Tooling & Production February 2005

"Shop Talk with Steve Rose"

The Author, Steve Rose

Sorting out ANSI, ISO inserts

There is often a lot of confusion between the two types of insert designation systems.  The American National Standard Institute and the International Standards Organization both assign meaning to each letter and number, but the ANSI is based on inches and ISO on the metric system.

The letters have the same basic meaning in both systems, the difference is in the numbers.

In the ANSI systems, the 1st number signifies the inscribed circle (IC) of the insert.  This is an imaginary diameter that is used to “construct” the insert shape.  The IC is measured in increments of 1/8” so a 4 means 1/2” (4 x 1/8 = 1/2”).  The problem is that the IC cannot be directly measured on many insert shapes.

Inscribed circle

ANSI System


Inscribed circle in 1/8 increments.

4 x 1/8” = 1/2”  

Edge length

ISO System

TNMG-22 04 08

Edge length in millimeters

22 = 22mm  



The ISO system provides a more functional description of the insert.  In the ISO system the first number represents the length of the cutting edge in millimeters.

Insert Thickness

The second number represents the thickness of the insert.  This is measured in 1/16” increments in the ANSI system (3 x 1/16” = 3/16” = 0.1875”).  In the ISO system the thickness is measured in increments of 1mm.

The Insert Nose Radius

The most critical dimension on an insert is the nose radius.  CNC programs are written around a specific insert nose radius size because the nose radius leaves its shape (form) on the part.  If you change TNR size of your insert you must also change the CNC program.

In the ANSI system, the third number represents the TNR.  The nose radius of an ANSI insert is based on 1/64” fraction increments.

A TNMG-432 insert would have a 0.031” nose radius.  The third number is 2 (2 x 1/64” = 1/32”).

The ISO system is a conversion from the ANSI system, in metric values. This conversion can be done by using the conversion factor of 1” = 25.4 mm.

To convert the metric number to the inch equivalent is easy.  For a CNMG-19 06 16 insert, 1.6mm ÷ 25.4 = 0.063”        

When using ISO (metric) inserts we must verify that the nose radius on the insert matches the nose radius used in the part program.  If in doubt convert the number to find the equivalent inch size.

In addition to these informative and descriptive numbers, many insert manufacturers are now placing speed and feed recommendations on the insert boxes.  This is very helpful and gives a good range for standard machining.

Another helpful piece of information I’ve always wanted to see on the insert box is the TNR size.  We know that the TNR value can be calculated as we’ve done above, but when you’re sorting through a cabinet full of blue, red or yellow boxes it would be very helpful to see the TNR with a quick glance.  This is the most important piece of information in terms of machining so it makes sense to make it handy for users.

Remember, the next time you see an ISO insert, it is not that different from an ANSI and may even be easier to understand.