In CNC milling, vibrations may arise due to the limitations of the cutting tool, toolholder, machine, workpiece or fixture. To reduce vibration, a number of strategies need to be considered.
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. In long overhang conditions this can cause the milling cutter to deflect; however, low axial forces are advantageous when milling thin-walled/vibration sensitive parts.
b) The 45° milling cutter is capable of producing evenly distributed axial and radial forces.
c) Circular insert milling cutters direct most of the forces upwards along the spindle, especially when the depth of cut is small. In addition, the 10° milling cutter transfers the main cutting forces into the spindle, thus reducing the vibration caused by long tool overhangs.
2) Choose the smallest possible diameter for the process.
3) The DC should be 20-50% larger than the ae.
4) Choose milling cutters with sparse and/or unequal tooth pitch.
5) Lighter weight milling cutters are advantageous, e.g. those with aluminium bodies.
For unstable thin-walled workpieces, use a large main deflection angle = small axial cutting forces; in long tool overhang conditions, use a small main deflection angle = high axial cutting forces.
The use of the CN CNC TOOLS modular toolholder system enables the assembly of tools of the required length, while maintaining high stability and minimum runout.
1) Keep the tool assembly as rigid and as short as possible.
2) Select the largest possible splice diameter/size.
3) Use CN CNC TOOLS that are suitable for oversize milling cutters and avoid using reduced diameter joints.
4) For small size milling cutters, use a tapered receiver if possible.
5) Switch to longer tools at predetermined positions in processes where the last tool travel is deep in the part. Adjust the cutting parameters according to each tool length.
6) If the spindle speed exceeds 20,000 rpm, use dynamically balanced cutting tools and toolholders.
03 Vibration reducing milling cutters
If the overhang is greater than 4 times the tool diameter, the tendency for milling vibration may become more pronounced and Silent Tools™ vibration damping milling cutters can significantly increase productivity.
04 Cutting edges
To reduce cutting forces.
1) Select lightly loaded groove type-L and thinly coated materials with sharp cutting edges.
2) Use inserts with small tip radii and small parallel edge bands
Sometimes the tendency to vibrate can be reduced by adding more damping to the system. Use a cutting edge groove type with a larger negative leading angle and a slightly worn cutting edge.
05 Cutting parameters and toolpath programming
1) Always position the milling cutter off-centre with respect to the milling surface.
2) For KAPR 90° long flute milling cutters or end mills, use a small radial depth of cut (max. ae = 25% x DC) and a large axial depth of cut (max. ap = 100% x De).
3) For face milling, use a small depth of cut ap and a high feed fz and a round insert or a high feed milling cutter with a small main offset.
4) Avoid vibration in corners by programming large circular walks, see internal corner milling.
5) If the chip thickness becomes too thin, the cutting edges will cut rather than cut, resulting in vibration. In this case, the feed per tooth should be increased.
The condition of the machine tool may have a large influence on the tendency to vibrate in milling. Excessive wear on the spindle bearings or feed mechanism will result in poor machining performance. Choose your machining strategy and cutting force direction carefully to take full advantage of machine stability.
Every machine spindle has areas of instability that are prone to vibration. The stable cutting zone is described by the stability diagram and increases with increasing speed. Even increases in speed as low as 50 rpm can change the cutting process from an unstable state of vibration to a stable state.
07 Workpieces and their fixtures
When milling thin-walled/base parts and/or when the fixture is less rigid, consider the following points.
1) The fixture should be close to the machine table.
2) Optimise the tool path and feed direction towards the position of highest machine/fixture strength in order to obtain the most stable cutting conditions.
3) Avoid machining along directions where the workpiece is not adequately supported.
4) Reverse milling can reduce the tendency to vibrate when the fixture and/or workpiece is less rigid in a particular direction.
Contact: Jacky Wang
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Tel: +86 14714816052
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Add: Floor 1, Shixi Industrial area, Canton, Guangdong, China. 510288