After you determine the fundamental type and size of the CNC machine you want to build, your next step is to choose linear bearings. Your choice of linear bearings is one of the most important decisions you’ll need to make. Linear bearings are the mechanical devices that allow parts of the CNC machine to slide with little friction, and little backlash. Bearings can be as simple and cheap as simple drawer slides from a hardware store, or bearings can be complex and expensive pillow blocks gliding on stainless steel rods.
Types of Linear Bearings
This is the generally the simplest and cheapest type of linear bearing you can buy, and it’s readily available at any Home Depot or Lowes hardware store. Some websites call these telescoping guides and jack up the price, so watch out for that. The one shown here is a Knape & Vogt 24″ 100 pound drawer slide. Note that the 24″ refers to the length before becoming extended.
- Readily available.
- Low amounts of give and backlash.
- The full extended length can NOT be used for CNC machines, because the drawer slides aren’t nearly stable enough to do even basic CNC work. For example, if your gantry is attached to a 24″ slide and the gantry is 5″ thick, This gives you a maximum stable motion of 19″ (24-5), even though it may be able to extend much further.
- Drawer slides are not designed with precise motion in mind.
- Short range of motion, can not be extended.
V-Guide wheels are common for woodworking CNC machines, and are very tolerant of any dust particles or debris that may be on the rail. For example, they are used on Shopbots. These are basically ball bearings with a V shape cut into it, allowing them to glide over a special rail. They are not readily available, but may be obtained my industrial supply companies such as Reid supply and Grainger.
- Simple to use
- Durable, not susceptible to particles.
- Medium cost. Each one can be $30 and it adds up, but it’s still cheaper than a lot of other options out there. Rails are also cheap.
- Depending on your design, may be susceptible to gravity!
- Cost of bearings can add up if you use excessive number of them.
Linear Support Rail
These are the most professional accurate, and most expensive type of linear motion system. The idea is to have a linear bearing pillow block guide on perfectly machines metal rods. Some rods are fully supported (as in the picture here). For these, an open pillow block glides on the rail. There are also partially supported rods , where the shaft is perfectly circular, and only supported at the very ends. In these cases, completely enclosed linear bearings may be used. Either way, these are usually the most precise linear bearing systems for CNC machines. They are available from industrial supply companies like Reid Supply and Grainger. But actually, eBay is a very good place to look for these at hugely discounted prices. For my latest machine, I bought 6 rails and 12 pillow blocks for $160 including shipping. If you look for SBR12, SBR20 (for 12mm diameter and 20mm diameter) etc. you’ll likely find cheap linear bearings from China, usually for one fifth the cost of Grainger.
- Very precise, smooth action.
- Very little backlash. In many cases, no human-detectable backlash.
- Very heavy loads can be supported.
- Motion can actually be choppy if you misalign the pillow blocks even slightly to whatever you fasten them to.