For this reason, the roller screw has lower friction and higher transmission efficiency and is more suitable for high-speed transmissions.

There is no limitation on the size of the shoulders. When they are larger than the minor screw of the screw, it is recommended to use turning or other processes to produce the lead screw.

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The raw steel bars supplied in good condition can be machined without pre-straightening, this will save a lot of costs. It is important to source the steel bars from a good supplier.

Let’s start by comparing the lead screws and ball screws. The lead screw has direct contact with the nut and generates sliding friction. While for the ball screws, there are rollers between the screws and nuts, so it is the rolling friction between the moving components.

Like the turning process, thread whirling is another metal removal process. The whirling ring has multiple cutting tips mounted on the internal diameter, it rotates at a high speed and cuts the threads on the stock bar (the workpiece) which rotates and moves forward (feeding) at a much slower speed. The workpiece moves forward at the distance of LEAD with each revolution.

Unlike turning or whirling, in thread rolling, the workpiece is actually being pushed forward freely by the rolling of helical lines of the roller dies, the feeding of the workpiece is less accurate. At the same time, the squeezing will cause some slight longitudinal deformation. Even if each pitch is 0.1% off the normal (like 0.004mm on a 4mm pitch), which does not seem a lot, it may translate into a 1mm deviation on a 1-meter span.

When we calculate the value by putting in the flank angle (α）with specific numbers (60°， 30° and 29°), the formulas can be simplified as follows：

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However, we can not get a quantitative result with go-no go gauges, so we will not know whether the dimensions fall in the optimized zone. Meanwhile, in practice, sometimes both d2 and D2 can be both made larger or smaller so they still match well together. When this is the case, the go-no go gauge will not be very useful.

Screw rolling is a cold-forming process that which 2 rollers (or sometimes 3 rollers) squeeze the bar stock to form the threads. It is the most popular manufacturing method for lead screws.

The bend of the finished screw bars comes partly from the raw steel bars, and partly from the machining process itself. In order to improve the straightness of the lead screws, it is recommended to straighten both the raw steel bars and finished lead screws. Usually, straightness of 0.15-0.3mm per meter can be obtained for most machining processes.

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In the screw rolling process, the major diameter of the screw will be squeezed larger than the diameter of the bar stock (while the minor diameter will become smaller), it is critical that the diameter of the bar stock is precise and falls in the required tolerance.

In the shop practice, the clearances between the outer and inner threads are often made slightly larger than the ISO standards. This means the pitch diameter d2 can be smaller and D2 can be larger. Besides, the OD of the threaded bars (d) and the ID the nuts (D4) can be off the tolerances a bit. After all, the pitch diameters are more critical for their performance. It all depends how the they perform in their actual application in terms of accuracy and operating life. You need to keep the balance between the manufacturing cost and quality, but not be restricted to the standards.

The lead screws usually have larger and flatter teeth than other screws, which allows them to be stronger to transmit torque or power. Based on different thread geometries, the lead screws are classified as

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An example: the tolerance for a mating screw and nut, Tr20*4-8H*8e (8H for the inner thread and 8e for the outer thread)

This is a non-contact inspection instrument, it can not be used to check inner screws. It projects the screw profile to the screen with a much-enlarged image, so you can look at the surface finish closely and check if there are any flaws like scratches, peels, etc. You can also measure the outer, pitch, and inner diameter, flank angle of an outer screw, but you need to capture the profile point by point manually, so the process is quite time-consuming, and sometimes not as accurate if not operated properly.

We have concluded the basic terms for the lead screws as follows. We put the terms in pairs (for the External and Internal threads respectively) for easier understanding.

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“Medium” can be chosen for the usual application. When machining difficulty exists or for lower transmission requirements,  “coarse” can be selected.

2. Screw rolling can only be used for large quantity production. Due to the higher initial tooling cost and the bar stock need to be precise in the diameter. For this reason, it is recommended to use the sizes of threads that the manufacturer is currently producing.

Turing is the most traditional yet effective manufacturing method for lead screws. it uses a formed cutting tool that has the shape of the thread teeth to cut the threads.

Unlike the IT grades (IT represents International Tolerance) which is widely used for general machining tolerancing, the lead screws have a different system for defining their dimensional tolerances. The reference standards are as follows:

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There are different kinds of lead screw manufacturers on the market, some of them have cutting-edge manufacturing and inspection equipment and are well managed, while others are small workshops, but their prices are 2 to 3 times different. Depending on the quantity you purchase and the requirements of your application, you do not need to choose the highest price every time. The key is to choose the appropriate tolerances for each inspection item. If you are not too experienced in this, we are here to help!

3. The shoulders on the 2 ends (for mounting and connection to the driven shaft) should be smaller than the minor diameter of the screw. Otherwise, there will be grooves left after machining of the 2 ends

It has a diamond tip that moves up and down along with the profile when the sample screw moves horizontally. It records its journey with precise 2-dimensional data. These data can be graphed as well to visualize the measurement result.

In order for the teeth profile to be projected correctly, the lead screw must be tilted by the helix angle (θ) so the teeth will face the camera lens vertically. Because of that, the screw profile projector is a bit different than others in that the head of the projector can be rotated to the desired angle to the worktable.

What are the basic parameters for the lead screw and nut? What are the optional lead screw manufacturing processes? How to inspect the lead screw and with what metrics? These are the questions you may have when starting to design a lead screw transmission.

The go-no go gauges can be used for checking the pitch diameters (d2 and D2) of both the screw bars and nuts. They are easy and quick to use.

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The 3-wire method is the contact type measurement method (meaning the measuring device has direct contact with the object to be measured) to check the pitch diameter of screws. It uses a disc micrometer and 3 wires (or called pins) to do the measurement. These 3 wires must have the exact same diameter. 2 wires are put on one side of the screw, and the other wire will be placed on the other side. Using the disc micrometer to check the M value, the pitch diameter can be derived from the following formulas:

Lead screw straightness: Because most lead screws are thin and long pieces, it is inevitable that they will be slightly bent. In the manufacturing of lead screws, the tolerance of straightness needs to be specified. It is also given as the deviation per unit length (or the total part length). For example: 1.5mm/meter, 0.005″/foot, or 1.2mm on the total part length.

When the steel bars are too bent, like over 1 mm/meter, it will cause excessive stress and heat in the screw rolling process, and may cause surface flaws like grooves and peels.

Among them, the trapezoidal or acme threads are most commonly used due to their manufacturability, achievable precision, self-centering property, transmission efficiency, and strength of thread roots.

The 7H and 7e refer to the tolerance bands for the pitch diameter only (7H for the inner thread D2 and 7e for the outer thread d2). The tolerances for the major and minor diameters are defined in other ways. Let’s break it down here:Ti

Obviously, we can use a caliper or micrometer to check the outer diameter of the screw bars (d) or the inner diameter of the nuts (D1), but that is certainly not enough. Here are the inspection devices and inspection methods to check the lead screws and nuts:

The lead screw is a simple but precise mechanism that turns rotary motion into linear motion. They look quite simple. However, it is still important to understand the basic knowledge behind them. In this post, we are going to introduce the basic concepts of lead screws and nuts, different machining processes, the metrics used to measure the quality of the lead screws, and how to inspect them.

Lead accuracy: The lead accuracy or lead deviation means the difference between the actual linear distance and the theoretical linear distance the helix (or the screw) travels. It is usually described by deviation per meet or foot. For example: 1.2mm/meter or 0.01″/foot. Obviously, the better lead accuracy, the more precise linear motion you will get from the lead screw.

Major diameter for Internal screw (D4)—only minimum dimension is defined. This is probably because the D4 is less important. Meanwhile, since internal threads (on the nut) are usually cut by a CNC machine, the geometry of the teeth profile is decided by the shape of the cutter, so the D4 can not be too large without a limit.

Please note dD does not have to be the exact value calculated by this formula as long as they are close enough, and again these 3 wires need to be precisely the same diameter.