Diamond wheels for honing scraping tools?

[Neilho]

I'm a little late in replying, but here goes....

I bought the 320 grit wheel, and after putting new bearings in the grinder and fixing several things the POs had butchered (why do all my projects go like this?) tried it on the scraper. It worked well for rough scraping, better than my previous green wheel and hand diamond hone sharpening, probably because the angles could be held more consistently, but as JTiers said, didn't achieve that "spangly finish". So I spent a little time making a cast iron lap, charged it with some diamond lapping compound from Mcmaster-Carr and and chucked it up in the lathe. It took a while to get the hang of how to charge the wheel with how much lapping compound and how much lube to apply when (the usual learning curve), but got it worked out, and used the microscope to check the edge. Next time I'll use some sort of angled table for honing- trying to maintain the angles by hand is just too much of a hit and miss operation. It's impossible to see what's really going on at the edge during honing, so the procedure was: try something, look at it under the microscope, try something slightly different etc till an acceptable edge emerged. But I did get reasonably fast at it and then the job was over. (Why do all my projects go like this? But it worked well enough for the first time, the planer table is now scraped to good-enough-for-woodworking standards, and my shoulders and hands are recovering nicely. Hopefully I'll still remember all this the next time I scrape something.

Thanks to Harold for stimulating the thread and asking good questions and especially to JTiers and JPFalt for the lapping and finish info. It's been a pleasure.

[J Tiers]

I'm a little late in replying, but here goes....

I It worked well for rough scraping, better than my previous green wheel and hand diamond hone sharpening, probably because the angles could be held more consistently, but as JTiers said, didn't achieve that "spangly finish".

I have not used a green wheel at all.... but the folks who ought to know have told me that they are not a good way to start, because the abrasive tends to (on a microscopic level) tear out pieces, instead of evenly abrading. That starts you out with a rough edge that has to be lapped longer. I think it is true, based on my use of coarse lapping sticks.

Once you get an edge going, lap before you absolutely have to, so you don't need to go as deep.

[Neilho]

Yeah, the green wheel was used mostly for very crude initial shaping of the edge. You're exactly right, it took a long time to get a real edge on it with a hand hone, and even then it was very hit or miss. The 320 wheel for shaping, then diamond lap for the real edge was a huge improvement. You're right, too, about the good effects of frequent touchups.

If I ever do another scraping job this big (16"x30") I'll do some R&D on handles, for both hands. I tried a lot of different grips and handles on solid and tubular scrapers, the best being old tennis balls and lots of friction tape on an Anderson ball handle on a tubular scraper, but still had a lot of trouble with my hands going numb. They'd recover after a day or so, so I cut scraping time to an hour per day but I'd like to be able to scrape till the job is done and have my hands intact. Maybe I'll buy some version of a power scraper for next time.

[Harold_V]

Green wheels have never been an option for me. I worked in one shop where they used them exclusively for sharpening turning tools and came to hate them. You can't get a decent finish on carbide, thanks to the wheel clubbing the particles off instead of cutting them. Surface finish suffered considerably when using such tools, so I think I can understand how important it must be when scraping.

From comments I've read here, I get the idea that a scraper must have a surface finish such that nothing adheres to it. The slightest degree of adhesion creates tearing when the tool is applied, thus the importance of a keen edge, and one that is very "slippery", which comes about from lapping.

Harold

[Michael_Az]

Harold, here is something I just experienced with the green wheels. About a year ago I bought Marty's B & S Microgrinder. I have never owned a surface grinder before so lacking in experience. When I went to grind the chuck, I chose a Norton AO wheel that was hard thinking the chuck was probably soft steel and couldn't get it to cut even a few tenths, wanted to burn. Yes, using flood. So, I tried a soft Norton and same thing. Just couldn't make it work. I finally got the chuck pretty flat but sure wasn't pretty. Some time later I got two sets of shear blades in to sharpen. First set went well with the soft AO Norton wheel. The next set wouldn't grind at all, wanted to burn! Same with hard wheel. After trying everything I could think of I thought the hell with it I'm going to try a green wheel. It worked beautifuly! I couldn't believe it but was very happy. After the job was done I got to thinking about trying the green wheel on the chuck and same thing, it worked like a charm. I kinda figured out the chuck must be cast iron instead of steel and that is probably why the green wheel worked so well on the chuck but to this day I don't understand those shear blades. Could one set have been some kind of stainless? Wouldn't be my choice for a shear blade. Grinding I'm beginning to think is a black art.
Michael

[jpfalt]

You're right about it being something of a black art. among the things that affect it are:
Size of grain
Shape of individual grain
Orientation of grains (standing up vs laying down. One supplier used to advertised that the grains were electrostatically aligned going into the wheel mold to present the best cutting edges.)
Friability of grains (if and how the grain breaks or chips before the grain is lost)
Material the grain is made from (type, purity, natural vs synthetic)
Bond type
Bond hardness
Bond toughness
Bond percentage in mix
Cutting speed
Depth of cut
Feed rate
Type of dressing
Type of coolant
Amount of coolant

And if any combinations of binders, grit complicates things even more. This doesn't even get started on the type of material you are grinding.

If a green wheel works for you, go with it.

[Michael_Az]

Yes, JP, there are so many variables it isn't funny. I think I have come to the conclusion to try what seems right and then experiment until it is right. One thing that doesn't help is sometimes something comes in the shop and you can't be sure exactly what the material is. Not hard to make an educated guess but those shear blades really threw me for a loop! I would at least expect them to grind the same way but they wouldn't.
Michael

[Harold_V]

The rule of thumb is to grind steel with aluminum oxide, but it's likely not beyond possibility that there are certain alloys that break the rule, due to their composition. Certainly in the tool steel family, especially if they are not iron based, you run the chance of finding an exception, which may explain the shear blades. I'm sorry to say I have no experience that parallels yours.

When the wrong abrasive is selected, one of the obvious signs is exactly what you experienced. The wheel quickly dulls and cuts hot. It often acts as if it's loaded, yet it has done little work. Dressing makes almost no difference. In the case of using aluminum oxide where silicon carbide is a requirement, it is chiefly due to the AO simply not being hard enough to withstand the abrasive action, so it dulls quickly. It's somewhat different when silicon carbide is used when it should not be. When you introduce silicon carbide to steel, where there's high temperatures, the silicon wheel is absorbed by the steel, almost immediately dulling the grain. In spite of the fact that silicon carbide is a great deal harder than aluminum oxide, it doesn't hold up as well when grinding steel, for that reason. That it held up on shear blades is a pretty good sign the blades are an unusual alloy, possibly not even iron based.

Normally, green wheels are bonded way too softly to be affective on almost anything aside from tungsten carbide. Because carbide is so hard, the green wheels are intended to break down quickly, exposing new, sharp grains, so they keep cutting. Had you tried a common (black) silicon carbide wheel, you'd have likely had even better luck. Such wheels are available fairly easily in the small surface grinder sizes (7" x ½" x 1¼") , due in part to the same wheels being used on cutter grinders.

I can't explain the phenomenon you've experienced with the mag chuck on your surface grinder. I've dressed a lot of magnetic chucks in my years, always with aluminum oxide, and never with any problems. I'd have to agree, there is something unusual about your particular chuck, or your aluminum oxide wheels are so poorly matched to the work that they make the silicon wheel look good. That boggles the mind.

As a parting shot to yours, about grinding being a black art. I'm not convinced the principles are, but I can assure you that the skill of grinding is. Few of the exceptional machinists I have known were what I would call good at grinding. My mentor, who was an exceptional grinder, was a lousy machinist. It's a strange procedure, one where you more or less become one with the machine and its activities. Skill, combined with gut feeling, or a seat-of-the-pants kind of thing is required. You don't learn it from a book, and you may not learn it under good tutelage. I firmly believe that good grinders are born, not created. Same thing as fine musicians.

Harold

[Michael_Az]

Thanks for the reply Harold, and all the good information. I do feel that I might have come across as a dummy with those experiences, because they sounded so wrong to me. I don't understand those blades and/or the chuck either. But just like you mention, the wheel loaded up right away with only a three or four tenths pass. Have you ever heard of a cast iron chuck? That would explain it. Anyway, I was trying a new Norton AO wheel, one of the 32A's and it sure wouldn't work.
Funny you should mention the black wheels, I think one trade name is "Crystolon". I knew I should have one or two on hand and have been watching ebay for months. Somebody put up several in seperate auctions and they all ended within minutes of each other. I didn't want to miss out completely so I bid on all the auctions. Yes, thats right, I won them all so now I've got 18 black Norton wheels. Oh well, average price was $2.25 or something like that.
Thanks again
Michael

[Harold_V]

Have you ever heard of a cast iron chuck?

I don't recall having seen or used one, but it would certainly explain the phenomenon you experienced.

I was trying a new Norton AO wheel, one of the 32A's and it sure wouldn't work.

A better choice would have been a 38A wheel, and might spell the difference. Hard to say.

Funny you should mention the black wheels, I think one trade name is "Crystolon".

Correct. Norton uses that trade mark for their silicon carbide wheels. The Carborundum Company uses the trade mark Carborundum. Their aluminum oxide wheels are designated as Aloxite.

Your ebay experience was interesting. Your luck must run like mine, but at the price you paid, you can afford to have them on hand. Early on I was bidding on saw blade stock and scored several 100' rolls for less than $15 each. That's a price even we old retired folks can afford. [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

Silicon carbide wheels don't get much use, but they're nice to have on hand. For grinding almost anything except for steel, they're the wheel of choice.

A reminder: Dust from silicon carbide wheels is very bad for your health.

Harold