ramaroodle
Member
How do you sharpen a negative rake carbide tool?
How do you sharpen a negative rake carbide tool?
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monophoto
Member
My response could trigger some contrary comments, but here goes - - -
Most everyone understands the marketing philosophy behind the Giletter razor - offer an inexpensive razor, and them make money by selling proprietary replacements blades. The same principle applies to carbide cutters. The original concept of carbide cutters was that they were disposable - they were fairly long-lasting, you used them until they became dull, and then threw them away. They were designed in a way that allowed the cutter to be rotated so that a single cutter could have a longer life, but in the end, carbide cutters were still disposable.
Then, along came a hack - someone discovered that flat-top carbide cutters could be resharpened using diamond hones - just rub the flat top on the hone. That's fine, and it does extend the life of those cutter with little or no set-up cost, but resharpened cutters aren't as sharp as new cutters, so it's just what I described - a hack, not a panacea.
Andy asked about negative rake cutters - cutters with a second bevel on top rather than a flat top. Sharpening the beveled top on square cutters is difficult because the bevel is tiny, and cutters themselves are very small, but sharpening round cutters would be impossible since that upper bevel is geometrically part of a cone. I suppose someone could come up with a jig that would allow those to also be tuned up with a diamond hone, but frankly, I don't see the point. And there is no incentive for negative rake carbide cutter manufacturers to invent a solution since they make money by selling replacement cutters. So I don't think there is a practical way to sharpen them.
There is a school of thought that 'negative rake' is also something of a hack - scrapers (and carbide cutters are actually scrapers) work better when they are held at a slight downward angle, so a marketing person at a tool company decided that they could generate a new market by offering scrapers with a top bevel. And that then led to the next logical step in the progression - carbide cutters with a top bevel.
But wouldn't the same effect be created by tapering the top of the bar on which a flat-top cutter is mounted? And because it has a flat top, it can be sharpened with a diamond hone.
Most everyone understands the marketing philosophy behind the Giletter razor - offer an inexpensive razor, and them make money by selling proprietary replacements blades. The same principle applies to carbide cutters. The original concept of carbide cutters was that they were disposable - they were fairly long-lasting, you used them until they became dull, and then threw them away. They were designed in a way that allowed the cutter to be rotated so that a single cutter could have a longer life, but in the end, carbide cutters were still disposable.
Then, along came a hack - someone discovered that flat-top carbide cutters could be resharpened using diamond hones - just rub the flat top on the hone. That's fine, and it does extend the life of those cutter with little or no set-up cost, but resharpened cutters aren't as sharp as new cutters, so it's just what I described - a hack, not a panacea.
Andy asked about negative rake cutters - cutters with a second bevel on top rather than a flat top. Sharpening the beveled top on square cutters is difficult because the bevel is tiny, and cutters themselves are very small, but sharpening round cutters would be impossible since that upper bevel is geometrically part of a cone. I suppose someone could come up with a jig that would allow those to also be tuned up with a diamond hone, but frankly, I don't see the point. And there is no incentive for negative rake carbide cutter manufacturers to invent a solution since they make money by selling replacement cutters. So I don't think there is a practical way to sharpen them.
There is a school of thought that 'negative rake' is also something of a hack - scrapers (and carbide cutters are actually scrapers) work better when they are held at a slight downward angle, so a marketing person at a tool company decided that they could generate a new market by offering scrapers with a top bevel. And that then led to the next logical step in the progression - carbide cutters with a top bevel.
But wouldn't the same effect be created by tapering the top of the bar on which a flat-top cutter is mounted? And because it has a flat top, it can be sharpened with a diamond hone.
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ramaroodle
Member
Just bought a round neg rake that will fit on my Rockler tool. I sharpen my regular carbide cutters using the hone method. Fast, easy, damn near good as new in 3 minutes but negative rake seems like it'd be a real pain to do. Thinking maybe I'd only use them on acrylics to save the sharpness. Although my regular carbides seem to do just fine on acrylics too. Might try using my Worksharp hone grit on the radius square one but the round one seems like it'd be a challenge if not impossible. Now that I think about it maybe I'll just send them back since I got them on Amazoo Prime.
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Curly
Member
I don't have any carbide much less a negative rake, however if I was going to sharpen the round ones here is what I would do. I would get a rod and drill and tap a hole in the end to match the screw for the insert. Then with the insert screwed to the end of the rod spin it in the lathe, drill press or even a hand drill. Touch a diamond hone on the right angle to it and it would hopefully sharpen it.
One of the big selling points of using carbide is to have a tool that doesn't need to be sharpened. Doesn't sharpening the insert kind of defeat the reason for them in the first place?
One of the big selling points of using carbide is to have a tool that doesn't need to be sharpened. Doesn't sharpening the insert kind of defeat the reason for them in the first place?
ramaroodle
Member
That's a myth. They do need to be sharpened. I sharpen mine all of the time. Your idea for the round one is a good one though. just don't know if it's worth the effort.
egnald
Member
As monophoto stated above, the same effect can be created by tapering the top of the bar on which the cutter is mounted. This is a tool that I made which mounts the cutter at 20-degrees so the scraper is presented with a negative rake angle. So far it is working very well.
Regards, Dave (egnald)
Curly
Member
Where I was going with that is many people get carbide tools so they don't have to get a grinder (cost or space) and learn to use it. They just have to turn the insert when it gets dull until there are no sharp edges the put in a new one. Somewhere in the process they realize the cost of inserts adds up and the search for ways to sharpen them comes to the conversation.
The interesting part for me is seeing the sharpening of inserts at all. I inspected structural aircraft parts in a machining plant for over two dozen years and never saw one sharpened or even touched up. In fact they are usually changed based on machining time and not sharpness so they were tossed before they got dull. Cheaper than having them get dull, fail and destroy the tool or parts.
Now back to the regular scheduled program.
The interesting part for me is seeing the sharpening of inserts at all. I inspected structural aircraft parts in a machining plant for over two dozen years and never saw one sharpened or even touched up. In fact they are usually changed based on machining time and not sharpness so they were tossed before they got dull. Cheaper than having them get dull, fail and destroy the tool or parts.
Now back to the regular scheduled program.
ramaroodle
Member
Now THATS what I'm talkin bout! A man after my own heart. What a great idea! Certainly worth a try.
I've had the same inserts for two years plus the ones I bought thinking the other ones couldn't be sharpened. I have a slow speed grinder and gouges that I haven't touched in 2 years. Maybe someday I'll turn a bowl or two. I use a round and 2"radius carbide tool and a 3/4" Benjamins Best skew and sharpen them all with a card-sized hone. I'll put the skew on my Work Sharp once or twice a year. I've made over 100 pens and they are always sharp as can be after a minute or 2 on the hone. PLUS you rotate the inserts 90 degrees so it's like having 4 sharp blades so I get 20-30 pens between sharpenings. But I digress.
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monophoto
Member
Exactly! Carbide is not inherently better than conventional HSS tools - in fact, carbide is arguably not as good as a sharp HSS tool. But it is more convenient because it isn't necessary deal with sharpening. And the price for that convenience is the need to periodically replace the cutters.
A lot of the tools we see advertised in the catalogs aren't nearly as essential as the manufacturers would have us believe. Instead,, they are improvements over more conventional tools that offer greater convenience.
Face it - a modern variable speed lathe makes a lump of wood rotate at a fast speed. An old fashion foot-operated pole lathe does the same thing. The difference is convenience. OK - that may be a bit of an exaggeration, but you get the point.
walshjp17
Member
"But wouldn't the same effect be created by tapering the top of the bar on which a flat-top cutter is mounted? And because it has a flat top, it can be sharpened with a diamond hone."
Which is exactly what Bob Marquis (Magic Bob) has done with his NRS (negative rake scraper) https://tshadow.com/turning-tools/9....html#/30-package-deluxe_package_short_handle.
Which is exactly what Bob Marquis (Magic Bob) has done with his NRS (negative rake scraper) https://tshadow.com/turning-tools/9....html#/30-package-deluxe_package_short_handle.
ramaroodle
Member
Well, it's nowhere near as pretty as Dave's but it works just fine! Great idea. Thanks for all of the input. Gonna send back the neg rake ones I got.
