Adrian, very annoying for you. Perhaps taking a small cut with the carbide insert was the problem. Inserts can lack the sharpness of HSS. Some are highly polished, I have no experience of them but some speak highly of those made for aluminium, when used on steel.

Prussian blue has been used for checking fit for a long time, and I have never found it abrasive, having used artists' oil paint before I bought the 'real thing.' So I doubt it is the cause of your problem.

Bill

Maybe repeat the experiment with a piece of scrap half blued half plain. Soon tell.

Seems doubtful though. Blue is bacically oil snd dye and designed to be non abrasive when bluing bearings etc.

As per other replies, carbide inserts do have a slight radius on the cutting edge to prevent premature breakdown. This means that they don't do well on small cuts. You can, however, sharpen them on a diamond wheel. This will give an edge that will take a very fine cut but won't be as durable. I keep a second tool holder that is used for recycled inserts.

As to the possible damage by the blue, well while I wouldn't rule it out, I doubt that it is abrasive as the idea of coating a precision surface plate with it and then rubbing what would effectively become a lap on it, would obviously not be great and I'm sure that this would cause wear that would have been flagged up by now.

Still, as you say, an interesting question.

Regards.

David.

Can you explain this ‘3 thou to go’? 3 thou on diameter or depth of cut?

It’s not too easy to actually measure the remaining cut on a taper, I would have thought? I usually just keep going until the wide end is either where I want it, or until I get a good finish and then shorten it to length. Final cut is then generally ‘in hindsight’.

I would guess you were simply trying to take a too-small cut for the tip, rather than the blueing causing the problem. But only a guess – and the cutter seems to have already been subjected to some very fine cuts already, so possibly the cutting edge had already been degraded a tad by taking too-small cuts before that final cut. Only way to sort out the real truth is to replicate those final cuts (from 40thou) several times with fresh tips, I would think. Or maybe just the final cut, several times, with new tips – with and without the marking dye as there may be another over-looked factor that caused that particular problem (lathe, cutter holder, supporting centre, etc).

I don’t think it has anything to do with the marking blue, it is most probably caused by taking very small cuts with tungsten carbide tips. If you want to take off a couple of thou at a time you would be well advised to use a piece of HSS.

The difference ,might be something as simple as the swarf not flowing off the tip smoothly. As most of us have no doubt experienced, even a tiny change in lube – a misting of wd40 or cutting oil etc – can have a huge effect on surface finish. A 3 thou cut will produce a very fine swarf and it could be nothing more than the blue causing the swarf to roll into a bunch rather than flow away from the tip.

I was cutting some 8.5 degree included-angle tapers last week and I had to experiment a bit do avoid the swarf rolling over and marring the finish. Once I got the speeds & feed right they came out very nice.

I tried smearing some blue on a piece of steel and taking light cuts just out of curiosity today. Couldnt get it to tear the tip off the ccgt insert.

I recon that you just ran out of luck when the edge went off at a critical time. I find these sharp inserts intended for aluminium will normally manage a 0.0005" depth of cut in steel when an ordinary grade will only rub.