Thread: Curious about PCD inserts

Originally Posted by RayG

I've had PCD inserts on the radar for sometime, but haven't yet bit the bullet and bought some to try... they used to be very expensive, but lately have come down in price to almost where I'd like to try one or two.

Has anyone tried them? I believe the edge sharpness can be many times sharper than you can get with HSS or carbide. So they might be good for light finishing cuts?

Ray

Anything capable of fluff cuts and I am interested. I notice that a few videos on Youtube seem to feature them for use with Aluminum. Are you thinking of steel or AL?

I'm pretty sure PCD inserts are only for non ferrous metals. A quick google search came up with this brochure from Kennametal.
http://www.kennametal.com/content/da...rts_metric.pdf

I copied this paragraph from it:
 Choosing between PCD and PCBN
It is primarily their thermochemical stability that determines the suitability of the two materials for specific areas of
application. For standard PCD, this means that it is restricted to workpiece materials where the cutting temperature
does not exceed 700 ºC and has no affinity with carbon if diffusion wear of the cutting edge is to be avoided at elevated
temperatures. Operations that satisfy these conditions are, for example, the machining of non-ferrous metals, wood,
plastics, and minerals where PCD mainly competes against tungsten carbide and, to some extent, high-speed steel.
By contrast, PCBN is thermally stable up to approximately 1200 ºC and has high resistance to chemical attack. As a result,
extreme temperatures, such as occur when machining hard ferrous metals, do not pose difficulties and can be used to
promote self-induced hot cutting. Therefore, technical and economic benefits can be achieved, particularly in comparison
with grinding, and in most cases, performance is significantly better than that attainable with conventional tool materials,
such as tungsten carbides and ceramics.

So PCD is no good for machining steel they are really good for aluminum. I've used PCBN inserts for finish machining hardened pins, they work really well but you need a really rigid machine and a fair bit of horsepower. If you want a really good insert for for light cuts I can recommend Sumitomo cermet inserts. I will have to look up the grade at work tomorrow. I've successfully taken off .01-2 before and had a really good finish. they have a positive rake and are hone finished so they cut well.

Thanks for that reminder, about not using PCD on steel, the thing that prompted me to ask the question was a post on PM by "carbidebob" who seems to know his stuff..

Where he says...

Originally Posted by Carbide Bob on PM

You sharpen PCD to values way past HSS or carbide.
On a nice polished carbide aluminum cutting reamer you would inspect at 50-100x for chips.
On PCD 750-1500x.
At these levels your Schick razor looks like a band saw blade.

Is 40 millionths a chip to you?
This is past the grinding marks groove depth on most "polished" carbide tools.

You will not chip all your PCD tools by checking them with an indicator.
But if you chip 10-30%, trust me you will cry and I have made the scrap parts when we did know better.

Done right these things last forever and are worth the money or people would not buy be buying them.
Some shops start with simple carbide, convert to PCD and never look back.

I'd never heard about the sharpness of the edge on PCD before, I wonder if you can get the same edge on PCBN?

Ray

Originally Posted by RayG

I'd never heard about the sharpness of the edge on PCD before, I wonder if you can get the same edge on PCBN?

Ray


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Looking at the Kennametal link, it seems both their PCBN and PCD inserts have ROUNDED cutting edges. See on page B143 step 2 edge preparation: the cutting edge is radiussed b honing to 0.02mm (note this is not the tip radius, this really the cutting edge itself that is being rounded). This means that you will not take off any "fluffy" chips with this. The minimum possible DOC is limited by this radiussed cutting edge and can physically not be less than the 0.02mm.
In fact, in step 4 on page B146 you see several graphs showing the DOC range as a function of different cutting edge radiusses. Now, probably you can hone the cutting edge sharper than the 0.02mm in order to take off very fine DOC (like in fluffy), but then the life of the insert will probably be dramatically shortened. I suspect the diamond is very brittle and subject to chipping and diffusion effects when ground too sharp.

Looks like PCD is suitable to turn very hard materials, such as some ceramincs and even tungsten carbide. They say it may replace grinding by turning. But I suspect it is not designed to approach a final dimension in very small steps like in grinding. This would probably make it less interesting fore home shop use, where usually older and not so rigid lathes are used and workpieces are mostly one offs.

Besides, the cheapest CBN insert I have seen on eBay, with one only CBN cutting edge fitted, sell for the price of a whole 10-pack of normal carbide inserts with 4 cutting edges each. That would make a CBN tip cost 40x as much as a standard carbide tip. At that price for a single and fragile CBN tip, you can buy yourself a toolpost grinder for the price of 10 pack CBN inserts. I paid $450 for my used Waldown nr0 toolpost grinder with 2 spindles, whereas 1 single tip CBN insert is $40+. Seems a no-brainer to me, at least for home/hobby use. A TPG has more uses.

Originally Posted by cba_melbourne

Looking at the Kennametal link, it seems both their PCBN and PCD inserts have ROUNDED cutting edges. See on page B143 step 2 edge preparation: the cutting edge is radiussed b honing to 0.02mm (note this is not the tip radius, this really the cutting edge itself that is being rounded). This means that you will not take off any "fluffy" chips with this. The minimum possible DOC is limited by this radiussed cutting edge and can physically not be less than the 0.02mm.

I'm sorry, but you asserted the same when recently discussing nose radius and apparently could not be convinced otherwise, but this is just not correct. In fact a cutting tool can take cuts much less than the edge radius, with figures varying between 5-30% of the nose radius, depending in quite a number of factors.

What happens is the effective rake of the tool becomes increasingly negative as the chip reduces below the edge radius. As we all know, a negative rake tool will still cut. However eventually the chip thickness will be so small such that the edge radius of the tool effectively skids across the top if the work and rubbing only occurs with no chip formation. It may be easier to imagine, say a ball bearing representing the cutting edge being pushed through the top surface of a sheet of foam. Even at half the bearing diameter it will still clearly push through the foam. Keep things rigid and it will still keep ploughing through that foam sheet until eventually either the ball will be deflected above the sheet, or the sheet will be deflected/deformed away from the ball ie it will just rub. I don't know if that analogy is helpful, sometimes they just confuse the situation, but it's the best I can come up with.

I've never seen it written, but I personally surmise that one of the reasons high rake tools, of equal sharpness to a conventional raked tool, can take finer cuts, is that the rake is high to begin with, therefore can take increasingly fine cuts as the cuts go below the edge radius, until they reach the critical point when rubbing only occurs. Again, not something I've ever seen written anywhere, but that's been my experience and it would certainly correlate with cutting tool geometry at these fine levels.

Pete, interesting theory. Just wondering, do you personally own a lathe that is indeed rigid enough to take off a DOC smaller than the cutting edge radius, without rubbing?

There was a video that showed under a microscope what actually happened at the cutting edge of a lathe tool, it was amazing, in that it showed the swarf being pushed up like a bulldozer and breaking off. I can't find it again, so if someone can remember... I think it was a video made by an insert manufacturer...

Found it...



Anyway, the way I like to think of the edge roundness question is to imagine using the handle of the knife to spread butter, rather than the blade, it's obvious (to me at least) that you can easily take a scraping much thinner than the edge. But finish might not be so good?

Ray

Originally Posted by cba_melbourne

Looking at the Kennametal link, it seems both their PCBN and PCD inserts have ROUNDED cutting edges. See on page B143 step 2 edge preparation: the cutting edge is radiussed b honing to 0.02mm (note this is not the tip radius, this really the cutting edge itself that is being rounded). This means that you will not take off any "fluffy" chips with this. The minimum possible DOC is limited by this radiussed cutting edge and can physically not be less than the 0.02mm.

Those figures are for hard turning and I doubt many here would have a lathe rigid or powerful enough to work the tips properly - the majority if not all the turning that people on this forum do is likely classed as soft.

Regardless of that I have always understood that it is the nose radius that practically influences minimum DOC, simply because the radius of the nose will tend to push the work away from the cutting tip. As the work becomes more rigid and able to resist these forces you will get closer to size but for long thin work it will be noticable as the work can bow away from the tool. Hence the need for spring cuts. In effect by doing a succession of these you are cutting at a decreasing DOC which then lessens the forces on the tool as the start of the cut becomes more tangential to the tool nose. That is not to say that the tool can not cut at less than it's nose radius but it is not optimum. Of course you then need a machine that is rigid enough to cope with that DOC too, which is why a lot of people with small lathes use carbide but complain about the finish.

An interesting experiment I did recently was to cut a taper starting at the small end. When the DOC was 'decent' - say over 20 thou, the finish was wonderful - smooth, shiny - all that you would wish for. As the DOC lessened (at the big end) the finish changed to that semi-torn look. That was with a tool with a 0.8mm nose radius. The tip still removed material but without the finish. I'm about to go traveling for work but when I get back in a few week's time (if I think of it) I can do this again and make a note of the diameter where this starts happening as well as take some surface roughness measurements.

Michael

Originally Posted by RayG

There was a video that showed under a microscope what actually happened at the cutting edge of a lathe tool, it was amazing, in that it showed the swarf being pushed up like a bulldozer and breaking off. I can't find it again, so if someone can remember... I think it was a video made by an insert manufacturer...

Found it...



Anyway, the way I like to think of the edge roundness question is to imagine using the handle of the knife to spread butter, rather than the blade, it's obvious (to me at least) that you can easily take a scraping much thinner than the edge. But finish might not be so good?

Ray

Hi Ray,

the chip that is being taken off in the video is several times larger than the cutting edge radius - hard to estimate, but I would think throughout the clip 5 to 20 times larger. There are two ways of taking off material. Cutting and shearing. In the video they are only cutting.

Shearing is when using negative rake tools, and this is most often done with on purpose heavily radiussed or chamfered cutting edge. It only works well on very rigid heavy and powerful industrial machines, because the cutting forces when shearing are much larger. Shearing also requires a substantial DOC. The advantages are a good surface finish and a very long tool life.


No, I personally do not believe that "that you can easily take a scraping much thinner than the edge". I am no expert at this. But I think of scraping when woodworking, with a negative cutting angle - in my experience it only works with a super sharp edge, if the edge is only slightly worn no more scraping. Same when scraping steel, it is done with negative cutting angle, and you need a very sharp edge do you? As the edge wears, you need to increase pressure do you? And it does not anymore take off any metal once the edge is worn, true?

> I'm sorry, but you asserted the same when recently discussing nose radius and apparently could not be convinced otherwise, but this is just not correct. In fact a cutting tool can take cuts much less than the edge radius, with figures varying between 5-30% of the nose radius, depending in quite a number of factors.

Lets take a lathe tool with a nose radius of 0.1mm. If we try with it to take a DOC of 0.05mm, the effective (negative) cutting angle would be -45 degrees. I think everybody can agree, that it is impossible to take off material in a controlled manner that way. The tool would be rubbing most of the time, and the obtained finish and accuracy would be the same as when using a 60 grit emery cloth.

If one wants to be nitpicking, then yes you can take a cut with a DOC of slightly less than the tip radius, maybe up to 20% less. But wether such a 0.08mm DOC can be a "controlled" cut with a predictable outcome will depend on many factors, such as the metal/alloy being cut, workpiece diameter, the overall rigidity of the lathe, the cutting tool material, if lubricant is being used and what lubricant is used, etc etc.

On the other hand, it will be easy to reliably take off a DOC of 0.2mm or larger with the above example tool. Without having to cope with intermittant tool rubbing etc. This is why I believe in my golden rule, that you cannot take off in a controlled manner a chip thinner than the cutting edge radius of the tool you are using. I agree the rule is not absolute, depending on conditions it may be possible to push the border a little. But I doubt it can be pushed by more than just a few % as suggested by Pete.

Originally Posted by RayG

Anyway, the way I like to think of the edge roundness question is to imagine using the handle of the knife to spread butter, rather than the blade, it's obvious (to me at least) that you can easily take a scraping much thinner than the edge. But finish might not be so good?

Ray

Yes Ray, I believe your supposition is correct, and eventually the finish deteriorates to the point below acceptable limits. Allied to that is the heat build up and low tool life.

As far as lathe rigidity, these are extremely small cuts and initially the cutting forces are very low. However as the cut falls below the edge radius, the tool is acting more like a "wedge" trying to force the tool and work apart. In my experience it is not the lathe that is the limiting factor in these situations, but in fact the work itself is deflected away from the tool. Whether something like a follow rest could be made to take up that deflection I couldn't say as I've never tried it.

Further to the above, Michael is also absolutely correct in his comments about nose radius, where a similar effect is occurring, but on another plane. As mentioned by Michael, the effect is to want to push the work away.

As far as this not complying with anyone's personal "rules", that's their problem to deal with. I do however think its unfair to spread incorrect information while others are trying to learn about the subject. I personally have never used PCD inserts, and always considered them the reserve of hard turning. However it seems my belief is mistaken and I'm very interested in how they may be used in general turning and look forward to reading the results from others.

The effects of minimum chip thickness and edge radius are well understood and papers discussing what happens at this level easily available for those with an open mind. In general engineering this is generally not an issue as no manufacturer is going to be "sneaking up" on sizes as we may want to. However in micro machining edge radius can be a very real issue to deal with and is well understood in materials ranging from aluminium to steels and through to exotics.

Originally Posted by Pete F

Yes Ray, I believe your supposition is correct, and eventually the finish deteriorates to the point below acceptable limits. Allied to that is the heat build up and low tool life.

As far as lathe rigidity, these are extremely small cuts and initially the cutting forces are very low. However as the cut falls below the edge radius, the tool is acting more like a "wedge" trying to force the tool and work apart. In my experience it is not the lathe that is the limiting factor in these situations, but in fact the work itself is deflected away from the tool. Whether something like a follow rest could be made to take up that deflection I couldn't say as I've never tried it.

Pete, in the case of very small workpiece diameters workpiece deflection is indeed the biggest problem. It needs a following rest.

But with larger workpiece diameters deflection of the workpiece itself is negligible. We are however talking here about very small DOC's. The sort of DOC's that one needs to turn for example a one off bearing seat to +/- 0.002mm or 2 microns. And here the lathe rigidity really matters. But it is another type of rigidity than flex in the lathe bed. It is for example, if the pressure caused by the tool cutting at a DOC near to or even slightly below its nose radius is sufficient to cause the spindle to move in its bearings by enogh microns to cause intermittant rubbing of the tool. The same is valid for the sum of play and flex in the cross slide, compound and toolpost.

Hi Pete,

Yes, I'm not so much interested in the theory, that's been studied and well understood by people with more resources and money.. what caught my eye was the claim that PCD (and perhaps PCBN) inserts are much sharper than HSS or carbide..

That's the question.. not the other question of DOC versus edge sharpness. No question that sharper is better.

For that matter I wonder what you would use to hone a PCBN insert?

Ray