Jeff Albro: The CNC Toolchain

The CNC Toolchain

CNC Machining is becoming more and more common for the hobbyist. Garage and basement machinists getting started don't always know how you go from idea to finished part. In my search to select a system for my own use I have done considerable research. Here is my understanding of the tool-chain:

CAD - Computer Aided Design... you draw the part on your computer, either in 2D or 3D.

CAM - Computer Aided Machining.... you use a program to generate a command list (G code) to make your machine cut the part.

Control Software - You use software to interpret this command list (G code) to a series of motor movements.

Break Out/Control Board - this connects the parallel port, USB port, or Ethernet Port to the individual motor drivers. They usually perform optical isolation, meaning if the drivers get miswired or zapped the computer won't lose it's "Magic Smoke".

Motor Drivers - these interpret the speed, step, or direction commands and actually turn the motors.

Motors - These motors turn the screws or pinions

Mechanicals - You need to convert rotary motion to linear motion. There are a few different ways of doing this with various advantages.

Machine - The linear motion pushes on a machine to move a cutter through the material. These can be milling machines, routers, plasma tables.... anything that can cut a material.

All these tools come together to create the finished part.

Now, some companies sell packages that will do the whole thing. Shopbot Tools makes CNC routers that come with everything you need. Most hobbyists working on mills tend to assemble their systems themselves. Here are some of the most common ways to traverse the tool-chain. I've picked tools from established companies commonly used by the hobbyist.

CAD

AutoCAD Lt - a simpler, cheaper version of AutoCAD.
TurboCAD - good value for the money, both 2D and limited 3D.
RhinoCAD - A very good 3D program. You can get it for about $600 from ComputerSculpture.com Handles complex surfaces well.
Alibre - A 3D parametric program, with a free version available. Parametrics make geometric parts much easier to edit later on as you can link a whole bunch of features to other features.
BobCAD - both CAD and CAM, 2D and 3D. Make sure to check around for a good price. Their sales force has a reputation for being pushy, but alot of people do good work with the software.

Almost all vendors offer trial versions. Download them and try a tutorial to see what you like.

CAM

BobCAD-CAM - Get used to the idea that many of the products you can buy can be used for more than one step in the toolchain. This can be a big source of confusion.
SheetCAM - a 2D tool path generator.
MeshCAM - a 3D tool path generator.
DeskCNC - a CAM and control combination. 2D and 3D
Vectric - Makes 2D software with an emphasis on graphics, as well as 3D software
MillWizard - a 3D toolpath generator by ArtCam
Hand Written G Code - G Code is the machine language to describe how a cutter should move to make a part. For simple programs and the fastest cut times, you can write a program by hand, skipping the CAD and CAM altogether.

CAM programs vary widely on how well they mange 3D parts.

Control Software

TurboCNC - A DOS based shareware program with a good reputation.
Mach (2/3) - Probably the most popular control software out there.
Linux EMC - A program that runs on Linux with a real-time kernel extension.
DeskCNC - Again, software that serves more than one function.

Break Out/Control Board

DeskCNC - DeskCNC also makes it's own hardware. I have their hardware and it works just fine. It currently only works with DeskCNC control software, which I am not fond of. Uses the serial port or USB port.
Campbell Designs A popular Break Out Board (BOB) that works with Mach 3. Uses the parallel port.
Gecko G-Rex A new product from Gecko, it takes care of the timing to allow the hardware to generate more steps per second than is possible with a parallel port.

There are many more options out there, but any board will be better than trying to solder wires from your driver boards to a parallel port plug.

Motor Drivers

Drivers are available in a wide range of quality and ease of wiring. Many of these companies sell individual components that need to be wired together as well as completely boxed up electrical systems with power supplies. Do not underestimate the complexity of the wiring job you will face if you assemble the components yourself. On the plus side, building it yourself allows you to control the quality and power of all the components as well as fix it yourself should a component fail.

Gecko - makes both servo and stepper drives. Their stepper drives are very popular and powerful. A key feature is mid range resonance damping which allows you to get much more power from the drives.
Xylotex - Stepper drivers and complete systems
HobbyCNC - low cost stepper drivers
Rutex - powerful servo drivers

Motors

Stepper Motors - These motors work by rotating in discrete steps. These are the most popular for hobby level machines due to low cost and simplicity. However, these drives have some disadvantages:
- If the motor isn't strong enough to move the machine, you will lose steps, meaning your cutter isn't where it should be.
- Stepper motors lose torque with increased speed. This means that gearing down to increase torque may not work at all.
- If there is no mid-band resonance dampening in the driver, the motors may not be able to achieve a fast jog speed without stalling.
Stepper motors come in various holding torque ratings and frame sizes. The most common ones are about 250 Oz-in and NEMA frame 23. Larger motors come in NEMA 34. You should get motors with dual shafts to make manual tuning and adjustment easier.
Servo Motors - These are more powerful motors, and combine optical encoders that tell the drivers if they aren't where they are supposed to be. More expensive and complex than stepper motors, but many people feel the extra complexity is worth it for more power and reliability. How well they are tuned makes a big difference in performance.

Mechanicals

Acme Screws - Everyone knows how a nut and bolt works... you rotate the bolt and the nut moves back and forth. Acme Screws are just like this with a thread profile made for motion. They can be highly accurate, but there is alot of friction in the system. Most systems have an adjustable nut to remove most backlash. Backlash is when you have reversed the direction of rotation of the screw, but the nut hasn't started moving, causing error. Again, it means the cutter isn't where it should be. The problem with anti-backlash acme nuts is they wear quickly and therefore need adjustment frequently. They are usually difficult to get to.
Ball Screws - In a ball screw, the nut has ball bearings in it, and the screw has spiral grooves that the balls roll in. Ball screw are highly efficient, meaning they run at higher speeds with smaller motors. Ball screw systems can't be used manually, as if you push on the table, it will move and back drive the screws. Backlash is still a problem, and you can "Pre-load" the ball nuts with precision balls, or use a two nut system to reduce the backlash. Ball screws make a big difference on how well a machine works for production.
Rack and Pinion - Rack and pinion systems are often used on larger CNC routers and plasma cutters. They are far less expensive and capable of moving faster over large distances than all but the most expensive ball screws. The accuracy is usually less than with screws.
Timing Belt - Timing belt systems are occasionally used in CNC routers. Accuracy and backlash are problems with these types of systems.

Machine

First of all, you have milling machines... Most of these are available as CNC or manual versions. Many companies make kits to convert Chinese and Taiwanese made manual machines to CNC machines. The main differences are stiffness (often related to weight) and size of work envelope (how big of a part you can make). Make sure you understand what kind of screws, thrust bearings, and backlash removal systems they come with.

Sherline - a small table top system
Taig - another small table top system, heavier than a Sherline
Sieg X1 - Sold by Harbor Freight and other companies. Conversion kits are available from CNCFusion and others. It is a very small mill.
Sieg X2 - Sold by Harbor Freight, Grizzly and other companies. Conversion kits are available from CNCFusion, KDNTool, Stirling Steele, and others. This is commonly called the "Mini-Mill"
Syil X2 - An X2 sold as a ready-to-go CNC machine by Syil America.
Sieg X3 - Sold by Harbor Freight, Grizzly and other companies. Conversion kits are available form CNCFusion, Syil America, KDNTool, and others. Quite a bit bigger than "Mini". This is the machine the author has his heart set on.
Syil X3 - An X3 sold as a ready-to-go CNC machine by Syil America.
Tormach PCNC - A dedicated CNC milling machine. Bigger than an X3 and three times as heavy. An excellent value. Ready to cut for hours on end.
Round Column Mill Drills - Available from Harbor Freight, Grizzly, and others. A pre-made conversion is available from Flashcut. Comparatively inexpensive, the round column means the quill must be driven, limiting range of movement and hurting accuracy.
Dovetail Column Mill Drills - Available from Harbor Freight, Grizzly, Lathemaster, and Industrial Hobbies. Conversion kits are available from Industrial Hobbies. Dovetail column machines are a better choice for CNC conversion as you can drive the whole head accurately over a large range of movement. These are also called bed mills.
Bridgeports - These are the old style heavy duty milling machines, also called knee mills. These are the largest machines usually converted by hobbyists. Conversion kits are common but expensive. The largest issue is the Z axis, as the knee is very heavy, and the quill more flexible than desired.

Then you have larger CNC routers and plasma tables. For these machines, you gain the ability to handle much larger size materials (4 foot by 8 foot for plywood is common) but you lose stiffness and accuracy. CNC routers can't handle steel, and need to be careful cutting aluminum. They are most commonly used on plywood and plastic.

Most CNC router manufacturers offer plasma tables as well. These use and electric torch to cut steel plate easily, but don't offer the accuracy of a milling machine.

Several set of plans are available to make your own CNC router from scratch.

CADCut - similar to a ShopBot in size and construction
Solsylva - a very simple, small cnc router.

Where to go from here? As you leave hobby level machines, you can gain large work spaces, high speed, high accuracy, and automatic tool changing. VMCs (vertical machining centers, a vertical milling machine with an enclosure, coolant system and tool changer) start around $30,000. Commercial CNC routers offer very high speed and accuracy for around the same price. Laser and waterjet systems allow you to quickly cut large slabs of material and cost even more.