We need consistent surface finish and wish to minimize subsurface damage and residual stress in the workpiece.
Additionally, we want all of this for minimum cost! However, there are many factors which may limit our ability to make the best use of a cutting tool.
When we're ready to start a job we typically open our catalog or launch our browser and make our selection from a nearly endless array of options.
At this stage, there are several important issues that are typically considered to arrive at a purchasing decision, such as:
- tool material – for carbide tools, the grain structure after sintering is important, for instance
- tool geometry – basic parameters include side rake angle, clearance angle, cutting edge radius, rake face geometry (it can contain chip breaking features, for example), and number of teeth and spacing (for milling); actual cutting tools, whether solid or inserted designs, include many additional geometric parameters
- tool coating – many coating types, application processes, and numbers of (alternating) layers are available
- edge preparation – the cutting edge may be sharp or a chamfer (or other preparation) may be added to help increase coating persistence
- tool holding – this influences runout, for example
- coolant – the cutting fluid type and application method affect the tool life
- CNC part path – the strategy used by the computer numerically controlled (CNC) part program can affect the time to machine and the tool life; for example, up or down (conventional or climb) milling and the path geometry, such as spiral in/out, constant radial engagement, trochoidal, and zig-zag strategies for milling, must be selected by the process planner.
There’s another critical issue, you may be overlooking: the tool’s vibration behavior when it is clamped in the holder and mounted in the spindle.
Why should you care about the vibration behavior? It’s because this “dynamic stiffness” of the tool-holder-spindle combination is responsible for the milling process stability.
In other words, is the cut stable or do we get chatter?
If the dynamic stiffness is known, then it is also possible to know which pairs of spindle speed and depth of cut will give stable cutting and which pairs will not. To collect this important vibration behavior information, which is referred to as the frequency response function (FRF), an impact, or tap, test should be completed.
There are four steps in an impact test:
- use an instrumented hammer to tap the tool and measure the input force;
- use a low mass accelerometer, which is typically attached to the tool by wax, to measure the corresponding vibration output;
- convert each time domain signal into the frequency domain; and
- calculate the displacement output to force input ratio. This frequency domain ratio is the FRF.
This video shows an example of impact testing for an aluminum bar. There are several hammer impacts that each result in a vibration response. On the laptop screen, the hammer impacts are shown at the top (blue line) and the corresponding vibration is shown at the bottom (red line). In the end, we average the results from the multiple hammer impacts. The good news is that you don’t have to figure all this out for yourself. A commercial product is available.
The hardware and software comes from Manufacturing Laboratories, Inc. (MLI). The product is MetalMax. The MetalMax kit from MLI comes with training which lets you perform the tap tests yourself. You can also get a consultant from MLI to measure a set of tools for you.
For the small to medium manufacturer, this technology reduces prove outs. Because margins are often low on small batch sizes, it can be unacceptable to make a bad part. By improving your ability to make the first part correct, it increases profit.
The same is true for the large manufacturer; tap testing becomes part of best practices and the shop floor culture. Additional benefits are more accurate quoting and a new condition-based maintenance opportunity. By measuring the same artifact (such as a stub length shrink fit holder) in the spindle at regular intervals, you can monitor the spindle health. If the frequencies shift or the stiffness changes, you’ve either experienced a crash or the spindle is due for a rebuild.
If you’re still reading and you’d like to understand this procedure in more detail, take a look at Figure 1, which shows the impact testing process in a step-by-step manner. The hammer applies an input force, f, in the x direction which causes the tool to vibrate. The input force applied by the hammer tap is shown in the top left panel. The corresponding tool vibration in the same direction is displayed in the top right panel. These are the time domain signals. We see that the short duration impact causes the tool to vibrate through many cycles. With low damping there are more of these vibrations cycles that occur before the motion stops. With higher damping, there are fewer cycles. Also, with high stiffness and damping, the peak-to-valley size of the vibration is small, while it is large for low stiffness and damping.
The lower two panels show the force and displacement in the frequency domain. The horizontal axis is now frequency (in units of Hertz, or cycles per second), rather than time (in seconds). On the left we observe that the impact excites the structure over a wide frequency range. You can think of this like striking a bell with a mallet. The short force input is very effective at causing the bell to ring regardless of the bell’s size or geometry. The right lower panel shows the displacement. We see a peak at the natural frequency; this is the frequency at which the tool “wants” to vibrate. The size of this peak depends on the tool’s stiffness and damping. With high stiffness and damping, the peak is small. For low stiffness and damping, the peak is large.
Figure 1: Signals obtained during an FRF measurement. (Top left) input hammer force in the time domain. (Top right) output tool displacement in the time domain. (Bottom left) frequency domain force. (Bottom right) frequency domain displacement.
We relate the FRF to machining stability using a stability map, or stability lobe diagram. This separates stable combinations of spindle speed and axial depth from unstable combinations (chatter). Figure 2 shows an example. In order to construct this diagram, we need the FRF from a tap test and a force model, which is based on the workpiece material. These curves move up when we increase the tool’s stiffness and damping (i.e., we can make deeper cuts without chatter). They move to the right as we increase the natural frequency.
Figure 2: Example stability map (or stability lobe diagram). The spindle speed-depth of cut combinations above the U-shaped lobes give unstable machining performance (chatter). The combinations below the lobes are stable.
Written By: Dr. Tony Schmitz, Professor of Mechanical Engineering and Assistant Director, Energy Production & Infrastructure Center, UNC Charlotte, Charlotte, NC