Modern Threading Technology
Introduction
Manufacturing engineers who are looking to increase productivity spend a lot time optimizing tool setups, selecting the right cutting tool grades for a workpiece, and finding the optimal feed and speed conditions for turning and milling applications. Drm Software Because of the "black box" attitude surrounding threading, they don't necessarily spend as much time optimizing this operation.
Today's threading technology has evolved alongside turning and milling innovations as far as tool grade and coatings. However, advances in insert design for threading chip control, and rapid strides in threadmilling technology have given manufacturing engineers more options for optimizing their productivity.
Thread turning
There are more than 40 international thread standards. Some are less popular while others are much more widely accepted. Many countries also have their own variations of the international standards to meet their manufacturing needs.
The threads are used in the following four categories:
Fasting: nuts & bolts
Included: Gas caps, lids for jars, etc
Connecting: Pipe couplings and fittings
Actuating: Use lead screws to transfer power or motion.
All industries use the ISO and UN standards. However, other standards have more specific uses.
BSW Gas and water fittings
NPT- Pipe fittings
BSPT- Water and gas fittings
ACME- Moving parts
Machine tool construction: Metric buttress-moving parts
Trapeze-moving parts
Fittings for the chemical and food industries in round tubes
UNJ & MJ - Aviation industries
API- Oil industry
The 60o Vee geometry is the basis of about half of all thread forms. They only differ in the sizes of the tolerances, root and crest radiuses.
Threading versus Turning
Threading operations require more effort than simple turning operations. Threading operations require greater cutting forces, and the insert's cutting radius is smaller and therefore less demanding.
When we compare the feed rates for turning and threading we can see that the feed rate in threading must match the thread pitch. For an 8 TPI thread the tool must move at 0.125 inches/revolution. The threading insert's nose radius is usually 0.015". For turning, the usual feed rate is 0.012 inches/ revolution with a standard radius 0.032 ". This example shows that the threading feed rate is typically 10 times higher than turning. The cutting forces at the tips of the threading inserts can be 100 to 1000 times higher than those used for straight turning operations. The threading insert's nose radius plays an important role in threading. It is limited by the radius at the root, as specified in the applicable standard. Thread distortion is not possible with turning, where the material can easily be sheared.
Furthermore, because the thread is formed through a series of passes along its length, the leadscrew is working too hard. It stops and starts, moves forwards and backwards, and this causes a limit on optimization potential.
Partially vs. Full Profile Inserts
Partial profile inserts are sometimes called "non-topping" inserts. They cut the thread groove without topping the thread or cresting it. These inserts can be used to make a variety of threads. However, the insert's nose radius (the most vulnerable part) must not exceed 0.5mm in order to produce the smallest pitch. A small nose radius can also affect the thread's depth. A thread with an 8 TPI thread will require a thread depth of 0.108", while a thread with a full-profile insert will not go deeper than 0.81". A full profile insert produces a stronger thread, and the thread will take four passes to make.
Multi Tooth Inserts
Multi-tooth inserts have multiple teeth that cut deeper into the thread groove than their predecessors. This reduces the number of passes needed to create a thread by as much as 80%. These inserts have a significantly longer tool life than single-point inserts, as the final tooth only removes half or a third the metal from a thread.
These inserts can be a huge boost to productivity. However, because of the higher cutting forces, they are not recommended to thin-walled parts due to chatter. It is important that the workpiece has enough thread relief to allow all teeth to exit the cutting.
Feed Per Pass
Threading is dependent on the depth of cut (or infeed) per pass. Each successive pass engages a greater portion of the cutting edge than its predecessor. A constant infeed per pass will result in increased forces and metal removal rates for each pass.
A 60o thread form will be produced using a constant 0.0.010" infeed. The second pass will remove three times as much metal as the first. The amount of metal that is removed increases exponentially with each passing pass. As a result, the pressure on the nose radius rises. To achieve realistic cutting forces, the depth of each cut should be reduced with each passing.
Infeed Methods
Radial - Not recommended for general usage
Although this method is controversially the most popular, it is not recommended. The tool is fed perpendicularly to a workpiece centerline, so metal is removed from the thread flanks. This creates a V-shaped chip. This type of chip can be difficult to break, making it difficult to control the flow. The insert nose is also subject to high heat, pressure and can cause tool wear.
b) Flank Infeed is generally not recommended
This method of infeed produces a chip that is very similar to conventional turning. It is also easier to form and guide away the cutting edge. This allows for better heat dissipation. This infeed, however, is parallel to one thread flank (30o) and the trailing edge does not cut along the flank. This causes burnishing and possibly, even chatter.
c) Modified Flank infeed - Highly recommended
This is similar to the flank feed except that the infeed angle for this method is slightly lower than the 30o. This allows for the flank infeed method to be used while avoiding the issues of insert training edges. The best results are usually achieved with a 29 1/2o infeed angle, but in practice, it is acceptable to use a 25o or 29 1/2o infeed angle.
d) Alternating Flank Feed - Not recommended
This method uses both the insert nose and flanks to create the thread. It also gives you a longer tool life. This method can cause chip flow issues, which can impact surface finish and tool lives. This method is often used for large pitches, such as ACME, TRAPEZE, etc.
Clearance Angle Compensation
One of the most important features of the laydown system is the ability to tilt the insert precisely in the direction of cutting by changing the angle of the helix.
This allows for a better thread quality because the insert does not rub against the thread form's flank. It also gives the tool a longer life as the cutting forces are evenly distributed along the entire length of the cutting edge.
The insert's cutting edge is on the left, parallel to the workpiece's centerline in the diagram. The clearance angles between the leading edge of the insert and the trailing edge are not equal. This can lead to the insert's flank rubbing against the side due to the use of many thread types, especially coarser pitches.
Laydown anvil systems (a helix angles chart is included in most catalogs to allow easy selection of the right shim for each application) will ensure that the insert's cutting edge (in the direction feed) is tilted in a plane parallel to the helix angle. The insert's trailing and leading edges will have equal clearance angles. This will ensure that the insert won't rub on thread flanks, and edge wear will not develop uniformly.
Thread Tolerances
Manufacturers specify the thread tolerance that their inserts are suitable for. It is important that manufacturing engineers take this into consideration.
Inserts that are compatible with other tolerance-defined threads can be made, but it is best to contact the manufacturer to order these tools by special order.
Miniaturization
Manufacturers are now looking for similar applications with smaller bore diameters due to the success of the laydown method.
The laydown system will typically allow threads to form in bores as small as 1/2" diameter. Interchangeable inserts are available with 2 or 3 edges for smaller diameters, around 0.3 inches.
Mini threading inserts have many advantages over traditional machining small bores. The thread quality is generally higher because the insert design allows for chips to flow out the bore without any damage. Also, indexing makes the tooling investment for machining much lower.
The tooling can be found in a variety of carbides and coatings, giving the manufacturing engineer flexibility to choose the best conditions.
These applications allow for machining at lower speeds than what is usually associated with threading. This is due to limitations in the machine tools rather than the tooling's capabilities.
The micro range of tools is available for bores smaller than 0.3 inches. It offers both threading and machining. This includes copying, turning, and both radial & axial grooving.
These Special Needs
Despite the wide range of tools available for seemingly endless applications, there are always those "special" problems for which standard tools don't provide the solution.