Did Stuarts marking blue really do that?

Today I was making a morse taper.

All was going great except the finish with my rather old CCMT insert was a bit rough. So with 40 tho to go, i swapped to a CCGT to see how it would go on steel.

Like magic the finish was perfect, several passes and I was down to 3 tho to go, still a great finish.

I then decided to check the taper in a socket. I marked it up with Stuarts marking blue, checked it with the socket and all was good enough.

I did not take the blue off the taper but started to cut the final few tho. Within a couple of seconds it started squealing and not cutting very well. Like a fool I continued and ended up with notches and bumps all along the taper.

Eventually I got the insert under high magnification, could not see any damage at first. Then I noticed a slight different colour on the front face.

From the tip for about 1mm along the face the insert had been ground away leaving a nice sharp edge but zero clearance.

All I can think is the Prussian blue in the marking blue is abrasive. I think I will be cleaning blue off more in the future.

Adrian

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

That is an interesting question. At first I thought you may be correct in supecting the marking blue reasoning that the ''blue' was probably some ground substance in a grease. Wiki says this:-

Prussian blue in an oily base, is the traditional material used for spotting metal surfaces such as surface plates and bearings for hand scraping. A thin layer of nondrying paste is applied to a reference surface and transfers to the high spots of the workpiece. The toolmaker then scrapes, stones, or otherwise removes the marked high spots. Prussian blue is preferable because it will not abrade the extremely precise reference surfaces as many ground pigments may.

This does suggest that Prussion Blue is not abrasive when used as per spec.

However maybe something else is going on here. It is quite sticky so any 'dust' from your fine turning is going to build up around the tip which could cause problems.

Don't know really, it's an interesting question though.

regards Martin

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.

On the final cut, presumably just 3 thou, the tool was pushed up against the job and tries to 'grab a bite' to get into the surface and start proper cutting. Possibly the initial friction heating on contact melts the surface a little to soften it. With lubrication of the grease in the blue it skates over the surface so its cutting edge is being pushed back and then maybe chips off like knapping a flint.

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.

It's possible that it has an effect. When scraping something that is heavily blued you can get a build-up of blue + scraping dust that makes the scraper skid, especially if it's dried out a bit. It doesn't happen often but it does happen.

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 was taking 2 tho off the diameter. I had cut that fine a couple of times before with no issue, and after putting a new insert in I did so again.

I was measuring the small end, and could repeat the measurements quite well. Also the having used the MT slieve a couple of times I had an estimate of X from diameter = Y depth in slieve.

Well these CCGT inserts are sold for non ferrous materials, so I guess they could be a bit fragile. Just such a coincidence it happened when I used the blue. I wonder if my blue or application pad are contaminated.

When I have broken inserts before they have been chipped, never seen one that looked like it had been ground away.

The MT is only for a tailstock die holder so is not super critical, but I do like to make things look good.

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.

.

That change of insert ^^^ is probably more relevant than the Blue.

MichaelG.

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.

Interesting phenomenon. Did you adjust the centre height when swapping inserts? Depending on insert they are often not completely interchangeable with regards to centre height. This may have caused the edge to fail prematurely. We use Ali geometry inserts at work for use on small diameter steel pins. The edges do degrade quickly, and once they start the finish can go from mirror to speed hump in seconds.

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.

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.