Machining methods to improve life - The importance of ‘onion skinning’

A massive factor of tool life and the quality of the finish on the end-product is the amount of vibration occurring in the machining process. You can see evidence of this vibration on the edge of machined boards. Vertical machine lines are an indication of chatter. It’s almost impossible to eliminate all chatter because machine systems are not perfectly rigid so you will always receive some amount of deflection in the tool, machine and workpiece.

Large amounts of chatter is a massive issue as it damages the workpiece and accelerates tool and machine wear; it is also very noisy. Chatter is the interaction between the rotation of the cutter and the properties of the workpiece material. The cutter oscillates periodically (vibrates side to side with each rotation) as each cutting edge impacts the workpiece. This is what produces the vertical lines on the machined edge. If you could look at the edge of the board from above you will see an almost perfect wavy line. If you can reduce the vibration you reduce the height of the curve and produce a smooth edge. Determining the correct feed rate and rotational speed to reduce chatter is a very complex process. The cutter must be both at the correct feed rate and the correct spindle rate. However, there is actually a common misconception that there is only one sweet spot to avoid chatter. There are many possible feed and spindle rate combinations for the same tool and the same material when it comes to chatter. It is completely possible to cut MDF at 100m/min and receive no chatter with the correct RPM, however, factors such as heat, machine bearings and OHS may prevent you from doing this. To check your machine is using the correct feed speeds read this article. (link calculating feed speeds article)

There are also plenty of ways to reduce chatter regardless of feed rates and RPM. You can also:

  • Use more rigid tooling and tool holders
  • Use tools with larger diameters and better mechanical properties to increase tool rigidity. Compression cutter geometries are designed to eliminate chatter and stabilise the board. Up-cut to down-cut ratios, helix and grinding angles all influence the deflection of the tool. Ensure chucks and collets are clean and tightened using a torque spanner  and use ball-bearing nuts to prevent collet distortion. There are also tool holder options which eliminate the use of collets such as thermo and hydro chucks (these are often expensive and only suit one shank size). Having fewer components and a more even distribution of pressure increases the concentricity at high RPMs.
  • Ensure good suction on the work-piece.
  • Look after your machine bed and skim the sacrificial board regularly. Ensure there are no dust particles between the bed and the workpiece. Cover the entire bed to ensure suction is not escaping.
  • Regularly service your machines

If your CNC machine is the lifeblood of your operation, make sure you’re maintaining it. Budget for the replacement of key parts and regularly monitor tolerances.

Here is an interesting example of machine learning, AI and innovation tackling chatter which we could see in the near future…

 As previously mentioned, chatter is caused by the oscillation of the cutting tool and the reaction with the workpiece material. Finding the feed rate and RPM sweet spot can reduce the amplitude of this oscillation. If you are milling a straight line through a high-quality board with a brand new tool, with a little trial and error, you can get the feed rate and RPM just about perfect. This becomes a very mean feat when you have different board consistency, complex shapes and an increasingly worn tool.

Introduced in 2012 by Okuma, there are now machines (more so in the metalworking industry) which use a feedback loop to increase and decrease the spindle speed based on systems which measure vibration in real-time. A microphone, multiple accelerometers and force dynamometers placed on the machine bed and spindle measure the vibration and optimise the speed.

If the machine is running the same program 100 times, it can use the data from previous iterations to optimise results and run the 100th program up to 50% faster than the first with no compromise in finish quality, tool and machine life. If you could graph the RPM of this program over time you would see the same oscillating pattern as the cutter would produce on the board if RPM was held constant. By oscillating the RPM at the same rate, you counteract the deflection of the tool, machine and workpiece producing a flawless result.

There are plenty of ways to always increase the efficiency of your operation!There are countless other ways in which machine learning, AI and innovation can improve your operation in the future.