Parting Off – Part 2: Best Practices
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 second of five posts relating to basic principles, best practices and troubleshooting of parting off operations.
Best practices and careful planning can reduce or eliminate most of the challenges mentioned in the previous article. Many factors, such as the insert width, geometry (right hand, left hand or neutral), chip breaker geometry, tool overhang, feed/speeds, center height and alignment, influence the success of parting-off operation.
Let’s look at each of these factors to analyze how they influence the outcome of the operation. Let’s also identify the best ways to optimize the input of each factor and maximize the operation efficiency.
Tool stability is obviously critical in all machining operations. Poor stability leads to vibrations which in turn reduces tool life. Inserts are subject to varying and unplanned stresses leading to catastrophic failure of the inserts due to fracturing or unpredictable/uneven wear. Vibrations also lead to variations in the dimensional accuracy and surface finish. Finished shoulders show witness marks and lead to increased scrap and down time. Hence in any given situation, the most stable tool possible should always be selected. This means you should always choose the tools with lowest overhang possible and the widest width of the blade or shank possible. Also, if possible, choose the tools that have a top screw clamping for inserts. Machine stability also plays an important role, but many times this factor cannot be controlled by choice.
The insert width selected should be as narrow as possible yet as wide as necessary for a robust operation.
When parting off exotic materials with higher cost/foot or in a case of high-volume production, even a 0.039” (1mm) reduction in insert width can lead to substantial cost savings in the long run. Hence a narrow width is always preferred. Usually, a narrow insert width leads to lower overall cutting forces, reducing overall energy consumption as well as tooling costs/insert.
Here is an example: high production parts run with a million parts/year are not uncommon. By going from a standard 3mm insert width to a 2mm insert width, the savings on material itself can be 0.6 miles in length! If this is an exotic material such as bearings steel or stainless steel, cost savings on workpiece materials can be substantial depending on the diameter being parted off.
However, wider inserts provide strength to the carbide, better support from the steel tool and prevent tool/insert failure. Therefore, this needs to be balanced against the advantages of selecting narrow insert width that was mentioned earlier. Some examples where the choice of wider inserts is beneficial are:
- older machines,
- unstable set ups
- workpiece materials prone to batch variations in material composition (eg. AISI 1010, AISI 1018 etc. low carbon steels)
Tool overhang and cutting depth
As a rule of thumb, for single sided inserts, the maximum cutting depth [Tmax] of the tool or the maximum clamping length of the insert holder should not exceed 10 × cutting edge width[s].
To improve face flatness and to reduce the vibration tendency and to improve tool life, the tool should be mounted in the machine with the shortest possible overhang. A lot of times, parting off inserts are ‘single edged’ like the SX inserts shown in the diagram below. These single sided inserts are the first choice for deep parting off operations for the workpieces of the order of 7-8” diameters.
If the maximum cutting depth does not exceed the second cutting edge, then the ‘double sided’ inserts, such as the GX inserts, are the first choice. Typically, these inserts have a wider and more stable base than the corresponding single sided inserts. The double-sided inserts have better clamping force in the tool holders and with two cutting edges per insert, they also tend to be more cost effective in the long run. However, as mentioned earlier, it is important to check the maximum cutting depth as a function of total insert length. As shown in the images below, if the part off depth “Tmax” is such that the back end of the dog-bone style insert hits the workpiece, then this can cause violent insert failure and damage to the tools. Thus, the double-sided inserts are always limited to a depth less than the total length of the insert. Although there are some other options, such as single-sided dog-bone inserts, twist style double-sided inserts or simply longer inserts with higher Tmax number, single sided inserts are generally the first choice for deeper depth beyond 1.34” (34mm).
Workpiece clamping can be yet another source of unwanted vibrations into the system. Clamping the workpiece at the shortest length possible is always recommended. As shown in the picture below, if the insert engages the workpiece farther away from the chuck, the workpiece will tend to deflect quite severely.
The deflection induced goes up as a cube of the distance from the chuck. So, if insert moves 2 x diameter away from the chuck, the deflection would actually go up by a cube, meaning 8 times the current deflection. In order to keep the insert as close to the chuck as possible, attention should be paid to the selection of holder as well. Just the change of selection from the right hand holder to the left hand holder can make a significant difference in the distance of the insert from the chuck, as shown in the diagram below.
Tool alignment can be yet another source of vibrations. It also affects the flatness of the finished face. The tool must be aligned 90° to the axis of rotation. Improper alignment between the tool and the workpiece leads to increased cutting forces and the tendency of the tool to ‘walk’. Since in parting off operation, there is not much space to walk, this leads to increased tool vibrations, leading to a poor finished surface on the finished shoulder. The flatness of the finished shoulder is also affected.
If the tool is positioned over or under the center, the effective cutting angles change during machining. Below is the recommended range for center height variation for consistent tool life and lower pip/burr formation.
As seen in the images below, if the cutting edge is above center, the edge will leave a pip on the last step near center. As the cutting edge is above center, the pip tends to hit the insert-clearance in an adverse manner, leading to insert chipping and catastrophic failure. Hence keeping the center height variation low allows a longer and more predictable tool life.
As seen in the images below, if the cutting edge is below center, the edge will leave a pip on the last step near center.
Retracting the tool
After parting off, do not retract the tool immediately. First, change to the axial direction and then retract. Retracting the tool directly back tends to create ‘witness marks’ on the finished surface. If there has been insert deflection due to machine stability or a misapplied combination of feeds and insert geometries, this effect of retraction-witness-marks is more pronounced. This further leads to scrapped parts or a larger pip left on the remaining bar stock that needs to be addressed in the next pass.
Parting off into pre-machined hole
When parting off a component with a pre-machined hole, the cylindrical part of the hole must be deeper than the position of the complete grooving tool. As shown in the diagram below, if the parting off tool crosses the taper of the pre-drilled hole, it will leave a very nasty and unplanned burr at the base of the finished component. The parting off insert should be planned to completely go past the contour left by the point angle of a solid drill or insert contour of an indexable drill.
This completes the second of the five-part series discussing best practices in parting off operations. In the next article, we will explore effective application of feeds and speeds. We will also explore effective sequence of operations for chamfering before parting off as well as the choice of handed inserts and their effect on success of parting off operations.