Threads are spiral grooves that allow easy fastening of different elements of an assembly, and Acme threads are one of the most important types of threads.
Acme screw threads are widely used for producing traversing motions for machines, tools, and heavy-duty work.
But what exactly are acme threads? What are its types? And how to cut Acme threads for your application?
Acme threads are screw threads with a trapezoidal form factor that are comparatively easier to manufacture than square threads and provide high accuracy. As a result, these threads are used for high-precision applications such as lead screw drives for CNC machines and other power transmission systems.
This article provides a detailed guide on ACME threads, types, methods of manufacturing ACME threads, advantages, and applications.
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What are Acme Threads?
Acme threads are a type of threads that are generally used in metal threading applications where accuracy and load capacity are of importance.
These threads are characterized by their distinctive trapezoidal shape with flanks undergoing a gradual inclination from the crest to the root.
The main aim of designing the ACME thread was to replace the square thread, which had straight-sided flanks and was challenging to manufacture.
Acme is of special interest to machinists due to its features like broader and thicker base that provide greater strength and durability when used for heavy-duty manufacturing applications.
Terms related to Acme Threads
In general, all types of ACME threads appear the same. However, they have their particular nomenclature, which helps the user distinguish the threads.
External and Internal ACME threads
External threads are also known as male threads. They extend from a cylindrical surface (cut outside the surface). Commonly available external threads include screws, bolts, etc.
Whereas internal or female threads are machined inside the cylindrical surface like an acme nut or a coupler.
It is a line running through the center of the cylinder on which the thread is formed. All measurements associated with screw threads are taken along the thread axis.
Peak points in a thread profile are known as a crest.
It lies at the bottom of the thread grooves between the two adjacent threads.
All sizes of the screw threads are not identical. Major diameter can be understood as an imaginary cylindrical diameter that makes contact with the external thread's crest or the internal thread's root.
Minor diameter is the diameter of an imaginary circle formed by the thread's root in external threads or by the thread's crest in internal threads.
It is the distance between two similar points on adjacent threads. For example, in a screw thread, a pitch is the distance between the two nearest crests. Acme thread pitches are calculated in TPI (Threads per Inch).
Pitch also helps you determine the mechanical advantage (the ratio of output to input force) of the screw nut assembly. A smaller pitch implies more mechanical advantage.
Pitch diameter is also known as the effective diameter.
It works as a frame of reference for the thread measurement and also signifies the compatibility between the internally and externally threaded parts.
You can consider it as the imaginary cylindrical diameter that passes through the midpoint of major and minor diameters.
The straight side between the root and crest is known as the thread flank.
It is the angle between the two flanks, measured along the plane of the thread axis.
Generallt, acme threads with a 29° thread angle are best suited for power transmission and carrying loads due to their broader and stronger form factor.
Thread angle influences the efficiency, wear rate, and strength of the screw. Therefore, friction and wear of the screw increase with the thread angle.
Lead is simply the axial distance advanced by a screw in one rotation.
Lead=Pitch ×Number of starts
Starts are the number of threads that run parallel to each other.
Generally, pitch and lead are equal for a single start thread, which is not the case for multi-start threads.
Multi-start acme threads provide greater rigidty, making them suitable for applications where strong power delivery is required.
Types of Acme Threads
Acme screw threads fall into three categories, summarized in the table below
|General Purpose (G)||Most commonly used Acme threads with minimal backlash in 3D and 4G class.|
|Centralizing (C)||Compararively more accurate and have less backlash due to tight clearance.|
|Stub Acme||Compact threads with a 43% shorter stub height than standard Acme|
General Purpose Acme Threads (G)
ANSI B1.5 standard suggests three classes of fit for General-Purpose (G) acme screw threads: 2G, 3G, and 4G.
Class of fit defines the relationship between the various attributes of internal and external threads.
Usually, Class 2G is preferred. However, if less backlash is desired, class 3G and 4G are adopted.
Centralizing Acme Threads (C)
These are quite similar to G-type threads. However, centralizing acme threads have limited clearance in the major diameter of both internal and external threads.
Restricted clearance prevents wedging and part sagging on the flanks of the bearing threads.
Centralized acme threads (C) have five main classes of fits that includes 2C, 3C, 4C, 5C, and 6C.
Out of these, a limited backlash is permitted in 5C and 6C but not advised for new designs.
Stub Acme Threads
Stub acme threads are available in many authorized varieties.
They are short and truncated, suitable for applications with limited space.
The approximate height of the stub acme thread is 57% of a regular acme.
Unlike standard acme class, stub acme thread permits only 2G class of fit.
Standard Size Chart for Acme Threads
According to the American National Standard thread size, ANSI/ACME B1.5-1977 is given in the table below.
|Tapping Drill Size|
|Thread per Inch (TPI)||Pitch|
|1/4” - 16 ACME||6.350||0.250||4.762||16||1.588|
|5/16” - 14 ACME||7.938||0.313||6.123||14||1.814|
|3/8”- 12 ACME||9.525||0.375||7.408||12||2.117|
|7/16” - 12 ACME||11.112||0.437||8.996||12||2.117|
|1/2” - 10 ACME||12.7||0.500||10.160||10||2.540|
|5/8” - 8 ACME||15.875||0.625||12.700||8||3.175|
|3/4” - 6 ACME||19.050||0.750||14.817||6||4.233|
|7/8” - 6 ACME||22.225||0.875||17.992||6||4.233|
|1” - 5 ACME||25.400||1.000||20.320||5||5.080|
|1 1/8” - 5 ACME||28.575||1.125||23.495||5||5.080|
|1 1/4” - 5 ACME||31.750||1.250||26.670||5||5.080|
|1 3/8” - 4 ACME||34.925||1.375||28.575||4||6.350|
|1 1/2” - 4 ACME||38.100||1.500||31.750||4||6.350|
|1 3/4” - 4 ACME||44.450||1.750||38.100||4||6.350|
|2”- 4 ACME||50.800||2.000||44.450||4||6.350|
|2 1/4” - 3 ACME||57.150||2.250||48.683||3||8.467|
|2 1/2” - 3 ACME||63.500||2.500||55.033||3||8.467|
|2 3/4” - 3 ACME||69.850||2.750||61.383||3||8.467|
|3” - 2 ACME||76.200||3.000||63.500||2||12.700|
|3 1/2” - 2 ACME||88.900||3.500||76.200||2||12.700|
|4” - 2 ACME||101.600||4.000||88.900||2||12.700|
|4 1/2” - 2 ACME||114.300||4.500||101.600||2||12.700|
|5” - 2 ACME||127.000||5.000||114.300||2||12.700|
Two of the most commonly used single and multi-start general purpose (G) acme threads are: 1 1/4” - 5 ACME and 2 3/4” - 3 ACME.
1 1/4” - 5 ACME
It’s a single start with a 1.25-inch major diameter and 5 TPI.
1 1/4” - 5 acme right-handed threads are used in steel hex nuts, which are commonly used in general-purpose fastening applications.
You can also find the application of these nuts in valves and power transmission systems.
2 3/4” - 3 ACME:
It’s a multi-start thread with a 2.75-inch major diameter and 3 TPI.
2 3/4” – 3 stub acme threads are widely used in ring plug gauges. It controls the workpiece’s internal thread pitch diameter and major diameter during machining.
How to Cut Acme Threads?
To meet high-quality thread standards while cutting any thread, main parameters like diameters, pitch, lead, and thread angle must be kept in mind.
There are various techniques to cut acme threads, but lathe threading is the conventional and common technique to produce acme threads.
Acme thread cutting is performed by making sequential cuts using a threading tool that matches the desired thread form.
The thread cutting tool must be grounded to the exact angle and contour to produce the precise thread profile.
Using compound slide, feed the tool into the plane of work. Locate the reference points of the tool using the cross-feed.
The type of toolholder determines the amount of back rake angle. Set the back rake angle to zero when a quick change or open side holder is used.
Otherwise, the back rake angle must match the tool holder angle. This will result into a combined effect of zero back rake angle.
Set the compound slide at an angle of 14 degrees and use an acme center gauge to center the tool into the work.
Machine the thread by feeding the tool using the compound slide.
Check the thread with the mating part or implement the one wire method to measure the threads.
In this method, wires which touch the thread at pitch diameter are considered to be the best size wire. The optimal wire size can be calculated by multiplying the constant, 0.4872 with the pitch of the thread.
For example, if there are 4 TPI, then the pitch, p=1⁄4=0.25, and the wire size will be 0.4872×0.25=0.1218 inch in diameter.
While cutting an acme thread, a clearance of 0.01 inch should be kept between the crest of the screw thread and the root of the nut thread.
However, this process can be simplified by inplementing CNC threading technique where a G32 threading cycle with appropriate parameters can be run on a CNC lathe to get the desired Acme threads.
Apart from lathe threading, rigid tapping, thread milling, thread grinding, thread with electrical discharge machining (EDM), thread rolling, and 3D printing are some other widely used methods for acme thread production.
Applications of Acme Threads
Acme threads are the most suitable option for power transmission and load-carrying applications.
They also function better in areas with dirt and debris thanks to their thickness and width.
Acme threads are best suited for industrial applications, and are frequently used as lead screws in split nuts, jacks, vices, and machine equipment.
Acme threads are also needed in applications where heavy torque and power transmission are required like lathes, milling, presses, CNC systems, etc.
In the oil and gas industry, acme threads are widely used where strength and durability are critical to ensure both human and equipment safety.
Such applications use acme threads in valve stems, hydraulic couplers, and connected fasteners for heavy load applications.
Some key applications of acme threads in the automobile industry include air conditioner refrigerant adapters, automobile fasteners, automobile screw jacks, hose connectors, bonnets on pressure cylinders, steering mechanisms, etc.
In the aerospace industry, acme threads are an excellent option in electro-mechanical actuators and aircraft flaps.
A few such actuators are inlet door actuators, landing gear unlocks actuators, seat, nozzle actuators, silencer actuators, etc.
Acme lead screws are great when it comes to delivering a quiet linear motion.
So, they are demanded in medical devices like patient-specific screw-type chairs, medical syringe pumps, automated hospital beds, and patient table lead screws in magnetic resonance imaging (MRI).
Advantages of Using Acme Threads
Self-lubricating nature of acme threads makes them damage-resistant and simple to clean even in a harsh environment.
Acme screw thread can transmit bilateral transverse motion and can also prevent back drive.
Due to their wider cross-section at the root, these threads are stronger in shear than square threads.
The acme screw has lower operating, manufacturing, and maintenance costs than the ball screw for the same application (where linear motion is necessary).
The working environment with acme thread is less noisy as compared to square thread.
Acme threads are ideal for vertical, light/moderate loading conditions and offer safer holding when operating under static load.
Acme threads are mostly associated with lead screws. When compared to other threads, acme threads present far superior wear properties, load capabilities, and tolerances than any standard threaded rods.
Each type of acme thread has its own function and application.
For example, G-type threads usually take a wedge shape on the application of an external radial load.
Therefore, they are compatible when both nut and screw are independently supported. Otherwise, it may increase friction and result in premature wear in screw nut assembly.
C-type acme threads are chosen where the load is held transverse to the screw axis. Here, the C-type threads act as radial bearings to support the load effectively.
Stub acme screw thread is used for those unusual applications where due to mechanical and metallurgical considerations, shallower depth thread is required, like in narrow diameter screws.
Frequently Asked Questions
How to choose acme threads?
The right combination of acme thread screws and nuts for a particular application depends on factors like axial loading (static, dynamic, reaction, and external), linear speed of screw rotation, length between the support, and end support type.
Commonly used materials for acme thread?
Stainless steel, plastics, and bronze are some of the common materials for acme screws and nuts. Alternate acme lead screw materials are aluminum alloy and medium carbon steel.
How efficient is the power transfer with acme screw nut assembly?
Acme Screw assembly efficiency ranges from 15% to 85%. It largely depends upon the material of nut, lead, thread start, and lubrication method.
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