Acme/Trapezoidal Leadscrews

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. In this post I’ll be going over leadscrews; what they are, how to read their specifications and what type of machine they are best suited to.

Leadscrews are the most common linear drive you will see on most DIY CNC machines. Leadscrews come in an imperial flavour called Acme, and a metric flavour called Trapezoidal.

Acme and trapezoidal leadscrews are very much the same thing, just different measurements. The only real difference to speak of is the angle between the threads (the thread angle), acme is 29 degrees and trapezoidal is 30.

Leadscrews are just essentially a very high quality threaded rod with larger more sturdy threads. Available in many different sizes and pitches, the ratings of leadscrews become very confusing when first attempting to understand them. Here are some things you’ll need to understand about leadscrews:

 

Major and Minor Diameters

The major diameter is often the first specification you are shown. This major diameter is the outer diameter of the leadscrew. The minor diameter is the inner diameter part of the screw, it’s where the threads end. I like to think of it as the meat of the leadscrew.


Right and Left hand Thread

Leadscrews are available in both right and left hand threads. Leadscrews will most commonly be seen in right hand threads.


Pitch and Lead

Pitch refers to the distance between the threads on the leadscrew. Lead refers to the distance the nut on the leadscrew will travel when the leadscrew is turned one complete 360 revolution. The lead is calculated by multiplying the pitch by the starts. If a leadscrew is just a single start screw then the lead is the same as the pitch.


Starts

Starts refer to the number of additional threads that run up the length of the leadscrew. The best way to demonstrate this is to show you.

First looking at the spiral on the left with the title: “1 start”. This represents a single thread leadscrew, also refered to as single start. This means there is one thread that spirals up the leadscrew.

Next look at the spiral on the right with the title: “2 starts”. This represents a leadscrew with two threads, these threads run together up the length of the screw.

Leadscrews with multiple starts allow faster speeds, more stability and provide more efficient power transmission. Due to the additional threads or starts, the pitch of the leadscrew needs to increase so that there is physically more room to fit in the additional threads.


Notation

Leadscrews are often notated something along the lines of this:

Acme: 1/2″-10

Trapezoidal: 12x3mm

There is a difference between reading these both so lets start off by reading the Acme leadscrew specifications. The 1/2″ rating refers to the outer diameter of the leadscrew, the outer diameter is the diameter they are “named” after.

The second part “-10” is read dash ten, not minus ten. This number ten refers to the Threads Per Inch (TPI), so for every 1″ there will be 10 threads, which means the pitch is 0.1″ (1 inch divided by 10 threads per inch).

The trapezoidal specifications are very similar, easier in fact. 12mm refers to the outer diameter and 3mm is the pitch.

 

Additionally the starts may be listed:

Acme: 1/2″-10 2 starts

Trapezoidal: 12x3mm 2 starts

Acme first – We know that with 10 threads per inch, the lead is 0.1″. However if there is more than one start we need to multiply the pitch by the number of starts, in this case there are 2 starts.

0.1″ (pitch) multiplied by 2 (starts= 0.2″ lead

The trapezoidal is very simple, same in-effect as above.

3mm (pitch) multiplied by 2 (starts) = 6mm lead


Conclusion

Leadscrews can be difficult to understand at first, especially the starts. But once you’ve got your head around it, it will help a lot with understanding ball screws and how a CNC is able to move in general.

So what is the best use of a leadscrew? Well leadscrews provide strength more so than speed or anything else. With something like a CNC router we need that kind of strength because a spindle tool or router going through wood or light metal at speed, can produce significant resistance.

In terms of speed, leadscrews don’t produce the best. Something like a belt drive or rack and pinion will produce great speed but they lack the strength of a sturdy leadscrew. Now of course there are exceptions to this, a very beefy belt drive or rack and pinion system will do just as good of a job.

The best home for leadscrews then is small to medium-sized CNC routers. Leadscrews will produce a fair amount of speed, good strength and power transmission to overcome the forces involved in cutting wood and light metals.

 

You can find leadscrews here.