Quite often, the machining forces are not the primary concern, but rather how to choose machining parameters which yield the shortest machining time while staying within the limitations of machine and setup. For this purpose, millalyzer provides four functions that can be reached from the Edit menu, Recommend AE (radial engagement), Recommend AP (axial engagement), Recommend feed and Optimal pocketing.
Spindle speed (N, VC) and feed (VF, FZ or HEX) will not be modified by the AP and AE recommendation functions, and this has a simple reason. With the models that are available today, an increase in speed will always improve productivity; therefore, any optimization that is unaware of further constraints would invariably select the highest possible spindle speed. In reality, speed is limited by tool heating (for steel, exotic alloys or dry machining); this limit depends strongly on coating properties and coolant flow. Therefore, it is recommended to follow the manufacturer’s published cutting speed recommendation to select spindle speed.
Due to the development of plastic stress with strain (strain hardening) and strain rate (rate hardening), an optimization that trades engagement against feed trends toward the maximum available feed, because cutting stresses in ductile metals decrease with increasing chip thickness. The magnitude of this effect differs between materials.
Before a parameter can be adjusted, there needs to be a limit up to which any increased loads can be accepted. Use the menu item Machine limits in the Edit menu to prescribe permitted mechanical power, a torque limit and maximum allowed peak and mean forces. The meaning of the parameters is explained in detail in the chapter on machine limits.
Unless more information is available, the recommended approach is to re-compute a know-good milling operation, and then use the loads computed for that case as limits:
- Setup a simple side-milling operation with a well-known tool using one of the workpiece materials available in millalyzer.
- Run at least three identical cuts along the same workpiece side to eliminate the effect of mean transverse tool deflection.
- Repeat the above at increasing feed (e.g. using manual feed override) until surface quality becomes unacceptable or noise indicate chatter.
- Perform a simulation with millalyzer using the highest acceptable feed and use the computed forces as machine limits.
Adaptive/dynamic milling: Recommend AE (radial engagement)
Many times, radial engagement (or optimal load, or stepover) is the only parameter that can be freely varied during programming, because the geometry of cut or tool do not permit any increase in axial depth of cut. The function Recommend AE will find the highest possible value of radial engagement that is possible without exceeding the configured limits.
Changing AE also has an effect on the uncut chip thickness HEX. Remember that you can set feed by prescribing a certain value of this chip-load (HEX). So, if the HEX constraint on feed is active, that is, the label Uncut chip [HEX] is displayed in bold, then the Recommend AE function will automatically adjust the linear feed speed when modifying radial engagement. (Sometimes, when the force limit is low with respect to material strength and axial engagement, then this can result in small radial engagement and very large feed values.)
Slotting: Recommend AP (axial engagement)
When cutting slots, only the depth can be varied because the axial engagement is locked to be equal to the tool diameter. The function Recommend AP will simply increase or reduce the value of AP until the first of the configured machine limits is hit. Of course, this function can also be used with any other values of radial engagement.
In a similar fashion, Recommend feed retains the set engagement dimensions and pushes the feed up to match the defined machine limits. This can result in very large feed values that exceed the available chip space of the flutes, as well as very small values that could cause rubbing.
Sometimes, more than one parameter can be chosen freely, for instance when a pocket is to be roughed out. In that case, only the total depth of the feature is fixed. It is therefore possible to run either more passes with smaller depth of cut (but possibly higher feed) or fewer passes with slower feed.
The pocket optimizer is locked to the user-defined AE value (which for conventional pocketing, is usually around 90% of tool diameter, but can be set to any value) and then increases the number of depth passes starting with the full feature depth. This means that the AP values used evenly divide the feature depth. For each axial engagement, the limit feed is computed and the resulting conditions are listed in the log window. In the end, the fastest set of parameters that is within the chip-load limits is selected and copied to the main window.
At present, the recommendations do not take the machine dynamics into account. That means that the recommended cutting parameters can cause chatter, unless the user-defined force limits are set sufficiently low.