We like talking about the wonders of 3D printing in metal and plastics, and no doubt we will continue to do so. But along with touting the benefits of additive manufacturing and how it applies to some rapid prototyping projects, we don’t want to lose sight of the fact that more traditional subtractive machining with CNC machine tools is still far and away the most common method by which things are made in the modern world, and this is likely to be true for a long time to come.
The first numerical controlled machined tools used punch cards or Mylar tape to control various hydraulic valves, cams and other actuators. True electronic controls, however, require step motors and an encoder to send varying voltage measurements to the control circuit, which compares this information to the initial set of values in the cutting program. This system, which is still essentially the same employed by all CNC machines today, was started in the late 1940’s at MIT in an effort to create more accurate parts for aerospace and defense purposes, though now of course CNC machined objects affect every aspect of our lives in the developed world.
We’re a bunch of machinists and gear heads around here, so in homage to the great machinists of the past we’d like to present some of the history and possible future of the top seven CNC machining techniques. Let’s start with first and oldest mechanical machining process.
Turning was originally done on a lathe, and the history and development of this profoundly useful device is itself a fascinating journey through the parallel cultural development of many ancient civilizations. The earliest workpieces were simply wooden sticks, suspended at their ends on some type of conical mandrel which was stationary. The workpiece was turned – reciprocated – back and forth by a cord of twine or leather wrapped around it, and as it turned the surface was variously shaved, sanded, cut or grooved as necessary.
The first known fully-rotating lathes, using a wheel device, were illustrated as far back as 1568, and no doubt were in active use in Europe before then. For our purposes, the modern sense of a lathe for turning and cutting metal on a commercial scale didn’t come into play until that watershed era of human invention commencing some time around 1800 and known as the Industrial Revolution. During this time, self-contained, powered, variable speed lathes became common and helped in turn to create more machine tools of ever greater types and uses. With the advent of computer numerical controls in the late 1960’s and early 1970’s, the turning of metal parts achieved even greater levels of accuracy, speed and repeatability.
Nowadays turning can be done on CNC machine centers, which are able to rotate the workpiece on multiple axes as required. The essential benefits of turning a metal part include concentricity – assuring that the workpiece is precisely circular in cross-section. Turning allows for many types of tools to apply a machined surface finish on a part, for sanding, grinding or polishing the outside face of any rotatable object. Turning can remove large amounts of material quickly, but this also implies an inherent risk factor. A large, spinning metal platter or cylinder carries with it a lot of mass and momentum. Holding the workpiece securely is essential, and great care must be taken to insure no article of clothing gets pulled into the machine or workpiece – with grievous consequences. Hence, on an industrial scale, most turning centers are fully enclosed inside of a cabinet for safety.
Still, turning a piece on a lathe is one of the fundamental machine skills, and there is a certain undeniable – even, dare we say, romantic – beauty in a fine, curling ribbon of metal or wood peeling away under the face of the cutting tool. Machinists speak of the necessity to control the cutting speed, force and angle just so, and when you get it right that lovely unbroken ribbon is the result. Now with modern machine controls, all such parameters are fed into a computer program which determines the right variables for each type of material, but of course the machine would never have known what to do if it hadn’t been for centuries of experimentation and the collective wisdom of anonymous millions of working men and women.
The other grandaddy of machining operations, milling differs fundamentally from turning in that the workpiece is held stationary while the cutting tool is rotated. This allows for many more operations, especially when working on a piece that is not cylindrical or even symmetrical in any cross-section.
The spindle on a milling machine is typically vertical, whereas in the lathe the spindle is horizontal. A mill will usually have a horizontal table on which is mounted the workpiece held securely with a machine vise. The table is able to move in a a plane on the X and Y axis, while the spindle moves up and down (Z axis), although there now exist CNC mills which can move on 5 or more separate axes.
Milling can also produce many cylindrical effects, like boring and drilling, but it excels at forming faces on a part using a variety of tooling inserts. Creating profiles, flat surfaces, notches, bevels, chamfers, lateral channels and other detailed geometries is the province of milling, and together with turning it is the foundation of the vast majority of items made in a machine shop.
As with turning, care must be taken with “speed and feed”, that is, how fast the cutting head is turning and how quickly it is moved across the workpiece, although detailed formulae now exist as a reference for virtually all standard material types and cutting tools.
Finally, as demonstrated in the picture above, lubricating fluid is used in all high-speed machining operations, for three essential reasons: to cool the workpiece and the cutting tool; to lubricate the cutting surface and improve surface finish while preventing metallurgical failure; and to carry away ‘swarf’, which is the leftover chips and flakes of material after cutting. The type of lubrication often varies depending on the type of material being machined.
Making a very flat surface on metal parts is important for many applications and the best way to make such a precise surface is with a grinder. The grinder is a spinning disk covered with an abrasive grit of a specific coarseness. The workpiece is mounted on a table and is moved back and forth laterally on a sliding table beneath the abrasive wheel or is sometimes held firm while the wheel moves.
In any case, different types of abrasives are used depending upon the material being ground. The heat and mechanical stress of the grinding process can adversely effect the workpiece so care must be taken to control tool speed and temperature. Surface grinding is still the best way to make very flat, smooth faces on parts.
Solid Sink EDM
It was the brilliant English polymath Joseph Priestley who first recorded the phenomenon of electricity dislodging particles from the surface of a material, way back in 1770. Much later, in the 1940’s, independent researchers from Russia and the USA were experimenting with electrical discharge techniques to work on tough materials in difficult to reach places.
Electrical Discharge Machining (EDM) works on the following principle: a conductive electrode is placed in close proximity to a conductive workpiece, separated by a thin barrier of an insulating dielectric liquid. Regular pulses of current through the electrode causes it to discharge once it surpasses the resistive barrier of the dielectric. This pulsed discharge is then repeated multiple times, up to thousands of times per second, in a controlled fashion.
Solid sink EDM is typically used inside of a cavity, to machine hardened tool steels. Both the tools and the workpiece are submerged in the dielectric and connected to a power supply. One of the challenges in this process is that, as the electrode sparks many times, material is eroded from itself just as with the work piece. Therefore the distance between workpiece and electrode must be controlled carefully, and there may be unintended discharges or shorting out of the tool if there is debris caught in the interface which creates a conductive path.
Also, the dielectric must be flushed regularly to remove discharged particles and to renew the insulating capacity.
Attempting to cut through hardened tool steel is very difficult, and requires the use of expensive cutter heads that wear out quickly and require replacement or constant sharpening. An alternative is wire EDM, which as its name implies uses a continues thread of wire as the electrode. The wire is held between two spools and, as the surface is constantly being eroded away, new wire must therefore be perpetually fed between rollers.
Wire EDM is capable of cutting through thick, hardened tool steel, and is most commonly used in rapid prototyping and low-volume production, for making samples or other tools. As with solid sink EDM, both the electrode and the target workpiece must be bathed in a dielectric liquid, usually deionized water.
If cutting from the inside of a part, a pilot hole must still be conventionally drilled and the wire threaded through that hole to begin the process.
As every schoolchild knows, James Watt was responsible for inventing the steam engine, the power source of the industrial revolution. But early versions of that engine were constrained by the notoriously unreliable cylinders which were out-of-round and unable to sustain high compression pressures. But in 1774 John Wilkinson invented a boring machine with a shaft that extended through the cylinder and which was supported at both ends. This allowed the mass production of accurate, round cylinders in cast iron, thereby greatly increasing the availability and use of combustion engines. Mr. Wilkinson also happened to be the brother-in-law of Joseph Priestly, mentioned above. It was an inventive age.
It is considered that Wilkinson’s boring machine may have been the first modern machine tool, helping to make him a very wealthy man but somewhat perverse in his obsessive devotion to all things made out of iron, including his eventual coffin. It was also an eccentric age. In any case, there are five types of cylindrical grinding: Inside diameter, outside diameter, plunge, creep and centerless grinding. The main benefit to cylindrical grinding is that it provides for extremely high precision of the finished piece.
So far we’ve been talking exclusively about the machining of metal, though of course many of the techniques discussed above can be used on other materials, including wood and some plastics. But let us not forget another very high precision application of computer controlled manufacturing, and that is the making of optics from glass.
The earliest known lenses for spectacles date from around 1280 in Italy, although the light-bending properties of crystal, water and gemstones were known long before that time. Isaac Newton wrote about the grinding of lenses back in 1666, while Galileo and da Vinci were experimenting with telescopes long before. Yet the practice was still hampered by the relatively poor quality of glass as well as the various abrasive media used to grind it.
Now we have an advanced understanding of glass making and the mathematical models needed to compute the precise curvature of a lens to achieve the desired result. Specialty machines are now used to grind the curved surface of a glass lens to a precision less than one quarter of a wavelength, while specialty evaporative thin-film coatings can enhance that precision even further.
Lens grinding typically makes use of a rotating head that oscillates over the entire surface in a circular motion. A lubricating paste mixed with water makes a slurry which further enhances the abrasive effect to within ever finer limits. Also, note that when we say “glass” we are actually talking about a number of different compounds of transparent silica-based vitreous solids – vitreous meaning ‘glasslike’, but also meaning that the compound does not form a crystalline molecular structure, and this property is valuable in forming unbroken curved shapes.
Bear in mind that when using additive 3D printing, the resulting part almost always needs one or more of the CNC processes mentioned above. Although 3D printing can make complex shapes it is still no match for the speed and precision of traditional machining. Did we forget any important ones, or leave out your favorite? Drop us a line and let us know.