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Threading demands smart choices. (Technology Notebook).

Once the purview of specialists and skilled machinists, threading is no longer an art. Today, it has become, in fact, more like a general turning operation. Advances in machine tools, CNC programming and cutting tools facilitate thread generation even for the smallest job shops. Positive geometry, more heat-resistant inserts, versatile tool styles and stable toolholding enable metalworking shops to automate their threading operations, while meeting today's demanding tolerances.

While CNC and DNC machines have increased in use during the last 10-15 years, only recently have job shops installed sophisticated CNC systems. These systems, now available with canned threading programs at lower cost, enable shops and production floors to run threading unsupervised with the proper tooling.

Many manufacturers today require greater precision and higher productivity in their threaded components in some of the newer, tougher stainless steels, Inconels, Hastelloys, and titanium alloys. The aircraft industry, in particular, has adopted System 22 and System 23 military specifications, which cut conventional aircraft threading tolerances in half on both angularity and precision. To meet precision requirements, proper threading tool selection is key. And there are a number of new tools to choose from, since cutting tool suppliers have responded to these demand side developments.

Let's examine some of these innovations to enable you to take a systematic approach to threading tool selection. First a look at the challenges.

Perils in selection

At stake when selecting threading tools are productivity, precision and tool life. All these concerns correlate directly to whether you can meet customer specifications and delivery requirements. Chip control is a major challenge, particularly in high-speed, unsupervised or limited supervision operations. Spiral or overly long chips often coil around chucks, conveyors, tools, and fixtures causing damage to the workpiece and equipment and loss of productive machining time.

Tool wear is another concern, though tool wear itself is not always negative. It's, in fact, expected in any chipbreaking operation. So it's not a matter of whether tool wear will occur, but rather when. Premature abrasive wear, for example, is quite commonplace and acceptable in threading. What you have to watch out for is premature edge wear. When you choose the wrong insert grade, premature wear increases cycle time, compromises surface quality and dramatically shortens tool life.

Typical premature wear patterns to avoid include plastic deformation, built-up edge (BUE), edge spalling, frittering and breakage. Plastic deformation typically occurs when the insert cannot withstand the high temperatures generated at the cutting zone, causing the edge to melt and lose its original shape. It is prevalent with low conductive and surface hardened metals. The remedies are usually to switch to more heat resistant insert substrates and coatings.

BUE, edge spalling and frittering, which often occur in combination, arise when the selected insert grade has insufficient toughness. This is a common failure mechanism when threading stainless steel materials and low-carbon steel workpieces. PVD-coated inserts are usually the threading tool of choice in these cases.

Insert breakage arises from a mismatch of insert grade to the material or from poor chip control. This type of failure indicates the need for a tougher insert grade or a switch to a different insert geometry. Highly abrasive workpiece materials and high cutting speeds can cause rapid flank wear. Such situations also require a grade with higher wear resistance.

Of material importance

Newer materials-for example, Ti 6-22-22, an alpha/beta titanium alloy, Aerospace-Met 100, an iron-based super alloy, and 15-5 PH stainless steel-are becoming more prevalent in the aircraft industry. These alloys are characterized by greater toughness, lighter weight and higher yield strengths. By the same token, these physical and mechanical properties also make them more difficult to machine with conventional tooling.

Stainless steels are typically characterized by low thermal conductivity and high surface hardenability, resulting in poor chip control, edge notching and BUS. The problem is exacerbated at higher machine speeds. Titanium alloys likewise possess low thermal conductivity and are prone to BUS. Bottom line is that these alloys require sharp, high positive-rake inserts with stable toolholding to counteract high cutting temperatures and cutting forces. Proper tool selection converts an otherwise difficult job into a conventional turning operation, shearing the chip off, rather than scraping it off. It will also facilitate chip control and evacuation.

Positive developments

So what's new in modem tooling? For starters, three geometries and one grade are usually all you need to thread 95 percent of the most commonly used materials. These inserts comprise general purpose, F, and C geometries.

The general purpose PVD (Physical Vapor Deposition) insert is optimized for extra toughness and BUE resistance. It features sharp edges and positive-rake angles to break small chips. It is suitable for both right- or left-handed threading. Combined with special edge treatment, it is a stronger insert for improved cutting action, chip control and security. In addition, chipbreaking dimples on the top surface of the insert direct chips away from the workpiece. This means that you get better surface finish with fewer passes, indexes and insert changes. The cumulative result is higher productivity and long, consistent tool life.

F and C geometries

The sharp F-type geometry in a PVD grade is ideal for softer, sticky materials prone to BUS as well as work hardening surfaces. It can be used for right or left-handed threads, depending on the infeed method. It is recommended for low carbon steels and Duplex stainless steels and narrow or small diameter components that are usually machined at lower cutting speeds. Uncoated grades, with a sharp F-type geometry, are optimized for work in Duplex steels and titanium alloys.

The C-type PVD grade is optimized for threading low carbon, carbon alloys, and easily machined stainless steels (cast, martensitic/ferritic). The C-geometry is symmetrical, which means that it can be used for both left and right hand threads. It is the insert of choice for unsupervised operations, especially internal threading. If the part has higher than Rc 47 hardness, use a cubic boron nitride threading insert. It generates consistently small chips, which are easily evacuated, particularly in narrow bores and blind holes.

These geometries are available in three different threading styles: full profile, V-profile and multi-point. Full profile styles are the most commonly used and offer the highest productivity because they form a complete thread profile, including the crest. V-profile inserts do not top the crests and require an adjustment to their outer and inner diameter settings for screws and nuts with different thread angles. Multi-point inserts are similar to full profile but have two or more points instead of one. They are the most productive for mass produced threads but must have extra stable toolholding because of longer cutting edges and greater loads.

Toolholder styles

As far as toolholder styles are concerned, the good news is that they are more versatile and accurate. The new styles accommodate a more extensive range of thread types and pitches and right-and left-hand versions for external and internal operations. There are essentially three styles: a high-productivity one for three edges (U-Lock system), a two-edge style (Top-Lok system), and a single-edge style for small diameter internal threading operations (Mini V-Lok).

The three-edge toolholder accommodates three insert sizes and 12 different styles for a wide range of thread types, including American UN, ISO metric, Whitworth, Round DIN, BSPT, and Acme. For close tolerancing, the two-edged TopLok style damps the insert in the holder in precision ground surfaces by means of a top screw. This kind of clamping not only locks the insert against the back of the pocket, but also against the side. Thus, it provides clamping integrity both axially and radially for accuracy, strength and rigidity. This style meets the threading tolerances required by the newer System 22 and System 23 military specifications.

For external threading, you can choose from offset, no offset, or a drop-head type of toolholder to suit your threading application. Offset toolholders are typically the first choice. However, when threading small shafts (32 pitch, 8 t.p.i threads), a toolholder with no offset can be a better option.

Drop-head toolholders have been specifically developed for use upside-down in external operations. Using a normal holder upside-down requires altering the clamping system to maintain center height. In those cases, a drop-head holder upside-down assists in chip removal and helps maintain center height without extra adjustments.

For internal threading, boring bars for both the U-Lock and Top-Lok systems accommodate inserts ranging from 1/4" to 1/2" and handle overhangs up to seven times the bar diameter. To avoid vibration, deflection and chatter in deep-hole threading, tuned antivibration boring bars are another option. In addition, you can fit some boring bars with exchangeable cutting heads, which supply coolant internally for better heat and chip control in smaller and medium-sized diameters.

Two newer insert clamping systems for U-Lock recently were added to supplement the previous eccentric screw system: a wedge clamping and a screw clamping system. Both provide greater positional accuracy and easier handling. The wedge clamping system is for applications where accessibility is of special importance. The screw system is for the very smallest bars.

Shims, which are used with almost all threading styles, except 1/4" widths, enable you to optimize the insert inclination angle for a wider variety of threading conditions. Shims come in 1-degree increments, ranging from -2 degrees to +4 degrees. They enable you to pull thread, which is useful when threading a blind hole. Negative inclination shims are also available so you can turn left-hand threads using right-handed tools, or vice versa. Another point about shimmed tools: they protect the insert. If there is a malfunction, the shim goes first and is much cheaper to replace.

Integral or modular?

With regard to toolholding systems, your choices include conventional integral shank holders and boring bars, or modular quick-change tooling systems. Integral toolholders perform well in external threading, especially if you only generate a few thread styles. But if you run high volumes, change threading inserts often or switch the toolholder from machine to machine, we recommend a modular system. Today's modular toolbolding systems are quick-change, rigid and truly universal. You can change inserts with a quarter turn of an Allen wrench in 10 to 15 seconds and continue machining. You won't have to measure or offset edges again, or do test cuts. The mating polygonal coupling cannot be clamped together wrong. Positioning repeatability is 0.000050". These time-savers add up when threading operations see a variety of thread profiles and pitches.

When off-the-shelf threading tools and toolholding do not satisfy a particular application, think of custom tools. These special tools have lately come down considerably in price and deliveries. If you supply the major OEMs with a part drawing of the threaded component, they will usually design the right tools for the job.

Making the selection

Having covered current threading tool innovations, let's get down to the actual selection decision-tree. Many factors drive tool selection. These include the shape of the component, accessibility, material, type and condition of the machine, and tolerancing requirements.

To optimize tool and toolholding selection for a specific thread turning application, ask the following questions:

* Is the thread external or internal? External threading can generally be accomplished with a general purpose tool. Internal is more challenging, and you might want to consider a general purpose insert with a higher-precision tool, such as a Top-Lok system.

* Is the thread right- or left-handed? You'll have to match the insert geometry and infeed method to the type of thread you're generating.

* What type of material are you threading? In general, default to a general purpose.

* For PVD-coated insert for steel, stainless steel and cast iron or more challenging alloys, consider a sharper F-geometry; for finishing operations in hardened materials consider a cubic boron nitride insert.

* What type of thread do you want to generate? For example, ACME, Stub Acme, buttress, API, NPO, UNJ, MJ, MM, etc. A general purpose insert will generate most thread types. For closer tolerances, use a higher-precision toolholder, such as a Top-Lok system.

* Are you having problems with chip evacuation? Make sure that you have a tool that generates small chips and directs them out and away from the workpiece.

* If you're performing internal threading, is the workpiece diameter small? Are the internal clearances limited? Use a boring bar style optimized to the workpiece dimensions.

* If threading is internal, do you have to reach down deep into the workpiece? If so, you'll need stable clamping and toolholding. If the depth of the hole is more than five times the diameter, consider an antivibration boring bar.

* Are you running the operation supervised or unsupervised? Choose an advanced insert optimized for your material and application.

* Are you observing premature tool wear? You may have to switch to a more heat resistant insert grade or coating, higher-positive rake angle, or sharper, reinforced edge.

* Are you generating a thread in thin-walled or soft stock? You may have to run at slower speeds, thus you may need an insert geometry or grade other than general purpose, such as an F-geometry.

* Are you generating threads upside-down for better chip control? Use drop-head cutter styles developed specifically for upside-down applications.

* Do you have a geometrically demanding workpiece or unusual thread style? You may want to consider a custom-made threading tool or toolholder.

Though threading has become considerably simpler, cutting tool manufacturers continue to develop newer tooling systems to meet material and machining innovations. The speed of change in a competitive market requires that machine shop personnel keep abreast of these developments so they maximize their existing capital assets.

Sandvik Coromant Co., Fair Lawn, NJ, or circle 382.
COPYRIGHT 2001 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2001 Gale, Cengage Learning. All rights reserved.

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Author:Pierce, Syd
Publication:Tooling & Production
Date:Oct 1, 2001
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