The ACME Lead Screw

The following sections will discuss the terms associated with the ACME lead screw. As with any piece of hardware it is best to know the terminology associated with that product before using or purchasing.

Major Diameter

The major diameter also referred to as the land or outside diameter, is the outer most diameter of the lead screw. For a ½-10 ACME lead screw, the major diameter would be 0.5 inches nominal.
acme lead screw major diameter

Minor Diameter

The minor diameter or the root diameter of a lead screw is the innermost diameter of the lead screw. The minor diameter is the diameter of the lead screw between the threads.
Thread Helix Types

There are two types or directions of the thread helix, left hand and right-hand threads. Most Common threads are right-hand threads. Always be sure to check if the lead screw is right hand or left-hand threads before purchasing. Right-hand threads are denoted (RH) and (LH) for left-hand threads. The saying “righty tighty lefty loosey” describes a right-hand thread and nut, where turning the nut clockwise or right moves the nut further along the lead screw. Left-hand screws are opposite. The image below illustrated how to identify left and right-hand threads.
left and right-hand threads

ACME Thread Forms

ACME threads are different than threads found on the standard threaded rod. The trapezoidal shape of ACME threads lends it well to power transmission. ACME threaded rod vs UNC or “typical” threaded rod.
ACME Thread Classes

Among ACME threaded rod or lead screws there are three major classes; stub, general purpose (G), and centralizing/centering (C). The stub is the rawest form of acme threaded rod. Although it is still possible to make a CNC drive system from this type of ACME rod, it is usually worth paying the extra price for a general or centralizing ACME thread form. ACME stub threads do not have the straightness or tolerance of other classes. The efficiency of this class of rod is significantly less.

The general purpose is a step above the stub. General purpose ACME threads can be finished and have decent straightness but depend primarily on the manufacturer. General purpose ACME threads may need to be de-burred before use.

Centralizing ACME threads have tighter tolerances on the major diameter than the other types of ACME lead screws. Thus these are less likely to bind under side loads. Centralizing threads are most common for power transmission purposes. Also, this type of thread will have a nice finish and will be the most efficient of the three.
Pitch

The pitch of a lead screw is the linear distance between the threads.
Lead

Lead is the linear distance the nut travels and the lead screw for every one revolution. The lead is equal to the pitch times the number of starts.

Lead = Pitch x Starts

lead screw starts

It is important to understand the lead of a lead screw as it is one of the most important aspects when choosing a lead screw. Check out the lead and pitch calculator here.

Screw Starts

There may be more than one thread “strand” on a single lead screw. These are call starts. Multiple start lead screws are usually more stable and efficient at power transmission.

You can identify the number of starts of a lead screw by viewing the end of the lead screw. The figure below represents the most common types, single, double, and four start lead screws.

Lead Screw Notation

Lead screw size and TPI are usually the first specifications given. TPI refers to the threads per inch. For example:
1/2″-10

This lead screw is ½ inches in diameter and contains 10 threads per inch. The information above does not specify the lead or number of starts. The number of starts should be stated elsewhere. This format is the most common.

Caution: Some manufacturers lead screw information is given by diameter and turns per inch, NOT threads per inch. It does not matter when discussing a single start lead screw. However, the threads per inch and turns per inch are NOT the same when discussing multi-start lead screws. The simplest way to check is to verify the pitch and lead. Always be sure to check.

Continue to the next section: The ACME Lead Screw Lead Accuracy and Straightness

Or Visit: Acme Threads vs Typical Threads