Grinding HSS Lathe Tools – Advice please

[HopperParticipant @hopper]

I've never had any problem using a standard 6" x 1" bench grinder fitted with the standard coarse and fine grey wheels. Then finish it off with a few rubs on an oilstone. Tin of water on hand to dip the HSS in to keep it cool while grinding and away you go. Never had a problem with the reported cracking of HSS from dipping in water. Maybe that is the cheap Chinese HSS? I use old HSS made in the West.

Yes it does take a while to grind 3/8 bits. But if you want instant gratification, this is probably not the right hobby for you. Take your time and enjoy the work.

[Chris TriceParticipant @christrice43267]

I've always used the recommended white grit wheels on HSS but I do get a bit of glazing. I've also dipped the tool being ground in water to cool and had no problem with cracking. I'm intrigued to know how they grind taps and dies.

[jaCK HobsonParticipant @jackhobson50760]

Posted by Neil Wyatt on 20/02/2015 15:41:22:

> Often said as in theory steel can absorb the carbon from the diamonds.

Neil

That's a new one for me. I'd suggest the theory doesn't apply at standard temperature and pressure.

I was sure I read in Sharpening by Lee that diamond could be too hard and fracture steel in a less optimal way than other abrasives. However, I cannot now find any such passage. On the contrary, he suggest diamond can be used for sharpening any steel. As I have just re-read this… a rule of thumb is that you should get polycrystalline diamond for paste, and monocrystalline for stones.

My personal experience is that you have to be careful using diamond stones on softer or tougher steels – for the stone's sake rather than the steels e.g. cheap stainless knives can strip the diamonds from stones very quickly. Using diamond stones on hard, brittle steel like HSS should be less of a problem in this respect. Don't press too hard. I have a lot of dull diamond slipstones

[Neil WyattModerator @neilwyatt]

I've got the science wrong. This is an explanation I found, but I don't think, in the hobby shop, it's a real issue (bear in mind the high temperatures are only at the point of grinding):

• Diamond will oxidize above 600ƒC into carbon-dioxide.
• Diamond has a tendency to react with the iron group metals (iron, cobalt, and nickel and its alloys) already at a temperature above 700ƒC. These metals are soluble in carbon, and diamond is just another form of carbon. Synthetic diamond is produced by catalytic conversion under high pressure, and metal inclusions of catalytic metal contaminate each crystal. Metal inclusions will convert the diamond back into a graphite type of carbon at temperatures above 700ƒC. Back-converted graphite expands inside the diamond crystal and creates microscopic cracks, which cause so-called "thermal weakening" of synthetic diamond.
• Diamond reacts with carbides (titanium-carbide, tungsten-carbide, and zirconium-carbide) already at a temperature above 700ƒC by converting back to amorphous carbon.
• Diamond will react with carbide formers (titanium and vanadium) above 800ƒC to convert into carbides.
• Diamond could be dissolved in transition elements of iron group metals or their alloys above 900ƒC
  In light of this, diamond cannot be used to grind steel or titanium alloys if the temperature exceeds 700ƒC. This makes it impossible for synthetic diamond to be used for the most common grinding applications, such as the grinding of hardened steel gears. This high rate of wear would make diamond uneconomical in comparison to CBN, or even other conventional abrasives.

[VicParticipant @vic]

About ten minutes in. He says it's well known in the industry that diamond "stones" aren't suitable for sharpening steel.

As Jack says, it's very easy to strip all the diamonds off the stone! I may try cubic boron at some point.

[steamdaveParticipant @steamdave]

Posted by Vic on 21/02/2015 09:54:38:

…As Jack says, it's very easy to strip all the diamonds off the stone! I may try cubic boron at some point.

Here you go Vic: A mere £95 for your 6" wheel from Axminster Tools.

Dave
The Emerald Isle

[VicParticipant @vic]

Or not then at £95! To be honest I don't use my grinder that much these days. Most of my tools are sharpened on a belt sander and I've recently been using well used 240 grit belts to touch up my Tangential tool bits.

[Rik ShawParticipant @rikshaw]

Chris – Taps are ground on a thread grinder and on the larger taps the flutes are ground between centres using a white saucer wheel. Smaller taps were not deemed worth the effort but sometimes I would regrind the lead in flute of a spiral point tap if things were desperate holding it in a collet head fitted with quick action rubber collets.

Rik

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