Tool Deflection values

The cutting strategy dictates that the finishing cut should be a full depth pass of 20mm with a small step over value. This value shall be 0.5mm. A large value for a finishing pass however it simulates a worse case scenario.

6061 T6 Aluminum alloy finishing pass data

                                            6061 T6 Aluminum alloy roughing pass data

 
                                                              MDF finishing pass data

                                                               MDF roughing pass data

                                         All four data sets were calculated using this tool:
                                                  http://zero-divide.net/?page=fswizard

Cutting strategy

To continue calculating and obtaining data for the parameters of this machine I need to define a method for tool paths (The paths the cutter will pass along)

The machine will need to be capable of high speeds and high feeds to machine MDF. These characteristics can be used to machine aluminum providing the correct tooling is selected.

Traditional CNC cutting strategies generally cut in small depths across the whole area when cutting a pocket or profile and then finish the outside or inside profile with a small cut width. They also incorporate what is known as a step over value where each cut is a given value of thickness. An example of this is a slot will be cut 10mm wide with a 10mm cutter the cutter will then move over 5mm and cut the slot width to 15mm this would be a 50% cross over or 5mm.

HSM (High speed machining) strategies are generally very different. Cutting a pocket will start with a circular pocket that then cuts in an axial pattern towards full depth. When at full depth it will then machine outwards with a step over value and finish the profile in the same manner as traditional CNC would. There are other aspects of variations within HSM that improve things further such as Waveform from EDGECAM.

The advantage of high speed machining is that it can dramatically increase MRR values. Heat produced within the cutting process is mostly ejected with the chip. Tool life increases as the tool is generally not cutting on just the end section of the tool and is cutting on the full flute length of the cutting flutes.

However there are some drawbacks to HSM. It increases the volume of each chip resulting in more aggressive chip clearance methods such as high pressure coolant being directed at the cutter. It also increases cutting force dramatically. This is something that needs to be avoided.

The cutting strategy that will be employed within this machine is a more traditional approach. However using tooling designed for HSM. This will allow shallow cuts with high spindle speed and high feed speed whilst steel keeping a suitable MRR value. It will also mean that less force is applied to the cutter and machine. Resulting in less deflection affecting accuracy and less magnitude of resonance.

Theoretical surface finish

Surface finish is dictated by the following equation

http://www.harrisonep.com/electropolishing-ra.html

Ra value is a measurement of surface finish, its the average of measurements taken from the peaks and valleys of a surface due to the cutting operation relative to a mean line.

http://www.rubert.co.uk/Ra.htm

This graph above shows a graphical representation of this data.

6061 T6 Surface finish calculation

Using the 6061 T6 aluminum speeds and feeds dictated in a previous post and the equation below its possible to estimate the surface finish of the cutting operation.








http://www.kanabco.com/vms/eng_surface/eng_surface_04.html

Tool data from
http://www.niagaracutter.com/solidcarbide/metric/nc_metric_catalog.pdf
3 flute 10mm solid carbide end mill

r = tool tip radius (Inches) 0.02 inches (2.D.P)
f = feed rate per tooth (Inches) = 0.003 inches (3.D.P)

Ra=((0.02-sqrt(0.02^2-(0.003/2)^2))*1000000)/2
     =((0.02-sqrt(0.0004-(0.00000225)))*1000000)/2
     =((0.02-sqrt(0.00039775))*1000000)/2
     =((0.02-0.01994367067517913062473309999688)*1000000)/2
     =(0.00005632932482086937526690000312*1000000)/2
     =(56.32932482086937526690000312)/2
     =28μin    
1 Microinch = 0.000001 inches therefore 28μin = 0.000028 inches
                                                                             = 0.0007112 mm
                                                                        Ra = 0.7112 µm

MDF theoretical surface finish calculation




r = tool tip radius (Inches) 0.02 inches (2.D.P)
f = feed rate per tooth (Inches) = 0.0079 inches (4.D.P)

Ra=((0.02-sqrt(0.02^2-(0.0079/2)^2))*1000000)/2
    = 196μin (3.S.F)
    = 4.9784µm

     

Axis movement required

Z axis height from the bottom of the workpiece should be material thickness + tool length + 50mm clearance for holding mechanisms.

The machine should be able to machine the full depth of a material with a cutter. As the maximum material thickness is 100mm the Z axis travel should be 250mm minimum.

The maximum material size in X direction is 1220mm. Allowing 40mm either side for clamping this should brings the required travel in X to 1300mm

Y direction will be 1220mm + 80mm for clamping + allowance for tool changer.


According to this image below an ISO20 tool holder is 30mm wide. Allowing a generous 100mm for the tool change brings the total Y axis movement to 1400mm.

http://www.spindel-shop.de/Shop/images/product_images/info_images/iso20-er16.jpg

Maximum electrical power and general requirements

The machine needs to be run from the electrical system within the workshop. This is a 240v 30A supply. However there is also a 2.25 KW compressor on the same circuit.

total power = 240*30= 7200w

This leaves 4950W available for the machine in its entirety.

The machine should be an automatic tool changing mechanism. The simplest way of performing this would be to add a stationary rack at the end of the table. The machine would then center over the top of the tool in a suitable holder. The Z axis come down to the tool holder and draw the tool holder and tool into a spindle. This will thus require pneumatics or hydraulics of some description.

Maximum tool size required in diameter: 13mm shank. This is partially dictated by the high spindle speeds as a result of MDF being the primary material being worked. But also as a function of the machine. Parts will be nested into a sheet and multiple parts cut at the same time. Therefore having a large cutter will be wasteful of material.

The machine needs to provide storage of some description of items like the compressor due to its foot print within the workshop.

The machine needs to be of a horizontal orientation despite the fact the this requires a larger foot print. However it does provide a lower center of gravity and thus is safer. It also allows materials to be easily loaded and positioned for clamping.

As this machine will be horizontal table height should be comfortable to lean across and position fixtures and other items. Work benches are around the correct height for this.

Many work benches are 900mm from top surface to floor. This will make for a suitable height.

http://www.bigdug.co.uk/shelving-c2/garage-shelving-c1248/rac-boltless-garage-workbench-pp13240?utm_source=google&utm_medium=cpc&utm_term=rac-professional-garage-workbench-2-levels-900h-x-1205w-x-605d-mm-30-load-kg-300-height-900-width-1205-depth-605-506-0134-387335&utm_campaign=product+listing+ads

The machine should have an accuracy of +/- 0.5mm in MDF and +/- 0.1mm in 6061 T6 Aluminum.

Smaller tolerance and repeatability would be beneficial. These are maximum theoretical values.

Work area criteria

The maximum work area in X and Y will be a compromise between my maximum machine size and available materials.

The maximum machine size will be dictated by a the size of a typical small single car garage with some room to move around the machine. A typical small garage could be considered to be 2400 x 4900 mm.

http://en.wikipedia.org/wiki/Garage_%28residential%29

The machine is being situated within a four car garage. With one quarter being dedicated to the a workshop area situating this machine as well as a work bench and other tools/equipment.

Thus the machine should have a maximum foot print of 2000x2000mm. Floor to ceiling height in this area is 2600mm. Thus the machine should be no more than 2000mm tall to not block the lights within the workshop.

With MDF being the primary material being machined the available sizes dictate the working area to some extent. MDF is readily available in sizes of 1220x1220mm in thicknesses up to 25mm.

However for this machine it needs to be able to machine tooling foam this is readily available in sheets up to 100mm in thickness with width and length small then that of MDF.

(Cutting power and force calculations are not going to be considered for this material as it requires the same cutter speed as MDF and is much easier to machine due to its low density)

Resonence from cutting operation and concequences

Some resonance from the cutting operation will occur within the machine.

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.