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CAD/CAM stands for (C)omputer (A)ided (D)esign / (C)omputer (A)ssisted (M)achining. It's the use of computers to assist in the design of objects in real space. Tasks that normally would take hours to do by hand with paper, pencil, and a calculator, can be done with the computer in a matter of seconds.
Let's look at the CAD portion a little closer before we move on to the CAM section. Computer assisted design is nothing new, companies have been using it since computers were still the size of a room. It was originally employed to solve three issues:

1. Increase the precision of drawings.
2. Reduce the man hours involved with design.
3. Reduce the cost of drawing blue prints.

The first CAD software developed was quite primitive, mainly used for 2D drawings. It was capable of drawing lines, some circles, and had the ability to resize lines. Today, CAD software had reached a much higher level of complexity. We can now work in 3D with lines, arcs, splines, and many types of surfaces (we'll look more at what this means later). A few examples of CAD software on the market are: AutoCad, AlphaCam, and MasterCam.

CAM (Computer Assisted Machining) is a relatively new concept to many manufacturing companies, due to the fact it was only made affordable 10 years ago. However, CAM is not only used in manufacturing. You may see CAM in use at auto mechanics and even food processing plants. CAM is a very valuable asset because it allows for few mistakes to occur, almost eliminating human error, altogether. Larrivée has been using CAM to assist its production in places where precision is a key factor, and where mistakes had most often occurred. Another valuable use of CAM is the safety aspect. In jobs where bodily risk is an issue, having a computer on your side can be a big asset. Here at Larrivée, we use CAD/CAM to free up human intelligence so that it can be used in more challenging jobs. Allow the machine to do the repetitive tasks. For example, the machine will cut fingerboards, cut inlay pieces, and cut kurfing. The people, on the other hand, will do the challenging jobs like buffing a guitar, fitting a neck to a guitar body, spraying the paint, and binding the body.

So how does CAD work?

Computer assisted design involves a lot of the basic geometric principles that you learned in high school math (and you said you'd never need it!). Today, designing CAD drawings requires you to know how to calculate angles, do some basic math, and, most importantly, think logically. Below, you can see a screen-shot of a CAD file.


What you see here is one of the guitar necks that is machined daily at Larrivée. First, we had to find a perfect neck, one that appeased both professional players and beginners. Next, that neck is digitized in three dimensions (using a laser probe), and the raw digitized neck data is brought back to the factory so we can begin the process of replicating it.


As you can see in this close-up view of the neck heel, it is made up of many lines and arcs. In CAD, a line is a two point "entity" consisting of a start and end point. Pretty basic, right? Now we move on to Arcs. Arcs make up the majority of a complicated object. An arc is made up of a three factors: a radius, a start angle, and a stop angle. So, if you wanted to create a half circle with a 5" radius, you could tell the machine to make an arc with a 5" radius with a start point of "0" degrees and an end point of "180" degrees. Seem a little complicated for a simple half circle? Well, that's where the computer comes in. It can find the start points and end points for you, it can use three points to determine the radius on the fly for you, you can take a single point and create circles around it, and so on. The options are almost limitless. This neck was the product of about 400 hours of programming and about 50 hours of tweaking to get it just right.

Before any project you design can be made into a real object, the geometrical drawings must be laid out for it. Here at Larrivée, we have CAD drawings for many pieces of our instruments: the bridge, fingerboard, kerfing, rosette, some inlays and more.

 

Ok, I've designed an item, what's next?

Now that we have the design worked out in CAD, it's time to make it into something real. That's where CAM and NC come into place. You can break down the entire process into three steps. The first is the design of the geometry (CAD), the second is the design of a tool path (CAM), and the third is the machining of an item (NC).

After the lines of a project are laid out in CAD, the next step is to tell it how to cut the object using those lines. Some of the factors you need to consider when creating the CAM portion of a project are:

  1. What tools are most suitable to use?
  2. What method should I use to machine it?
    - Contouring
    - Pocketing
    - Drilling
    - Surface Machining?
  3. What material am I using to make the item?
  4. At what speed should I cut ? (feed rate)
  5. Do I need to make rough cuts?

All of this information (and many other questions) is very critical when deciding how to machine something. The difference between entering a material from the left or the right can mean the difference between a piece of wood flying ten feet out of a machine later on. Below you can see a picture of what a tool path might look like. It is a picture of our fingerboard cutting program. You can see the lines are attached vertically, and each line spans 6 fingerboards. We increase the efficiency by machining six fingerboards at once.

And, below here, you can see the final machined product. Remember that the drawing above is a top view, and the one below is a front/top view.

As you may have guessed, CAM is where most error occurs because it has the most room for human error. The program you are designing now is going to be cut on a machine worth $100,000, you cannot afford to have a single error. Once the tool path is known and laid out, we can move on to the final stage.. NC.

NC? What the Heck is NC?

NC, or Numeric Control, is a technique for controlling machines. It is a process that uses coded command instructions. The NC Controller [in the CNC machine] interprets these instructions and then converts them into two types of control signals: Motion Control Signals and Miscellaneous Control Signals.

Motion Control Signals are a series of electrical pulse trains that are used to control the position and the speed of the machine table and spindle. Each pulse activates a motion of one "basic length-unit" (BLU) which is the minimum increment size of the NC Control system. The typical increment for one BLU in the old generation of NC Controls is 1/1,000ths of an inch, while today the standard is 1/10,000ths of an inch. Miscellaneous Control Signals control items such as how fast the machine travels, whether it should accelerate or decelerate, and how fast the spindle should spin.

Our CNC machines have the programs uploaded to them by Dummy-Terminal through what's called a Null modem connection. We then transmit the files from the dummy computer to the CNC machine using two protocols. A straight ASCII (or plain text) transfer, and a more advanced method called X-Modem. The program sent to the CNC machine is no longer a drawing like the ones you've seen above. The Final NC file is a series of numbered lines like the ones here.


 

The lines in this case are sequentially numbered in the format "N--". This tells the machine the order of the tasks. What I'll do here is explain a line or two to you so you can get an idea of what happens. If you look at line number "N8" you see the following:

N8M6T2(Tool 2 is 0.0394 bur)

If we break this into sections, it becomes a little more clear. The N8 is the line number. Next, you have two joint commands, M6T2. The command M6 is the command to tell the machine to change tools, T2 is the tool number it is changing to. CNC Machines are capable of storing multiple tools, and certain machines are capable of changing tools in the middle of a program. That way, you can do complete operations away from the machine.

If we look at another line, N15, you should read the following:

N15G1Z-0.065F10.

This command is, again, more easily explained when you break it up into parts. The N15 is the line number, next the G1 is telling the machine to move in slow feed mode. The Z-0.065 is telling the machine to move the Z Axis to -0.065 in real space, and the F10. is telling the machine to move at a feedrate of 10 inches per minute.

That's a basic introduction to Cad/Cam for you. Feel free to use it for personal use but let me know by mailing letters@larrivee.com. The pictures are all copyrighted to Larrivée but, if you want to use them, please send an e-mail to jeannette@larrivee.com and we'll authorize use depending on the requested usage.

 

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