Precision Lead Screws & Nuts

Working closely with our customers to supply a ready to fit machined product not only reduces their scrap rates, but also ensures that the lead screw and nut fit tolerance is maintained every time.  With a machining capability that is enjoyed by numerous customers, an excellent journal concentricity product is delivered, regardless of diameter. Working directly from dimensional drawings, or if they do not exist, from physical examples that enable us to reverse engineer from. Combine the lead screw and nuts with a full range of linear guides and rails will provide load support to any linear mechanism. Offering excellent rigidity and parallelism during operation the guides can be used in both vertical and horizontal planes. The whole linear program allows the supply of complete mechanical systems, through knowledge staff.

Our latest machine, the ROMI GL250M, transforms our machining scope and capability. The ROMI GL250M CNC Turning Centre offers a unique integral C axis milling capability. The new machine is part of the long term vision of the company and its progressive UK manfacturing goals.

ROMI GL250M

A2-6” Spindle 65mm Bar Capacity
Fanuc 0i-TF HMI CNC Control with 15” Touchscreen display
C-Axis Milling Capabilities
Servo Driven Tailstock
The package also comes complete with a bespoke external bar support system for longer length screw stocks. 

Types of screw threads

There are 3 basic ways to make the lead screw thread, those being, rolling, whirling and grinding. Each method has its advantages and disadvantages so let’s look at each method individually. Lead screws can be found in many industries including Automation, Medical, Defence and Transport applications to name a few.

Rolled

The thread form is squeezed into the bar of material using dies under high force. This method makes a thread with a really good sliding surface finish and the process is quick. As the rolling process compresses the material it also superficially hardens the working area of the thread. The majority of our threads are rolled. The disadvantage to a rolled thread is at that very high precision isn’t possible. Most rolled threads are 2G, with care we can roll most sizes to 3G but 4G is too much for rolling.

Whirled (Machined)

The thread is machined rather than formed as it is in rolling. This means the thread is high accuracy but doesn’t have great surface finish nor is it hardened. This thread (with natural finish i.e. uncoated) is not suitable for use of plastic nuts due to surface finish.

Ground

Very precise thread can be created in hardened materials but you have limited choice of sizes and thread forms. The process is also labour intensive which makes them a higher price and longer delivery time than the other methods. A ground thread is normally used where accuracy is of upmost importance.

Screw Materials

As standard, most rolled lead screws are made from 300 series Stainless Steel. These materials form well, give a great surface finish and have good corrosion resistance. For a ground screw, something like 440C or 17-4pH works well as it is a grade of stainless which can achieve sufficient hardness. But these types of Stainless doesn’t have the corrosion resistance of 300 series. Customers like the thought of 440C or 17-4pH because of its hardness but unless it’s a ground thread, it is difficult to machine and doesn’t leave a great surface finish.

Nut Materials

Several types of material can be used for the lead screw nut. The material you choose for the nut depends on the application. Broadly speaking, you have 3 nut materials; Steel, Copper Alloy or Polymer (Plastic)

Steel - Used for high loads at low speed.

Copper Alloy (Bronze) - Bronze nuts are very popular for lead screws. There is a vast choice of bronzes to choose from, such as Leaded (Pb) Bronze, SAE660 or Sigma 7 Bronze are also popular choices. (The Pb in the material reduces the sliding friction between nut and screw). Materials with higher strength include Tin Bronzes (PB102) and Alu Bronzes (CA104) but these have more friction than the Pb Bronze, less tolerant of lack of alignment,  lubrication and shock loads.

Polymers - The number of plastics suitable to make lead screw nuts is almost endless however, there are several that have become the industry norm. Plastics are useful because in the main they are low friction and require little if any lubrication.

Lead Screw Coating

Various coatings can be applied to the screw to reduce friction of the mating part. PTFE based coatings can have very low coefficient of friction but can be quite soft and can be rubbed off by abrasion of even a plastic nut with bronze nuts wearing through it quickly.

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