TM.5.1 CAD modelling
It is a rather fundamental choice if moulds for gluing or laminating are made by hand or designed by CAD. Most luthiers have a series of tools for every model they make and with many models, the space requirement becomes an issue. In industries the specific tools and spare parts for each product are stored and space and retrieval demands led to digitization. Some industries even have such methods for each item sold. For instance, the aircraft industry used to store pegboards on which the cable configurations for each individual aircraft were produced. These boards are the size of an aircraft and for thousands of aircraft the space requirement was impressive. Today, cable configurations are produced on a single board on which the individual layout is projected in an augmented reality. The data fit in a chip. The space issue is for the luthier not as pressing, but space and archiving are not ignorable. Another argument in favour of CAD is that automated production processes using CNC machines simply require a CAD model to work with. Or if you order travel cases from a company elsewhere, chances are that they will ask for a CAD file to produce the interior. There is thus reason to switch to CAD at a certain point of sophistication of your production process. Large guitar producers made the switch a long time ago, in particular those producing solid bodies.
CAD is done with a competent program (AutoCAD, SolidWorks, FreeCAD and many other, see https://en.wikipedia.org/wiki/Computer-aided_design#Overview_of_CAD_software for an extensive list) and one needs to learn to create shapes by means of geometrical operators. The strategy to create objects must be mastered because seemingly efficient design strategies may turn out later very laborious when trying to make changes, or they may exceed the capabilities of the software. Most software deals better with simple geometrical forms, bodies made of flat surfaces for instance, than complex curved surfaces. The greatest advantage of designing by CAD is its shape consistency. It is easy, once the model has been built, to make perfectly fitting parts, to apply recesses, mounting holes, fitting marks, etc. to the model, which may be difficult to position and mark afterwards. Also curved shapes fit exactly, which is hard to achieve by hand. Another advantage is that exact copies may be made for comparative studies or to keep the sound similar. Also, not unimportant these days, it enables you to cooperate with somebody located elsewhere and smoothly assemble parts later.
CAD software can be very expensive, depending on the purpose of the software. Some, like Cathia or AutoCAD, are developed for industrial applications with many concurrent users. For our purposes simple programs will do and if you don’t need the support that commercial suppliers offer, there is free software like FreeCAD or a limited version of Fusion 360. For FreeCAD several tutorials are available on the internet (www.freecadweb.org). Fusion 360 is provided by AutoCAD and has instruction manuals.
TM.5.2 Dreadnought outline definition
The typical outline of most guitar tops is defined by 8 circle segments and 7 tangents connecting them. The circles are as in Table TM.5.2.a. This top is for a dreadnought and is 500 mm long, 380 mm wide at the lower bout, 260 mm at the waist and 288 mm at the upper bout. Within limits these dimensions can be adjusted to produce other forms, resulting in the familiar types of modern and classical guitars. Surprisingly, just 12 independent data define a wide range of guitar shapes, demonstrating the power of a formal geometrical approach. There are more than 12 numbers in the table, but symmetry is creating some redundancy. This defined outline can be combined with dome shape, wedged sides, cutaway, etc.
Table TM.5.2.a. Description of the circles making up a dreadnought top, defining the centers and radii of the circle segments involved (in mm).
TM.5.3 Mould production
CAM, Computer Aided Manufacturing, which is the physical counterpart of CAD, is using milling machines to create shapes in solid materials or 3D printers to build shapes. In series production of solid body guitars this is very common as it involves only one production step towards the final shape. For acoustic guitars the value lies more in solid parts or in making tools. The tool is a mould which is used to shape materials, such as plates and sides, and hold them in place during glueing. The mould will often consist of two parts, a positive and a negative, with an enclosed space for the materials. A mould for a domed plate could for instance have a part with a hollow side to receive the top plate and another part with a convex side to press the plate. The latter side may have recesses for the bracing. The plate and braces can then be cut by the mill to the desired outline, after which the plate is bent in shape and the braces glued on in a single step. This may seem overkill, but note that the order of processing is reversed. The plate is cut to size and thickness and the linings routed in one run and then mounted to the bracing using the mould. This is possible by virtue of the reproducibility of the CAM process, so that parts and accessories always perfectly fit. Also the refining of parts with markers, mounting aids and perfect excavations is no trouble at all. The violin making in https://www.youtube.com/watch?v=5iP2bkFUj94 is acoustically rather odd, but gives an interesting insight in the functionality of a CAM process.
The available cutter tools set a limit to shapes. Usually undercuts and depth are limited by the feasible shapes of the router bits, but this depends on the flexibility of the mill. For sophisticated products mills with up to 6 degrees of freedom are used. For the luthier larger or more complex shapes may be composed of parts, each of which can be worked on from several sides.
A CNC router is a costly tool. Factory products may cost $20,000. Parts for a DIY router, which has a working space of 1000*500*120 mm, cost about 1600 $€£, including spindle (www.openbuildpartstore.com or www.ooznest.co.uk/Openbuilds for mechanical parts, aliexpress.com for motors and electronics). This is in line with the data on CNC kits from https://www.scan2cad.com/blog/cnc/best-cnc-kits-beginners/. The price is heavily dependent on the work space size, because the machinery overhead is more or less the same for all sizes. Small machines may cost over $200 per square dm ($12.5 per square inch) but for sizes over 13 square dm (200 square inch) the cost per square dm drops to $50 and gradually lower for larger sizes. At this point one has to decide if it is ment for engraving decoration or making small parts or for tooling, necks or solid bodies. In the latter case a workspace size of 80 by 50 cm is required, costing less than $2000.
A CNC mill is not indismissible for tool making. The alternative is hand modeling the original product and producing a negative copy as the first part of a mould. This can be done by a more dedicated router design, which needs no stepper motors and is considerably cheaper than a CNC router. In fact, copying to a negative is the handiest way to handle surfaces. In the first step a negative is made from the original. The negative is covered with a layer of material with thickness as desired and this assembly is then negative copied again, to obtain the counterpart of the mould (Fig TM.5.3.1). The two copies encapsulate a space with the shape of the original surface and with the thickness as applied.
Fig TM.5.3.1. Making a two part mould from an original shape by making twice a negative copy.
This method leads naturally to casting plates or components. In most examples of carbon compound parts a one sided mould is used, while a suction bag is pressing at the other side. This leaves a slightly uneven inner surface that may not nicely match other parts when assembled. In case parts are designed to snap in during assembly a 2-sided mould is required.
TM.5.4 CNC machine
Although I am not sure if a CNC machine belongs to the thrifty luthiers outfit, it is an interesting option. In the usual configuration it consumes capital, but also space. Since I do not have so much space, or have to rent another workshop, I decided to build one upright, on the wall. The space saving is enormous since the footprint is now much smaller, but also the space to walk around it is strongly reduced. Mounting it to a brick wall assures stability. However, there is a mixed blessing in the effect of gravity. No longer the work piece is lying on the table, it hangs on it and may need improved fixation. The usual fixation methods with double sided tape or vacuum suction are also strong enough in this setup, however. Another thing to solve is that the moving frames of the cutter must be lifted by the actuators against or with the force of gravity and that is too much variation for their capacity. Static forces must be avoided. So the weight is compensated by a counterweight, but now the mass to move has doubled and the speed of acceleration is only half of what it used to be. Stronger motors help, but as this machine is not intended for speedy production, I am happy with slower cutting. A further significant advantage is that the dust does not require a vacuum collector, but falls by itself in a container at the floor. This again saves space and, very important, the sight on the cutting process is not lost. The routing is easier to observe than when bending over a table. It is also easier to cover up to keep the workshop clean.
In Fig TM.5.4.1 the CNC cutter is shown. It has been built from aluminum parts, designed by OpenBuilds.com. The profiles have the size two by four units. A unit has the size 20*20 mm and all profiles are multiples of this unit. Each unit has at its outsides rails which can be used as a wheel path and a concealed mount alike. The material has been made to high accuracy to make the wheels run smoothly. Wheels are mounted on a plate to make a carriage, on which another rail can be mounted or something else. The vertical rails (y) in the picture have 6-wheel carriages, carrying a horizontal rail (x), on which a 6-wheel carriage holds a cross rail (z). On this rail the router is mounted (Fig TM.5.4.2, left). The y-rails together move the largest mass and each rail has a stepper motor. The x-rail and z-rail each have one motor. Motors directly drive a spindle which sits safely inside the rail and drives the carriage with considerable force, due to the high gear (Fig TM.5.4.3, left). An exception is the motor for the z-axis, for which a belt drives the spindle (Fig TM.5.4.2, right). This serves a more compact build so that the motor does not stick so far forward.
In Fig TM.5.4.1 you may see the counterweights at the sides. The weights are balanced separately for each side because the mass is not completely symmetric. Another counterweight is in the can of my favourite coffee brand, which is filled with lead. This can sits on a carriage which moves counter to the whole z-rail to keep the center of mass of the x-rail in the middle. Otherwise the balance over the y-rails would be lost when the x-coordinate changes.
The cables have to move with the router position and there are quite some, for the router power, for the stepper motors power, for the safety switches, for auxillary power, etc. Cables are held in sleeves which have a hard cover but can flex. Their motion is confined to avoid that the cables get in the way of the workpiece or router (Fig TM.5.4.3, right).
The electronics consist of power source, microprocessor board, breakout board, drivers and interrupt handling (see section TM.5.5). This has been mounted on top and receives ample ventilation because it is power equipment. The controllers need to be handling timing and power, adjusted to the capability of the system, not just to the motors, but also to the mass they drive and to the balance of acceptable speed and missing steps. This must be tuned for the machine to work well, by microswitch settings. If something does not work out as expected there ought to be a safety switch and end switches because the cutter is not supposed to eat its way into the wrong material or trying to run out of bounds (Fig TM.5.4.4). A later modification is a machine on/off button set which warrants that after interruption of power the cutter cannot restart by itself. This is a legal requirement for products on the market. Additional features such as a zeroing device are recommendable. This is a sensor which tells the machine the exact position of the work piece.
Those interested may look at https://openbuilds.com/projects/ for all kinds of inspiring projects, although I have never seen a vertical machine there. Also the forum is a good source of information.
Fig TM.5.4.1. Vertical CNC machine, hanging on the wall.
Fig TM.5.4.2. The z-axis with the router (left) has a reverse action. Not the router is moved, but the whole axis with stepper motor and router. Its spindle is belt driven to obtain a more convenient form factor.
Fig TM.5.4.3. Stepper motor with spindle drive and end switch (left). The cables fold and unfold with the motion (right). Here cable sleeves are used to avoid wear and damage by the continuous motion.
Fig TM.5.4.4. The electronics is mounted on the top board and protected with a vertical board, while receiving enough cooling. In front is a safety switch which is interrupting the cutting process and setting all drivers in a safety mode.
TM.5.5 CNC process and software
The idea of computer controlled production is conceivable, but the real process is complex and involves many steps
- Production of a CAD model. The various software have proprietary formats, but these are convertible to the exchangeable formats IGES (old), STEP (newer), STL, OBJ and other. IGES, OBJ and STL are for surface models, STEP for solid models. Surface models build the surface from many flat polygons. With limitations referring to defects in the model, surface models can be converted to solids.
- Also bitmaps can be used as a source for a CAD model. Visual interpretation software like Inkscape convert the bitmap to the vector based format (SGV or DXF) that CAM software can read. In this software the flat image is extruded to a 2.5 D CAD model or converted to an engraving CAD model.
- Conversion of a CAD output file into G-code. The G-code describes the coordinate system and the milling procedure in terms of coordinates, steps and tools. G-instructions convert a move through the CAD model to incremental actions of the cutter. The conversion to G-code is done by CAM software (Fusion 360, FreeCAM, Dolphin Partmaster, Mastercam and many other). The software starts with a strategy to calibrate the position of the raw material and to remove material, called the tool path. Along this path material is removed in parallel layers of deminishing thickness. For this purpose the software wants to know all about the router bit used. The toolpath is a string of G-codes. G-code is an industry standard (NIST RS274) and supposed to be milling machine independent, but in reality many machines have custom features. For this reason postprocessors may enrich the G-code for specific machines. Typical file extensions are .gcode or .nc.
- Other maker methods are laser cutting, 3D printing, lathe cutting etc. These follow the same steps but the settings of the machine are different. For 3D printing, for instance, the extrusion material, temperature, extrusion speed or layer thickness are typical settings. These are also expressed in G-code commands.
- The G-code file is passed to milling control software like the industrial Mach 3 and 4, Universal Gcode Sender, OpenBuilds Control and other and sent to the mill. Sometimes the sender is integrated in CAM software. The controller takes care of the calibration, movement speed, acceleration, start/stop signals and tool changes and thus allows coworking of the command file and the operator. It is often featuring a preview of the milling procedure. The milling process is involving mechanical limitations of acceleration and speed so that the controller needs to know what vehicle it is riding to anticipate upcoming moves. The machine instructions are fired to the milling machine and controlled from the graphical interface on the host computer or from a handheld connected to the milling machine. There is a knack. Milling machines have parallel strands going. This was easily served by a parallel port as interface. Modern computers do have serial ports only and using these instead may cause timing errors in the milling procedure. In this case a microprocessor (often an Arduino board or Smoothieboard) is used as an intermediate storage and signal generator for the flow of pulses, which then are sent to the hardware, exactly timed.
- A control board (shield) at the mill receives the signals, splits them out to the various motors and receives interrupts from sensors on the machine. Usually there are three to five axes to control, the spindle must be switched and the speed set, end stops protect the machine and dust collectors and lights are switched on and off. If the machine is used as a 3D printer there are additional controls for compound temperature and extrusion. If provided with a laser cutter the laser power is modulated. Many mills will have in addition a zeroing device (probe) to match the calibration in the software with the actual position of the raw work piece.
- Finally the signals to the stepper motors are powered through driver circuits, which are fed by a competent power supply. The drivers have several dipswitches which set maximum current, step size and pulse timing.
Many software packages cover more than one step in this series, for instance designing a CAD model, taking the CAD file as input for CAM, producing a toolpath and building a G-code file. Other import a CAD file and integrate the rest, including execution of the milling control. Some of this software is really expensive and only pays off in series production. For experimentation free or cheap software is available, but not as a fully integrated package. Several communities work on the development of public domain software, such as FreeCAD/FreeCAM software (www.freecadweb.org), GRBL interpreter and control software which supports the Arduino board (https://github.com/grbl) and LinuxCNC. Although active members of these communities make great personal efforts to help newcomers get started, it should be kept in mind that they do not provide helpdesks and that the documentation is not as user friendly as a product manual. Many affordable corporate programs like ESTLCAM, HeeksCNC and Easel are user friendly and well suited for the not so demanding user.
You may find a useful brief overview of CAM software at https://all3dp.com/2/best-free-cam-software/ and CAM controllers at https://all3dp.com/2/cnc-router-software-find-the-tool-for-you/
TM.5.6 Using a CNC machine as a 3D copier
Like the production of a product, the copying is also fairly complex. Copying is interesting when a product has been hand shaped and can not easily be converted into CAD algorithms. There are basically two methods. The first is to control the router by hand and skip motion driving by a computer model. The second is to use a CNC machine as a 3D scanner to create a computer model, which can be subsequently milled. The steps involved are:
Hand method
- The master model is lined out to be in parallel with the raw material.
- A ‘sensing’ stick is mounted in line with the router, pointing backward to the master model. The master surface is sensed and at the most protruding sections the router cuts into the raw material at the opposite side. The distance between router tip and stick tip is stepwise increased, while repeating the sensing, until the master surface makes contact (through the router assembly) with the raw material everywhere. Now a negative has been carved in the raw material.
- A principal difficulty is that the sensing tip size and the milling tip size may cause erroneously cutting of raw material. The sensing tip is in the space that should not be cut. It means that in the approach to the final surface ever finer tips and tools must be used. In case of steep slopes it still may go wrong and finishing by hand may be required. Strategies to obtain a good approximation could be developed, while rotating the router/sensor module relative to the master model, so that vertical flanks become slopes. This is rapidly becoming so complex that scanning from more points of view and combining the scans in a database is more logical. The hand method then becomes a 3D scanning method.
- There is a bypass for these problems, be it a laborious one. We have assumed that the router moves with the sensor as one unit. If the router, by a mechanism, moves opposite to the sensor an exact mirror copy is obtained (M). These mirror copies can be made smaller (M–) or larger (M+) at will. Larger means that an imaginary surface layer is put over the model, by using a router tool larger than the sensor. A direct copy (C) is always a tool tip sized layer larger than the original, as explained above. If for the top mould the transformation M–CM is used (meaning that M– is direct copied and this half product is mirrored again) the surface is kept, but the material is at the other side, thus casting the hollow below the original surface. For the back mould the transformation M+M is used, resulting in the original with a layer off. The top and back fit together, leaving a space with the thickness of the layer. The transformations satisfy the conditions that the surface is not mirrored and that the space is below the original surface (like a plate is below the top surface).
3D Scanning method
- The master model is lined out to be in parallel with the raw material
- A local action depth sensor is mounted in line with the router, pointing backwards to the master model. When the sensor is contact less, the CNC head must cover the whole surface moving in one plane, while the depth data are read out and sent to a data base in the host computer. If the sensor is contactless the XY motion can be used to move a camera chip. Pictures are taken following a grid.
- The images are stored and overlapping areas are processed photogrammetrically to retrieve depth information. This may be done by Autodesk 123D Catch. Autodesk Memento is even more versatile and does the complete process using the computing power of the cloud. It allows you to upload very many pictures, retrieve a mesh, clean it up, change it and put it out as an STL file that can go into the CAM software. Because the motion system is in place this feature is relatively cheap and may provide high resolution data. An example is shown by Whitney Potter at 3ders.org/articles/20160128-build-your-own-arduino-powered-desktop-3d-scanner.html
- The photogrammetric method is also available as an app, using your phone as a moving camera. The motion path is free and the model is processed as you go around. The accuracy is probably less than with professional scanners which apply the same technology. These cost more, but prices have come down over the years to less than 1% of what mechanically driven laser scanners used to cost.
- If the sensor is contact based, care must be taken not to move the sensor sideways against a wall in the master. The movement strategy is not obvious and in any case time consuming.
- The STL file may be used for reverse engineering, which is the construction of a CAD model that fits the surface. This is a feature that most CAD softwares offer. Automation is limited, since much of the structural information (parts, how they fit together, degrees of movement freedom) is absent in the surface model.
- The remainder of the procedure is as in the CNC process. The negative mould is easy to produce as it is the reverse of the scan (swapping inside and outside). The positive mould is not an exact copy of the scan. A layer must be taken off in the CAD software.