What is chatter when milling?

Chatter is a resonant vibration in the machine or workpiece. Chatter in machining is very bad for your tool life, interferes with the accuracy of the machining operation, and can shorten the life of your machine too. Chatter is not always a huge problem, sometimes you can still machine perfectly, maintaining the tools life and your machine.

Here's 8 ways to reduce chatter when milling.

1. Reduce The Number Of Flutes Sometimes, chatter can come from having too low rigidity and accuracy when machining, if you lower the number of flutes, you will increase your rigidity and therefore reduce vibration. 

2. Decrease The Chip-Load Per Tooth You can do this by reducing the feed or increasing the speed or RPM. Even though this will reduce chatter, slowing down the cutting process is not always the best course of action, and reducing the chip-load can be detrimental to the cutter. 

3. Check Your Work Holding Check that sufficient pressure is being applied to the workpiece by the chuck, vice, vacuum table or other work holding device. Try to apply clamping pressure to the part as evenly as possible using the right size work holder for the job. Avoid clamping just one end of a long thin piece of stock material (think about what happens when you twang a ruler on the edge of a table). If this is a likely problem consider using a larger work holding device or additional clamps on a mill. On a lathe consider using a tailstock or a steady rest. 

4. Find The Balance Vary your feed rates and spindle speeds by small amounts to find stable operating points. 

5. Rigidity Check Ensure that every part of your CNC machining set-up is as rigid as possible. If your Z axis, which holds the spindle, is just a little bit loose where it attaches to the gantry or bridge, that will translate to a much larger movement down at the end where your tool hits the material. If that's significant, the cut will waver, even if the tool is perfectly stiff. And if it's big enough, it will snap the tool, especially if it's small in diameter. Similarly, flex between the gantry and the rails it runs on will be magnified at the tool tip due to leverage effects. Your frame cannot be too rigid; any little bit of slop anywhere in the structure will be evident where the cutter meets the material. 

6. Go Big Or Go Home If possible, try using a slightly larger end mill with a larger core diameter. This will help improve the tools rigidity & stability. 

7. Longer Isn't Always Better Try to use a stub drill or a short length end mill as much as possible. The shorter it is in length, the higher rigidity it has. It also reduces the chances of the tool breaking. 

8. Uncoated = Sharper Use sharper inserts to reduce the cutting force of CNC milling machines. Clamp inserts are divided into coated and uncoated inserts. Uncoated inserts are usually sharper than coated inserts because if the inserts are to be coated, they must be passivated (ER-treated) at the edge. 

To summarise, chatter is bad. It can and most likely will damage your tooling & your machine. If you see any sign, or hear chatter or vibrations, attempt to reduce it immediately. There are many ways to reduce it, generally aim to ensure everything is tightened and locked in, and you make the tool & machining process as rigid and stable as possible.