Chatter in the form of vibration and noise is a frequent challenge when end milling. It can cause scalloping and uneven finishes.

To reduce chatter, try the following:

1. Ensure that the starting places for speeds and feeds are correct for the workpiece material and the cut.
2. Increase the feed, or chipload per tooth/tool.
3. Make the workpiece as secure and rigid as possible.
4. Reduce excess overhang between the workpiece and spindle.
5. Select an end mill with more flutes.
6. Check the tool run-out.
7. Review the tool geometry to ensure the cutting face, relief, fluting and helix angle are appropriate for the workpiece material.

If conventional milling, try climb milling.

Technical Tip #12 - Reducing Chatter in End Milling

When chatter occurs, it can be self-sustaining until the problem is corrected. Chatter causes poor finish on the part, and will damage and significantly reduce the life of end mills. Carbide end mills are particularly susceptible to damage.

Typical methods to reduce chatter include reducing cutting forces by:

1. Reducing the number of flutes.
2. Decreasing the chipload per tooth by reducing the feed or increasing the speed or RPM.
3. Reducing the axial or radial depth of cut.

Though these steps will reduce chatter, slowing down the cutting process is not always the best course of action, and reducing the chipload can be detrimental to the cutter.

Better first steps are to improve rigidity and stability:

1. Use a larger end mill with a larger core diameter.
2. Use end mills with reduced clearance or a small circular margin.
3. Use the shortest overhang from spindle nose to tip of tool.
4. Use stub length end mills where possible.
5. Use balanced tool holders.
6. Rework fixture to hold the workpiece more securely.
7. Reprogram the cutter path to shift cutting forces into stiffer portions of the workpiece.
8. Look for ways to improve spindle speeds then adjust feed accordingly.

Chatter is common when machining corners. As the end mill enters the corner, the percentage of engagement increases the number of teeth in the cut. This drastically increases the cutting forces, causing chatter.

To reduce chatter when machining corners, consider using circular interpolation to produce a bigger corner radius than indicated by the part print. Then remove the remaining stock with a smaller end mill using circular interpolation.

Technical Tip #32 - Chatter and Vibration in End Milling

Because end mills are supported only at the shank end, they are subject to deflection, which can reduce the accuracy of the milled part.

Several factors affect the amount of deflection that will occur:

• Overall Length and Length of Cut: As the length of the mill increases, difficulty in maintaining dimensional accuracy also increases. Rigidity decreases in proportion to length of cut to the 3^rd power. Thus, a 4" length of cut is 1/8 as rigid as a 2"length of cut. A regular length end mill cutting 7075 aluminum can deflect <002", while an extra-long end mill can deflect >.006".
• End Mill Diameter: Rigidity increases in proportion to diameter to the 4^th power. A 1"-diameter end mill is 16 times more rigid than a 1/2" end mill. A 1"-diameter end mill over a 5/8" length of cut in 1040 steel will cut to size, while a 3/8"-diameter end mill may deflect to >.003".
• End Mill Material Composition: Solid carbide is about three times more rigid and resistant to deflection than high-speed steel end mills, but not as tough.
• Radial Depth of Cut and Axial Length of Cut: Heavy radial cuts as well as long axial lengths of cuts will deflect the end mill much more. A light-finishing pass is generally required to produce accurate parallel cuts.

Tips:

• Always use the shortest tool possible.
• Shorter tools can reduce chatter.
• Increase coolant.
• Try left-hand spiral end mills.
• Try using higher helix end mills.
• Increase overall system rigidity.
• Reduce overhang.
• Conventional milling can resist deflection better than climb milling.

Dull tools deflect more than sharp tools.

Technical Tip #115 - Accuracy and Deflection in End Milling