Oscar DavidsonUnit: CAD/CAM

Title: CAD/CAM reportTutor: Simon Clarke

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Contents! ! ! ! ! ! ! ! ! ! !Introduction! ! ! ! ! ! ! ! ! ! 3!Computer Aided Design/Drafting - CAD! ! ! ! ! 4!Modern CAD! ! ! ! ! ! ! ! ! ! 4!Different CAD types!! ! ! ! ! ! ! ! 5!CAD modeling!! ! ! ! ! ! ! ! ! 5!Computer Aided Manufacture - CAM! ! ! ! ! 5!Example use of G code CNC milling! ! ! ! ! 6!Rapid prototyping! ! ! ! ! ! ! ! ! 7!Stereolithography - SLA! ! ! ! ! ! ! ! 7!Applications of rapid prototyping! ! ! ! ! ! 7!Conclusions! ! ! ! ! ! ! ! ! ! 8!Recommendations! ! ! ! ! ! ! ! 8References! ! ! ! ! ! ! ! ! ! 9Appendix!! ! ! ! ! ! ! ! ! 10

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Introduction

For an engineering idea or concept to be recorded, the best way is with a technical drawing, showing dimensions, and annotated with materials used. Before a standardised set of rules was adopted for technical drawings, many different companies would produce different drawings. The British Standards Institute (BSI) released BS 308 in 1927(1). This standard set out how technical drawings should be laid out, what dimensions to use, and how to show different materials and their intersections. The standard was falling out of compatibility with the more used ISO drawing standards and so in 2000 it was withdrawn and replaced by BS 8888. This was a standard that collaborated many different ISO drawing standards into a managed standard set. There were several important differences however, such as dimensions used to be shown with commas and a dot for the decimal, e.g. “1,234.123,4”, but in BS 8888 that changed to commas and spaces, e.g. “1 234,123 4”.

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Computer Aided Design/Drafting - CAD

CAD had the promise of manipulation of a drawing once drawn. This proved a huge advantage, as before this any mistake or revision that could not be reworked into a drawing meant redrawing. If the diameter of a tube with lots of corresponding dimensions was to be changed it would mean a complete redraw on paper, but could be relatively easy on CAD, with modern more powerful packages at least.

The first CAD package was Sketchpad(2), created by Ivan Sutherland in 1963 as part of his PhD thesis. Sketchpad was run on one of the worlds most advanced computers of the age at MIT’s Lincoln Laboratory, see Fig 1. One of the most amazing features of Sketchpad was that it used a ‘light pen’ to interface with the computer, in a similar way to a modern day graphics tablet, see Fig 2. This was a great feature as it allowed engineers familiar with using a pen, rather than a computer, to use the system more easily. The system also had features found on modern systems such as rubber-banding, templates and master drawings. If the master were to be changed, all subsequent drawings would carry that same change.

Modern CAD

Modern CAD packages such as AutoCAD and SolidWorks allow for very complicated 2D as well as 3D drawings to be created. These packages also allow different modeling environments in 3D, with different materials and lighting.

An example of 3D rendering is depicted in Fig 3 left. A single spot light is used to illuminate the scene. Many more lights of different properties can be used. Spot lights with different angles of light and intensity, flood lights, and backlights. This scene also shows how light bounces off objects to illuminate other objects.

3D representations of objects can be very useful, however a technical drawing benefits best from clear 2D drawings, as it is much easier to show dimensions, materials, and detail, as usually 2 or 3 views are used, sometimes many more for complex shapes.

Fig 4. shows a 2D drawings of a 3D object, showing dimensions and the profile of the object. The scale of the drawing, units, date, title, drawing number, drawing type, and author are shown as well.

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(Above) Fig 1. Sketchpad in operation(Below) Fig 2. Light pen for interface with the computer

(Above) Fig 3. A 3D rendering of different types of objects using AutoCAD(Below) Fig 4. A 2D representation of a 3D object, created with AutoCAD

Different CAD types

There are many different types of CAD with different applications. The most common is that of engineering technical drawings, as discussed earlier in this report. Another common CAD type is that of electrical CAD. These packages can be used to design circuits and wiring systems, circuit boards also known as PCB’s, and dies of integrated circuits (IC’s) such as processors and memory chips. These packages have a great advantage over conventional design techniques, with hand drawing or breadboarding, in that the circuit can be tested in different ways. Current flows can be analysed, and connections automatically checked. Very complex calculations can be performed such as thermal analysis and RF interference created and absorbed.

Simulation PRogram with Integrated Circuit Emphasis (SPICE) is a suite of tools used to analyse the characteristics of a circuit. Originally released in 1973 as open source software (freely editable source code). SPICE can determine the characteristics of a circuit in terms of AC and DC electrical signals. This is something very useful for complex power and audio circuits, such as amplifiers.

CAD modeling

The aforementioned types of CAD employ modeling to determine certain things about whatever is being designed. SPICE is a type of modeling, or simulation. Computer simulations can be exceedingly useful.

Fig 6. shows how a beam can be simulated to show internal stress. This can provide engineers with detailed information about where high stress points are likely to be, and so allow the addition of supports and braces where they are needed in the early stages of design, when it is easiest and cheapest.

Computer Aided Manufacture - CAM

CAM allows for very complex shapes to be created automatically, accurately, repeatably, quickly and without much if any human interaction. In this way blank material can be automatically loaded into a CAM machine, machined to its final properties, and transferred into a receptacle completely automatically. In automated factories like this the lights are even switched off to save power as they are not required!

CAM machines are controlled in way called Computer Numerical Control. The various movements of the machining operations are controlled by stepper motors, which have accurate and powerful control over a number of ‘steps’ per revolution of the motor. In this way a milling head in a milling center can accurately be moved from one known position to another with a lot of force, therefore accurate cuts can be made quickly and through tough materials, provided the cutting surface is lubricated.

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Fig 5. A circuit board for data logging designed with CadSoft’s EAGLE package

Fig 6. SolidWorks modeling of a the forces in a beam with uniformly distributed load applied and central point load

Example use of G code CNC milling

Fig 8. shows a block of wood that I have machined a pattern into the top of using a CNC milling machine. The G codes are attached in the appendix.

The G codes used are explained in table 1. These G codes are based of the Fanuc system. Fanuc was originally a section of the Fujitsu company, the Computing Control Division became Fujitsu Fanuc in 1972(3). Fanuc have been the leaders of Direct Numerical Control (DNC) as well as CNC. They developed the first of each system. The Fanuc series of G codes are the widest used.

G codes themselves are essientally a very simple programming language, without any variable, loops, sequences or other operations similar to any other programming language. They perform a simple ‘step by step’ program, which is exactly what a CNC machine requires.

T9 Selects tool 9, as a multi-head milling machine was used

M6 Used to index the head against the 0 point

G43 Sets the tool offset height, to account for the length of the bit, the value is stored in the machine

M9 Sets the coolant not to flow

G54 Used again to tell the machine where the work is in relation to the 0 point

G90 Absolute programming. All further movement coordinates given will be in absolute relative position

M03 Starts the spindle spinning in clockwise direction

G0 Moves at fastest possible speed to the given coordinates

G01 Moves the tool head in a linear path to the given coordinates. The F number the follows is the feed rate, in X Y movement 300 in this case, and 200 for Z movement. Units are mm/s

M05 Stops the spindle

M30 Ends the program

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Fig 8. A block of wood with a pattern machined using a CNC milling machine

Rapid prototyping

Rapid prototyping is a set of processes for rapidly creating a model from a 3D computer drawing. 3D printing is the most common of them, which is a means of ‘zero loss’, or additive production. This means that instead of the product being cut out of a block of material, the material itself is laid down to form the product, and so no material is cut away or wasted, hence ‘zero loss’. One of the advantages of this is that an incredibly complex shape can be constructed with hollow portions and other parts that would essentially be impossible to machine, and would have to be cast out of metal.

The material is layered down in slices of the 3D model. This has the disadvantage that each of these steps becomes a visible line of resolution, and so a curve must be made of multiple steps resulting in an imperfect curve.

The resolution 3D printers is typically less than 100 micrometers (0.1 mm) even for the lower end machines. However the Z axis resolution is sometimes greater than this, sometimes as low as 0.5 mm for the cheap kit cheap 3D printers, leading to large steps. In practice the Z axis resolution is usually decreased in software to give an even courser step, even on the higher end 3D printers. The reason for this is that it would take many hours to complete a relatively small part even at the software reduced resolutions. If a part is required to be very accurate and time is not an easy then the machines can be set to run at full resolutions, but with quite a time penalty.

Stereo Lithography - SLA

Stereo lithography uses a laser beam to cure a liquid resin into a solid, in a process similar to 3D printing. The bed is raised up to the surface, or just below it. The laser beam is pulsed on and off in the correct areas as the head passes over the top of the tank of liquid resin. Each layer is built up by the head passing back and forth building up joining solid material. When a full layer is complete the bed is lowered by one step. A blade like wiper then passes over the fresh layer coating it in fresh liquid. The laser head begins another pass building up another solid layer on top, see fig 10.

STA has a very high strength and can be produced quicker than 3D printing. The resolution is also considerably finer than that of 3D printing typically 0.01 mm in the Z axis. The costs involved however are a lot more. The liquid resin can cost around £150 per litre. The cost of the machines is enormous and as such parts to be made by SLA are contracted out of house usually.

Applications of rapid prototyping

Rapid prototyped parts can be used as a final product, usually after finishing and any support structures removed. However they are usually used as design prototypes to test fit and check alignment. It is invaluable to hold a product prototype and manually figure out locations of components before it is sent for mass production.

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Fig 9. A 3D model is translated into a series of steps to be 3D printed

Fig. 10

Conclusions

CAD packages have great possibilities for engineers. They allow for complex designs to completed relatively quickly and to incredibly high levels of detail and accuracy. Once a design is created in CAD it is extremely useful to be able to model it. If it is an object that has parts that are going to be stressed and strained by normal operation then those forces can be modeled and displayed in highly detailed ways. If we take for instance a battery box which is to be bolted onto a product. The areas through the bolts will pass will undergo stresses which could be quite high. By modeling these areas in CAD it can be shown if the corners where the bolts pass are close to breaking, will break, or will be perfectly fine. Until the factor of safety is truly known the component could fail at any second.

Modeling is not just limited to the world of engineers. 3D video production using CAD at the heart of the creations. Indeed, some objects may need to be drawn accurately by engineers to get the look, textures, and even physics correct. This is the same with games and other 3D designed parts.

When engineers are designing products and components multiple revisions and gone through. There are always going to be faults, bugs, and improvements. This can be a costly process, especially if the product is released to the public before the revisions are made. Wooden mockups used to be built for this purpose, or one of prototypes made. Each of these methods has disadvantages however. Wooden mockups are generally bulky, heavy, and not functional. One of functional prototypes are good, however to make a single product that is designed for mass production, such as injection forming, can cost many thousands of pounds.

Rapid prototyping allows a partially functional model to be created quickly, and relatively inexpensively, especially for larger companies than can justify the cost of the rapid prototyping machines. Having a prototype in your hands to test fit and play with is invaluable, and can save huge amounts of time and money down the road, by making revisions early when the cost is only that of another 3D printed prototype.

Recommendations

It is recommended that whenever any product is being developed that it first be rapid prototyped. The risk of not doing so is too great too ignore, as third party companies can make these models inexpensively and courier the part within a few days.

3D printing is the cheapest rapid prototyping method, and yields good results for mockups and even semi-functional prototypes. The strength of these materials is quite high, and can withstand being machined, ground, and polished if a fine surface finish is required.

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References

1. Iain Macleod Associates. (2009). BS 308 and BS 8888. Available: http://www.g-tol.co.uk/in1.htm. Last accessed 19/11/12.

2. CADAZZ. (2004). CAD software - history of CAD CAM. Available: http://www.cadazz.com/cad-software-Sketchpad.htm. Last accessed 13/11/12.

3. Fanuc. (2011). FANUC’s History. Available: http://www.fanuc.co.jp/en/profile/history/index.html Last accessed 19/11/12.

Figures

1. http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-574.pdf Figure 1.22. http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-574.pdf Figure 4.23. 3D rendering of different types of objects using AutoCAD4. 2D representation of a 3D object, created using AutoCAD5. A circuit board for data logging designed with CadSoft’s EAGLE

package6. http://yuji.files.wordpress.com/2011/03/solidworks.png7. http://www.reuk.co.uk/OtherImages/a-small-stepper-motor.jpg8. A block of wood with a pattern machined using a CNC milling machine9. http://upload.wikimedia.org/wikipedia/commons/9/9c/Rapid_prototyping_slicing.jpg10. http://upload.wikimedia.org/wikipedia/commons/1/1e/Stereolithography_apparatus.jpg

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Appendix

T9 M6G43 M9G54G90M03 S4000G0 X0 Y0 Z10G01 F300 X20 Y10G01 F200 Z-1.5G01 F300 X20 Y5G01 F300 X25 Y10G01 F300 X30 Y5G01 F300 X30 Y2G01 F200 Z1.5G01 F300 X50 Y20G01 F200 Z-1.5G01 F300 X45 Y20G01 F300 X40 Y25G01 F300 X45 Y30G01 F300 X50 Y30G01 F200 Z1.5G01 F300 X30 Y50G01 F200 Z-1.5G01 F300 X30 Y45G01 F300 X25 Y40G01 F300 X20 Y45G01 F300 X20 Y50G01 F200 Z1.5G01 F300 X2 Y30G01 F200 Z-1.5G01 F300 X5 Y20G01 F300 X10 Y25G01 F300 X5 Y20G01 F300 X2 Y20G01 F200 Z1.5G01 F300 X10 Y20G01 F200 Z-1.5G01 F300 X10 Y10G01 F300 X20 Y10G01 F300 X20 Y15G01 F300 X25 Y20G01 F300 X30 Y15G01 F300 X30 Y10G01 F300 X40 Y10G01 F300 X40 Y20G01 F300 X35 Y20G01 F300 X30 Y25G01 F300 X35 Y30G01 F300 X40 Y30

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G01 F300 X40 Y40G01 F300 X30 Y40G01 F300 X30 Y35G01 F300 X25 Y30G01 F300 X20 Y35G01 F300 X20 Y40G01 F300 X10 Y40G01 F300 X10 Y30G01 F300 X15 Y30G01 F300 X20 Y25G01 F300 X15 Y20G01 F300 X10 Y20G01 F200 Z1.5G0 X0 Y0 Z40M05M30

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