Drive Options for CNC

Selecting Drive Components for CNC machines

When designing a CNC machine from the ground up one of the very first decisions is drive methodology.  It is worth taking a moment to think through the pros and cons of each option.  For the DIY hobbiest and the industrial machinist alike, there are four basic options for moving the axis of a CNC machine:

1. Ballscrews and Ballnut:  Ballscrews are an amazingly efficient mechanical device.   When I first laid hands on one of these and felt the smooth recirculating bearings and the zero backlash nature of the design I was truly mesmerized.  Truly ingenious!  Here is an exploded diagram of how they work.  Ballscrews are found in CNC mills, lathes and routers and are a very popular choice.
2. Belt and Timing Pulley:  Timing pulleys are a great solution for quick, inexpensive, low backlash setups.  In a timing pulley design, the motor turns a timing belt directly and a “carriage” attached to the belt moves the axis along its linear bearings.  Most inkjet printers use some version of this setup.
3. Rack and Pinion:  In longer length cnc machines, a rack and pinion setup can work well.  The basic idea here is that the rack (the long section of toothed track) follows the axis while the pinion (the gear) is turned by the motor which is connected to the axis needing motion.
4. Magnetic Linear Motors:  These motors work of the same principles of magnetic repulsion and attraction to move a precisely controlled electromagnet up and down a rail of permanent magnets.  The forces at work are incredibly powerful and precise.  These are the super precision, super precise, super expensive components.  

• Cost:  Generally speaking the drive systems of each axis will be one of the most expensive part of any CNC machine and the more precise, the more you pay.  For example, there are several grades of ballscrews with various costs associated with each of them.  The Ball Screw Accuracy Grade scale is divided into eight classes: Precision Grades – C0, C1, C2, C3, C4, C5, and the General Grades: C7 and C10.  Higher the number, lower the accuracy grade.  The accuracy of a ball screw is defined by three major parameters, namely: Lead accuracy, Mounting interface accuracy, and Preload-torque variation in percentage.   So a lot of the cost is determined by how accurate the components chosen.  On the other end of the spectrum timing belts and pulleys are relatively inexpensive.  By far the most expensive in this category is linear motors which can easily be $10,000 per axis or more.

Conclusion:  In descending order from most expensive to least expensive:  Linear motors, Ballscrews, Rack and Pinion, Timing Belts.

• Precision:  Again depending on the grade, precision will vary.  For example, extremely expensive and precise ballscrews can have tolerances within 5 microns and will even come with an accuracy map that reflects the error tolerance along the length of the entire screw.  By contrast “sloppy” ballscrews are just as common.  Ballscrew accuracy is a function of both pitch and manufacturing precision.  The most accurate in this category is again the linear motor which can obtain accuracies below 1 micron.  Rack and pinion as well as timing belts have similar accuracies.  Both of these options share the common problem that in order to obtain accuracy some sort of mechanical reduction must take place at the motor itself.

Conclusion:  In descending order from most precise to least precise:  Linear motors, Ballscrews, Rack and Pinion, Timing Belts.

• Acceleration:  This is mostly a function of how much mass a particular drive option has to accelerate.  For example, in a ballscrew there is a lot of mass to accelerate relative to some of the other drive options and as such, ballscrews perform sluggishly in this category.  On the other end of the spectrum, linear motors have very little mass to accelerate.  Also it is important to note that in most cases acceleration is a tradeoff to precision and power.  An important exception is linear motors.  Extremely fast acceleration can be paired with extremely precise tolerances.   This is kind of the holy grail of drive designs.

Conclusion:  In descending order from fastest to slowest acceleration:  Linear motors, Timing Belts, Rack and Pinion, Ballscrews

• Speed:   This refers to top speed of the drive option.  Ballscrews can move surprisingly quick, however, there is certainly an upper limit due to the friction and mechanical forces at play.  The more course the screw the quicker it will travel but this is at the expense of precision and power.  Magnetic linear motors are by far the fastest option here.  With no moving parts to slow things down, the theoretical upper limit is far faster than any linear bearing could ever handle.  Rack and pinion and timing belts can be lighting quick but will give up accuracy and precision to do so.  Linear motors are the clear front runner here.  After that, the options bleed together.

• Power: This refers to the amount of force a drive option can apply in a linear direction.  This is where ballscrews really shine.  The spiral incline plane is hard to beat in terms of raw force and this is the reason that ballscrews are used in powerful milling machines that require strong forces.  The tighter the pitch the stronger they become.  Because of the direct drive methodology of rack and pinion and timing belts, these are inherently weak unless geared down significantly with some sort of gearhead at the motor.  Linear motors can be very powerful but require tons of current to obtain these kinds of forces.

• Resistance to Debris:  This is an important category for dirty work environments.  Any of these drive options can be covered to prevent debris from entering so this evaluation will be done on an “uncovered” linear drive.  Probably the most bulletproof option here is the timing belt.  It is hard to foul one up with debris.  A close second is the linear motor, in fact, it would be the front-runner if it were not for the pesky problem that ferris metals cling to the rails!  In a non-ferrous metal or wood enviornment these would be impossible to kill as the debris simply falls through the rails with no disturbance at all, but any iron swarf is a nightmare for this solution.  Most ballscrews come with some sort of squeegee to wipe away debris before it enters the ballscrew; however, these can become clogged and that is a significant issue.

• Scaleable:  This refers to how easy or difficult it is to lengthen an axis using a particular drive method.  Ballscrews are generally expandable to several feet but then begin to experience limitations.  The longer they get the more heavy they become and suffer from a whipping motion (picture a jump rope).  Practical upper limits for ballscrews are somewhere in the 6-8 foot range.  Linear motors are expensive to manufacture and for this reason have a similar upper end but not because of any mechanical limitations.  Timing belts can go a very long way but will suffer from belt drooping and at that point, backlash begins to enter the equation.  Rack and pinion has a virtual infinite scale-ability.

• Life Cycle:  Virtually any drive option has a life expectancy.  The wear and tear of use will require periodic maintenance and eventual replacement.  Some options wear sooner than others.  Rack and pinion probably wears the quickest due to the metal on metal, poor lubrication nature of the setup.  Ballscrews typically wear unevenly since the centers of the screw are used more frequently than the extremities.  Linear motors have no wear and tear since there are no moving parts.  These have life cycles that exceed yours!