How to reduce vibration
Vibrations can arise due to limitations in the cutting tool, the holding tool, the machine, the workpiece or the fixture.
The cutting tool
- For face milling, the direction of the cutting forces must be considered:
– With a ψr 0° cutter, the dominant forces focus in the radial direction. This creates deflection of the cutter at long overhangs; however, the small axial force is advantageous when milling thin walled/vibration
– Cutters with ψr 45° generate evenly distributed axial and radial forces.
– Round insert cutters direct most of the forces up the spindle, particularly at small depths of cut. Also, the CoroMill 210 with ψr 80° transmits the forces primarily into the spindle, which reduces vibrations generated due
to long tool overhangs.
- Choose the smallest diameter possible for the operation
- Dc should be 20-50% larger than ae
- Choose a coarse pitch and/or differential pitch cutter.
- A cutter with a low weight is advantageous e.g, CoroMill Century with aluminum body.
At long tool overhangs, use a large lead angle=high axial cutting force. With thin walled, unstable workpieces use a small lead angle = small axial cutting force.
The holding tool
The Coromant Capto® modular holding tool system enables tools to be assembled to the required length, while maintaining high stability and smallest run-out.
- Keep the tooling assembly as rigid and short as possible.
- Choose the largest possible adapter diameter/size.
- Use Coromant Capto adapters with oversize cutters to avoid reduction adapters, see picture.
- For small milling cutters, use a tapered adapter if possible.
- In operations where the final pass is located deep in the component, change to extended tools at pre-determined positions, see picture. Adapt cutting data for each tool length.
- For spindle speeds over 20 000 rpm, use balanced cutting and holding tools.
Oversized cutters allow the largest possible coupling size.
Always use the shortest possible tool length. Extend length successively.
Silent Tools dampened milling cutters
For overhangs greater than 4 times the tool diameter, vibration tendencies can become more apparent, and Silent Tools damped cutters can dramatically improve the productivity.
The cutting edge
To minimize the cutting force:
- Choose a light cutting geometry, -L, with a sharp edge, and a grade with a thin coating.
- Reduce cutting forces by using inserts with small corner radii and small parallel lands.
Sometimes adding more damping to a system can decrease the vibration tendencies:
- Use a more negative cutting edge geometry and a slightly worn cutting edge.
Cutting data and tool path programming
- Always position the cutter off-center in relation to the milled surface.
- With ψr 0° long edge cutters or end mills use low radial immersion – max ae = 25% Dc and high axial cut – max ap = 100% De.
- In face milling, use a small depth of cut, ap, and high feed, fz, with round inserts or high feed cutters with large lead angles.
- Avoid vibrations in corners by programming a large path radius
- If the chip thickness becomes too low, the cutting edge will rub rather than cut, causing vibration. In such instances, the feed per tooth should be increased.
The machine tool
The machine condition can have a large influence on vibration tendencies. Excessive wear on the spindle bearing or feed mechanism will result in poor machining properties.
- Choose machining strategies and cutting force directions to take full advantage of the machine stability.
Each machine spindle has natural areas which are prone to vibration. The areas of stable cutting are described as stability lobes, and increase as the rpm increases.
- Even small increases as low as 50 rpm can move a cutting process from unstable, with vibration, to stable.
For weak fixturs, feed direction into machine table
Workpiece and its fixture
Milling components with thin wall/base and/or when the fixture is weak.
- The fixture should be close to the machine table.
- Optimize the tool path and feed direction towards the machine's/fixture’s strongest node to obtain the most stable cutting conditions.
- Avoid machining in directions where the workpiece is poorly supported.
- Up-milling can reduce vibration tendencies when fixture and/or workpiece are weak in a specific direction.
Note that the first step should be made at half the depth of the second, third, etc.