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Linear Drives

March 29th, 2012 | Posted by Joshua in Hardware

CNC machines require a way to transmit the power made from the actuators to drive its linear axes. The most popular linear drives used for CNC machines are: Leadscrews, ball screws, rack and pinion and belt drive. Each of these linear drive components has their own pro’s and con’s and I’ll be highlighting them, along with what machine they would best suit.

Leadscrews

Leadscrews are probably one of the more commonly used linear drive system on DIY CNC machines. There are some important things to know about leadscrews in order to understand how they work and how they are different to a plain threaded rod you might see at your local hardware store.

I cover leadscrews in a separate post that you can find here. I advice reading through it, even if you don’t intend on using leadscrews on your CNC machine. It always helps to have a broad range of knowledge.

Pro’s:

  • Good power transfer
  • Good speed, better with multiple start screws
  • Relatively inexpensive

Con’s:

  • Friction and wear of the nut introducing backlash
  • Over long lengths the screw can whip and resonate

 

 

Ball Screws

Ball screws are very similar to leadscrews in the way that they resemble a threaded rod.

Ball screws however, instead of having threads have ball races. These ball races are like a channel that spirals down the length of the screw.

These ball races fit ball bearings, which are a very close fit to the size of the ball race channels.

 

The ball bearings fit inside something called a ball nut. This ball nut allows the bearings to ride inside the ball screws ball race and after a number of turns around the ball screw, recycles them to start the journey all over again.

As opposed to a leadscrews nut which rides on the threads directly causing friction and wear, the ball bearings roll along the ball race, separating the nut from the screw – hugely reducing friction and wear. The ball nut is packed with grease which allows for even smoother movement.

 

Very smooth movement is what ball screws do. The ball nut has so little friction with the ball screw that the nut will sometimes fall under gravity. Despite this lack of friction, the ball screw still maintains great anti-backlash properties and will provide great precision.

Ball screws just like leadscrews support different major and minor diameters, pitches, leads and multiple starts. If you’re not sure what these terms are, read my post on leadscrews here.

Pro’s:

  • Very little friction and wear
  • Great anti-backlash properties
  • Offer incredible speed while maintaining good power transfer

Con’s:

  • Expensive, anything that’s good usually ain’t cheap!
  • Ball nuts must always be greased and seals replaced when worn (minor problem)
  • So friction free that counter weight systems may need to be integrated to keep heavy axes from failing when machine is powered off
  • Over extreme lengths screw can whip and resonate. Problem goes away with thicker diameter screw

 

 

Rack and Pinion

Rack and Pinion is another great linear drive system for CNC machines. First thing to note about R&P (Rack and Pinion) is its capacity for amazing speed.

With great speed comes the trade-off that it isn’t as sturdy and effective at power transmission as an equally price ranged leadscrew or ball screw.

Don’t read into that statement too much, they still suit CNC machines very well. They particularly suit CNC routers extremely well. A CNC router, unlike a milling machine won’t require huge amounts of strength and sturdiness to operate. CNC routers can do away with some of that strength and power and trade it in for speed, especially if it is cutting primarily wood, plastics and other light materials.

R&P fits to a CNC router by running a rack of teeth down the length of an axis. Now a R&P drive comes in, this drive will have the pinion attached and will pull or push that pinion into the rack so the two mesh together tightly. It will do this push or pulling usually with a spring under tension from a turn buckle. The drive will also usually gear down the pinion from the drive actuator (Stepper or Servo) to trade some speed for additional torque.

Pro’s:

  • Can provide great speed
  • Easy to maintain – Spring keeps constant tension and automatically adjusts itself to an extent
  • Huge speed will allow quicker rapids of the CNC machine, overall speeding up cutting time – More productivity

Con’s:

  • Trade-off some strength and sturdiness for speed
  • For axes with two contact points like the Y axis on a CNC router, two racks, two R&P drives and two actuators will be required

 

 

Belt Drive

Belt drive shares the same speed advantages as Rack and Pinion, and the same negatives. Everything is a trade-off, with belt drive we’re gaining a lot of speed but loosing strength and power. But this is perfect for different types of CNC machines.

CNC machines such as plasma cutters, laser cutters, pick and place machines and 3D printers all require much less torque and power than a router or mill. This is because they won’t have the same forces at play as a CNC router cutting wood, or milling machine cutting metal.

A plasma torch fly’s over the material, force isn’t transferred from the plasma beam to the machine. Same with laser cutters, pick and place machines and 3D printers.

 

The goal then is to have a sensible amount torque and power that suits the machine well and trade the rest of the potential torque off for speed. Speed in these CNC machines is more important than torque, the faster they can go the better because they don’t need to worry about increased forces and resistance at higher speeds.

A belt drive system common in CNC machines will use a special type of belt, called a timing belt. This belt has teeth, which are all precisely and equally spaced. The teeth on the belt mesh with a pulley that has the same teeth, with the same spacing embedded into it. The timing belts are usually made from flexible rubber with lengths of tensioning materials running through the length of the belt such as Kevlar.

Pro’s:

  • Offer extreme speed capabilities
  • Relatively low maintenance, other than occasional tightening of belts
  • Precision movement
  • Fairly inexpensive

Con’s:

  • Trade off strength and sturdiness for speed
  • Over long lengths belt can whip and cause resonance

 

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