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What came before 3D Printing? Way, way before . . . SUBTRACTIVE Image credit: Pixel B. MANUFACTURING A sculptor creating a statue from a rock by chiselling away unwanted pieces is a classic example of subtractive manufacture, albeit a manual form. Another such process which will be familiar to many readers is the chemical removal of unwanted copper from blank PCB laminate by the chemical action of ferric chloride or ammonium hydroxide, to produce the desired circuit pattern. Subtractive manufacturing in a production environment (or increasingly, a home workshop) typically involves using various machine tools. In the past, these were under manual control of an operator, but today are usually under computer control. This is known as CNC or computer numerical control, or just NC for numerical control when a computer is not used (up until about 1978). A machine tool is a powered tool, fixed in place, used for shaping various materials that are held by the tool. Basic operations which can be performed with machine tools include turning, boring, milling, broaching, sawing, shaping, planing, reaming and tapping. The raw materials used as a starting point are typically solid blocks of plastic, metal, timber, composite or ceramics. The tools used to perform the shaping include lathes, milling machine, broaching machines, pedestal drills, slotters, hand or mechanical saws, shaping machines, grinders or planers. Milling machines have mostly by Dr David replaced shaping and planing machines. 10 Silicon Chip More recently developed processes to perform the above operations are electrical discharge machining, electrochemical machining, electron beam machining, photochemical machining and ultrasonic machining. This article discusses subtractive manufacturing processes, with a particular emphasis on techniques and automation. We’ll start with a brief history of subtractive manufacturing machines. The entire history could (and probably does) fill a book! Lathes, mills etc In case you don’t know the difference between the different types of machine tools, here is a quick rundown. Probably the two most common types are lathes and mills. A lathe is normally used to work cylindrical objects like logs. They are clamped by one or two sets of jaws which spin the object, then a cutting tool moves along its length and towards the axis of rotation. Items made on a lathe include table legs, vases, chess pieces etc. A milling machine is similar to a 3D printer in that (at least in its basic form), the object is essentially fixed, and a cutting head moves overhead, dropping down to make cuts into the workpiece. By moving the cutting head in a zig-zag fashion, it is possible to make a flat surface aligned with the plane of the mill, ideal for placing another item on Maddison top of for accurate machining. Australia’s electronics magazine siliconchip.com.au Additive (eg, 3D printing) Subtractive (machining processes) Material is added layer by layer Material is removed from a solid block of starting material, usually in several passes. Requires suitable materials such as thermosoftening plastic or metal powder Can be applied to almost any solid material; special techniques are required for extremely hard or brittle materials. Little or no material is wasted, except for possible small amounts of material used for temporary supports. Scrap materials can be recycled in some cases. Material that is removed is wasted, although most metals can be recycled. Shapes of almost infinite complexity can easily be produced, including those with hollows, even if closed-off like a hollow sphere The complexity of shapes is limited by geometric factors such as the accessibility of an area to a cutting tool. A hollow sphere would be impossible to make subtractively in one piece. Typically a relatively slow process. Automated CNC production can be very fast. Table 1: summary of differences between additive and subtractive manufacturing Mills can also be used for drilling, by merely inserting drill bits into the tool holder and plunging them into the workpiece. Drill bits are designed mainly to cut at the tip; other types of milling bits have cutting surfaces on the sides, so they can be moved sideways through the workpiece to make slots and so on. There is a large variety of milling tools available including end mills, slab mills, hollow mills, ball mills, fly cutters, dovetail cutters, face mill cutters, bevel angle cutters and so on. They suit different types of material and making different sorts of cuts. Differences between additive and subtractive manufacturing There are important differences between additive and subtractive manufacturing processes and so neither process can fully replace the other. These differences are outlined in Table 1. The main differences are in the types of materials that can be used, the shapes that can be made, the amount of waste that is produced and the speed with which items can be made. having previously had a hole drilled through it. This provided an accurate bore in terms of diameter, straightness and roundness. Wilkinson’s machine is regarded by many industrial historians as the first machine tool and was a critical development for the progress of the Industrial Revolution. Later models of the boring machine were powered by steam engines, whose cylinders were made by the machines they were powering! This led to the development in 1794 of the first enginepowered lathe by Henry Maudsley, which was later developed into a screw-cutting lathe in 1800. The availability of the steam engine to power machines led to the development of other machine tools such as the planer, invented by Richard Roberts in 1817 and the horizontal milling machine, invented by Eli Whitney in 1818 (Fig.1). History of subtractive manufacturing Lathes, which enabled the production of axially symmetrical parts such as pots and vases, have been known since ancient times. But precision parts such as steam engine components (eg, pistons) could not be made on such machines due to their limited precision and accuracy. In 1774, John Wilkinson developed the first waterwheel-powered horizontal boring mill. This enabled him to supply James Watt and Matthew Boulton with accurately bored cylinders for their steam engines in 1776. For the first time, these had minimum leakage due to the accuracy of the bore. Unlike previous boring machines, the bar that supported the boring bit was supported at both ends, the workpiece siliconchip.com.au Australia’s electronics magazine Fig.1: The first horizontal milling machine by Eli Whitney, from around 1818. July 2020  11 Fig.2: a drawing of Thomas Blanchard’s original copying lathe of 1818, with a photo of a later development of that machine made in Chicopee, Massachusetts and sold to the British Government in the 1850s. It was used at the Enfield Armory for the next 100 years. An early use of precision machine tools was Eli Whitney’s manufacture of muskets for the US government. At the time, parts for devices like firearms and steam engines were custom-made for the individual unit, and were not interchangeable. Whitney’s idea to win a US Government contract was to produce firearms with interchangeable parts using a precision lathe and milling machine. This would lower costs and reduce the necessity for highly-skilled machinists, who were in short supply at the time. The experts did not believe this was possible, so he went to Washington DC in 1791 and took the parts of ten muskets he had produced, mixed them all up and then proved that the performance of the muskets was not noticeably affected by using the mixed-up parts. This principle of interchangeability now applies to virtually all mass-produced machine-made objects today. The development of the lathe, the planer and the mill led to the ability to make more and better copies of these same machines, plus different machines and more products. Today, the function of the planer is mostly but not totally replaced by the milling machine, broaching machine and grinding machine. It is important to note that machine tools can be used Learn CNC machining free, online Titans of CNC (https://academy.titansofcnc.com/) is a free USA-based online training academy that teaches CNC machining to people in all countries. It was established by Titan Gilroy, who is a reformed prisoner. Read his fascinating story and why he established the academy at http://siliconchip.com.au/link/ab0w See also https://titansofcnc.com/about/ and the video titled “Titan Gilroy’s Powerful TESTIMONY - CNC Machining” at https://youtu.be/WMQT1YvcQ38 12 Silicon Chip to make better versions of themselves, hence the ongoing improvement in the quality and precision of such tools. Machines were typically powered by a water wheel before 1775 and steam engines from about 1775 (many made by Boulton & Watt, a partnership between James Watt’s company and the engineering firm of Matthew Boulton). Nikolaus Otto produced four-stroke gasoline stationary engines from 1876 to power lathes and other small machines, although some coal-gas powered internal combustion engines preceded that. Electric motors were also used from about 1890. Early machine tool automation Industrial mass-production required ways to control machine tools that would enable hundreds or thousands of identical parts to be produced with minimal or no manual input. It was also desirable to be able to alter designs with minimal effort. Machine tool automation started in the 19th century with the use of cams to move parts of a machine tool in a particular sequence. Thomas Blanchard developed the “copying (or duplicating) lathe” in 1818, for reproducing gun stocks and any other irregular shape in wood (Fig.2). The cutting tool was guided by a cam that represented the shape to be cut. It was regarded as one of the most significant tools in American industrial history. See the video “Blanchard Lathe at Asa Waters Mansion” at https://youtu. be/ITNEHqW0hyQ The turret lathe is designed for automatic production of multiple duplicate parts using an indexing tool holder with multiple different cutting tools, each designed to do a different job (Fig.3). When one part of a machining operation is finished, before the next part of the operation starts, the tool holder is rotated to the next tool by a cam or other mechanism. The first turret lathe was built by Stephen Fitch in 1845, Australia’s electronics magazine siliconchip.com.au Fig.3 (above): the turret lathe of Stephen Fitch from 1845 from “Report on the Manufactures of Interchangeable Mechanism” US Government Printing Office, p.644, 1883 (siliconchip.com.au/link/ab0u). The indexed head with different cutting tools is still used today. Fig.4 (right): a “brain wheel” (instructions encoded on a cam) on a screw-making turret lathe of the type invented by Chris Spencer in 1873. From the same US Government Printing Office document as Fig.3. with others making similar designs around the same time. In 1873, Chris Spencer of New England, USA patented the first automatic lathe, but he failed to patent a vital component which he called the “brain wheel”. That was a cam that coded ‘instructions’ for movement of the tools on the lathe, and others quickly took up the idea (Fig.4). The “brain wheel” can still be found on some mechanicallycontrolled automatic lathes today. The beginning of numerical control These earlier automated machining approaches using templates or cams made it relatively difficult to change the “program”, since new templates or cams had to be produced. The modern era in subtractive manufacturing started in the 1940s with the introduction of numerical control or NC. It was then relatively easy to change the program be- Fig.5: a ball screw with external ball return as used on CNC machines, to precisely convert rotary motion into linear motion. The balls are the only contact surfaces between matching helical grooves. There are several variations of this design. Source: Barnes Industries, Inc. siliconchip.com.au cause early NC programming used punched cards, paper or magnetic tape to control servomotors which operated machine tools. Changing the program on the punched cards or tape was easy compared to making a new template or cam. There were earlier programmable machines such as the Jacquard loom, which used punched cards, but this technology was never applied to machine tools. Early NC machines were connected to computers as soon as they became available, and today the process is fully computerised and known as CNC (computer numerical control). Important CNC inventions Before NC and CNC could be developed, certain enabling technologies that had to be invented first. These include punched paper tape, punched cards, magnetic tape, the ball screw and servo motors. Fig.6: the elements of a simple hobby servo motor. Screengrab from the video “How Servo Motors Work & How To Control Servos using Arduino” at https://youtu.be/LXURLvga8bQ Australia’s electronics magazine July 2020  13 Fig.7: the first experimental NC milling machine, developed by the Servomechanism Laboratory at MIT in 1950. It involved automating an existing commercial milling machine. Fig.8: the Kearney & Trecker Corp. Milwaukee-Matic II from 1958. Punched paper tape was initially used to control weaving looms, with the first known usage in 1725 by Basile Bouchon. Paper tape was later used as a data storage medium for CNC machines in the 1970s, among many other computer-related uses. Punched cards were first developed by French weaver Joseph Marie Jacquard in 1804 to control weaving looms by encoding the pattern that was to be woven. In 1890, a punched card system was developed by Herman Hollerith at MIT (Massachusetts Institute of Technology) for encoding and analysing data from the US Census in the new science of data processing. He founded a firm which became a part of IBM, and the cards were known as Hollerith cards. Punched cards were also used in computers associated with early CNC. Magnetic tape was invented in Germany in 1928, and was used to record analog and later digital signals. Tapes were used in the first commercially-successful CNC machines. Paper tape was often used in early NC machines because the reader was smaller and less expensive than punched card or magnetic tape readers. Rudolph Boehm invented the precision ball lead screw in Texas in 1929. He called it an “antifriction nut” (see siliconchip.com.au/link/ab0t and Fig.5). This is not vital for CNC machines, but it is a highly desirable and precise method to convert rotary motion into linear motion with minimal friction and play, with much less maintenance than the traditional Acme screw. A servo motor is a rotary or linear actuator that provides accurate rotary or linear position placement. It comprises an electric motor, a sensor to detect the position and a controller. When the appropriate signal is sent to it, it moves to the commanded position (Fig.6). Servo motors are responsible for various motions of CNC machines. Parsons, Sikorsky and MIT The origins of modern NC are usually attributed to John Parsons and Frank Stulen of Parsons Corp in Michigan, The smallest and cheapest CNC machines One of the cheapest five-axis CNC mills is the PocketNC (https://pocketnc.com/). Prices start at around US$6,000, ramping up to US$9,000 plus accessories. That doesn’t include delivery to Australia or GST. You can run a simulator of this machine, which also shows the G-code, at https://sim.pocketnc.com/ We have not tested this ourselves. See the video titled “World’s Smallest 5 Axis Milling Machine - Pocket NC V2” at https://youtu.be/vMY06dzf7UA CNC routers (often incorrectly referred to as three-axis CNC machines), can be bought relatively cheaply from online sources such as eBay. They start at a few hundred dollars, but they are really only suitable for working with softer materials. Some will apparently machine aluminium, but do so slowly. See the video titled “Sainsmart 3018 PROVer Mini Cnc Build, Test and Review” at https://youtu.be/fT8dv1Eanps The video author says it is good for wood, acrylic, PCBs and aluminium. The manufacturer’s website can be viewed at siliconchip.com.au/link/ab0x 14 Silicon Chip Fig.9: a Knuth KSB 40 CNC drill press for drilling, reaming and thread cutting. A typical workpiece is shown inset above. Australia’s electronics magazine siliconchip.com.au CNC machine languages APT Fig.10: a Giddings and Lewis milling machine attached to a Numericord numerical control system around 1955. The magnetic tapes it used were prepared elsewhere on the Numericord Director. USA. In 1942, the Parsons company became involved in the production of helicopter rotor blades for Sikorsky. Sikorsky sent the shape of the ribs in the form of 17 coordinate points which defined the outline. The space between the points had to be interpolated with French curves. The original manufacturing process as required by Sikorsky had deficiencies, so it was decided to stamp the ribs from metal rather than build them with trusses. The 17 coordinate points were interpolated to make 200 points using an IBM 602A punched-card calculator, and these were tabulated and used to guide, by hand, on a milling machine, a cutting tool to make the stamping die. One person controlled the X-axis and the other the Y-axis, to guide the milling machine in a straight line between the 200 points; enough to emulate the desired curve. This was NC but with humans rather than machines providing the guidance! Parsons then had the idea for a fully automated machine, but had trouble getting people interested. Then in 1949, the US Air Force funded Parsons to build machines. His early ones had problems related to the requirement for a feedback mechanism to control power to the cutting head. Otherwise, it made rough cuts, as the cutting forces changed as the direction changed, so the power had to be adjusted. This feedback mechanism turned out to be a very important development for CNC. Parsons approached the MIT Servomechanisms laboratory, and they became involved in the project to build a better machine based on Parsons’ ideas. They automated an existing commercial Hydrotel milling machine using vacuum tube electronics and a tape reader in 1950 (Fig.7). A remarkably advanced machine from Hughes Products in 1958 There is a video showing an early CNC machine operation from 1958 titled “The History of Numerically Controlled Machine Tool - NC and CNC” at https://youtu.be/TdoaHK5TRh8 All the essential elements of a modern CNC system are present, except perhaps the CAD software to design the part. 16 Silicon Chip APT or Automatically Programmed Tool is a computer language developed under the leadership of Douglas T. Ross of MIT in 1956. It and its derivatives are still in use today. The language defines the path a cutting tool must follow using sets of coordinates (see listing 1). The program output is converted into a CL or Cutter Location file, which controls the machine. This latter control code is often produced in a standardised set of instructions defined by RS-274, known as G-code. APT can be regarded as a high-level English-like language that produces the lower level G-code that provides instructions for the machine. It is also possible to directly program in G-code for those so interested; however, most modern computer-aided design (CAD) packages can turn a three-dimensional model directly into the required G-code instructions for the CNC machine. Such programs are known as G-code generators. G-code can be used for additive manufacture (eg, 3D printing) as well. Listing 1: PARTNO / APT-1 CLPRNT UNITS / MM NOPOST CUTTER / 20.0 $$ GEOMETRY DEFINITION SETPT = POINT / 0.0, 0.0, 0.0 STRTPT = POINT / 70,70,0 P1 = POINT / 50, 50, 0 P2 = POINT / 20, -20, 0 C1 = CIRCLE / CENTER, P2, RADIUS, 30 P3 = POINT / -50, -50, 0 P5 = POINT / -30, 30, 0 C2 = CIRCLE / CENTER, P5, RADIUS, 20 P4 = POINT / 50, -20, 0 L1 = LINE / P1, P4 L2 = LINE / P3, PERPTO, L1 L3 = LINE / P3, PARLEL, L1 L4 = LINE / P1, PERPTO, L1 PLAN1 = PLANE / P1, P2, P3 PLAN2 = PLANE / PARLEL,    PLAN1, ZSMALL, 16 $$ MOTION COMMANDS SPINDL / 3000, CW FEDRAT / 100, 0 FROM / STRTPT GO/TO, L1, TO, PLAN2, TO, L4 TLLFT, GOFWD / L1, TANTO, C1 GOFWD / C1, TANTO, L2 GOFWD / L2, PAST, L3 GORGT / L3, TANTO, C2 GOFWD / C2, TANTO, L4 GOFWD / L4, PAST, L1 NOPS GOTO / STRTPT FINI Australia’s electronics magazine siliconchip.com.au G-code G-code (for geometric code) is the low-level command set that provides instructions to perform motion procedures, such as moving the workpiece and cutter in the desired path. A list of typical G-code commands is shown in Table 2. G-code comes in various “dialects”, which are slight variations according to the manufacturer. G-code is written in the form of commands which start with a letter and are followed by a number. The letters stand for: • • • • • • • • • N: line number G: motion and function X, Y, Z: position F: feed rate S: spindle speed T: tool selection M: miscellaneous functions. I, J: incremental centre of arc R: radius of arc Using the above form, an example of a G-code program line provided by Autodesk is G01 X1 Y1 F20 T01 M03 S500. This will generate a linear feed move G01, to position 1,1 with a feed rate of 20, tool 01, spindle on CW rotation and spindle speed 500. (See Table 3 for M-codes.) G00 G01 G02 G03 G04 G17 G20 G21 G28 G40 G43 Table 2 – example G-codes Rapid traverse (positioning) Linear interpolation (eg, feed in a straight line) Clockwise movement (CW) Counterclockwise movement (CCW) Pause or dwell Select X-Y plane Imperial format (inch) Metric format (mm) Return to machine zero Tool cutter radius compensation off Apply tool length compensation The shape defined by the APT program listing of Listing 1 (Wikipedia). siliconchip.com.au G54 G80 G90 G91 G92 G94 Work coordinate system Cancel canned cycle Use absolute dimensions Use incremental coordinates Set the origin Feed rate Apart from G-code, there is also M-code (Table 3), where M stands for miscellaneous. While G-code instructions tell a CNC machine where and how to move, M-code instructions are for miscellaneous functions such as starting the cutter or turning coolant on or off. These instructions are incorporated into the overall program code.    Table 3 – example M-codes M00 Program stop M02 End of program M03 Spindle clockwise rotation M04 Spindle anti-clockwise rotation M05 Spindle stop M06 Tool change M08 Coolant on M09 Coolant off M30 End of Program, rewind and reset modes A sample of a more sophisticated G-code program, courtesy of HelmanCNC, is shown in Listing 2. Note that program code structure is a little different than the one-liner above. The part produced by this code is shown below. Listing 2: \O1000 T1 M6 (Linear / Feed - Absolute) G0 G90 G40 G21 G17 G94 G80 G54 X-75 Y-75 S500 M3 (Position 6) G43 Z100 H1 Z5 G1 Z-20 F100 X-40 (Position 1) Y40 M8 (Position 2) X40 (Position 3) Y-40 (Position 4) X-75 (Position 5) Y-75 (Position 6) G0 Z100 M30 The part produced by the simple G-code program shown in Listing 2. Australia’s electronics magazine July 2020  17 Fig.11 (above): a Knuth Turnstar 300C horizontal CNC lathe for mass production. You can see the control screen, the chuck to hold the workpiece, the tool holder to the right of the chuck and coolant nozzles with orange tips. Fig.12 (right): an Okuma Genos M460V-5AX entry-level fiveaxis machining centre. Its capabilities include workpiece size of up to 600mm diameter, 400mm height and 300kg weight, a tool magazine with a capacity of 48 tools, spindle speed up to 15,000rpm and a power of 22kW. It weighs 8,300kg. Then Parsons was locked out of the work, despite it being his idea! Many of the team left after this, and in 1955, they went on to develop the Numericord NC system, and other companies started producing NC systems as well. By 1955, several NC machines were on display Chicago Machine Tool Show (Fig.10). This led to the development of the first commercial NC machining centre with an automatic tool changer and workpiece positioning, the Kearney & Trecker Corp. MilwaukeeMatic II of 1958 (Fig.8). You can view a very satisfying original promotional video titled “The Numerically Controlled Machining Center 1950s Educational Documentary” at https://youtu.be/ Y3YrbEGWE04 Conventional and unconventional machining processes Virtually all machining processes can be automated with CNC technology, but processes where material is removed by mechanical force are generally considered ‘conventional’, while those which use little or no mechanical forces are ‘unconventional’. The conventional machining processes most commonly used with CNC include the lathe, the milling machine The origins of precision machining and measurement There is an interesting video titled “Origins of Precision and first project introduction” at https://youtu.be/gNRnrn5DE58 It discusses the true origins of precision measurement. It all comes from being able to make a very flat surface, which you can make with no other tools but a great deal of handwork. All other measurements can be derived from that. Another video shows the world’s first precision all-metal lathe, titled “The 1751 Machine that Made Everything” at https://youtu. be/djB9oK6pkbA You can also read a book about how civilisation could be restarted in the event of a catastrophe; measurements and tools would have to be developed from scratch. It’s titled “The Knowledge: How to Rebuild Civilization in the Aftermath of a Cataclysm” by Lewis Dartnell. There is a related video, “How to rebuild the world from scratch | Lewis Dartnell” at https://youtu.be/CdTzsbqQyhY 18 Silicon Chip Fig.13: an Okuma lion made by an Okuma machining centre. See the video “Okuma GENOS M460V-5AX Leo the Lion” at https://youtu.be/A49l8ljcPis This shows that the machining process is much like the inverse of an additive process like 3D printing. Australia’s electronics magazine siliconchip.com.au Fig.14: the matching parts of a component manufactured by EDM. The components match so precisely that when one is inserted within the other, the boundary between the two is almost invisible. Source: Reliable EDM. and the drill. Of these, the milling machine is the most versatile. A milling machine with CNC controls is usually referred to as a “machining centre” (see Fig.12). Electrical discharge machining (EDM): electrical energy is used to remove material from a conductive workpiece. This is often used for hard metals which are otherwise difficult to machine (see Figs.14-16). In operation, the workpiece and electrode are immersed into a dielectric fluid and the electric field increased until dielectric breakdown occurs, resulting in melting and vaporisation of the desired workpiece material. No mechanical stress is applied, but heat is generated, which may affect the material being machined. Excellent surface finish can be achieved. Electrochemical machining (ECM): electrolysis is used to remove material and so, in a sense, this is AUSTRALIA’S OWN MICROMITE TOUCHSCREEN Since its introduction in February 2016, Geoff Graham’s mighty Micromite BackPack has proved to be one of the most versatile, most economical and easiest-to-use systems available – not only here in Australia but around the world! Now there’s the V3 BackPack – it can be plugged straight into a computer USB for easy programming or re-programming – YES, you can use the Micromite over and over again, for published projects, or for you to develop your own masterpiece! 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Voltage/Current Reference (Oct16 – siliconchip.com.au/Series/305) Energy Meter (Aug16 – siliconchip.com.au/Series/302) Super Clock (Jul16 – siliconchip.com.au/Article/9887) Micromite Boat Computer (Apr16 – siliconchip.com.au/Article/9977) V3 BackPack: Ultrasonic Parking Assistant (Mar16 – siliconchip.com.au/Article/9848) * JUST $7500 See August 2019 (Article 11764) P&P: Flat $10 PER ORDER (within Australia) *Price is for the Micromite BackPack only; not for the projects listed. siliconchip.com.au Australia’s electronics magazine JULY 2020 19 Fig.15: an EDM machine. Note how the workpiece is immersed in a dielectric fluid. Source: NezzerX. the opposite of electroplating. The workpiece is attached to a positive power source and the tool used for removal of material, the negative power source. ECM can be used to produce small holes accurately and for 3D micromachining (see Fig.18). Electron beam machining (EBM): a high-energy electron beam in a vacuum chamber removes material from a workpiece by vapourisation (Figs.17&19).    The electron beam can be controlled very accurately, to within about 0.002mm.    Hard or heat-resistant materials can be machined, and the beam is extremely accurate, but it is relatively slow and only really suitable for removing small amounts of material. Also, the equipment is expensive. Applications include drilling holes in synthetic jewels for the watch industry, welding small pieces of refractory metals, drilling cooling holes in aerospace gas turbines or space nuclear reactors, and drilling small holes in wire-drawing dies. Laser beam machining (LBM): a laser beam vaporises material from the desired area (Fig.20). Tiny feature sizes can be produced, a wide range of materials can be machined, there is no tool wear and machining times are rapid. But longer holes tend to be tapered, blind holes of a specified depth are hard to achieve, and the maximum material thickness is restricted to about 50mm. Photochemical machining (PCM): chemicals and a photoresist material are used to etch a workpiece selectively. A Fig.16: the basic configuration of an EDM system. simple example is the selective removal of copper from a blank PCB.    A pattern is photographically printed onto a surface to be machined using a photoresist layer, and unexposed parts of the workpiece are then removed with an etchant chemical.    Highly-detailed parts can be produced such as circuit elements, grids for batteries, optical encoders, jewellery, signs etc. Ultrasonic machining: a cutting tool vibrates at a high frequency (18-40kHz) with a low amplitude (0.05-0.125mm) in the presence of an abrasive slurry to remove material. This is useful for machining brittle materials such as ceramics; however, the material removal rate is low, and the tool or “sonotrode” is subject to wear.    Ultrasonic machining is suitable for substances such as glass, sapphire, alumina, ferrite, polycrystalline diamond, piezoceramics, quartz, chemical vapour deposited silicon carbide, ceramic matrix composites and technical ceramics. Abrasive jet machining (AJM): small abrasive particles are suspended in a stream of air and directed at the workpiece at a high pressure to remove the desired material. The process is suitable for brittle or soft materials, and good cutting accuracies can be achieved. There is minimal surface damage. Abrasive water-jet machining (AWJ): similar to AJM but using water instead of air; almost any material can be cut with no heat damage to the workpiece (see Fig.21). DIY machining projects There are lots of websites devoted to DIY CNC machining, including converting existing equipment such as lathes or mills for computer control. One video describes a DIY water jet cutter. It is titled “Waterjet cutter built with a cheap pressure washer” and can be viewed at https://youtu.be/Lg_B6Ca3jc Note that such a machine could be quite dangerous to operate. A video describing DIY electrical discharge milling can be found at: “Drill through anything (conductive) with Electrical Discharge Machining”, at https://youtu.be/rpHYBz7ToII (also see photo opposite). Again, this involves significant hazards. 20 Silicon Chip Australia’s electronics magazine siliconchip.com.au Fig.17 (above): a combustion chamber component made by an electron beam machine manufactured by PTR Strahltechnik GmbH. The material thickness is 1.1mm, and there are 3748 0.9mm-diameter holes. It took one hour to make. Ice-jet machining (IJ): was developed as it is difficult to filter out and reuse abrasive particles from a water jet. So this is like AWJ, but as ice is used as the abrasive medium, used water can be re-frozen and re-used. Plasma cutting: used by some CNC machines to cut sheet Fig.19: the electron beam machining process. The electron beam is controlled much as it is in a conventional cathode ray tube (CRT). The entire electron gun mechanism and workpiece chamber are held under vacuum, because the electron beam will not travel through air. metal, plate or pipes. An electrically ionised and conductive gas, a plasma, is created between the workpiece and the cutting torch and the electric arc established melts or vapourises the material that is to be cut.   A compressed gas is used, and as it passes through the cut area, it blows away molten or vapourised material. The Maslow open-source CNC machine The Maslow (www.maslowcnc.com) is a DIY, open-source CNC machine able to cut out large flat sheets of soft, thin materials such as timber or plastic up to 1.2m x 2.4m (the size of a standard sheet of plywood) – see photo below. The manufacturer suggests applications such as building a “tiny house, a kayak, a tree house, some furniture, or anything else you can imagine”. It is unique in that it is vertically orientated and only about 1m deep, so it occupies relatively little floor space. The free software and designs work with Mac, Windows or Linux. (Some support plans require payment.) Note that the basic kit does not include all the parts such as timber pieces, a router and possibly other components. Please do your own research if you want to build it. See the video titled “Maslow CNC Introduction Video” at https://youtu.be/gtJ5Z3phDhs You would have to find a seller that ships to Australia. One that we found (but did not purchase from) sold a basic kit for US$399 plus US$80 delivery to Melbourne. See siliconchip.com.au/link/ab0y Fig.18: the electrochemical machining of cooling holes in a nickel-alloy gas turbine blade. Nitric acid is used as the electrolyte solution, and the machining electrode (cathode) is made of a titanium alloy, machined to exact dimensions. A high current passes between the workpiece and the machining cathode, resulting in the dissolution of the workpiece material. Source: Tokyo Titanium Co., Ltd. siliconchip.com.au Australia’s electronics magazine July 2020  21 Fig.20: the configuration of a typical laser cutter, a type of laser machining device. The workpiece and/or the laser can be moved under computer control to cut the desired pattern. LBM is good for sheet metal parts, making holes from 0.005mm to 1.3mm, cut-outs of various shapes, features in silicon wafers for the electronics industry and thin or delicate parts. Number of axes for CNC machining CNC machines are partly characterised by the number of axes they have, which is usually between two and five, but possibly more. A two-axis machine cuts only in the one plane using two axes, X and Y. An example of this would be a basic laser cutter. A 2.5-axis machine also cuts in one plane, but the height can be changed in the Z-axis direction (not simultaneously with X and Y movements). Examples are a very basic milling machine or a drilling machine. A three-axis machine can simultaneously move the cutting tool in three directions, X, Y and Z. A true fouraxis machine adds rotary movement around the X-axis, referred to as the A axis. This rotation allows the material to be cut around the B-axis. A five-axis machine allows extremely complex modes of movement, with two axes of rotation (A & B, B & C or Open source CNC software LinuxCNC (http://linuxcnc.org/) is an open-source CNC software suite. It is described as being able to “drive milling machines, lathes, 3d printers, laser cutters, plasma cutters, robot arms, hexapods, and more”. Fig.21: glass is a difficult material to machine by normal methods. Here it is being cut with abrasive water jet machining. Source: Water Jet Sweden AB. A & C) around the X, Y and Z axes. Some milling machines are available with six or more axes, but the five-axis type is the most common. Extra axes beyond five allow certain transitions to new positions and tool movements to be executed more quickly. For a comparison between five-axis and six-axis machines, see the video “Zimmermann FZ100 Portal Milling Machine” at https://youtu.be/wOPt0dMP6ZA – the job completes far more quickly using six axes compared to when it is restricted to five. What accuracy can be achieved? The positional accuracy and the repeatability varies between machines, but a positional accuracy of 0.02mm is typical; it can be as good as 0.003mm for a jig boring machine. Repeatability is a measure of how accurately the machine can return to the same point, and this is typically half the positional accuracy, so 0.01mm. Dutch tool maker Hembrug has a range of CNC lathes such as the Mikroturn 100, designed explicitly for ultraprecision work, that have a positional accuracy of 1µm (0.001mm) and repeatability of 0.1µm for workpieces up to 380mm diameter. See the video “Soft turning, drilling & milling on a Mikroturn 100” at https://youtu.be/MtrJDBBmONo Some CNC milling machine videos • “Look what this excellent CNC milling machine do” https://youtu.be/peuvASjUsJI • “Building my own CNC Mill” https://youtu.be/q0RE-h1VDIg • “Fastest CNC Lathe Machine Working” https://youtu.be/W0E1aX6vVWw • “5 Axis OneCNC CAD CAM CNC Turbine Blade Manufacture” https://youtu.be/Vk_lhNTO6z8 SC 22 Silicon Chip Australia’s electronics magazine siliconchip.com.au