Grit value for honing carbide tools

I have a few carbide brazed tools. The carbide type is P30. I can make them very sharp with a special green grinding wheel. But I was thinking to improve the cutting edge with a diamond wheel. What would be the best value for grit?

I'm thinking on a cup grinding wheel like this one.

I've only ever used a green grit wheel because if I made my carbide too sharp it was prone to chipping at the cutting edge

However, you have a very good machine and if your carbide tools are of sufficient quality you may just get away with it, but it's usually only hss which gets the razor edge treatment

Definitely around the 150/180 grit size, any finer and it won't remove much material

I would go for a finer grit because you don’t need to remove much material, the 320 should be a good compromise, if you can afford two get a coarse and a fine.

I'll buy only one. They are not cheap and it is not a necessity, more a curiosity. I have one of those small USB microscopes and I'm curious how the edge of the tool will look after honing.

Also look at the edge after using it to see if the fine edge you get from honing will hold up under machining. Let us know what you find so we can use the information to make decisions on our own carbides.

Depending on the size of the tool, there is an interesting approach to using diamond wheels to hone small scale tools at Taig Lathe: Lathe Tool Sharpening Jig (toolingaround.ca)

I would be inclined to suggest that you start out by getting one of the inexpensive diamond "sharpening stone" sets (available widely on the internet for around €10-€30) which you can use to work the edges by hand (either with the "stone" secured on the bench and the tool hand-held like a wood chisel, or the tool in a vice and the "stone" handheld like a slipstone or scythe-stone).

That will both allow you to dip your toe in the water to see if the results are good enough to justify investing in a proper wheel, and because most come as a set of 3-4 grits, give you an opportunity to compare to see what works best to help you pick a wheel that suits your uses best.

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Carbide doesn't necessarily benefit all that much from an extremely sharp edge, for exactly the reasons Ady1 describes, but it can benefit a lot from a really smooth surface finish on the surfaces that intersect to create the edge, which helps to reduce the tool-pressure more than you'd think.

On one or two occasions I have used diamond laps/hones/stones of a comparatively fine grit to polish up the points behind the cutting edge on a tool, then taken one or two strokes with the very lightest pressure to just slightly dull the knife-sharp edge created, which is something my grandfather taught me.to do, and results in a ground tool which very closely mirrors the design approach used for modern insertable tooling.

A diamond cup wheel costs around 20€. So the advantage with the diamond stone is only that you have more grits. Tomorrow I'll check the local flea market, I saw last Sunday a few wheels. If it is a good price I will not wait one month to get one from China.

Meanwhile I had a look with a microscope on what is going on in the cutting edge of tools. I have an unused old communist HSS 20x20mm tool that has some rust. It doesn't look too bad:

Until you put it under the microscope:

It reminds me of underwater stones. I tried first to polish it with a felt disc:

But the rust has made small holes and the surface is not even anymore. Then I used a slow turning wet stone, with maybe 120 or more grit, and it looks better:

And finally I polished it with the same felt disc + a blue paste:

Here I draw the first interesting conclusion. Polishing does nothing to improve the surface. It make it mirror like, but those vertical lines from the grinding wheel are too deep. You can see that the top side is polished better but the surface is still not smooth.

The second conclusion is that those cheap USB microscopes are toys. They are just 2Mpixel web cameras with a different lens. I think I can make better pictures with my phone.

Next is a P30 20x20mm brazed carbide parting toll. I used it to make a stainless steel handle for my tail stock locking bar. The first picture is with a new one. It cut a 20mm bar with it. The only thing that I discovered here is that re-sharpening was not necessary. And that I used a wrong angle anyway.

Im afraid that using a microscope for looking at tool tips will have you chasing your tail.!

Yes, in the end the results matter. But still is interesting to have a look.

Not quite right - some steps are missing!

The process starts by removing most of the metal quickly with a coarse grit as you did. However, as coarse grit leaves deep scratches, it's then necessary to remove them with a finer grit. This also leaves scratches, albeit finer, so an even finer grit is applied. Scratches are removed in stages, not in one go.

I quite often start with 30 or 100 grit, then go down through 200, 400, 800, 1200, 2000 and maybe finer still before applying the final polish. Light scratches don't matter on an HSS tool, so I probably wouldn't bother below 400 or 800.

Dave

Before throwaway tips became popular we had to use Wimet brand brazed on carbide tools, these were sharpened with the usual green grit wheel then put on a diamond lap, a fine diamond wheel to put a decent edge on. When the tool became dull it was just touched up on the diamond lap. We didn’t have microscopes or bother polishing the tools, just grind to shape and lap the front and side clearances and a quick rad applied, we didn’t faff about, we were on bonus, that was just how it was done.

A green silicone carbide wheel can not produce the correct finished tool surface just like your photos show. Yes they will still cut, but the surface finish won't be the best. Honing does make a difference in HSS for tool edge durability and possibly even more important to the parts surface finish. As G.H. Thomas mentions in his Model Engineer's Workshop Manual, a roughly finished tool straight from a grinder can not produce a finely turned surface finish. He also pointed out that a honed HSS tool would stand up for much longer before it required resharpening.There were and are multiple slow speed diamond wheel lapping tools produced and specifically designed to power lap both HSS and Carbide. If it wasn't a viable and useful method, then those expensive slow speed lapping tools wouldn't even be available. While most replaceable carbide tips aren't honed, have a look at there cutting edges. There formed in molds that are highly polished so they create very smooth cutting edges.

Those un-honed tips obviously aren't razor sharp, but most carbide tips are designed to be used on high horsepower and rigid machines. At there average cutting speeds, the high heat generated at the tool tip helps to plasticize the material. So there's more plowing or flowing of the material than actual cutting in the traditional sense. There are some softer material specific carbide tips that are honed, and they can be made to work with steels at the expense of some cutting edge life. But most of the custom shaped carbide I hand hone are Micro 100 brand and the braised carbide type. It's properly formulated since the manufacturer knows it will be rough ground and then lapped just like they already finish hone there tools. Cheap and poorly formulated carbide might well break down very quickly compared to industrial quality, but that's the fault of trying to save where you shouldn't.

I couldn't wait and I bought a diamond cup from the local Sunday bazar. It is unused. The only drawback is that I don't know what grit value it has. It could be 100. But the word in front of it translates into "concentration" ... so who knows.

By the looks of the label could be 100 grit

The label says

"Granularity No. 2 - model Kang Abrasive JR Concentration. Binding agent Specification 2"

I think it is important to know here that the coarseness of diamond wheels is not given in the same numbers as other grinding wheels.

The usual system goes from small numbers = coarse to big numbers = fine. So grit 100 is (much) coarser than grit 1000.

In diamond (and CBN) it is the other way round. Usually the grit is preceded with a D (or B for CBN). The numbers are in steps, which I can't write down out of my head, but:
C40 is very fine, C1xx is coarser, C2xx is very coarse.

But as with most things, there are exceptions. I have a couple of Chinese grinding discs (galvanic bound on 1 mm steel, 120 mm dia.), which use the grit system. Mine go from 800 to 3000. They are wonderful to hone carbide scraping blades, and naturally turning tools also. Used on a very slow grinder, driven from a windscreen wiper motor.

Regards,
Hans

Postscript: polishing lathe tools with a felt disc and polishing compound is nonsense... Sorry for being so blunt.

Edited By Versaboss on 01/05/2023 22:53:30

And one more: found a list of the grit sizes here:

Grit size

Edited By Versaboss on 01/05/2023 23:01:25

Could you explain how you arrived at that translation, Jon? It's not only hard to interpret but very defective.