Mark Gottschalk has been cutting metal with his 8 foot long blackToe CNC Machine for a while now. How does he do it? It's a balance of depth per pass, feedrate, the correct end mill and cooling; however, it is possible to cut metals without cooling.
Here's the thing: specifically with aluminum, this metal melts at approximately 1200 degrees Fahrenheit, and when you apply an end mill rotating at high rates (i.e. 13,000 RPM), the end mill will get really hot, and melt the aluminum during the milling process. Aluminum is a low melting metal. Compare this to steel, which melts at 2100 degrees Fahrenheit and some machinists will say that mild steel is easier to cut that aluminum simply because the end mill can mill at a slower feedrate and dig out the material.
There are many ways to control the temperature of the end mill. First, and the most widely used method, is to pour a liquid on the end mill while the end mill is cutting. This is a special liquid that is combined with a cutting fluid to provide the best cutting efficiency. Second, the end mill can be sprayed with just a cutting fluid which is typically done by hand. Mark initially used Isopropyl Alcohol. Third, cold air can be directed to end mill. Mark uses this method, which uses a vortex system that sends -50 degrees Fahrenheit air from one end of the nozzle, and very hot air out of the other end. The final method is simply finding that right balance of depth per pass, spindle RPM, feedrate and end mill flute and helix angle to achieve dry cutting. This final method is actually desired by the manufacturing industry (** add source **).
Achieving this balance is not easy, and by the last statement that the industry is heading in that direction gives you the impression that the industry hasn't figured it out yet either. Well actually, it is being practiced, but not with the perfect parameters and finding these perfect parameters is the holy grail of metal cutting.
Let's talk specifically about aluminum cutting and how you can get good results. The balance: running the CNC Machine with with a fast feedrate, and very small depth per pass will allow you to keep the end mill cool. The end mill will pass over new aluminum fast enough to cool the end mill, but if you dwell too long (slow feedrate and deep depth per pass) in the same place, the end mill will heat up and melt the aluminum through friction. CNC machine of all types can successfully cut aluminum with this in mind. Let me describe an analogy: an adult can dig a hole pretty fast and with a good amount of dirt in the shovel each time. A child can dig dirt too, but only scratching the surface each time. The child will eventually get to the same depth as the adult, but it will take a bit more time. The problem: the child is not using the shovel most effectively because the sharp tip of the shovel will get dull faster than the upper part of the shovel whereas the adult will evenly wear the entire shovel. This is the case with end mills as well. The deeper you can make the end mill pass, the more evenly the end mill will wear, extending the life of the end mill.
So, what parameters should be the starting point? This is an important question, because the outcome could be costly. Well, we have a good example from Mark Gottschalk. Remember, he is using the vortex air system to blow -50 degree temperature air on the end mill. The aluminum that he cuts is 6061 which is a structural grade of aluminum and the thickness is 3/16", but the thickness is not important since the cutting is done with many passes. The thicker the material, the longer it will take to get to the final pass. He uses a Porter Cable router set at 13,000 RPM. The feedrate (the speed that the end mill is moving through the cut) is set between 12 to 17 IPM (inches per minute). Any slower, the aluminum will start to melt. The depth per pass is the critical parameter and must be chosen carefully. Onsrud, a great manufacture of end mills, recommends that the depth per pass should be 1/2 of the diameter of the cutting end of the end mill. On a 1/8" end mill, that would be around .0625", but Mark likes .03" (about 1/4 of diameter) for his cuts since it leaves a good finish.
With end mills, plunging is generally the most harmful to the end mill, so pecking (having the end mill pull out at specific depths), or a slow plunge rate should be employed. Mark sets his plunge rate to 6 IPM for aluminum. If deep material is needed to be plunged, it may be better to mark the holes with the end mill with a slight plunge and finish it off on the drill press, or do the drilling portions of the work using a drill mill (an end mill that has a pointy tip). For plunging into the beginning of a profile, it's best to ramp into the material (having the end mill in a horizontal motion while the z-axis is moving down or plunging).
When cutting metal, vibration becomes a issue that must be addressed. Mark uses drywall screws to secure the aluminum to the spoilboard. Whatever clamping or securing method is used, make sure it will not move at all and that the clamping source (screws, clamp) is as close to the cutting as possible.
Last words from Mark: This has turned out to be one great machine. It's allowed me to do things that I would never have been able to do without breaking the bank. I love working with it. It puts home based manufacturing within reach of almost anyone and lets a little guy like me do business with the big guys. It is fun to be a "Maker"