How to Select a CNC Machine

Here are some questions to investigate prior to making a buying decision.

1.  Is the machine cast iron, aluminum, or polymer composite? Cast iron construction offers a higher level of rigidity and longer wear, but is heavy.  Will you move the machine around a lot?  If you will, consider aluminum, it is lighter and almost as rigid. The polymer composites are light, also. 

2.    Does it use industry standard ISO G&M codes? 
Fanuc^® is currently industry standard in the US and many parts of the world. 

3.    Stepper or servos, what's the difference? 
The axis motor drive types on the market are called stepper and servos.  Servos are more accurate than steppers and cost much more.  The true servo system strength is that the system checks its position at each move against an independent measuring device, such as a glass scale.  This is a closed loop system. Steppers are open loop systems executing a chain of commands without checking their position against an independent device.  There is no question that servos are more accurate, however, steppers could be adequate, it depends on how repeatable and accurate your final product needs to be. 

4.    Does it provide Unlimited Program Lengths through drip feed capability?
Precision machining may call for more complicated, longer programs.  Drip feed allows longer programs to be run. 

5.    How big is the work envelope? 

This is the total area that the mill can possibly cut.  Perhaps a more accurate definition could be the largest possible part that could be cut.  Is it big enough to accommodate the work you envision?  Many small CNC machines boast Y-axis travels of 4+ inches, when in reality it is much less if a vice or stock over 2" high is used.  If you want to use clamps, t-nuts, vices, fixtures, vacuum tables, etc., make certain they fit in the work area. 

6.    What is the axis feed rate? 

Feed rate is how fast a machine can move while cutting stock. High feed rates might be crucial to the success of your program, as the production schedule usually must fit into a 50 minute class period.  For example, the Techno DaVinci's maximum machining feed rate is 140 IPM (inches per minute), while some small CNC mill's maximum machining feed rates are in the 16- 30 IPM range.  You need to determine how long it will take to mill the pieces you plan to make.  If a CO₂ car body takes 15-20 minutes to machine at 80 IPM;  at 16 IPM, one car could take well over a class period to complete.  

7.    How about spindle speed? 

For nonferrous metals, wood plastics, and prototype material, high spindle speeds are recommended.  Without high spindle speeds on soft materials, the flutes on the endmills will load up with stock and ruin the part.  The only way to avoid gumming up the cutters in soft materials at low spindle rpm is to lower the feed rate.  Is that a problem?  See #6 above to determine if it is.