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The Job Shop’s “Go-To” Handbook for Choosing CNC Turning Insert Shape

Choosing CNC Turning Insert Shape

It’s often overlooked, or perhaps misunderstood, but selecting the correct turning Insert shape for an operation is the most fundamental first step to success. In this post, we will focus on the “go-to” workhorse insert shapes that are the most popular for job shop CNC turning operations:

  • 80° Diamond, or “C-Style” inserts
  • 80° Trigon, or “W-Style” inserts
  • 60° Triangle, or “T-Style” inserts

As a quick note: when referring to the number of edges on an insert, we will use the count for double-sided negative rake inserts. Naturally, if your applications need smaller, screw-down positive rake inserts, just consider that will mean half the number of edges as what is mentioned in this post.

Let’s start with the most popular insert shape:

C-Style 80° Diamonds


C-Style inserts have long been the most popular shape due to their high versatility. They have a strong cutting edge with very secure seating in the insert pocket. The 80° corners can be used for both turning and facing operations. The opposite 100° corners can be used for general roughing applications (especially facing), providing a maximum economy of 8 total cutting edges. The only real caution is that, with only 5° of clearance between the trailing side of the insert and the workpiece, chip jamming can occur when boring.

W-Style 80° Trigons (the other 80° Diamond shape)


Six-cornered 80° Trigon shaped inserts have the same included angle presentation to the workpiece as “C-Style” inserts. If your shop normally only uses the four 80° corners on a “C-Style” insert, the six edges offer cost savings potential. W-Style inserts are generally used on more moderate depths of cut and feed rates compared to C-Style inserts. It’s good to be aware that the seating of the insert in the pocket is not as stable as C-Style inserts. The included angle of the toolholder clamping pocket is 140°, which creates the opportunity for the insert to “move” slightly in the pocket. Also, remember that W-Style inserts cannot take as deep a depth of cut as similarly sized C-Style inserts.

T-Style 60° Triangles


Triangles are a very versatile insert shape. They can be used for turning, facing, boring, copy turning and basic profiling. There is a very broad assortment of toolholders available with many different lead angle options. Triangles offer good economy thanks to their 6 cutting edges. T-Style inserts are an excellent choice for general boring due to very stable seating of the insert in the boring bar pocket and the extra side clearance between the insert and the workpiece bore (greatly reducing the risk of chip jamming). Keep in mind the 60° edge is measurably weaker than 80° diamond shaped inserts. Be sure not to use a triangle insert that is too large for the application, as the cost per edge can increase. For example, a 3/8” IC (Inscribed Circle) triangle insert (TNMG-33x) can manage up to .375” depth of cut in most situations with nearly the same insert strength—but a much lower cost—than a 1/2” IC triangle insert (TNMG-43x).

Considerations for selecting the right insert shape for roughing and semi-finishing cuts

Now that we’ve established the baseline characteristics for our 3 workhorse shapes, are there any additional secrets, tips or rules to help choose which workhorse we should select for an operation? Not quite. It’s not possible to have 100% steadfast rules, as each shop and application has its own unique needs. But there are 3 considerations that can help you choose.

Which to choose: 80° Diamond or 80° Trigon?

In this case, it’s a good idea to consider the size of your turning center versus the workpiece. If your lathe uses ¾” Shank tools (or less than ¾”), then you can lean much more toward 80° Trigons. The depths of cut are going to be lighter, as well as the cutting forces. If your lathe uses 1-1/4” shank tools, stay with 80° Diamonds. Your workpieces are big and you have a lot of HP at the spindle—therefore, you want the strongest and most stable 80° style insert. If your machine has 1” shank tools, then things are a bit more in the middle. The decision line is likely based on how much power you’re going to use on the cuts. If the cuts aren’t going to move the power needle much, then Trigons are fine. But if you’re going to run things hard and heavy, then it’s better to go for 80° Diamonds.

Do you frequently run jobs that have a lot of facing cuts?

Almost all CNC Turning Centers in job shops are horizontal. That means facing cuts are more stressful on the insert. The direction of cut is in the X-Axis, which is not as stable on a lathe as Z-Axis cutting. More importantly, the workholding stability is much lower in X-Axis cutting. This is the perfect time to utilize the 100° corner edges on C-Style inserts—especially considering the economics.

Will there be lots of passes?

On jobs where you’re going to have a large number of roughing passes, normally the best choice is a Triangle insert. You’ll have plenty of cutting edge length to maximize the depth of cut and maximum economy with 6 cutting edges. You’ll also have the benefit of that extra clearance on the trailing edge side of the insert—which gives just that extra bit of room for the chips to escape the cutting zone. This is especially important for internal machining.

As a final note, most of these recommendations are framed in terms of roughing and semi-finishing cuts. But what about finishing cuts?

Tips for selecting the right insert shape for finishing operations

If the finishing operation could use any of the 3 workhorse shapes, which one is best? One way to look at is to use whatever you’re happiest with. Over time, we all develop our favorites. And that’s fine. Perhaps, though, it’s worth considering that finishing cuts are what it’s all about. That’s where we complete the workpiece to the specs on the drawing—size as well as surface finish. That means the biggest payoff will come from stability. We never want to be chasing specs because of our insert choice. Therefore, it makes some sense to lean heavily toward C-Style 80° Diamonds. They are very stable in the toolholder pocket and their indexability is excellent (meaning the new cutting-edge position will repeat very closely to the previous edge). Better indexability means less chasing dimensions with control offsets. Less chasing means a more stable process.

In Summary

In the complex world of CNC turning operations, the right turning insert plays a vital role in ensuring success. It's not a one-size-fits-all decision, your unique shop and application needs dictate the right fit. Make the smart choice for your operation today, and let the right turning insert shape unlock your shop's full potential.