This will be a result of the cutting teeth impacting the work.
Using the example cutting conditions its possible to attain the frequency of the resonance created by the cutting operation.
This image shows a climb milling pass on the edge of a workpiece. The circle with a gradient shows the magnitude of the cutting force with relation to its cutting path relative to one tooth.
As the load is at peak (The start of chip formation) is where the force is the highest dictated by dark red. As the cutter cuts the chip the force decreases with relation to the cross sectional area of the chip decreasing. This is depicted by the fading gradient where finally all force is released and thus the force circle turns white.
This creates a resonance in the machine that transmits through the machine. If there is much backlash within the machine this will transmit back to the cutter and thus into the workpiece causing an undesirable surface finish.
Backlash is considered the clearance between components that is taken up upon movement. Within climb milling the force generated by the cutter will be in the same direction as the workpiece direction.
Thus, this will cause the clearance between components to open and close as the cutter takes a cut on one of its teeth opening the clearance then the clearance being closed as a result of the feed mechanism in the time between cuts of each tooth. This will happen in a frequency relative to the amount of times per second the cutter impacts the workpiece. Using the speeds and feeds parameters set out in the "More criteria" post its possible to calculate this frequency.
induced frequency = (Spindle speed in rpm/60)*cutter teeth
6061 T3: 19385/60 * 3 = 969.25Hz
MDF: 24000/60 * 2 = 800Hz
Backlash when conventional milling is less of a problem because of how the force is applied. With climb milling the peak force is on the initial section of the cut and tapers off. With conventional milling it tapers up to peak force. The force is on a radial motion at a tangent to the cutter with force being applied in an opposing direction to the cutter direction when the cutter face reaches a perpendicular tangent to the feed direction. Thus the clearance is kept closed reducing resonance.
Although climb milling will increase the issue of resonance being transmitted back to the workpiece finish. It does provide a multitude of benefits these include:
Better surface finish due to less material being in shear at end of cut, resulting in less deflection nearing material surface.
Increased tool wear due to the tools cutting rather than rubbing and then digging into the workpiece, tools rubbing causes dulling of tool edges and increase in heat throughout the workpiece and cutter due to friction.
Less re cutting of chips as the cutter leaves chips behind the cutter relative to the feed direction rather than conventional milling where it will place the chips in front of the cutter causing it to drag the chips through the cut wedges between the cutter and the workpiece causing gouges to the workpiece.
Force is directed down into the workpiece as opposed to up resulting in less clamping force required of thin components when machining. This is evident when climb milling. The chips are tall and thin as they are ejected from the cutter. This means the force of the chips is in an upward direction and therefore any force in in the z axis must be directed downwards. This also results in the heat of the cut being spread over a larger area of the cutter.
The only real downside of climb milling as opposed to conventional milling (other than backlash issues as described above) is that in very hard materials it can cause a great deal of shock to the cutter causing micro fractures ultimately resulting in tool failure due to the chip load being highest at the start of the cut resulting in a higher impact. However this machine is only designed to cut materials as hard as aluminum and as a result this is not really a consideration.
As a result of this the machine must be designed in such a way to eliminate backlash as much as possible in as many areas as possible.
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