Parting Off – Part 1: Basic Principles and Challenges
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 first of five posts relating to basic principles, best practices and troubleshooting of parting off operations.
Definition of parting off
Parting off or cutoff operation is a lathe application in which an orthogonal cut in the radial direction is used to split a round workpiece into two pieces. In many cases this operation is used to remove the finished end of the workpiece from the parent bar stock that is still clamped in the chuck.
Because the tool must go deep into the cut all the way to the center of the workpiece, parting off operations present some unique challenges. First and foremost, is the problem with zero cutting speed.
Vc= cutting speed in surface feet/minute (SFM)
Dc= workpiece diameter in inches
n= revolutions/minute (RPM)
As the parting off insert radially goes towards the center, the Dc of the workpiece reduces, and the required RPM linearly increases to maintain the same cutting speed (SFM). Regardless of the capabilities of the spindle, at some point the spindle RPM cannot keep up with the requirement to maintain a constant cutting speed in SFM. As the insert goes closer to the center of the workpiece, the machine RPM maxes out and cutting speed starts reducing. At the center of the workpiece, the cutting speed is zero SFM. Cutting metal at zero -or nearly zero- cutting speed means that the insert must push the material without shearing it. This requires the insert grade to be very tough; that is why typically PVD (physical vapor deposition) grades are better suited for cutoff applications. CVD (chemical vapor deposition) grades are also available for cutoff and they usually tend to have very tough substrate for the inserts.
The second challenge for parting off operations is chip evacuation. If the chips are not evacuated efficiently, the chips get re-cut and eventually the operation stalls, with chips plugging the groove. Usually this leads to insert failure as well as severe damage to the cutting tool. Many times, the tool gets ‘friction welded’ to the workpiece.
As the tool goes deeper into the groove, coolant delivery at the cutting edge becomes a challenge. Correct selection of tool with high pressure thru coolant systems including clearance face coolant delivery can be critical for the success of the operation. Additionally, correct insert chip breaker as well as process parameters (feeds, speeds etc.) are required for effective chip evacuation. These topics will be covered in detail in the later postings.
The third challenge with parting off operation is efficient chip curling. If the chips are not folded lengthwise into nice ‘clock springs’, they are hard to evacuate; leading to a groove plugged with chips, insert failure and a stalled spindle.
A good cut off insert also needs to be able to efficiently contract or ‘fold’ the chips sideways. If the chips engage on the side of the finished wall, they leave witness marks on the finished surface leading to scrapped parts. Here is an example of good cutoff geometry that shows ideal chip contraction.
The fourth big challenge with cutoff operations is burr or pip left at the center of the workpiece. Insert geometries, tool alignment, cutting parameters as well as machine conditions all play a very big role in reducing the pip leftover at the end of operation. It is also important to plan the operation in such a way that the pip is left over on the rough component rather than the finished component.
We just looked at the four main challenges encountered during parting off operations. Issues relating to these challenges can be avoided by following the best practices and selecting the right insert and holder combination. In the next article we will look at some of these best practices.