Parting Off – Part 3: Feeds, Speeds and Chamfering Considerations
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 third of five posts relating to basic principles, best practices and troubleshooting of parting off operations.
The only chips that are good in a parting off operation are the chips that come out without plugging the groove. If the chips don’t get out of the groove, we risk insert failure and even stalling the spindles- in case the tool gets accidentally ‘friction welded’ to the workpiece. Process parameters such as feeds and speeds are always critical for the success of any machining operation. In the case of parting off operations, they are especially crucial; because they can determine chip curling and chip breakage and hence the success of the operation. In this post we will look at effect of feeds and speeds on the success of a parting off operation. Towards the end of this article, we will also look at the best practices in planning chamfering strategies.
Feeds and speeds are among the largest influences in any machining operation. In grooving and especially in parting off operations, feeds and speeds are very critical for chip breakage, chip evacuation and tool life. As explained in the first post of this series, towards the center of the workpiece, 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 insert machine, RPM maxes out and cutting speed starts reducing and eventually reaches zero at the actual time of parting off. This requires the insert grades to be especially tougher for parting off operations, because towards the center of the workpiece, the carbide is basically just ‘pushing’ the material rather than shearing the material.
Feeds need to complement the speeds as well, because an excessive feed with near zero speed will mean that the material is pushed off in the wrong direction, causing premature parting off, fracture of the pip and a large and un-managed burr at the center of the finished workpiece. In a high volume, high precision operation, this will result in an uncontrolled ‘throwing off’ of the finished component, rather than a disciplined, consistent operation that can predictably catch the finished component. For this reason, at a diameter of 1.5 × insert width [S] or above, reduce the feed [f] by 75 %.
Additionally, it is not recommended to groove past the center, as there is a risk of insert fracture. It is possible to groove past the center by the amount equal to the maximum of corner radius, usually 0.004 in [+0.1 mm].
Speeds also play a critical role in parting off operations. The best chips in parting off operations are the ones that come out without damaging the finished shoulders (folded in axial direction) and are ‘curled tight like a clock spring’ (folded in radial direction). Excessive speeds prevent the chips from ‘setting into’ the chip breaker groove as shown in picture below. As the cutting speed (Vc) increases, the chip gets less and less time to conform to the chip breaker shape, thus leaving a thin flat chip rather than a thicker and conformed chip. Thus, the chip loses the ability to be molded into tight clock spring curl, leading to the finished groove to be plugged with chips and eventually stalling the spindle. Also, as the chips don’t conform in the axial direction, they tend to damage the finished shoulder of the workpiece.
An example is shown in the picture below. For the same feed of 0.004 inch/rev, just by changing cutting speed (Vc) from to 656 SFM to 492 SFM, the chips got longer time in the cut to conform to the chip breaker. This lead to significant improvement in chip control.
Feeds when grooving on inclined surfaces
When grooving on inclined surfaces, inserts tend to deflect toward the direction of the incline. This leads to high vibrations and consequently, inaccurate part dimensions with poor surface finish on the shoulder. To avoid this, the feed for the chamfering operation must be reduced by approximately 20-50 %. A sharper insert geometry should be used as it will shear the material better and reduce the tendency of vibration or ‘insert walking’.
Chamfering and parting off
Many times, a chamfer is required before the parting off operation. The chamfer can be either a component feature or it can be just a small corner-break on the finished as well as raw workpiece. For a low volume operation and in some cases for high volume operations, it is possible to start chamfering and then directly plunge into a parting off operation in the subsequent GoTo command. However, in most high-volume operations, this causes the parting off insert to simultaneously cut from the side as well as the bottom of the insert at high feed rates. This leads to unpredictable forces and poor chip control (in the form of spiral chips), as well as the introduction of vibrations in the subsequent parting off operation. So, it’s recommended to pre-groove a plunge cut past the distance of the chamfer height. For example, if there is a 0.010” x 0.010” chamfer, then first create a plunge cut past the 0.010” mark, say about 0.020” plunge cut (shown in step 1 of diagram below). Then perform a separate chamfer cut from one side. This will create a clean chip (shown by step 2 in the diagram below). Then, proceed to the third and final step, with a chamfer and continue to part off the tool.
This completes the third of the five-part series discussing best practices in parting off operations. In the next article, we will explore the advantages and disadvantages of using inclined cutting edges (also called handed inserts) in parting off applications.