Home Shop Machinist

Cool tools for lathe work.


Just about every kind of machine needs to be “tooled up” to some extent in order to start making stuff. Vertical mill tooling is relatively simple. Start with a collection of two-, three-, or four-flute high-speed steel (HSS) or solid carbide end mills. Add a boring head, a fly-cutter [“Fun with Fly Cutters,” February 2015], maybe a chamfering bit, and a set of round-over bits, and you’re good to go.

Brazed carbide tools and carbide insert tooling come in many different shapes and sizes.

Lathe tooling is a bit more complicated. It’s so complicated that I’ve rewritten this column a number of times trying to condense the information in a sensible way. The best one column can do is barely scratch the surface.

HSS vs. Carbide

Old-school high-speed steel (HSS) tools are perfectly fine for home shop work. But it takes skill and experience to shape and sharpen them. Brazed carbide tools and carbide insert tooling eliminate that frustration (sharpening). On the other hand, they come in so many shapes and sizes that it seems to trade one frustration for another. Yet the advantages of carbide make it worth the effort.

You can still buy HSS, either blanks or pre-ground, but their popularity has faded because hardly anyone is trained to shape or sharpen them anymore. It’s not that they are hard to shape or sharpen, it’s just they are never used with CNC machines. CNC machines use tungsten-carbide (usually shortened to “carbide”) insert tooling almost exclusively. Insert tooling lasts longer than HSS and, most important, they allow worn or spent cutting edges to be replaced without affecting the machine setup. This saves hours of tedious labor. On top of that, most inserts have two or more cutting edges per side. Inserts with zero relief angle can be flipped, doubling the number of tool edges. A good example is the WNMG “trigon” insert. It has three cutting tips and can be flipped. It’s like getting six cutters for the price of one.

Decipher the Code

Not including parting, threading, or grooving tools, insert manufacturers have standardized at least 17 common shapes. The first letter—of the four-letter code—is the shape (the W in WNMG). Some of the letters make sense: D for diamond (55 diamond, to be exact), H for hexagon, O for octagon, P for pentagon, R for round, S for square, and T for triangle. The rest are random. For example, A is an 85 parallelogram, C is an 80 diamond, L is a rectangle and W is the aforementioned trigon shape. I mostly use diamond shapes (C, D and V). I occasionally use square (S) and triangle (T) inserts. I have found that these shapes work for 99 percent of my turning requirements.

Example of a negative-rake trigon insert, style WNMG.

The second letter (WNMG) defines the clearance, also called relief or rake angle. The letter N is for zero (also referred to as negative rake) and the list goes from A, which is 3, to G, which is 30, with the letter P (out of alphabetic sequence for reasons unknown to me) for 11 relief angle. There are nine potential options for relief angle. The letter C happens to be the code for a 7 relief angle, which is a good all-purpose angle for small inserts for home shop machining. The general rule for CNC machining, where the goal is to machine parts as aggressively as possible, is positive rake angles require less horsepower, but are more prone to fracturing when machining hard materials. Zero rake inserts work better in hard materials, but require more horsepower. However, I use positive-rake tools all the time, including on hard steels. I rarely, chip an edge. Manual machining in the home shop is not about throughput, so tools should not see that kind of stress.

Examples of CCMT style inserts: 80 diamond shape with a 7 relief angle. The red line is the “inscribed circle” that defines the size of an insert. The one on the left is a 3/8-inch insert (the diameter of the circle) and the one on the right is a -inch insert. The numerical code for “size” comes after the four-letter code. (Example: CCMT-3 for 3/8 and CCMT-2 for .)

The third letter (WNMG) describes several tolerances to which an insert is manufactured. The information is important for production work as part of the tolerance stack, but I did not include a code chart because it’s not critical for manual lathes since you’re “touching off” and measuring each part, usually more than once and during each machining operation. Google it if you’re interested. Some manufacturers may grind the cutting edge of their inserts to make them sharper. The third letter G may be (but not always) used to designate that feature. A ground edge is sharper and therefore desirable for low-power home machines. They also produce the best finish on most materials.

The last letter (WNMG) describes the hold down scheme and chip breaker style. There are 17 options from N which is no hole and no chip breaker (OK that makes sense) to Z which describes an insert with a cylindrical hole and a double-sided high-double positive chip breaker. Again, Google it if you’re interested.

We haven’t even gotten into the different sizes, carbide grades, or coating options. If we count up just the shapes, relief angles, tolerances and chip breaker combos, we get thousands of possibilities. For practical reasons, tool companies and their catalogs don’t offer every option. But you can see why trying to wade through a catalog to pick out just the right one can be confusing.

Typical boring bar configuration: The insert fits in a pocket and is held in place with a screw.

Tool Holders

Insert tools require compatible tool holders. The holders are an integral part of the insert tooling system. In addition to securing the inserts, the holder will have the correct angles built in, so all you need to do is line up the cutting edge on center and you are good to go.

Inserts can be secured with a hold-down clamp (top) or a center screw. Good quality holders will have a snug pocket for the insert to assure repeatable positioning. The large triangle insert is a 3/8-inch size, the small 80 diamond in the middle is a -inch. The bottom insert is an internal threading insert.

One good way to start out with insert tooling is to buy a kit or assortment set that includes holders with inserts. Once you have a collection of holders, you simply buy new inserts when you need them. Depending on how much machining you do, inserts may last a few weeks or years. Use the web to browse for sets on sites like Little Machine Shop, KB Tools, eBay, and Amazon. I recommend the Micro100 set of turning tools with 3/8-inch shank sold by KBC Tools (www.kbctools.com). Many small home shop lathes have tool posts that limit the shank size to -inch or smaller, so check the capacity of your lathe to verify compatibility.

Boring out this ring of 7075 aluminum with a CCGT-2 insert (-inch inscribed circle) in a 3/8-inch shank boring bar produces excellent results.

I use mostly -inch or 3/8-inch shank holders for outside turning. For boring I use 3/8-inch and -inch shank holders. I have various holders that accommodate CCGT and DCGT inserts for both -inch and 3/8-inch inscribed circle cutters. A CCGT-21 insert costs $10 to $25 depending on the brand, coating and carbide grade. A compatible 3/8- or -inch shank holder costs $25-$45. Most tool resellers offer discounts on multi-packs of 10 inserts or more.

Getting Started with Carbide

Brazed carbide tools are an alternative to carbide inserts. They come in many shapes traditional to HSS tools. The downside to them is they can’t be re-sharpened on a regular grinder. If they become dull or the edge gets chipped, you toss them out. (A useful item to have is a loupe or magnifier to inspect the edges for cracks or chips.) I own several Micro100 brazed tools. They are super sharp and hold up well for most turning. My standby Micro100 bit is the AR6 (right-hand cutting) with a 3/8-inch square shank. It sells for $13-$18.

Example of a brazed carbide tool.

For the home shop, carbide tooling should be run completely dry with most materials (titanium being one exception). In other words, do not use coolant or any cutting lubrication at all. According to an article in Modern Machine Shop [“Machining Dry is Worth a Try,” October 1, 2003], uneven cooling caused by too little coolant can cause the edges of carbide tools to fracture and chip. The article cited how advances in coatings and carbide grades have eliminated the need for coolant for most applications.

I realize the above information is in the realm of “just enough to be dangerous” so please consider it just an introduction to carbide.

Bob Hadley is the R&D manager for a California-based consumer products company. He holds a Sport Pilot certificate and a Light-Sport Repairman certificate with inspection authorization for his Jabiru J250-SP.


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