Threaded rods vs. ACME rods
There are many methods of accomplishing linear power transmission, otherwise known as linear movement. Anything from a simple wheel, to a gear on a rack, to rubber belts will work. However, for hobby CNC machines, the most common method to accomplish movement is using threaded rods, or ACME rods.
Threaded rods are readily available from most hardware stores like Lowe’s or Home Depot. Keep in mind they are primarily intended to be used for fastening, not linear motion. Also readily available at hardware stores are nylon nuts which are compatible with threaded rods. Most threaded rods in the US have three parameters; diameter in inches, threads per inch, and length in inches or feet. Threads per inch is commonly abbreviated as TPI.
For example, if you buy a 3/8 inch – 16 TPI rod, be sure that the nylon nuts you buy are also 3/8 inch – 16 TPI. Always double check that the nylon nut has the correct specifications for your rod.
Threaded rods in hardware stores are single start, which means 16 threads per inch also means exactly 16 rotations will equal one inch of movement. When buying these, consider the accuracy of motion you need, as well as the maximum speed you’re able to run your stepper motor. If you buy a rod with too many threads per inch, movement can be painfully slow. Also note that using standard nylon nuts with threaded rods can result in quite a bit of backlash. Meaning that the nut is still able to traverse a little bit back and forth without it or the rod rotating.
While nylon nuts are mentioned a few times, please be aware that they can have quite a bit of backlash. To reduce backlash, you might want to look into buying simple rod coupling nuts. These are usually made of metal, are longer, and have more friction than nylon nuts. But the advantage of rod coupling nuts is that they have less backlash then standard nuts. As always, be sure the diameter and TPI match your threaded rots. Rod coupling nuts are common, and can be found in many hardware stores.
ACME threaded rods
ACME threaded rods may appear similar to threaded rods, but have very different characteristics. For one, ACME rods are much more efficient for linear power transmission, which means less energy is wasted, which means you might be able to run your motors faster. Special ACME nuts must be used with ACME rods, regular nuts at the hardware store won’t work. To obtain ACME threaded rods, your best bet is the internet. Industrial suppliers like Reid Supply, McMaster-Carr, and Grainger carry ACME threaded products, as well as many smaller companies. Unlike nylon nuts for threaded rods, ACME nuts can cost 20 dollars or more, even though the ACME rod itself may not be much more expensive than a threaded rod.
ACME rods can be either left handed or right handed, as well as ACME nuts. This is commonly abbreviated as LH and RH. If you have a LH ACME rod, be sure to get a LH ACME nut.
ACME threaded rods can have multiple starts, which effectively act as speed up ratios. For example, if an ACME rod has 16 TPI and 2 starts, this means that 8 revolutions will equal one inch of movement. Some sites specify TPI, while others specify the number of starts and the number of rotations per inch. So be sure to keep that in mind. Whatever nut you buy must have exactly the same number of starts as your ACME thread, otherwise there is no hope that it will work.
Backlash is unwanted motion that happens even when neither the nut nor the rod are rotating. There are a few ways to get rid of backlash. The cheapest solution is to just compensate for backlash with software. Another solution would be to have a spring constantly pulling your moving parts in one direction. The most common solution for backlash is to use anti-backlash nuts. These nuts usually have some sort of spring built into the nut which clamps tightly around an ACME rod. Obviously, these rods are more expensive than normal nuts. But if you’re doing precise metalworking where you need very good precision, it may be worth it to invest in some anti-backlash nuts.