A Bit of Puzzling News from Phys.Org

Phys.org is an online physics journal and not in my regular reading rotation, but there's an article that got picked up in the Woodpile Report (no. 538) about a pretty bizarre discovery having to do with machining aluminum and some other "gummy" metals.  Their machinability can be improved by coloring them with a Sharpie, or applying a glue stick. 

Say what? 

Let me back up for a minute.  I machine mostly aluminum these days, and I've seen the result of machining alloys like 6061-T6 (so-called aircraft aluminum) many times.  The freshly cut metal welds onto the cutting edge, eventually rendering the cutter useless.  It has become standard advice to home machinists to use coolant when machining aluminum to prevent or minimize this.  Spray water with aqueous coolants added are popular, it's why I have my Fogbuster system, but I've seen many guys say that either WD-40 or kerosene are better.  Copper is notorious for gumminess as well, and I've heard the standard cutting fluid is (or used to be) milk; presumably full fat, not skim.

Your everyday permanent markers, glue sticks and packing tape may offer a surprisingly low-tech solution to a long-standing nuisance in the manufacturing industry: Making soft and ductile, or so-called "gummy" metals easier to cut.

What makes inks and adhesives effective isn't their chemical content, but their stickiness to the surface of any gummy metal such as nickel, aluminum, stainless steels or copper, researchers at Purdue University and the University of West Florida find in a study recently published in Physical Review Applied.

These adhesives help achieve a smoother, cleaner and faster cut than current machining processes, impacting applications ranging from the manufacturing of orthopedic implants and surgical instruments to aerospace components.

The first question that came to my mind is whether or not I'd have to machine one pass, and stop to apply more marker or glue before the next pass once I cut past the layer I'd applied.  They don't specifically say so, but it seems to be the case.   Still, if this really improves the production rates of machined pieces, the technique should reduce the costs of things machined out of aluminum or the other metals they mention.

It turns out the gumminess of the metal is because of the way it moves in response to a cutter.  At the microscopic level, most metal machining is peeling away a surface layer of the stock; the metal shears and temporarily flows like a fluid.  A non-gummy metal flows in straight paths; a gummy metal doesn't.

"Gummy metals characteristically deform in a very wiggly manner," said Srinivasan Chandrasekar, Purdue professor of industrial engineering. "This wiggly flow involves significant energy consumption, which means that these metals require more force to machine than even some hard metals. We needed to find a way to suppress this wiggly flow."

Photographically, it's rather visible.  Especially when "flow lines" are included.

Gummy metal on the left, easier machining metal on the right.  Photo from Phys.org/Purdue University.  There's a video at Phys.org which shows details like this photo captures. 

Marking with ink or attaching any adhesive on the metal's surface dramatically reduced the force of cutting without the whole metal falling apart, leaving a clean cut in seconds. The quality of the machined surface also greatly improved.

Stickiness didn't initially stand out as a solution that permanent markers, glue sticks and tape have in common.

"We looked at the chemical ingredients of the permanent ink, isolated each of those on the metal's surface, and there was no noticeable effect," said Anirudh Udupa, lead author on the study and a postdoctoral researcher in Purdue's School of Industrial Engineering. "So we realized that it's not a particular chemical but the ink itself sticking to the metal through a physical adsorption mechanism."

Somehow the ink reduces the energy in the atoms at the surface?  Beats me.  They don't say. 

I have to wonder how important this is really going to be, since all commercial operations I'm aware of use flood cooling, most often with streams of coolant forced into the cutter at substantial pressures to clear chips from the cutting operation and prevent them from welding to the cutter.  To develop ways to cut, apply another layer of (whatever), cut more, and so on would take some detailed cost/benefit studies.