Instructables

Step 2: CNC Router Design

One of the aspects of any home built CNC machine is the use of each material in the construction of the machine vs the quantity of that material you have to buy. You are only building one machine so you don't want have to buy more material than you need to build that machine. You especially need to consider this when deciding the length of travel you want for each axis, because this decision effects almost every other part of the machine. This was the general design process I went through for my CNC machine
  1. Decide what length of travel you need for each axis (if you have a specific project in mind for your cnc then start with it's sizes requirements)
  2. Decide what type of linear motion system you will use for the machine
  3. Decide what kind of linear drive you will use for each axis
  4. Decide what type of drive motor and controller you will use
  5. Decide the material you will use to construct the machine
  6. Based on the previous decisions, design a machine on paper or a CAD software of you choice (this does not have to be a complete design, just enough so you know the total quantity of the materials you'll need)
  7. Determine if you will need any special tools for your design
  8. Determine the overall cost of your design, which includes the cost of tools you may not have
  9. Decide that you can't spend that much money on the machine and return to step 1
I went through this process 5 times before coming to a final design.  The pictures show the different versions of the router as my design progressed. I know most people would consider this to be overkill but for me doing all this important. I knew that once I finished actually building the machine I would have something that fit my needs and my budget without any headaches do to poor planning.

Here is my thinking for each one of the design steps I outlined:
  1. Travel: My first thought for a CNC machine was to build molds for the vacuum forming machine I have already built. So I decided to build the machine with roughly 12”x24”x6" of travel because that how big the forming platen is on my vacuum forming machine.

  2. Linear Motion: There are many options to choose from for linear motion. Commonly used methods for CNC routers include, drawer slides, skate bearings, v-groove bearings, round linear rail and profile linear rail. These are ordered in terms of cost, I would recommend going the best system you can afford. You can save some money in other areas of the machine but getting a good motion system will pay off in cutting quality. I chose to use round linear rail. This system uses precision ground and hardened steel shafts and linear bearings that use small steel balls that roll on the shaft and re-circulate through channels within the bearing. This offers smooth low friction movement and has good resistance to forces placed on the bearing in any direction. There are many different manufactures of these types of rails and bearings and costs can vary quite a bit. I got my rails and bearings from a reseller in China on ebay. The ebay store is linearmotionbearings and the prices were the best I found online. They often sells kits with three sets of rails and two bearings for each rail, which is what is needed for a 3-axis CNC. The kit I got uses 20mm x 800mm long rails for the x-axis, 16mm x 500mm long rails for the y-axis and 12mm x 300mm long rails for the z-axis. This kit cost me $223 dollars shipped.

  3. Linear Drive: The three basic options to drive each axis of a CNC router are ribbed belts, screws, and a rack and pinion. The most common on DIY CNC routers are ACME screws, ball screws and rack and pinion setups. Screw drive systems work by attaching a nut to the movable part of each axis, a threaded rod is then fed through the nut and locked into position at both ends. The screw is turned by the drive motors and the nut moves along the screw. ACME screws have trapezoidal threads that are either cut or rolled into a steel rod. ACME screw threads are used on common C-clamps. Their thread shape makes the screw stronger than the threads on standard bolts. When these threads are precision cut they are perfectly suited to drive a CNC router. Probably the most common and cheapest ACME thread size is 1/2"-10. That means1/2” in diameter and 10 threads per inch. Ten threads per inch means that if the screw in spun around 10 times the attached nut will move 1 inch along the screw. For any screw size multiple individual threads can be cut on the screw, this is referred to as the number of starts the screw has. A single start screw has one thread a 2-start has two threads and a 5-start has five threads. What is the significance of multiple threads on a screw? Well there are two things that make multiple start screws better for CNC machines. First multiple start screws are more efficient at turning the rotational force on the screw into linear force on the nut. This means it takes less torque for the drive motors to move each axis. Second, multiple start screws increase the lead of the screw, which is how far a nut would move if the screw was rotated once. To determine the lead for a screw divide the number of starts by the number of threads per inch. For example, a 1/2”-10, 5 start, ACME screw would have a 5/10 or 1/2” lead. This means for every rotation of the screw the nut moves 1/2”. This is important because the electric drive motor can produce the most torque at low speeds, and with a higher lead the nut will move farther per revolution of the screw and that means the motor can spin at a lower speed to move the axis of the machine. For my machine I chose to use a 1/2”-10, 5 start, precision ACME screw from Mcmaster Carr for all 3 axis.
Another important thing to note is how precise the fit between the nut and the screw is. A standard nut on a bolt will wiggle a small amount back and forth and in CNC terms this is known as backlash. You want to reduce the amount of backlash you have between the nut and the screw because every time the screw changes rotation direction that small amount of play in nut will throw of your CNC position off and your parts might not come out correctly sized. There are ways with both hardware and the software you use to reduce the amount of backlash you have. On the software side there are simple settings that can compensate for backlash and on the hardware side you can use an anti-backlash nut. I purchased  anti-backlash nuts from dumpsterCNC  and again you can find part numbers on the parts list. Typically the effects of backlash can be reduced to the point that parts can be made to within a few thousands of an inch.

  1. Drive Motor: For CNC routers two basic options exist, stepper motors or servo motors. Stepper motors are used in the vast majority of DIY CNC routers. CNCroutersource has some excellent information comparing these two types of motors. The key difference in these motors is servo motors provide position feedback to ensure proper positioning while stepper motors do not. I chose to use stepper motors for my machine mainly due to cost. Servo motors are more expensive and require more expensive controllers then comparable stepper motors for the sizes that are commonly used on CNC routers. Also stepper motors are highly supported in the DIY router community and are available from many different retailers. When looking in to stepper motors and controllers I found many options and price ranges from less than $100 to more than $500. When deciding what to get for my machine I came to the conclusion that these systems are so universal that I could use my controller and even steppers for other CNC projects in the future. Knowing that I wanted to get good performance and long term reliability I decided to go with American made components from Gecko. I purchased a Gecko G540 stepper controller which can control up to 4 stepper motors at once and connects to a computer through a parallel port. I also purchased 4 280oz-in, NEMA 23 stepper motors from Gecko which are also made in America. The control software I decided to use is called Mach3 and it uses a computer's parallel port to send signals to the G540 which controls the stepper motors. Mach3 CNC control software can be downloaded and used for free, but is limited until you buy the software for $150. Mach3 is probably the most widely used software for DIY CNC machines and is well supported.

  2. Construction Material: Most DIY CNC routers are built using either MDF, aluminum extrusion, or steel. MDF can be easy to work with and cheap to buy and many first time builders use this material. Slotted aluminum extrusion, commonly from a company called 80/20, is used on many DIY CNC router design plans available on the internet. It offers many design options due to the large amount on mounting brackets and configurations the slotted design allows. Aluminum extrusion would also be the most expensive of the three methods I listed. Steel is also used to construct many DIY routers. Square tubing, angle, and flat stock are common and can usually be locally sourced. In most cases steel machines are welded together so a welder and the ability to weld are necessary. Steel is generally going to be less expensive per foot than aluminum extrusion. I chose to use 1”x2”x0.065” steel tubing to construct my CNC router. I was able to purchase a single 24ft piece from a local steel supplier, Industrial Tube and Steel. They even cut it in half so I could load it in my car. If you don’t have a local steel supplier I would suggest looking at speedymetals, I have purchased from them before and they have good prices and deliver fast. I have experience welding and a flux core welder, which is similar to MIG welder but doesn’t require shielding gas. If you want to get more information about welding take a look at this great instructable from Phil B, Learning to Weld. Using steel also requires the use of metal working tools. I used a small horizontal band saw to cut the tubing and a small bench top drill press to drill holes. I have included a few tips about working with metal and some tools that make life a lot easier in this instrucable.

  3. Design: You can use what ever software you are comfortable with when designing the machine. You could even just draw your machine on paper.  123D from Autodesk and SketchUp from Google are both free 3D modeling software programs you could use.  Many of the parts I used on this machine came from McMaster-Carr.  Their website provides drawings for many of the items they sell including 3D models which can be downlaoded for free. 

  4. Tools: I used a number of tools to build my CNC machine and they are listed on the Tools step. Some of the tools are specific to working with metal and are essential to getting the best results. I also made a few of my own tools to make building this machine much easier.

  5. Cost: I estimated my cost for the complete machine and electronics around $1500.

You now know my decisions and hopefully understand my reasoning. I think I have a pretty good combination of parts that has exceeded my expectations. If you decide to build a machine based on my plans I have everything laid out in the following steps.

Great layout!! 1500.00 is good price if it was backed product...warranty..etc. DIY'ER are like the craigslisters....we want it all for little to nothing. If you get busy selling in Dallas area...lemme know if I can help!!

Dan