Carbide Insert 101

I am a home shop machinist. I have a SB 9" lathe. I am interested in using carbide inserts and making my own tool holders. I know that HSS
is easier to work with but I still want to experiment with carbide. I have been doing my homework but have a few questions and need some clarification on a few points.
1. Any insert with a "N" in the second position is a negative insert. Any insert with any letter other than an N in the second position is a positive insert.
2. I am assuming that the positive and negative are refering to back rake.
3. Negative inserts if kept horizontal, really have no back rake on their own and also no end clearance or side rake. Positive rake inserts if kept horizontal, really have no back rake on their own just end and side rake.
3.5 Negative inserts are more economical because both sides can be used.
4. Tilting down a negative rake insert gives end clerance and negative back rake. This is why they are called negative inserts when really they have no back rake on their own.
5. Tilting up a positive rake insert *should* give end clerance and some positive back rake would seem to be the ideal situation for a small lathe. However this creates an interesting paradox, all of the sources that I could find, seem to want to operate a positive rake insert perfectly horizontal. This would make it a zero back rake tool (neither positive or negative). Why does everyone want zero back rake?
6. I found a source suggesting using a TNMP insert. The chip breaker goes clear out to the edge of the insert. Tipping this insert down 5 degrees gives you 5 gegrees end relief and 5 degrees positive back rake. I think that this insert has chip breakers on both sides so it could be turned over and used again. DOes anyone know for sure? I think that this is the best way to go. Has anyone tried this?
7. What effect does the tip radius have. I beleive that the larger the raduis the smoother the surface finish on turned work. However, the larger the radius the more power required.
Any help would be greatly appreciated. Thank You
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You can forget using negative inserts on a SB 9" unless you just love chatter. Those inserts require three things: rigidity, power, and speed.
I admit I use carbide some on my SB 9" but it often doesn't work that well. I'm always better off grinding the appropriate HSS bit and using that - better finish, less chatter, etc. Mostly now I only use carbide for cutoff, and on those I use the GT?-2 which are .087" wide (GTN-2, GTR-2 ..).
You're correct on tip radius.
Grant Erwin Kirkland, Washington
John Albers wrote:

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Yes. I think it was Ted Edwards (apolgies if this is wrong) who makes his own holders for these inserts. He likes that setup a lot. If I did not already have a set of tpg221 holders from valenite, that's the way I would go.

Yes. Also coated inserts invariabley have a "honed" edge to retain the coating at the cutting point. Because of this they are not dead sharp and don't cut on smaller machines quite as well.
Jim
================================================= please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================
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--------Yes, except as the rake is modified by grooves and such on the top face.

--------Typically true.

----------For edge strength.

----------You can use TNMPs on both sides. I like TNMGs better than TNMPs, but I have a stout lathe. I find it hard to get chips to break with TNMPs.

-----------Big tip radius is bad for a lathe like yours. You'll get chatter and pushback from the work.

--------I suggest trying some of the Rouse or Borite screw-down sets that MSC, ENCO & others sell. These use TCMM, TT221, or TPMT or similar inserts. The trick is to get good quality, name-brand inserts like Kennametal, Carboloy, Iscar, Valenite, Sumitomo, etc. Another approach is to get one of Circle Machine Co's square shank holders that lets you use their boring inserts for outside turning. Their stuff is pricey but can be found on ebay. The edges are typically very sharp, nose radii are small, and they sell carbide grades specifically for low surface speeds. Carbide likes to run at 250 ft/min surface speed and higher, but you will have a very tough time doing this with your lathe.
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The three rules of carbide: 1. RIGIDITY 2. RIGIDITY 3. RIGIDITY
Any slop or flex in ANYTHING and you won't enjoy it!

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I don't understand how that's any different than using HSS tooling. Those rules still apply. Rigid, more rigid, and even more rigid than that!
Jim
================================================= please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================
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I'd say HSS is significantly more lenient in regards to a lack of ideal cutting conditions.
Generally, anything slower than *too fast* is fine with HSS. I've run a HSS cutter such that it (3/8" square) was hanging out a good 1" from the toolpost and flexing about 1/8" down each time the work piece hit the cutter (interrupted cut on O1, massive feed and DOC, very low surface speed).
A carbide insert or brazed cutter would have snapped on the first hit. In that situation, carbide would have been a slow method of material removal because of the interrupted cut.
Of course, I'm not saying one can throw rigidity out the window with HSS, but when you're running carbide so hard that the spindle is close to stalling (not usually possible with HSS), rigidity becomes a big issue.
Regards,
Robin
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Things carbide does not do well under *any* circumstances: interrupted cuts.
Things that carbide does well, under any circumstances: turning hardened items.
I use carbide instead of HSS on smaller machines, when having to turn something that cannot be annealed.
Jim
================================================= please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================
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wrote:

Because the crux of the matter is, Jim, HSS properly done, is heads and hands sharper than carbide.
I tend to use a lot of HSS with my lathes and shapers, and have a decent Baldor grinder with a set of diamond wheels. Ive played with both, and the HSS wins 90% of the time under normal turning, particulary if you can flood cool with oil. This is only true if you keep your tools sharp, which is labor intensive. Which is one of the reasons production shops use a Lot of carbide, because they have rigid machines and labor is a significant part of production costs.
A goodly number of the tool makers I know, still use a shitload of HSS, rather than carbide in MOST one off applications as they can do more, with the HSS, than with carbide.
Gunner
"You cannot invade the mainland United States. There would be a rifle behind each blade of grass." --Japanese Admiral Isoroku Yamamoto
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I totally agree with Gunner's comments. You can really tell the difference between guys who actually have small machines and have tried carbide, and the ones for whom this is merely a "thought experiment".
Really, 9" SB lathes are all about HSS. You can certainly use some carbide some of the time, but negative inserts? forget it. - GWE
Gunner wrote:

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    Hmm ... does a 5" swing machine, used almost entirely with carbide count? It is my Compact-5/CNC, and I have some *very* sharp (uncoated) carbide inserts for it. They are a win in part because it *is* CNC, and the ability to turn or replace an insert and to not have to change the offsets throughout the program (at least at every tool change -- since that machine is too old to have a single place to insert the offsets for a given tool once and for all.)

    The ones with the chipbreaker designed to give a positive rake with an otherwise negative insert may work for him. They work for me, in the 12x24" Clausing -- but admittedly, that is a more rigid and more powerful machine than a 9" SB. Maybe the heavy 10 (10l) might be closer to sufficient.
    Enjoy,         DoN.
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Normal turning is anything you would do on a lathe, including grooving, forming and threading. Carbides can be very "hungry", but unless you've got lots of power and rigidity, it comes second to a good high speed tool. A solid 14 or 16 inch lathe would be about the minimum I'd even look at a carbide for, the smaller ones just don't have what it takes. Most of us have smaller machines or imports, and carbide is the way to ruin both your budget and your machine. Very few of the bench machines are heavy enough to run carbide without either shortening it's life, or making your life miserable. But, (Giggle, snort), I don't use carbide, and I'd rather work 4140 or 4140HT than the gummy soft stuff. May take me a while longer, but normally I get a good finish, hold size easily, and if I want to polish it, nothing else comes up as nice.

Ummmm, yes. If a job has something, like an interrupted cut, it can raise hell with inserts, and I've seen it pull the brazed tools apart. However, the only time I ever stopped the spindle on the 7A J&L was with a 1/2 inch radius on a 1 inch square piece of Congo. However, one of the common things that I've seen is people wasting time trying to make a carbide work, because "High speed would take too long." Usually, by the time they give up and use the HSS, they've already wasted more time than it would have taken them to use the HSS the first time. Carbide can remove a lot of metal in a hurry, but power and rigidity have to be there or you're wasting time.
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On 15 Oct 2003 22:27:57 -0700, snipped-for-privacy@lycos.com (Lennie the Lurker) wrote:

Damn..something else Lennie and I can agree on. There is a (insert Diety of your choice) afterall!
Gunner
"You cannot invade the mainland United States. There would be a rifle behind each blade of grass." --Japanese Admiral Isoroku Yamamoto
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When I first started metalworking I insisted on trying carbide in spite of what I was told. It never did work well for me on my atlas lathe.
I have a friend who has been a machinest, model maker, and tool and die maker for years. He still grinds and uses lots of HSS. He has taught me a lot and NOW I listen to him. One time I was making a crankshaft out of 4140. I sort of figured I would need to use carbide because 4140 is pretty damn tough stuff. Surface finish resembled sandpaper. I changed to HSS with cutting oil at 50FPM and it worked much better. Just like my friend said it would! chuck
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That's only true if you're using coated carbide inserts, and using them right out of the box. On smaller machines, I only use uncoated inserts, and hand hone them for an even keener edge. I'll put the edge I can get on those inserts up against anything you can achieve with HSS (and maintain for more than one second of cutting).
Carbide gets a bad name by people using *coated* inserts right out of the box on small lathes. That doesn't work well at all. But you can put a *very* keen edge on uncoated carbide with a diamond hone.
About the only time I don't use carbide is when doing interrupted cuts (everything you've heard about that is true), or when I need to grind a special form tool.
I even routinely run carbide tooling on my Taig. Lathes don't get much smaller or underpowered than that. Of course carbide performs much better on my larger machines, which have the rigidity and power to really get the most out of it. But the fact that you *can* get a keen enough edge on it to work on a Taig puts the lie to the idea that you can only get a *sharp* tool with HSS.
Gary
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snipped-for-privacy@bsu.edu (John Albers) wrote in message

Hi John,
I've been a CNC programmer for the past twenty three years, and can offer a few general comments about carbide inserts.
Always use negative G style inserts when you can - they're cheaper and stronger than positive P style - which aren't true positive inserts, but normally cut cleaner and break chips better than the G's - they just won't take high depths of cut/high feedrates as well.
Negatives usually work fine for turning on rigid machines and with rigid workpieces. The most widely-used rough turning insert in industry is undoubtedly the CNMG series, which is an 80 degree diamond. It's very useful on CNC machines because it's quite strong and can turn or face equally well in the same toolholder. They're double-sided, so you get four corners. SNMG (squares) are stronger but less versatile, TNMG (triangles) are weaker but can get into odd places or close to a tailstock center.
The previously mentioned geometries also come in an M style, which is single-sided. They're designed for heavy roughing - the 100% contact on the bottom gives better support than the G style has.
The P styles are economical for light roughing (especially where chip control is a problem) or finishing. They are double-sided and have four corners.
Positives generally cut cleaner than negatives, but negatives almost always work well enough. Negative inserts often don't work well at all for milling.
Recognize that there are lots of sub-categories. Insert salesmen will gladly sell you ten slightly different configurations of chip breaker if you let them. Most of them can be ignored.
Larger radii theoretically give a better finish at the same feedrate, but in practice the reverse sometimes happens, particularly when machining a thin-walled or otherwise unstable workpiece where the extra tool pressure from the large radius is detrimental. Most shops have standardized on a 1/32 radius for finishing, but 3/64 and 1/16 are around too. Sometimes the blueprint will force you (maximum inside corner radius callout) to go to 1/64 or even smaller. Generally, tool life suffers as the radius size decreases.
That's my five minutes worth. Most of it's pretty standard stuff, some might be debatable.
Mike
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snipped-for-privacy@bsu.edu (John Albers) wrote in message

Yup, the TNMP is double-sided and cuts well if tilted at the right angle. I used to make all my own toolholders years ago, though not with the fancy cams and so on; just a socket-head capscrew with the underside of the head turned at a taper to set the head lower in the insert so it doesn't interfere with chip removal. Larger tip radius gives better finish, but unless the radius is large (like a small, round insert) the power increase needed is marginal. Chatter will be more of a problem if the radius is large and the lathe is light or worn. A larger radius runs cooler, too, which is important in carbide, as it suffers thermal shock easily. For the same reason, either use no coolant at all, or flood it. Intermittent splashing will cause thermal shock and cracking. Get TiN coated inserts, and they'll last longer.
Dan
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    Yep. At least for lathe tool inserts. Not too sure about those for milling cutters and such. And, of course, there are things like the threading inserts (There are two styles, one is the "laydown" which have the triangular inserts mounted more or less flat (there are solid carbide anvils with various angles to allow you to tune the angle of the insert to match the lead of the thread, since you can't just grind more relief on the typical insert.)
    The other style of threading insert is also triangular, but is mounted on edge. I know of no way to tune this for the needed clearance angles.

    Basically.
    End and side clearance -- not rake.
    The positive ones have whatever back rake may be contributed by any chipbreaker which they may have -- if it comes close enough to the edge.

    They also are stronger, because there is more meat behind the edge. The edges on positive inserts are more fragile.

    Right.
    They don't -- except for turning things like brass. But they depend on the chipbreaker to generate the effect of rake.
    And if you tilt a positive insert, you actually *reduce* the tip and side clearance -- which might not be too bad, depending on the feed rate.
    The tip clearance on a positive insert is formed by the combination of the clearance angles on the two sides which join to make the tip -- and it is affected by the angle at which they join. There are *lots* of shapes. On my 12x24" Clausing, I use the triangular form, while on my little Compact-5/CNC, I use the 55 degree diamond. The choice for the Compact-5/CNC was made for be by the tooling which came with it.
    The triangular ones on the Clausing are the TNMP, and usually uncoated, which gives a sharper edge. These give a total of six corners which can be used (assuming that the wear to a given corner is not sufficient to destroy the use of the flip side one).
    The diamond shaped ones on the Compact-5/CNC are positive rake ones, and the diamond only allows two of the four corners (the sharper two) so there is a 3:1 ratio in favor of the triangular ones on the Clausing. Most of my inserts for the Compact-5/CNC are also uncoated, though I have a certain percentage of TiN coated ones.
    The diamond shaped ones have three formats (in the collection which I have). In one (the first ones I had were all of this type), the chipbreaker ran around the full diamond shape. The other two either have the chipbreaker only on the left edges (for right-hand-side turning tools), or on the right edges (for left-hand side turning tools and for boring bars). The corners are a bit stronger on the ones which have only the half chipbreaker. I presume that there are also diamond-shaped inserts with *no* chipbreaker (good for brass), but I don't have any of them. I have a large collection of the other styles, thanks to an offer from someone else in this newsgroup a few years ago. He was willing to send a few samples free to anyone in the list, and as I was the only one (at the time) who could use them, he wound up offering to sell me the rest (several thousand, I think) for what I considered a reasonable price -- which I gladly accepted.

    Yes -- the chipbreakers are on both sides in the TNMP inserts. These are what I use in the Clausing -- both in normal holders held in Aloris style quick-change holders, and (lately) in the special holders made by Aloris which directly accept two inserts -- one on each end for the turning and facing operations. This makes for a more rigid setup, and reduces chatter. There are two forms of this -- one for the positive inserts (16P), and one for the negative inserts (16N). I have the latter only. I won a couple of lots on eBay which had 100 of the TNMP inserts and a straight holder (I later got the left and right holders via MSC to go with the inserts which I already had. All of these are uncoated, and seem to work well for what I normally do -- though for certain materials, the TiN coating could improve wear life and friction, while reducing the sharpness of the available edge (which isn't that sharp on a TNMP anyway. :-)
    Note, however, that most of my threading inserts *are* TiN coated.

    And -- the more likely you will get chatter, because of the higher forces.

    One consideration for true negative rake inserts (not the TNMP) is that it takes a more rigid machine to hold them without chatter. And chatter can destroy an insert where a HSS tool would not have problems, because the carbide is more brittle. I'm not really sure whether the 9" South Bend is rigid enough (and has enough power) for true negative rake insets, but it is probably sufficient for the Positive/negative ones which we are discussing. The true negative rake ones take *lots* of power, and are better for cutting really hard materials, where the clearance for the positive inserts weakens the edge too much, as does (to a lesser extent) the chipbreaker groove that turns a negative insert into a positive/negative one.
    I hope that this helps. Now off to read your other responses, to see how many agree or disagree with me -- or offer other things that I didn't think of.
    Good Luck,         DoN.
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On 15 Oct 2003 23:35:11 -0400, snipped-for-privacy@d-and-d.com (DoN. Nichols) wrote:

From past experience..a Logan 10" in good condition, is barely rigid enough for a negative rake insert, at any DOC. My 13x36 Clausing gear head, eats it up like it was made for it. Ones 250lbs..the other is 2000 lbs <G>
Gunner
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John Albers wrote:

If you haven't already found it, check out http://www.metalwebnews.com/howto/toolholder/toolholder.html
Since I wrote that, I have written some more stuff on these. Not quite sure what to do with it.
Ted
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