In CNC milling, vibrations can be generated by limitations of the cutting tool, toolholder, machine, workpiece, or fixture. To reduce vibrations, there are a few strategies to consider.
01 Cutting Tools
1) For face milling, the direction of the cutting forces must be considered:
a) When using a 90° milling cutter, the cutting forces are mainly concentrated in the radial direction. This can cause the milling cutter to wobble in long overhang conditions; however, low axial forces are beneficial when milling thin-walled/vibration-sensitive parts
b) A 45° milling cutter is able to generate evenly distributed axial and radial forces
c) Round blade milling cutters direct most of the force upward along the spindle, especially at smaller depths of cut. In addition, 10° cutters transfer the main cutting forces into the spindle, thus reducing vibrations caused by long tool overhangs
2) Select the smallest possible diameter for the operation
3) DC should be 20-50% larger than ae
4) Select cutters with coarse and/or unequal pitch
5) Lightweight cutters are advantageous, such as those with aluminum alloy bodies
For unstable thin-walled workpieces, use a large lead angle = low axial cutting forces; for long tool overhangs, use a small lead angle = high axial cutting forces.
02 Toolholders
The Coromant Capto® modular toolholder system enables the assembly of tools of the required length while maintaining high stability and minimal runout.
1) Keep the tool assembly as rigid and short as possible
2) Choose the largest possible adapter diameter/size
3) Use Coromant Capto® adapters for oversized cutters and avoid reducing adapters
4) For small cutters, use tapered adapters if possible
5) Switch to extended tools at predetermined locations in operations where the last pass is deep in the part. Adjust cutting parameters for each tool length
6) Use balanced cutting tools and toolholders if the spindle speed exceeds 20,000 rpm
Oversized cutters
Always use the shortest possible tool length and gradually increase the length
03 Vibration-damped cutters
If the overhang is greater than 4 times the tool diameter, the tendency for milling vibrations may become more pronounced. Silent Tools™ vibration-damped cutters can significantly increase productivity.
04 Cutting Edges
To reduce cutting forces:
1) Select lightly loaded geometries with sharp cutting edges -L and thinly coated grades
2) Use inserts with small nose radius and small parallel lands
Sometimes, vibration tendencies can be reduced by adding more damping to the system. Use cutting edge geometries with more negative rake angles and slightly worn cutting edges.
05 Cutting Parameters and Tool Path Programming
1) Always position the milling cutter off-center relative to the milling surface
2) For KAPR 90° long-edge milling cutters or end mills, use small radial depth of cut (max. ae = 25%×DC) and large axial depth of cut (max. ap = 100%×De)
3) When face milling, use small depth of cut ap and high feed fz with round inserts or high feed milling cutters with small lead angles
4) Avoid vibration in corners by programming large arc passes, see inside corner milling
5) If the chip thickness becomes too thin, the cutting edge will scrape instead of cut, causing vibration. In this case, the feed per tooth should be increased
06 Machine Tool
The condition of the machine tool can have a large impact on the milling vibration tendency. Excessive wear of the spindle bearings or feed mechanism will lead to poor machining performance. Carefully select the machining strategy and the direction of the cutting forces to fully utilize the machine tool stability.
Every machine tool spindle has an unstable region that is prone to vibration. The stable cutting region is described by the stability diagram and increases with increasing speed. Even a speed increase of as little as 50 rpm can change the cutting process from a vibrating unstable state to a stable state.
07 Workpiece and fixture
When milling thin-walled/base parts and/or when the fixture is less rigid, consider the following:
1) The fixture should be close to the machine table
2) Optimize tool path and feed direction towards the position of the strongest machine/fixture to obtain the most stable cutting conditions
3) Avoid machining in the direction where the workpiece is not fully supported
4) When the fixture and/or workpiece are less rigid in a certain direction, reverse milling can reduce vibration tendencies
When the fixture is less rigid, use the feed direction towards the machine table
Note that the first cut should be performed at 1/2 the depth of the second cut, the second cut should be performed at 1/2 the depth of the third cut, and so on.
