Machine Construction and Materials


Building to Reduce Chatter


Chatter

Milled with 5/16-18 and tee nuts.
Chatter caused by low quality leadnuts.

This image shows the results of chatter on the edges of aluminum flat bar.
Chatter is simply a vibrating bit. It is the result of machine components that are too flexible or loose to carry the cutting forces.

The bit's cutting edge will dig into and/or bounce off of the stock because the cutting forces are not supported or damped by the stiffness of the machine. This creates vibrations that cause the bit to carve jagged or rough cuts.

Virtually every component of a CNC machine is a point of potential weakness that can contribute to chatter.

The following observations are based on the building of (too) many prototypes including this first clunker.
 

Overall Design

The overall design of the machine determines its solidity and resistance to chatter.

A gantry with tall end plates is more likely to rock on its rails than a short gantry with its beam closer to its supporting rails.

This limitation is addressed in fixed gantry machines by removing the bearings on the gantry end plates. The gantry to base connection does not have to move, so the gantry can be firmly secured.

A carriage that is high above the cutting surface also permits the bit to chatter because of the flexing of the long Z axis.
In both cases there is a trade off between high clearance and adequate rigidity.

A long gantry that is driven with a single leadscrew can offset under the cutting forces when the bit is near the end of the gantry. This can be resolved with leadscrews or racks at each end of the gantry, and with solid bearing systems.
 

Materials for the Machine Body


Modulus of Elasticity
The table lists materials from the least to most stiff. Note that MDF is not as stiff as plywood. Also note that well chosen pieces of southern yellow pine can be as rigid as any other wood, including oak and maple.

Aluminum is 5 to 10 times stiffer than wood, and steel is 3 times stiffer than aluminum.

Plywood is about the same as solid wood. It is better than solid wood across the grain, but worse with the grain. This is logical since plywood’s grain orientation alternates between layers.

Tests in this shop indicate that there is as much variation between sections of one sheet of plywood as there is between species and types of plywood. Lower cost framing stock can be as rigid as multi-ply hardwood cabinet stock. Again this follows because better quality wood is used for the veneer while poplar or similar is used for the core of higher priced plywood.
 

Rails

The rails need to be rigid and solidly anchored. Aluminum, brass, copper and thin wall tubing are too soft and flexible, and will bend under heavier cutting loads.

Unsupported rails will also bend under the cutting forces. Even rails that seem stiff, such as 1-¼ inch rigid electrical conduit and galvanized plumbing pipe, will flex with a trim router when not supported along their length.

Well tensioned bearings will press tracks into unhardened steel rails such as steel pipe and flat bar. The tracks will stabilize with use and can be filed or sanded if pits develop.

Hardened rails are superior, but they are costly and can be challenging to mount. Their hardened surface has to be ground off before the rails can be drilled and tapped.
Standing on Gantry
Rails should be well supported.

In the image above I am standing on the gantry of one of the machines. Machines that cannot support this load will not be able to push even a trim router to its potential.

Supported rails make a big difference in the capability of the machines. This is an important feature I would consider when deciding what kind of machine to buy or build.
 

Bearings

Bearings need to be well tensioned against their rails. Any looseness at the rail to bearing connection will permit chatter.

The tension can be maintained in a variety of ways. These include tension rods that pull opposing bearings into their rails, and offset bushings in the bearings' bores that are rotated to adjust tension.
The bearings need to be well supported.
Wood can flex around the bearing's axle, especially with V bearings, which require high tension to perform well.

The bearings can be mounted to metal that is attached to the wooden machine. This compromise gives fairly good stability while remaining inexpensive. There is more information about DIY bearings on the Bearing Trials page.
 

Leadnuts and Leadscrews

Leadnuts can be another weak link in a CNC system. Any looseness between the nut and screw will contribute to backlash and chatter. Also, the nuts need to ride securely on the leadscrew without causing excess drag.

Inexpensive options like tee nuts on threaded rods will have ~0.003 inch of play between the nut and rod. Slightly crushing the nut so it is oval rather than round can tighten the tolerance somewhat, but it is challenging to find the balance between precision and drag. These softer metal components also will wear sooner, which will degrade performance.

Anti-backlash nuts resolve this problem by clamping the nut's threads around the leadscrew. As the nut wears, the clamping action continues to press the threads into the screw. Anti-backlash nuts are often made of Delrin. This plastic machines well but is flexible. This flexibility will contribute to chatter with more aggressive cutting.
The plastic nuts are usually rated for loads in the 12 to 25 lb. range. This holds well against basic cutting forces, but it is not enough to inhibit chatter with aggressive cuts.

A full sized router will likely not be pushed to its potential with plastic leadnuts. Bronze nuts are another option, but like tee nuts they will have play between their threads and the leadscrews' threads. This is resolved with the split leadnut that can be tensioned by distorting the threads, but these too are challenging to adjust between too much drag and too much play.

Quality ball screws address all of these problems, but good ones are costly, and using them on machines that are made of wood and aluminum may not take full advantage of them.

There is more information about leadscrews and nuts on the Acme Upgrade page.