If you machine aluminum it is only a question of time before aluminum builds up on the cutter. This usually happens when you are plowing through material with a deep cut to make a slot. Unless you have a powerful stream of coolant to blow chips away from the cutting area.
A look at the above picture gives a lot of clues about what is going on. The biggest tipoff is the molten metal at the rear of the cut. That aluminum stuck in the flutes is molten aluminum that has cooled until solid. It has to be driven off the cutter with a punch. Why did the metal melt? The feeds and speeds were fine for awhile so it isn’t turning too fast. It melts because the cutter is not able to eject chips, even though it keeps trying. With nowhere to go the chips ;pack into the flutes and the friction of spinning starts the melting process.
But what keeps the chips from ejecting? This is the key question and the least understood part of this phenomenon. The answer can be seen in the picture above. The chips behind the molten area have been crushed together as ejected chips are jammed together trying to get out. The reason for this is that milling machine chips tend to take up three times the volume as the uncut material. We know this because makers of chip briquetting equipment publish chip volumes for lathes, mills, etc. In fact 3 to 1 is about the least volume of any of the other types of chips.
This means that the chips that fill up the slot behind the cutter are about one third and the rest needs to leave the area, usually by flinging up and out. In fact, videos of cutter galling show a sudden drop in the chips flying out just moments before failure. The sound changes as well. In a case where a 1/4″ diameter endmill enters into uncut material at a depth of 0.4″ and a five inch per minute feedrate, we can spot exactly what triggered the event. In the first inch of the cut, a stream of chips shoots down the length of the slot and out to the surroundings. These chips are 0.4″ long and the slot is 0.25″ wide. The chips can easily bridge across the slot, forming a log jam. A video of the cut shows the chips flying out then, all of a sudden, the chips build up above the top surface and the sound changes. The chips surrounding the cutter tend to stop the chips from flinging upward and out. The cutter keeps moving forward but isn’t able to remove material and the cutter will break if the feed is not stopped quickly enough.
Notice that none of this involved too high a cutting speed or inappropriate feed rates or the wrong lubricant. It is because chips swell up in volume because they are curly and wrinkled. If trapped behind the cutter the chips get mashed together as newly ejected chips are shoved into the crowded space. At some point the chips are so packed behind it that the cutter can’t get rid of them and the flutes become packed. The cutting edge doesn’t stick out a little to grab a new slice of material. Instead the face of the packed material gets rubbed around and around until it melts the material. This melting and solidification is about the only way to have the chips equal the volume of cut material.
So what can be done?
Flood coolant has a long and proven history of preventing this kind of failure by blowing chips away from the cutting area. Air blast may also work. If the slot goes through the material thickness then the part can be jacked up off the table so the chips can fall down. There are possible solutions available from programming tricks. Plunging to remove the main material tends to remove chips vertically. Cleaning up the slot will be less risky because most of the material has been removed. Reducing the depth of cut to 0.3″ worked in this example. Perhaps the shorter length of chip is less likely to create a jam. Another possibility would be to back up the cutter a little, periodically, to make room for chips. The type of lubricant and feed and speed don’t seem to do anything to help. Even using a corncob cutter to produce granules instead of chips, does not work.
Aluminum cuts just fine most of the time, as long as there are places for the chips to go. It is situations like making a slot where you must start by plowing into material, you are generating a lot of full sized chips and a narrow corridor to fling them out and clogging is to be expected. Another instance where chip welding occurs is when machining a pocket. Chips build up around the edges of the pocket and the pile may become so high that chips hit the pile and slide back down. Chips fill the pocket up pretty fast and you get galling.
Steel doesn’t seem to suffer the same problem. It might be that the depths of cut aren’t as great. It might be because the melting point of steel is much higher than aluminum.