioned abrasive paper, I was thinking of how a (steel) star wheel dresser can shape a SiC wheel. Should've said so from the get go - sorry. Some of that must be due to the bonding agent. I imagine a bed of diamonds bedded in a sea of molasses - you could easily level them with a piece of fir.

Yeah, ol' Dan is "free at last". Thank God.

Jeez, you're in High School now, right? I hope you mention r.c.m. when you get your Nobel Prize. Of, course, all of the smart guys here will be dead by then .... I'll still be around, though, and I'll be keeping you honest.

R, Tom Q.

Reply to
Tom Quackenbush

Probably so. I should have mentioned the context. Someone on rec.woodworking asked about the deleterious effects of lapping on a glass plate. Also, I noticed that Lee Valley sells some sort of PSA plastic sheet to stick on your surface plate, but they don't specify what advantages it confers.

Excellent. I hadn't known about that function. About 2,370 hits. That'll take me weeks and weeks - hey, waitaminnit! I sense that you have an ulterior motive here, Ed.

Ok, I will. Thanks for the info.

R, Tom Q.

Reply to
Tom Quackenbush

Meh, use some spray adhesive. But then you have to make sure that layer of adhesive stays within a few thou, no bubbles, and so forth.

Can always try some wet/dry paper taped down at the edges. There's no reason the paper nor the grit are as even as the glass or the adhesive above, but I have a nagging feeling it is...

Heh heh.

I dunno about the star dresser, I haven't seen one working in person actually. I would imagine your description is correct. Similar to how you can sharpen carbide (well, you probably could if you wanted to) on a diamond-coated steel type sharpening stone, yet anything which can cut the steel base can abrade layers off the side, taking the previously-thought-to-be-indestructible diamond layer with.

Actually, I officially graduated homeschool this year. :)

Heh heh. Not sure you guys would want the attention... I mean, we've already got Cliff...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

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Reply to
Tim Williams

Well, we're looking forward to hearing the results of your hard work and research. The Scholar function is in beta, and has only been around for a couple of weeks, I think.

But I'm not looking for story ideas, honest. I write about medicine and drugs now, not metalworking.

Ed Huntress

Reply to
Ed Huntress

Star dressers are actually an impact device, which is one of the reasons they are not so great for truing a wheel to run smoothly. On the positive side, a wheel so dressed is generally a far better cutting wheel than one dressed with a diamond. They are just difficult to use for achieving good finishes when precision grinding. With today's finish requirements, diamond is pretty much the only way to go.

When a star type dresser is used, the impact of the steel wheels chip away at the bonding agent, typically a vitrified wheel. In the process, some of the abrasive is split, but the vast majority of the bits are simply broken away by impact, exposing new grains that are sharp already. Bottom line is the dresser is far softer than the abrasive, and would lose the battle fairly quickly if it didn't spin, hammering the wheel as it did so. Try applying one sideways and see how long it lasts.

Harold

Reply to
Harold & Susan Vordos

That's pretty much the system I use now for sharpening & "lapping". I've got a half dozen pieces of plate glass covered with different grades of wet/dry paper. I used to use 3M spray adhesive to attach the paper, but now I just use a glue stick. Easier to apply & remove and no lumps. The glue stick doesn't make a very strong bond, but that's fine for my purposes.

Actually, the diamond/molasses was a terrible analogy. I'll to do better next time. Fortunately, Harold has already posted a very good description of how a star wheel dresser works.

I missed that. Congratulations!

R, Tom Q. Remove bogusinfo to reply.

Reply to
Tom Quackenbush

Thanks for the good explanation (I knew my analogy was pretty crappy, even as I was hitting "send"). I haven't used a star wheel dresser in years.

R, Tom Q.

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Reply to
Tom Quackenbush

Hmm, I hadn't thought of that. I just thought you were trying to get rid of me for a few weeks.

R, Tom Q.

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Reply to
Tom Quackenbush

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