Drive Systems


Belts, Chains, Racks and Leadscrews


The axes of CNC machines can be driven in a number of ways. The following is based on the building and testing of multiple prototypes.

Belts and Pulleys

Belts are among the most efficient drive systems with efficiency approaching 95%.

This makes them useful in low load systems such as pick and place machines and laser cutters, where cutting forces and kick-back are minimal.

Belts are also useful for attaching steppers to axles and leadscrews, and for gearing systems as shown here.

However, on CNC machines that use router bits to cut the stock, driving the axes directly with belts can result in rebound and chatter due to the belt's resilience. This is remedied by slowing the feed-rates and reducing the axes’ acceleration.

Since belts will stretch, they are not suitable for long axes. The longer the belt, the worse the rebound.

Gearing with Stepper
Belt and pulleys for gearing.
 
The pulleys that ride the belts have to be large enough to keep six teeth in contact with the belt, otherwise the belt’s teeth will ratchet or tear.

The above photo shows an idler that wraps the belt around the pulleys so the belt will ride firmly in the pulleys' teeth.

Ten tooth pulleys are usually not the best choice for the smaller pulley because it is difficult to wrap the belt far enough around to give six tooth contact. Therefore, larger pulleys are required.
A problem is the larger pulleys will move the belt a long way with each pulley rotation. This is similar to driving a vehicle in only high gear. The acceleration will be terrible, and the motor will be likely to stall.

Another disadvantage with the pulley is the resolution of the axis will be too large. When the pulley is driven directly, the stepper will not be able to move the axis a tiny amount.
 
For example: A 1 inch diameter pulley would move the axis 3.14 inches per rotation. The stepper, which has 200 steps per revolution, would give a resolution of 3.14 inches divided by 200 steps, which is 0.0157 inches per step. This can be improved with micro-stepping, but using micro-stepping to improve resolution is not recommended by some drive suppliers.
Therefore, to increase the cutting forces and to improve resolution, gearing between the stepper and the drive pulleys, as shown above, is necessary.
This is not a severe problem, but it does increase complexity and cost.
 

This image shows two idlers with a pulley that drives the axis. As with the above system, the idlers are required to keep the belt securely wrapped around the pulley.

In my experience, timing belts can serve well for connecting motors with leadscrews and axles, but the rebounding that results when belts are used to directly drive the axes makes long belts less appealing than leadscrews or racks.
Idler Tensioners with Belts
Belt and idlers.
 
Regarding using a belt to attach a pair of leadscrews to a single stepper:

The problems with belts are not as pronounced in this configuration because the cutting forces are carried by the leadscrews and leadnuts.
The drag between the leadnut and leadscrew helps to prevent the leadscrew from being back-driven by the cutting forces.
The belt does not take the direct cutting load from the router bit, but helps to hold the leadscrew steady against the cutting force that is applied to the leadnut.
With extremely efficient leadscrews, such as ballscrews, the belt's resilience would be more of a limitation, but with Acme and All Thread leadscrews, the limitations are minimal.

I prefer dealing with the shortcomings of belt coupled leadscrews over those of slaved steppers.
More information about slaved steppers is on the Drives and Steppers page.
 

Flanged Idlers

Idlers used with belts and pulleys sometimes need to be flanged to prevent the belt from derailing.

Flat belts tend to travel to the higher sections of idlers rather than to the valleys; therefore hour-glass shaped idlers can cause the belt to wander from edge to edge of the idler.
The two shop-made units shown here have worked well.

The top image shows a flanged idler that is made from plastic electrical conduit couplings and 608 (skate) bearings.

The unit is made by pressing a bearing into each end of a 1/2 inch coupler.
A bolt and nut with large washers can be used to press the bearings into place.

Flanges are made by cutting 1/8 inch sections from the ends of a 3/4 inch coupler.
The bolt also can be used to clamp these rings into place.

The fits can be tight. To prevent the plastic from splitting it may be necessary to file the plastic before pressing the parts together.

Conduit glue is needed to hold the flanges in place. Wrap the body of the smaller coupler with tape to mask the surface during the glue-up and assembly.

After assembly, file the flange's edges smooth to prevent them from cutting the belt.
Flanged Idler
Shop-made flanged idler.

Flanged Idler
3/4 inch box adapter.

The image directly above is a 3/4 inch electrical conduit box adapter. The single flange is part of the off-the-shelf unit.

608 bearings can be pressed into these with a bolt and washers as described for the other idler.
 

Chains and Sprockets

Chain systems are similar to belts. The components tend to be a little less costly, but they have the same problems as belt drives. They have another disadvantage of not being as efficient.

Backlash with chains is a problem. When the chains are tensioned enough to remove most lash, the system’s drag is increased, and performance suffers.
Chains stretch and rebound, and like belts, they are not suitable for long axes.

The chains' drive sprockets have the same resolution problems as belts' drive pulleys, so gearing between the stepper and the drive sprocket is required.

Though chains are used in some DIY CNC systems, in my opinion, they are not worth the trouble. There are simpler and more accurate options for a similar price.
 

Racks and Pinions

Rack systems, like belt and chain systems, require gearing to give good resolution and force. Unlike belts and chains, racks do not stretch, and they can be abutted for virtually infinite axis length.

The first image shows a spur gear, a pinion, on a rack. The gear is turned by the stepper; this moves the axis.

Backlash between the rack and pinion can be a problem, and the pinion can ratchet out of the rack, so a tensioning system is needed to keep the pinion in the rack.

As shown in the next images, a rather complex spring loaded bearing system is used to pull the pinion into the rack on this shop’s metal 4x8 table.

The rack option of the 24x48 machine pulls the rack into the pinion by moving the racks' supports, which is simpler than a spring loaded system.

The pinions on the 18x24 machine are pulled into the racks with an adjustable offset bushing system, which is also simple and tight.

A range of rack and pinion sizes can be used. This shop's metal 4x8 table uses 3/4 inch wide, 12 pitch, 14.5 degree pressure angle rack. This has ~3.8 teeth per inch, which is rather coarse, and is the lower limit of teeth per inch that I would use.

The plans' 18x24 and 24x48 machines use 1/2 inch wide, 20 pitch, 20 degree pressure angle racks, which have ~6.4 teeth per inch. This size was chosen because of its availability from mcmaster.com.

Other racks that are in this size range are 24 pitch with ~7.7 teeth per inch, and 16 pitch with ~5.1 teeth per inch.

Racks are available from different suppliers including use-enco.com, macmaster.com, sdp-si.com, and mscdirect.com.
The suppliers' international shipping varies. I would begin construction only after having the parts in hand.
Rack with Spur Gear (Pinion)
20 pitch pinion spur gear on rack.

12 pitch rack and pinion
12 pitch pinion and rack.

Pinion Tensioner
Spring loaded tensioning system.

Pinion pitch diameters of 3/4 to 1 inch have served well when gearing is used between the steppers and pinions.

20 Degree pressure angle is often recommended over 14.5 because it is supposed to be less likely to ratchet. However, for these DIY machines and their low loads, the difference does not seem to be noticeable. Again, it is important to firmly tension the pinions into the racks to prevent ratcheting and backlash.

Racks and pinions are a popular option because of their speed, relatively low cost and their ability to be used with very long axes.
Their downside is they require stepper gearing, and pinion to rack tensioning.
 

Leadscrews and Leadnuts

Leadscrews are a popular choice with DIY builders for a number of reasons.

They are available in a variety of sizes and types, from inexpensive hardware store threaded rod, to precision Acme rod.

Leadscrews can be attached directly to the steppers’ shafts since no gear reduction is required.

Initial leadscrew alignment can be challenging, but it remains true once it is set.

Over time, anti-backlash leadnuts self adjust to remove play that would otherwise be introduced by wear.
This is in contrast to the other drive systems, which all have to be adjusted to remove slop due to use.

Two and five turn per inch leadscrews have served well in this shop. 3/8 Inch diameter works well with axes under 3 feet, and 1/2 inch diameter works well with axes over 3 feet.

See also the Acme page.
 

Back-Driving

Back-driving of leadscrews can be a problem when the screws are very efficient, such as with ballscrews, or when the turn count is low, as with two turn per inch Acme leadscrews.

On a Z axis, for example, the weight of the router can cause the leadscrew to turn, be back-driven, when the stepper is not powered, which allows the router to fall.

This loss of Z position can be a nuisance, but it can be resolved by using a higher turn count leadscrew on the Z axis.

Ten turn per inch rods are available in 12 inch lengths. This also can remove the need to cut down higher priced precision rods, which are often only available in 3 or 6 foot lengths.

The slower speed that results from the higher turn count of the Z axis' leadscrew is seldom a problem because the travel of the Z axis is usually much less than that of the other axes. It moves slower, but it does not have as far to go.

There is more information about leadscrews on the Acme Threaded Rod page.