View Full Version : Carbide rake

loose nut
06-11-2010, 08:25 PM
It seams that most carbide inserts are negative rake, is it better than a positive rake or is it that you can get more cutting edges that way.

My understanding of it is that negative requires more HP and positive less while freer cutting. I use TMNG inserts on a 13" lathe with a 1 1/2 HP (???Chinese 1 1/2 hp) motor and they work fine but would positive inserts work better on the same types of steel. They have a molded chip breaker so they not have to much of a negative rake.

What is the reason for using a neg. rake and would a positive one be any better.

Jim Shaper
06-11-2010, 08:46 PM
Has to do with the cutting dynamics of turning. When you're turning, you're not "slicing" the material at the cutting edge, you're peeling it off just prior to the edge of the tool.

There's a really slick video we saw in my machining program that was excellent in demonstrating what's actually happening between the tool and the work. I wish I could share it here.

06-11-2010, 08:49 PM
Okay, I'm far from an expert on this one. To date, I don't even own a single neg. rake toolholder...

Everytime someone seems to ask about rake (at least on PM), the mantra ends up: positive, positive, positive. Want a face mill? Get the high-positive one. Want a lathe toolholder, get the positive one, etc. etc.

Yet, when you look in namebrand catalogs, most lathe tools (to take an example) are negative. Wha?

My understanding is the negative tools are best for hogging material. The insert is supported better because of the pressure angle. Positive would seem to me to put more uneven stress on the carbide itself rather than compressive force supported by the thickness of your toolholder backing it up. So 1) you can take larger DOC, and 2) your inserts last longer. Both of which only matter in high production environments.

As I said, though, my qualifications on this subject are slim to none. We'll see if others prove my thoughts wrong or right :) I will be curious to hear an informed perspective on when negative rake actually excels in any area other than efficiency.

loose nut
06-11-2010, 10:15 PM
Would a positive rake insert be better for lighter finishing cuts compared to a negative rake insert, would it give a better finish.

06-11-2010, 11:22 PM
I put positive rake rake WNMG inserts (kennametal WNMG431-FW) into my negative rake insert holder. I still get 6 cuttting tips... and a better surface finish on lighter cuts. On heavy cuts, negative rake gives me better results, and longer life.

Surface finish is complex... and minimum DOC complicates things on lighter machines. Look at Kennametal catolog -you can get inserts in both positive and negative rake for negative rake holders that have a minimum DOC of 75 thou (or more), and down to 2 thou.

06-11-2010, 11:29 PM
This is the last test cut I did on the "Wreck" after installing new rear spindle bearings. The first 2" or so were done at 1500 RPM and then the speed was increased to 3000. DOC was .0075" feed .006". Insert is TPG 321 with a molded chip breaker. Material is 1018.

Doc Nickel
06-11-2010, 11:30 PM
Everytime someone seems to ask about rake (at least on PM), the mantra ends up: positive, positive, positive. Want a face mill? Get the high-positive one. Want a lathe toolholder, get the positive one, etc. etc.

-It's my impression that the positives are recommended for home-shop and relatively small machine use. They take less HP and work better with less-than-rigid machines.

The negatives are better for high-production-rate, high-metal-removal applications with heavy machines and rigid automated machines.

That's not a hard-and-fast rule, of course, but that's generally it, and why people here tend to recommend the positives.


06-12-2010, 12:30 AM
A carbide inserts cutting edge is pretty fragile. By having it at 90° this strengthens the edge as much as possible. The holder’s pocket is then angled down to give the insert clearance.
An insert that has a 10° chip breaker and is raked forward 5° in the holder gives you 5° double positive cutting configuration.
The edge prep on the insert determines the minimum depth of cut (along with feed rate). On a insert with edge prep, if you feed slow with a light cut you are actually cutting below center on most of the inserts cutting edge. This will destroy the insert and give you a real nice finish :D (just kidding on the nice finish)
You do get more edges out of a negative insert also, and they do require more horsepower over a positive. Another advantage is they are usually less expensive.
If they work for you, use them for roughing down and then finish with a positive.


06-12-2010, 02:57 AM
Negative or neutral rake inserts are also best on graby materials like brass where a posative insert will just give you a bad day.

06-12-2010, 08:57 AM
The inserts are negative to give the edge more strength - carbide is brittle and the acute angle formed by positive rake doesn't last well. Negative does require more cutting force meaning more hp and rigidity (hence the common recommendation its not the best choice for light home shop machines), but creates longer lasting tooling when using carbide

06-12-2010, 10:42 AM
This has become an interesting subject to me. Beckley23's photo shows what I have found about inserts. He didn't say whether the insert was negative, neutral or positive to the work. Speed always produces a better finish with carbide.

I have mostly used 0 rake HSS and brazed carbide cutters and when roughing I didn't worry much about the sharpness. When taking finish cuts I was careful to get a sharp edge and radius on the nose that related to the feed I would use. The best finish on finish cuts with carbide was at .005" more or less depending on the spindle speed and I found high spindle speed was important with carbide.

Now that I am starting to use inserts more I have some issues with them. For one thing inserts are NOT sharp and looking at them with a magnifying glass will show you that. I guess you can buy inserts that are sharp but you would have to order them that way and probably a higher price.

Using an insert held at a neg. rake works ok for heavy roughing cuts but is worthless for finish cuts. I also don't buy that an insert held at a neg rake has better strength of the cutting edge. Draw it on paper and then tell me the cutting force on the downward angled insert doesn't tend to try to break the edge off. The strongest edge is at 0 rake with minimal front clearance where the cutting force is straight down on the insert.

Inserts held at 0 rake cut well under most conditions I have used them in, but still require high speed and sometimes slightly fast feeds.

I have not had good luck with HSS or carbide at positive rakes except for aluminum and sometimes brass/bronze.

It's for the above reasons I stick to inserts at 0 rake as they perform better for what I want. For manual lathe operations a cutter at center line 0 rake does a better job for me than anything I have tried. That holds true for brazed or insert carbide and HSS.

As to a chip breaker, on the inserts I have with a chip breaker there is a flat area after the cutting edge and then a dropping radius to make the chip break. That is, the chip breaker radius is NOT right behind the cutting edge on the inserts I have and have seen. On the other hand, I have not seen all the inserts that are made and really don't care to. If I need something special I inquire about it.

Since I am not into high speed production or CNC I can't say what would help or hurt there.

The problem is that there are so many different inserts it is hard to decide which one will do what you want without calling the makers. Even then you may get false info. One maker will say this type and grade is best and another will say something different. I even had two different salesmen at the same company give different recommendations, go figure :confused: .

It seems experimenting was the best test for me but that can get expensive with inserts for special alloy metals. Some companies will send samples and that helps.

All I can say is EXPERIMENT, your results may differ from others and nothing is written in stone.

06-12-2010, 12:37 PM
For one thing inserts are NOT sharp and looking at them with a magnifying glass will show you that.

Very true. This is part of the reason for the heavy DOC needed, to my understanding. I have never seen a specifically noted "sharp" insert for steel, but there are ones for aluminum. I can attest that they are indeed sharp! :) The copy sounds a little too good to be true, but they do work very very well. This is one insert geometry example: http://latheinserts.com/product.sc?productId=9&categoryId=88

Paul Alciatore
06-12-2010, 04:02 PM
Very interesting thread.

I have to wonder about the sharpness of inserts. Is the lack of sharpness on most inserts deliberate or is it just a consequence of how they are made? I suspect that they are made from powder that is first formed in some kind of mold and then baked to fuse it into a solid. I would appreciate corrections or further insite into this from anyone knowledgable.

But that kind of process would make forming a really sharp edge difficult. A further sharpening operation would be needed to produce a truely sharp edge and that would add to the expense. So are most inserts made to a price point and the slightly dull edges just a result of this? Or is there a real advantage to a slightly drll edge?

I may have to sharpen the edges of some truely dull inserts I have to see how they would perform in steel.

06-12-2010, 04:21 PM
Sharp edges are much more prone to chiping. more unsupported edge.

Try an interrupted cut in mild steel with a 'supersharp' aluminum insert.. then check how 'dull' it is after the edge chips off.

06-12-2010, 05:04 PM
In "Design and Use of Cutting Tools" by St. Clair, he mentions that carbide fails more quickly when ground to a sharp edge. Carbide is hard but brittle so it tends to develop a micro-chip at the cutting point; this then expands,leading to failure. He found that dulling the edge very slightly on carbide led to much longer tool life.

St. Clair also found that negative rake tooling can be advantageous with carbide because there is more carbide material adjacent to the edge. That is, the edge is 90 degrees and relief is provided by angling the whole bit down. On positive rake tooling the included angle is less than 90 so there is less carbide adjacent to the cutting edge for support and heat conduction.


loose nut
06-12-2010, 05:10 PM
OK, got that, now for the next bit.

Does different insert geometry work better with different grades of steel IE: does one shape and/or rake work better with the 1020/1045 range as opposed to the 4140/4340 type and does free cutting (not that I ever get any) steel need a specific shape for good results. The couple of times I have turned it it seemed pretty forgiving but maybe I could do better.

Sometimes if I use HSS it is better than Carbide and sometimes not, I don't know if this is the material or how it is ground, different radius, relief and rake etc.

06-13-2010, 12:43 AM
I can't tell you about different geometry but different grade/coating inserts are made for different alloy steels. Look in the Enco catalog at the carbide grade chart. There are charts at most if not all the insert makers online. Using the right grade of insert makes all the difference in the world as well as nose radius, design of the chip breaker and clearance angles.

06-13-2010, 12:26 PM
In "Design and Use of Cutting Tools" by St. Clair, he mentions that carbide fails more quickly when ground to a sharp edge. Carbide is hard but brittle

Carbide has higher hardness than HSS, and lower toughness. "Sharpness" is how positive the edge is. So as McGyver and GagetBuilder say, if you grind a carbide insert with a sharp edge, the edge breaks down very quickly. That's why the ultra-positive, "Upsharp"-ground inserts are designated for finishing passes (shallow DOC) on aluminum. They literally disintegrate before your eyes on steel.

Negative rake inserts (which have flat sides) support the brittle edge much better, and therefore last much longer than positive rake inserts, which is why the overwhelming majority of pro shops run negative rake tooling.

06-13-2010, 03:20 PM
the Geometry of an insert plays an important roll, but only in the fact that it lets you pick diffrent lead angles and side relief angles and affects number of edges, as well as where you can physicaly fit the insert while cutting.
As well as overall strength of the insert while roughing.

For example: a square cutter with a 45 degree lead makes an AWSOME rougher... to anything thats not a square corner, or just roughing it and leaving enough to make the corner square with some other insert.

45 degree lead causes 'chip thining' that lets you take off more steel with less effort/heating.

a square insert however can not turn to a square shoulder because you'd have 0 side relief. Squares are also very strong due to the 90 degree angle, lots of carbide. 4 (or 8) cutting edges is nice too.

Triangular, let you get into square corners, but are much weaker, less cutting edges, 3 (or 6)

80 degree Rhombic let you basicly have the same advantages of a square (80 degrees insted of 90 degree cutting tip for high strength/heat disipation), and can even turn/face with the same insert/toolholder/mounting position.
But they only have 2 (or 4) cutting edges.

55 and 33 rhombic afaik are for making tight grooves. Places where the insert must be really narrow to get into

Basicly, the bigger the angle at the tip, the stronger the insert, but the more clumsy it is to get it into tight areas and turn to shoulders in a narrow groove, etc. And more likey it is to have many cutting tips.

I use a square mounted at 45 degrees for general roughing (or 15 degrees if I need closer to a shoulder), and sometimes for finishing OD's, Rhombic for tight work, And my old triangles.. well, I don't use those anymore except for one in a facing holder that seems to allways give me mirror finish facing cuts. Im sure other tools could give me mirror finishs, but then id have to try them and find what one does, and maybe swap it around in a tool holder.

PS: there are some face mills and lathe tooling that actualy let you use the other two 'points' of a rhombic, though its a little ackward to use an insert with more the 90 degrees for much more then roughing/flat work since it doesnt work too well near a shoulder, So rhombic can have 4 (or 8) cutting edges, just need 2 tools to do it.

loose nut
06-13-2010, 05:24 PM
Sounds like I should stay with my TMNG inserts instead of positive rake inserts for general turning, square might be stronger but I think the triangular shape is more useful for my needs and use HSS for fine finishing.

06-13-2010, 07:46 PM
Should you stay with the TNMG inserts? Clearly....it depends. What kind of material are you cutting, what machine (and what HP) do you have, how much of it do you have to do and how much time do you have?

Most of what Black_Moons has written is spot on, but perhaps I can expand upon that. The 55º and 35º diamond-shape inserts, whether they be positive or negative rake, are most-often used for profiling on CNC lathes. I use all shapes other than WNMG. I use it all on a 16x40 manual lathe. I have holders for CNMG, DNMG, VNMG, RNG and several grooving and cutoff tools. They all have their specific applications. I haven't needed to use HSS tooling for quite some time now, and that's fine with me. ;) Yes, I know how to grind a tool and have a surface grinder to do it very nicely of need be.

The negative rake insert vs. positive rake insert debate is really one on inclination angle. Fundamentally, if you screw the insert down with a screw down the middle and no top clamp, it's probably a positive rake. If you clamp the insert with a top clamp, no screw and sometimes a mechanical chip breaker (shim) on top, it's probably a neutral rake. (TPG's are most commonly used this way.) If you have an eccentric screw in the middle and a top clamp, it's probably a negative rake insert. (These have 0º sides and can be flipped over.)

Almost all inserts all types are actually positive rake because of the chipbreakers molded into the top surface. The correct term may be "top form geometry", but the term chipbreaker is the one used most. Without top form geometry, the holder's inclination angle determines positive, neutral or negative top rake. I have negative rake inserts with high-positive top form geometry that cut with less force and much better chip control (and life!) than a "positive" rake TPG insert.

Here are some basic mantras I try to follow in selecting carbide tooling:

Match the grade and coating to the material and cutting speed.
Match the chipbreaker to the feed rate and material's resistance to breaking chips.
Match the tool nose radius to the depth of cut, minimum part corner radius and desired surface finish.
Match the overall material removal rate to the machine HP, speed capability, rigidity of setup and cutting conditions in terms of safety.

An insert's "true geometry" can usually be found in the maker's catalog or website. From looking at the manufacturer's data on recommended materials, depth of cut, feed rate, chipbreaker profile, coating and grade, you can usually get valid starting points. Your rates and success will vary because they never seem to be cutting your material on your machine, but you get something to start with. When you buy $2 no-name inserts from Ebay, you save money but you may not find usable data.

I have gathered a ton of manufacturer data on my PC here, so if you have any specific questions I may be able to help. I may not be able to help sometimes, but I'll always try. At least I'll tell you if I don't know the answer, and you can feel free to tell me I'm full of sh*t anytime. ;)

06-13-2010, 08:08 PM
The negative rake insert vs. positive rake insert debate is really one on inclination angle.

You can incline a positive or negative rake insert to the same inclination angle by tipping the pocket.

The primary difference between positive and negative rake inserts is how sharp (and therefore, how fragile) the edge is. The sharper (more positive) the insert is, the less horsepower and rigidity it requires, but the edge breaks down much quickly. Great for home shop types, lousy for production work.

Black Moons went off on a tangent about lead angle, which really only applies to milling inserts.

loose nut
06-13-2010, 08:08 PM
I should have quantified my last post.

Blackmoons is right about the square inserts being strongest for roughing out but since most everything that I turn has a shoulder the triangular shape seems a better choice for me. My lathe is a 13" Chinese model 1 1/2 HP so taking deep hogging cuts (I don't like to overload it, can't afford too repair even if I could get parts) isn't in the plan but the TNMG inserts work pretty good on this machine, some times the finish could be better that's why I was wondering about Positive rake inserts. Buying holders and inserts of a different type to experiment with isn't in the budget. I am going to stick to TNMG inserts with HSS for fine finishing for now. Thanks for the help.

06-13-2010, 08:13 PM
What workholding do you have on your 13" Chinese lathe? A common 3 jaw scroll chuck? What's the maximum speed?

There's no reason why you would be able to get the same or better finish on your machine with the right TNMG (TNGG?) insert. What material and how much are you leaving for the finish cut?

BTW, I've never done any "tipping the pocket" other than that of the wait staff at the restaurant. OK, just kiddng, I do that only with the 16ER lay-down threading inserts. Edge breakdown is a function of grade, material and cutting speed. I have some VERY sharp inserts that have done plenty of work and are still nice and sharp.

Lead angle also applies to turning. I have a MSSNR 16-4C holder that holds a square SNMG43x insert at a 45º lead angle for roughing.

06-13-2010, 08:22 PM
TNMG inserts work pretty good on this machine, some times the finish could be better that's why I was wondering about Positive rake inserts. Buying holders and inserts of a different type to experiment with isn't in the budget.

Try the TNMP inserts -- they have a 10° positive chipbreaker, so when they're tipped 5° downward in the typical TNMG pocket, they have a 5° positive rake.

In other words, you can use the so-called "Positive-Negative" inserts in your negative rake TNMG toolholders.

06-13-2010, 08:31 PM
If this is any use to you that are researching inserts...here is a link to the Sandvik site that lists the different grades and where they are best suited:


06-13-2010, 09:02 PM
Black Moons went off on a tangent about lead angle, which really only applies to milling inserts.

... Since when? Lead angle is lead angle, milling or turning.

Try some serious roughing on mild steel with 0 or 15 degree lead angle and note the chip tempature (color), then try it with it at 45 degrees at the same settings/insert, the 45 degree chips are cooler, and to me that means free to incress feed/rpms even more.

Plus.. cooler chips = less burns on my hand from stray black chips.

06-13-2010, 09:19 PM
... Since when? Lead angle is lead angle, milling or turning.

The OP is asking how to improve surface finish on his lathe by trying positive inserts. Since lead angles are fixed on lathe tools (you can't buy a triangle insert holder with a non-standard lead angle, as opposed to milling cutters, where you can buy square insert facemills with 0°, 30°, 45°, 60° and 90° lead angles), that's a Red Herring.

On the other hand, I do agree with your comment about insert shape: the greater the included angle, the stronger the cutting edge. So you could confuse the issue even more by comparing a 90° positive insert against a 35° negative insert :p