Selecting Tungsten Carbide Turning Inserts for a Small Manual Lathe (Long)

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Oct 28, 2009
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Pardon me, as a new contributor to this site, for plunging into the topic of using tungsten carbide inserts on small manual home lathes. This has probably been hashed over before. And let me make it clear that I’m not trying to convert anybody away from HSS. If HSS does everything you ask of it, you have the most cost-effective cutters. If I had spent 10 years problem solving CNC applications with HSS, I’d be writing about it instead of carbide inserts.

I agree that most tungsten carbide inserts are not suitable for small lathes. But some of them are. I'd like to contribute some of my experience to those who DO want to use tungsten carbide inserts on small lathes. They have become an integral tool in my workshop, but they are not right for everybody or for every situation. Allow me to share what I have learned, and what works for me. I think that too a large degree people have bad experiences using tungsten carbide on small manual machines because they did not know how to select the inserts with correct features. And, unfortunately for the hobbyist, “economy carbide” is generally not available with the right features. If you are happy with HSS, don't waste your time reading any further, for changing to carbide would be a waste of money for you. Personally, changing to carbide made my hobby a lot more fun and productive. Your mileage will vary. Being a jack-of-all-trades kind of guy, I'm not an expert in any one field, and can only claim amateur abilities in my home shop. Almost my entire career involved some aspect of using tungsten carbide. I began my career with 11 years as a product evaluation engineer for a major manufacturer of oil field rock drilling bits using tungsten carbide teeth, and finished it with 11 years as a tungsten carbide tooling sales rep for Iscar, Valenite, and Mitsubishi Carbide. I sold tungsten carbide inserts to manufacturers and machine shops in Washington, Oregon, Idaho, Montana, Wyoming, Colorado, and Utah. The most common materials used in these states are steel, stainless, aluminum and high temperature alloys, so I have very little experience with cast iron. This was the great job for a home shop machinist, for many days I couldn't tell when the hobby stopped and the work began. I always took a problem solving, applications approach to my customers, and most of them appreciated that I machined for a hobby. When I faced particularly difficult or unfamiliar materials, I commonly took scrap samples home to learn how to machine it on my own lathe. I couldn't duplicate what their CNC machines could do, and I do all of my machining dry, but what I learned with my 1.5 HP lathe could often be scaled up to their 20 HP machines. I retired about 4 years ago. I have no ties with, and receive no retirement benefits from any of my former employers.

First, my experience with HSS. My exposure to sharpening and using HSS bits is far from professional level. My grandfather was a tool room machinist, and I have inherited hundreds of HSS bit that he had ground to a wide assortment of configurations. My dad taught me to run a lathe and to sharpen HSS bits. And, believe it or not, sharpening and using HSS bits was a requirement for my BS Mechanical Engineering degree at Cal Poly, San Luis Obispo, CA, whose motto is "Learn by Doing". However, since I've learned to use tungsten carbide inserts, my enjoyment level has soared while my frustration level has dropped. I now use HSS for less than 5% of my lathe work. Chip control is much better. Surface finishes are much improved. Dimensions are more predictable. I don't have to use coolant no matter what SFM I choose to run. And I no longer wrestle when I pick up a piece of material of unknown alloy. I primarily machine steel, stainless, aluminum, brass, wood and plastic, but I've successfully machined on my lathe the toughest materials that my customers encountered: spin-cast stainlesses, titanium, nitronic, inconel, hastelloy. Indexable carbide inserts have become my first choice for turning, facing, boring, grooving, threading and partoff in all of these materials - even plastic.

Some thoughts on using tungsten carbide in a home shop- (I refer to the products of Iscar, Valenite, and Mitsubishi due to my familiarity with them - most of the other major manufacturers offer comparable products. Ask them for details.)

- Lathe Horsepower, Drive, and Rigidity
If you have less than 1 HP, your machine is probably too small to use the same inserts tha I do. You'd probably be better off with HSS, or positive, single-sided insert holders. I agree with most of what is said in the website But I prefer not to use the TiN coatings or utility grade inserts on a small lathe because they don't have what I feel is a sharp enough edge. I have a Taiwanese 12" gear-driven Birmingham lathe that I inherited from my dad. It replaced an old belt-driven 12" Logan. I couldn't recommend carbide tooling for the Logan since it was worn and sloppy, and I don't think the belt would have transmitted the required power for partoff. The Taiwanese lathe came with a motor rating of 1.5 HP, but the starting windings burned out one day as I made repeated partoffs with a carbide insert partoff tool (see more below). The replacement US-made motor also has a rating of 1.5 HP, but appears to have much more power than the original, so my experiences are based upon this new motor, and this is probably a good lower horsepower limit to my recommendations here. I commonly limit my maximum DOC to .040", but if I recall correctly, I've pushed it to .070" DOC in steel without a problem. Carbide doesn't like flexible setups, or chatter, so if your machine is old and loose, you might as well stop reading this. And, as a rule of thumb, if your manual lathe is too small to hold at least 3/4" shank tooling, stick with HSS or single-sided inserts. The sharp carbide I'm recommending doesn't tolerate chatter, heavy interrupted cuts, or the unfortunate goof.

- Brazed Carbide
My comments here relate to indexable tungsten carbide inserts. Brazed tungsten carbide stick tools have their place, but they don't have any place in my shop. Inserts offer repeatablity, accurate indexability, positive geometries, chip control, coating options, accurate corner radii, no grinding, and they are not subjected to the heat from brazing - features I've never enjoyed from brazed carbide. If you want to use brazed carbide, maybe someone else here can give some pointers. I threw all of mine away years ago.

- Costs
I use 3/4" and 1" shank OD turning tools. A holder will cost about $80 to $100. The insert styles I prefer cost about $14 to $18 apiece when bought in boxes of ten. Most distributors don't sell partial boxes of inserts, but some do, so ask. MSC sells single inserts at a higher individual cost. You'll have to evaluate for yourself if you can justify the expense. In my case, I feel that if can't afford the inserts, I can't afford the hobby. I'd avoid the no-name carbide sold by the lower echelon suppliers. This is sometimes made from re-ground used carbide, unpredictable, and unlikely to come with the top-face geometries and sharp cutting edges that a small lathe needs.

- Carbide Grades
Wow - how to summarize such a broad topic in few words. In general terms, carbide grades are a compromise between toughness and heat/wear resistance. Most inserts are optimized for use in high production CNC environments, where cost savings are often found in higher feeds and speeds, which result in higher temperature. Like most materials, tungsten carbide is sensitive to high temperatures, so most industry research is in the direction of increasing heat resistance. That's not a factor for most hobbyists. When my face is 18 inches or less from a spinning chuck, I tend to keep the feeds and speeds down. I'm looking for toughness in an insert because my manual lathe doesn't have the rigidity of a CNC, and my boneheaded handle-cranking is by no means repeatable, or predictable. I prefer to have an insert wear rather than chip. The demands of industry have lead to grades specific to certain materials, but where I may be turning steel, stainless and brass all on the same day, or same hour, I want a general purpose grade. A C2 grade is a good compromise for most metals, or a C5 if you turn mostly steels. Coatings help
primarily by protecting the inserts from the heat of cutting, but it is a little talked about fact that the act of applying the coatings in hot furnaces degrades the toughness of the base carbide. Some recent advances in carbide technology have focused on minimizing that effect. In many cases the home shop machinist would be better off with an uncoated insert in order to optimize toughness and edge sharpness, and you also save a couple of bucks per insert. I'd change to a coated grade if I were experiencing built-up edge (where the material adheres to the insert), or if I did a lot of stainless steel. If I were to suggest a coating for general home use, it would be one that uses a PVD, rather than a CVD, process, due to the lower coating furnace temperatures, and the typically thinner, more lubricious coatings. The PVD process can also coat up to a sharp edge, where the CVD process requires a rounded honed edge. My favorite coating materials are TiCN (titanium carbo-nitride), and TiAlN (titanium aluminum nitride). Counter-intuitively, TiAlN is a wonderfully temperature resistant coating initially designed for use without coolant, but it also offers a high lubricity factor, which helps prevent built-up edge. To the best of my knowledge, the toughest coated insert in the industry is Iscar's IC3028 (IC328 for milling and partoff). I can't verify it, but I have been told that it's toughness, measured in Transverse Rupture Strength, while not equal to HSS, is at least in the ballpark with HSS. This has a PVD TiCN coating. The TiAlN coating version is called IC9028. The uncoated version of this is IC28 grade. This is classified as a non-ferrous grade, and is not optimized for steel but will work in steel, too. All major manufacturers offer something competitive with this, and what they have on their conversion chart that's equivalent to Iscar's grades are what you want. ( Grade numbers are added and deleted constantly. I left Iscar 10 years ago, so specifics may have changed since then, but IC 28/IC328/IC3028 have been been benchmark grades for them.) Unfortunately, the toughest grades generally aren't available with the chipformers designed for light depths of cut. Even though I call it roughing on MY lathe, .040" DOC is generally considered a finishing application, so these inserts tend to come with wear resistant grades to handle the higher SFM's commonly used in industry. So when I choose a finishing chipformer, I have to compromise and select the toughest grade available in that configuration.

- Insert Shape
The most commonly used insert shape and size is the double-sided 80 degree diamond, such as the CNMG432. With its 80 degree included angle, it can cut up to a 90 shoulder. Its popularity ensures that it is available in the most configurations and grades that are of interest to me, and at the most competitive costs. EVERYBODY makes CNMG's. There is valid ground to argue for a narrower angle, like a DNMG-style, if needed, but these will cost more, and offer fewer choices. I use a VNMG-style insert for profiling and reaching into tight corners. There is also a very valid argument for using single-sided "positive" inserts. They are freer cutting, and if you need the freest cut, they are the way to go. But the cost per edge is higher than a double-sided insert with the same features, and there are fewer options available if you need to address a specific application problem in the future. I can't remember any instance where the "up-sharp" double-sided "negative" inserts didn't provide a free enough cut for me, but cutting a long, slender shaft might be more of a challenge. Since I'm an amateur and more likely to chip an insert than wear it away, the double-sided, four-cornered CNMG-style is my choice if I could only have one. The third letter of the ANSI/ISO nomenclature (M in this case) denotes the tolerance class. The two
most common options are "M" and "G". It helps a home shop machinist to equate the "M" with "as molded", and the "G" with "as ground". As an insert comes out of its molding process, the cutting edge has a "hone" of .005" or more. This is well illustrated in Mitsubishi's product bulletin #B036A.

Zoom in on the photos on page 2 where they distinguish between the "M class" and "G class" inserts. This edge hone is advantageous in most applications in order to keep the cutting edge from chipping. To get an "up-sharp" edge or tighter tolerance, the insert must be peripherally ground. Most manufacturers call these CNGG's or CNGP's. This adds several dollars to the cost of each insert, but this is a necessity for most aluminums, titanium, plastics, or wood. I use “up-sharp” inserts for taking depths-of-cut from .0005” to .040”, even in steel. But when I recently turned cast iron for the MLA Diesel, I changed to an insert with an edge hone. For aluminum and plastic, I prefer (but seldom use) inserts that are both ground and polished. The top faces of these are polished to practically a mirror finish, which greatly helps to prevent builtup edge from forming. They are quite pricey, and are hard to justify for the small quantities I produce. When dragged by hand across the top of your thumbnail, a proper CNGG or CNGP will easily shave off some thumbnail material. A CNMG will not – I save those for cast iron or heavy hogging in steel. The third digit designates the cutting tip corner radius. For a CNMG432, this is the "2", indicating a 1/32" radius. A "1" indicates a 1/64" radius. CNMG's are available with corner radii down to 1/64", but most inserts below 1/64" must be ground, so can only be made in a CNGG or CNGP style. It isn't a hard and fast rule, but it is generally recommended that the tip corner radius is the minimum depth of cut for that insert, in order to ensure that the radius is full engaged in the work. Applying this rule, the minimum DOC for a CNMG432 would be .031". Where I generally don't cut more than .040" DOC due to horsepower limitation, a CNMG431 is a much better choice for me. I won't explain the holder nomenclature here, but for manual turning (and facing) TOWARD the chuck, the proper insert holder for a CNMG or CNGP or CNGG431 is MCLNR12-4 (for 3/4" shank tooling) or MCLNR16-4 (for 1" shank tooling). These holders hold the insert at a 9 degree negative angle toward the work piece, thus giving a 9 degree front relief angle.

- Chipformers

Ahhh - the blessings of chipformers! To be able to make little chips in the shapes of "6"s and "9"s instead of a rats nest of entangled razors! But keep in mind that if you are machining at low feeds and depths of cut, or a tenacious material like some aluminums or stainlesses, it may still be a challenge to break the chips. Subtle differences in chipformer shape can make a big difference, and proper chipformer selection was often the hardest part of selecting the right insert for a customer. Chipformers also can give you a POSITIVE cutting edge at the material even when the insert itself is held at a negative angle by the MCLNR holder. Chipformers are not standardized between manufacturers like the insert sizes and shapes are, so every manufacturer has their own nomenclature. For the depths of cut that I use, I need a chipformer designed for finishing or super-finishing, even when I consider .040" DOC to be roughing. And even the super-finishing chipformers aren't expected to control the chips at DOC below .020". Chipformers usually have a two letter designation, and it is common for finishing chipformers to include the letter "F". I prefer to use what are known in the industry as "high positive" chipformers. For DOC below about .005-.010", I sometimes use a ground insert like a CNGG or CNGP with a finishing chipformer like Mitsubishi's "FJ" (with a positive top rake that varies from +12 to +20 degrees). When held in a 9 degree negative holder, a 20 degree positive top rake will enter the work with an 11 degree positive angle. Since these inserts have an "up-sharp" edge, they may chip if used too aggressively in interrupted cuts. They are not recommended for heavy cuts in steel, but they have worked well for me at very light passes in steel, and everything I do in aluminum. Since these inserts are commonly used for high temperature alloys like titanium, the grade choices are limited to the higher hardness end of the spectrum, but tend to be PVD coated or uncoated. I pick the toughest grade available. These are designed to work at about .002" to .010" feed, and .004"-.050" DOC. I use inserts similar to this for about 85% of my turning. If I want to feel more secure about not chipping the insert, I’ll use an "as molded" insert. Examples are the Mitsubishi CNMG 431-MJ (+12 to 20 degrees), and the Iscar CNMG 431-PP (+13 degrees). The honed edges can handle steels very well, and can even handle moderate interrupted cuts. These are available in more grades than the CNGP or CNGG, and once again I'll select the toughest PVD or uncoated grade.

To summarize the key words, I mostly use the toughest uncoated or PVD coated grade available in an insert with a “high positive” “finishing” chipformer and an “up-sharp” grind.

Bob G
Lot's and great information and well written.
Thanks for sharing your experience with us.

A very informative write up and I can't thank you enough. This without a doubt the best post about carbide from a HSM perspective I've ever read. It appears tho that my carbide selection I have been buying is 99.9% wrong. I'll definately be using this information the next time I order.
Would you make many changes to these recomendations if you were using flood coolant?

Pete -

I can't think of anything in these recommendations that I'd change if I were to begin using flood coolant on my lathe, a manual 12"x36". But be aware that, although I used to sell coolant to professional machine shops, and had access to try all the different "flavors", I have never used flood coolant on my lathe, and all of my customers that used coolant had either CNC machines or much larger manual lathes. The only recommendation that I can think that I may have changed for a customer running dry versus coolant is to slow down the SFM when running dry. Theoretically, using coolant would allow higher SFM and spindle speeds while returning similar insert life, but on an open lathe, higher RPM would throw more coolant around, and I find that my personal selection of spindle speeds is limited by factors other than insert life. Tungsten carbide is more susceptible to thermal shock than HSS is, so don't suddenly turn on the coolant when the insert is in the cut, and you are likely to find that using coolant on interrupted cuts will lead to shortened insert life due to micro-cracks from the hot-cold-hot-cold stresses. You can see these cracks under a microscope. This is the same reason most indexable milling cutters are run dry. If you feel you must use coolant on an interrupted cut, the rule of thumb is to use less than 300 SFM and use the toughest (i.e., least wear resistant) carbide grade available.

Bob G
Again a great post. I really apreciate the information.

Again thank You for giving us information on recent tool trend.
I only use HSS in a tangential holder and for boring and threading where speed cannot be high enough for carbide.
First picture show a cylinder I needed as jig for manufacture of a tangential HSS holder.
The piece of cheap steel was found in a scrap container and was 100.5 mm and rusty.One pass with 1300 rpm or more and cut by the CCMT holder shown and it is now 99.61 mm top and bottom apart from jaw zone.
I have measured it more than once as I still do not really believe it.
The inserts come from a technical school where they use the 80 degree corners for milling.I spoil the 100 degree corners and give them back.
Only Iscar loose on that.
Next picture is a close up of shipsteeler holder and last picture is lathe after working.
On the face of the cylinder one can se the degradation of surface quality when cutting speed goes lower.
Simply dared not run faster.

Diamant primo Oct2010 013.jpg

Diamant primo Oct2010 003.jpg

Oktober ved Struer 021.jpg
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Niels -

I like how you made your insert holders! Very clever. I also like to see people use the 100 degree corners, when you don't need to turn up to a shoulder. I don't have any holders for using the 100 degree corner, but your photos have given me a good example of how to make one. Kudos!


Bob G
Dear Bob G,

I'm from Sri lanka and engaged in marketing. I'd like to learn more on these carbide inserts and your post which was really important for me.

Could you please tell me how to learn more on identification of inserts since there are many codes given by manufacturers.


Arjuna P
Hello Arjuna

Try to google carbide inserts code and look at the answer from carbide depot.

Kind regards

Niels Abildgaard
I have decided to make the jump to Carbide Inserts and came across this thread. For the first time I can understand some of the terminology.

I found a link to a great catalog on Mitsubishi (it has picture and charts)

The chip breakers start on page A036 and the CN type inserts start on page A062.

Chip breaker Cross

Mitsubishi FJ = Iscar SM = Sandvik KM = Kennametal MF

Mitsubishi MJ = Iscar PP = Sandvik KF = Kennametal RP

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Try to google carbide inserts code and look at the answer from carbide depot.
Kind regards

Niels Abildgaard[/QUOTE]
Hello there
Sent U a PM requesting a couple of Your BLADES for the ISCAR Inserts.
Do U still have these available or can Your EDM Shop run a couple ???
We like the TANG GRIPS for our Stainless and Tool Steels.
Which would U recommend we get ???
Your esteemed input would be immensely appreciated.
Thanks in anticipation.
Kindest regards
Most of my machining is done by the tangential holder shown.

The tool is a piece of carbide made by wiresparking from a broken endmill.
It can be sharpened with my grinding wheel of siliziumcarbide.
When my wiresparking friend stops working normal CCMT 09 inserts will be the rescue.
The lower piece of iron will serve to illustrate how to make a very rigid CCMT holder without having a milling machine.
Machining up to this level is anglegrinding , turning with the tangential carbide stick and swearing when hot swarf hits me.
At this stage I have used less than two hours

Kind regards



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The intended upside down,damaged,useles guiding inserts have one of their 100 degree corners removed by grinding.This is for clearance.If You play a little before,the nessecary amount can be judged.Clean and glue
Drill a 4 mm hole central in the two upside downers.
I made a small guiding bush after several mishaps.

less than one hour.




Countersink 7.5 mm for M4 Unbraco screws on underside of holder.
Take a deep breath and tighten the screws.
So hard as You dare and do not have the upside downers move.
They shall never come apart again and be carefull that the upsid downers do not move.
This time I was lucky.
Time for shining up and a beer.



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Cutting clearances are now OK due to some grinding.
A ligth hammerblow and the active insert comes of.
The commercial toolholders use some very expensive Torx M3.5 screws to fasten CCMT09inserts.
I make my own M4 and have had no problems.
We want the screw to push the insert hard against the guides.
Again the magic bush helps.
It keeps the active insert one mm over normal working position while we drill and cut the M4 thread.When insert comes down the seven degree slope of the guides M4 thread and insert hole is not concentric.Exactly what we need.




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This has been really interesting Niels! Lot of useful info there I'll be copying down the track to make some myself mate!

cheers, Ian
Drilling through insert and magic bush and cutting thread M4 fixed solid in drilling machine.Safe and the way to do it.
Next picture shows a professional and very expensive M3.5 Torx screw bougth from SECO,and a cheap M4 made by Niels and the very simple bush to machine it.
Last picture is valid until monday where I will buy some more M4 Unbraco screws of 12.9 quality and finish the other end.
Have a nice sunday.




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Decided to try another procedure.
Drill and tap M4 for active insert.
Let active insert hover 1.2 mm over base.Magic washer.
Glue upside downers in place and drill and countersink for through bolts.
Tigthen bolts.Remove magic disk and we are there.
It feels simpler that way.
CCMT 09 comes in two thicknesses,do not ask me why and I future I will use thin version (CCMT 0903) for upside downers and thick (CCMT09T3) for the active.Having the new edges of the active rubbing shoulders with thrash ones sounds wrong to me.


Hello Ian

Thank You for interest and excuse the late answer but I wanted to try the FIX ACTIVE FIRST method before anybody including You wasting time on the not so good method.
The new way is much better and it says a lot about getting older that I had to make 9 ends the old and not so smart way, before seeing the shining ligth.
I found a new home for the superflous holders.
Just like kittens. Much to nice to eliminate.


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I started on HSS in metalwork at High School 1966, then as apprentice fitter and machinist 1971.
Retired now and have a "OPTIMUM TU2506V" lathe 12 centre height 1.2kw 240v motor
Brought at toolholder with 10 WNMG Trigon tips off ebay
Best thing since sliced bread!!!!!!
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