How to choose the thread turning insert and shim

When choosing the most suitable thread turning insert for your application, you need to consider the insert type, flank/radial clearance, and insert geometry. These are all factors that influences chip control, insert wear, tool life, and thread quality.

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How to choose the insert type

When turning a thread, there are three main insert types to choose from: full profile, V-profile, and multi-point inserts. Each type has its advantages and disadvantages.

Full profile insert

The full profile insert is the most common insert type. It is used to cut a complete thread profile, including the crest.

Advantages

• Ensures correct depth, bottom, and top profile for a stronger thread
• No deburring of the thread profile needed
• Fewer passes required compared to a V-profile insert thanks to the larger nose radius
• Gives a productive threading performance.

Disadvantage

A separate insert is required for each pitch and profile.

Note: Extra stock/material should be left on the workpiece for topping the finish diameter of the thread (0.05–0.07 mm (0.002–0.003 inch)).

V-profile insert

V-profile inserts do not top the thread crests. Therefore, the outer diameter for screws and inner diameter for nuts must be turned to the right diameter prior to threading.

Advantages

• Flexibility: the same insert can be used for a range of pitches, provided that the thread profile angle (60° or 55°) and radius are the same
• Minimum tool inventory needed

Disadvantages

• The insert nose radius is smaller to cover the range of pitches, which reduces tool life
• Burr formation can be a problem

Multi-point insert

Multi-point inserts are similar than full profile inserts but have more than one insert point (NT>1). A two-point insert doubles the productivity, and a three-point insert triples the productivity, etc.

Advantages

• Requires fewer passes, which gives better tool life, productivity, and lower tool costs

Disadvantages

• Stable conditions are needed as a result of increased cutting forces as the cutting edge has a longer contact length
• Needs sufficient room behind the last thread to clear the last tooth of the insert, generating a full thread depth

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How to choose the insert geometry

Selecting the correct insert geometry is important when thread turning. The geometry influences chip control, insert wear, thread quality, and tool life.

Flat geometry

• All-round, can be used for most materials
• Edge rounded cutting edge for edge strength

Sharp geometry

• For sticky or work-hardening materials, e.g., low-carbon steel, stainless steel, non-ferrous materials, and super alloys
• Sharp cutting edge for low cutting forces and good surface finish

Chip-breaking geometry

• For long-chipping materials, e.g., low-carbon materials. Can also be used for stainless steel, alloyed steel, and non-ferrous materials
• Chip-forming geometry that enables a more continuous and unsupervised machining
• Not to be used with radial infeed

Read more about threading inserts and grades

Insert clearance angles

Angular clearance between the insert and thread is necessary for precise, accurate thread turning. There are two types, radial clearance (ALP) and flank clearance (ALF).

Radial clearanceFlank clearance

Flank clearance

Cutting edge clearance between the sides of the insert and thread flank is essential to ensure even tool wear and consistent, high-quality threads. The insert should therefore be tilted to gain maximum symmetrical clearance from the flanks (flank clearance angle) and get the correct thread profile. The tilt angle of the insert should be the same as the helix of the thread.

Flank clearance

Selection of shim

Insert shims are used to give different tilts to the insert, so that the angle of insert inclination (λ) is the same as the helix of the thread. See table below for methods on how to select the correct insert shim.

• The standard shim in many holders is 1°, which is the most common angle of inclination
• Negative shims are used when turning left-hand threads with right-hand tools, and vice versa

Lead (pitch) mmThreads/inch Workpiece
diametermm
inch

Example:

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• Pitch = 6 mm and workpiece = Ø 40 m: a 3° shim is required
• Pitch = 5 threads per inch and workpiece = Ø 4 inches: a 1° shim is required

Threads with small profile angles

ALP = Radial clearance
ALF = Flank clearance

For ACME, trapezoidal, and rounded threads, it is especially important to choose the correct shim to tilt the insert, because the pressure on the cutting edge is higher, and the flank clearance is smaller.

Flank clearance (ALF) depends on profile

Flank clearance
(ALF)Flank clearance
(ALF) Metric, UN60°7.6°5° Whitworth55°7.1°4.7° Trapezoidal30°4°2.6° ACME29°3.8°2.5° Buttress10°/3°2.7°/0.8°1.8°/0.5°

Radial clearance

To set the correct radial clearance, the inserts are tilted 10° or 15° in the tool holder. It is important to use internal inserts with internal tool holders, and vice versa, to ensure that the correct thread form is achieved.

Insert sizes:
11, 16, and 22 mm
(1/4, 3/8, and 1/2 inch)

Insert size:
27 mm (5/8 inch)

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For more information, please visit Carbide Inserts.

Forgive me, I think in Imperial rather than Metric. I could do conversions but the range we are discussing cannot be done in my head. I would need to calculate each number and that would just take too long. I have delved into threading with carbide tips on several occasions in the past. Without links to the particular posts and my poor attitude tonight, I won't go deeply into the subject. For cutting, to most people moderate, thread pitches say 16 TPI or M1.5 metric, carbide threading tools or even triangular cutting tools work fairly well enough.

For smaller pieces, such as 2-56, M2X.4, things start to get tight. Carbide is a pressed material, a powder put under pressure and heated. As such, there is a small "nose radius", even on threading tools. The Unified thread form allows a small misformed portion at the bottom of the "V". And a corresponding "flat top" on the outside. This is to allow for production tooling to lose its' keen edge and still produce a usable product.

With carbide, the smallest nose radius I have ever encountered was 1/64th of an inch. There may be smaller radii, I just am not aware of them. At 1/64 radius, the diameter is 1/32 inch. At 32 TPI, the pitch is 1/32 of an inch. The resultant thread is more of a "U" shaped groove than a thread pitch.

My feelings on that subject are simply that there are some things that carbide won't do. I use HSS for threading. All threading. In truth, I attempt to cut "V" sharp threads and smooth the surface after cutting. Much of my threading is 56 TPI, I do small work. I have done smaller, down to 80 TPI. But at and below that, I use a die. I'm pretty good, but I'm not that good.

.

Forgive me, I think in Imperial rather than Metric. I could do conversions but the range we are discussing cannot be done in my head. I would need to calculate each number and that would just take too long. I have delved into threading with carbide tips on several occasions in the past. Without links to the particular posts and my poor attitude tonight, I won't go deeply into the subject. For cutting, to most people moderate, thread pitches say 16 TPI or M1.5 metric, carbide threading tools or even triangular cutting tools work fairly well enough.

For smaller pieces, such as 2-56, M2X.4, things start to get tight. Carbide is a pressed material, a powder put under pressure and heated. As such, there is a small "nose radius", even on threading tools. The Unified thread form allows a small misformed portion at the bottom of the "V". And a corresponding "flat top" on the outside. This is to allow for production tooling to lose its' keen edge and still produce a usable product.

With carbide, the smallest nose radius I have ever encountered was 1/64th of an inch. There may be smaller radii, I just am not aware of them. At 1/64 radius, the diameter is 1/32 inch. At 32 TPI, the pitch is 1/32 of an inch. The resultant thread is more of a "U" shaped groove than a thread pitch.

My feelings on that subject are simply that there are some things that carbide won't do. I use HSS for threading. All threading. In truth, I attempt to cut "V" sharp threads and smooth the surface after cutting. Much of my threading is 56 TPI, I do small work. I have done smaller, down to 80 TPI. But at and below that, I use a die. I'm pretty good, but I'm not that good.

.For hobby use, I think threading inserts have little useful application, in manual machines they cannot be run at speeds that they like and the half nuts closed and opened easily, the main reason that I think they are used by hobbyists is that all too many of them are afraid to grind a HSS tool, but there are ways to avoid that and use HSS, as I suggested in an earlier post, by the use of Aloiris threading bits.