Sharpening TiN-coated HSS turning tools

Lataxe - titanium nitride specs at 85 rockwell C. Ceramic alumina just at summary glance covers a range from 80-90 rockwell C. I don't know the mohs numbers, or any other basis, but I'm sure they can be found. 

I use a lot of the micron ceramic belts for toolmaking, and of the alumina belts- or really anything reasonably priced, they are by the far the fastest and coolest grinding, but my understanding is the difference is they are a friable pile of micron sized or so little bits. First, the TiN is a coating on these tools, so you will be able to grind them. will it be as easy as diamond would be? No, but diamond at speed and with spark is going to erode unless it is cooled, so I have little confidence you would get long belt life on a $200 belt and would guess they are specialty polishing equipment for certain steels that have really hard carbides in them. 

What is the chance that you could buy a mandrel for your lathe or drill press and put inexpensive 200mm diamond discs in it? I've had great luck with those for the purpose of lapping while toolmaking, and they are much longer lived in terms of distance honed vs. the high pressure low speed of hand diamond honing. 

Silicon carbide belts are also an option for your sorby, but they are in the same ballpark as the TiN, and probably also variable (further look suggests a step harder than TiN. The problem with SiC is I've never had a belt with it that has much longevity. 

An interesting question  -Bill Tindall on this forum long ago had a bunch of cold work tools TiN coated (or some, I shouldn't state quantity - I don't know how many it actually was), but for cold work, they didn't make much difference. For turning with less acute angles and rotary cutting, the benefit potential sounds more reasonable.

Hello David,

Thanks for that very useful reply.  As per your remarks, I'll try a ceramic belt on the Sorby Proedge to see if it manages to cut the very thin TiN layer atop the M2 HSS of a small turning tool bit.  The ceramic belts are around £5 for a 120 grit or £6 for a 220, which are the recommended grits for touching up turning tool edges. It won't be a disaster if I rub the grit off one.

If that doesn't seem to work well, the diamond disc notion sounds a good way to go. I can put together an MT2 arbour with a 16mm threaded end on which to mount a 100mm 150 grit diamond disc sandwiched between two M16 washers with 16mm holes and locked-on with an M16 locknut, for £27. This will fit in the lathe drive centre. The arbour will also be useful for mounting other things, such as a sanding disc, as well as for the diamond disc.  It should be a simple matter to fashion a block of wood of the right height to lock to the lathe bed rails, with an 10 degree slope on which to rest a turning tool so as to present the tip to the diamond wheel at the right angle for touching up the edge.

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No TiN-coated tool bits are yet in the turning tool box, though. The HSS bits work perfectly well ...... but I do like to have a play with this and that.  Since a new TiN-coated tool tip is only a couple of quid more than the uncoated HSS versions (£15 rather than £13) I'll give one a try when attempting to turn something needing a strange-shaped tip - of which there are many, as you may know.  They're purported to keep their edge "at least 3X as long as HSS bits". I doubt that there's any other advantage .... but perhaps the TiN also provides a finer as well as stronger cutting edge?  But fine edges seem to be redundant in turning. :-)

My recollection from Bill's experiment was that TiN could be fractured due to hardness. Those are my words summarizing what he said. it's difficult to make something that hard that also has impact toughness, and I don't know if there are conditions regarding adhesion to the base metal (as in, can impact hit the edge and break some loose at the tip but also a little further back because the adhesion allows for it). 

It'll be interesting to hear what you find. There are steels like 10V and 15V and others that go up the ladder in edge life, but they get expensive. I believe 15V may have gone extinct with the demise of Crucible steel, but it had been used in (probably eye wateringly expensive) laminated tools for production turners. Laminated because it's got so much carbide in it that it is fragile and will break easily if it's just a single "log" of steel like M2 tools. 

If you find sorby's claims accurate in use - even if 1/3rd or 1/2 of the claimed additional interval occurs, it's a big gain and nearing 10V steel edge life at half the cost.