Parting Off – Part 4: Grooving with Neutral Versus ‘Handed’ Cutting Edge
Parting off is one of the most common lathe applications in a shop. In this series, we will discuss various challenges, tips and tricks to make your parting off applications more productive and trouble free. This is the fourth of five posts relating to basic principles, best practices and troubleshooting of parting off operations.
Overview of handed inserts
There is quite a bit of confusion and misapplication in the market about inclined or handed inserts. First rule of thumb is to always use a neutral cutting edge when possible. A neutral insert leads to improved chip formation, lower resultant cutting forces and, as a result, longer tool life. Use of inclined or handed insert always has a negative effect on insert life. However, there are specific applications where handed inserts have an advantage. First, let’s look at the differences between neutral and ‘handed’ inserts.
Both tool holders as well as inserts can be ‘handed’. Currently, we are looking at handed inserts rather than tool holders. The hand of the inserts (right or left) can be determined by looking at the cutting edge from above. Meaning, by looking at the rake face of the insert rather than the base of the insert. Depending on the inclination of the cutting edge, the inserts are categorized as shown in the diagram below.
Handed inserts have two advantages over neutral inserts:
First advantage is in reduced exit pip/burr formation. A properly applied handed insert always leaves smaller pip than a neutral insert. However, the hand needs to be selected carefully. If the wrong handed insert is used, then the pip gets left on the parted off component; which is usually the finished workpiece. A simple rule of thumb when selecting the hand of the insert is as follows:
Direction of rotation of the machine spindle:
Clockwise –> rotation –> right hand cutting insert
Anticlockwise –> rotation –> left hand cutting insert
The picture below shows three cases. First is a correctly applied insert- a left-handed insert for anticlockwise rotation of the spindle. This leaves the least exit pip. The second is a neutral insert. This leaves a large exit pip on the finished component. The third is the least desirable, as it leaves the burr on the finished component, and usually the pip left with a handed insert is quite pointed.
Second advantage in using a handed insert is seen in parting off tubes and thin walled components. When parting off tubes, the handed inserts reduce the tendency to form an ‘exit ring’ as shown in the picture below. If the exit ring remains on the finished component, it can interfere with the rest of the manufacturing processes. Reduced burr formation on the ID of the parted off tube is also an advantage of using the handed inserts in parting off tubes.
When an inclined cutting edge is used for the parting off operation, the lead angle or inclination angle is detrimental to chip formation. The chip always rolls of at 90O to the cutting edge. So, if the cutting edge is inclined, the chip is formed in a helical shape rather than a nicely wound clock spring shape which is formed by a neutral cutting edge that we discussed in part three of this five post series.
Pictures below show the examples of chips formed by neutral edge as well as an inclined edge.
One option of guiding the shape of the chip while using an inclined insert, is to interrupt the operation after a depth of 1 or 2 x width [S] is reached. Once the operation resumes, the new chip then flows within the formed groove. This method, however, still risks damaging the shoulder of the finished workpiece with the helical chips.
Another application to consider when using inclined cutting edge is the tendency of the tool to ‘walk’ or run off-center with the feed force. With a neutral insert, the feed forces Ff are in the direction of the feed.
With an incline cutting edge, there is an additional component of force Fp involved that tends to induce vibrations in the parting off operation as well as pushes the cutting edge away from the 90O shoulder. This leads to creation of a ‘dished face’. Thus, feed should be reduced by approximately 30% (or more) to reduce the vibrations.
It is important to note that the benefits and adverse effects both increase as the inclination angle increases. So, a neutral cutting edge will have the largest burr, but the least tendency to ‘walk’, because the additional component of force (Fp) is missing. As the inclination angle increases, the burr formation reduces, but the tendency to ‘walk’ increases steadily. So, additional care should be exercised when using inserts with aggressive inclination angles. Below are the typical inclination angles seen in the market and corresponding effect on burr or pip formation.
From all of the above discussion, it is clear the inclined or handed inserts should be used only to trouble shoot in certain applications. Below is the summary of applications considerations between neutral and inclined cutting edges.
This concludes the fourth of the five-post installment discussing best practices of parting off applications. In the next article, we will look at some of the common problems and troubleshooting recommendations for parting off operations.