Dec. 02, 2024
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Advancements in coatings and material grades have also positively impacted tools designed for thread turning. Additionally, improvements in thread turning inserts have been made to enhance chip control. Nevertheless, many manufacturing engineers overlook opportunities to optimize their threading operations, viewing the process as a 'black box' that resists incremental enhancements.
In reality, the thread machining process can be adjusted for greater efficiency. The initial step involves understanding some fundamental principles of thread machining.
Thread turning poses greater challenges than standard turning activities. Higher cutting forces are typically involved, and threading inserts have a smaller cutting nose radius, which makes them weaker.
The feed rate in threading must closely match the thread pitch. For instance, with a pitch of 8 threads per inch (TPI), the tool must progress at a feed rate of 8 revolutions per inch, or 0.125 inches per revolution (IPR). In contrast, typical feeding in conventional turning may revolve around 0.012 IPR. Thus, the feed rate for thread turning is significantly higher by a factor of ten, leading to cutting forces at the tooling tip potentially being 100 to 1,000 times greater.
The cutting radius under this pressure is generally 0.015 inches, unlike the 0.032 inches seen in conventional turning inserts. The threading insert’s radius is constrained by the thread form's root radius defined by standards and the cutting action necessary; otherwise, thread distortion could occur.
As a result, threading inserts endure considerably more stress compared to standard turning inserts.
Partial profile inserts, known as 'non-topping' inserts, carve out the thread groove without finishing or cresting the thread. A single insert can create a variety of threads down to the coarsest pitch permitted by the nose radius strength.
For finer pitches, the nose radius is smaller, signaling that the insert must penetrate deeper. For example, when machining an 8 TPI thread, a partial profile insert requires a thread depth of 0.108 inches, as opposed to just 0.081 inches with a full profile insert, which consequently generates a more robust thread in fewer passes.
Multi-tooth inserts possess several teeth arranged in a series, where each tooth cuts deeper than its predecessor. These inserts can significantly decrease the number of passes needed for thread production by as much as 80 percent. Moreover, they typically have an extended tool life compared to single-point inserts because the final tooth removes a fraction of the metal in a particular thread.
However, due to their elevated cutting forces, they are unsuitable for thin-walled parts as they might cause chatter. Additionally, the design of a workpiece being machined with a multi-tooth insert needs ample thread relief for all teeth to exit appropriately.
Critical to threading is the depth of cut per pass or infeed per pass, as each successive pass engages more of the insert’s cutting edge. Maintaining a constant infeed per pass can lead to dramatic increases in cutting forces and metal removal rates.
For example, when creating a 60-degree thread form using a constant 0.010-inch infeed, the second pass may remove three times the metal of the first. To manage the cutting forces better, it’s wise to reduce the cut depth with each pass.
There are at least four infeed techniques available. Many operators are unaware of how significantly these choices can influence threading efficiency.
This method is widely used for thread production but is not necessarily the most efficient. The tool is fed radially (perpendicular to the workpiece centerline), which can create a V-shaped chip that complicates chip flow. Additionally, since both sides of the insert’s nose experience high pressure and heat, tool life can suffer compared to other methods.
Here, the infeed direction aligns parallel to one of the thread flanks—typically on a 30-degree line. Compared to radial infeed, this method generates a chip easier to form and remove, aiding heat dissipation. However, since the trailing edge rubs along the flank instead of cutting, this may produce poor surface finishes or cause chatter.
Similar to flank infeed, this technique employs an infeed angle of less than the thread angle, which negates the drawbacks of the trailing edge rubbing. Ideally, an angle around 29.5 degrees is optimal, with any angle between 25 and 29.5 degrees likely producing satisfactory results.
This approach feeds the insert alternately along both thread flanks, effectively utilizing both sides of the insert. It tends to extend tool life as both sides engage in forming the thread, yet it may lead to chip flow issues impacting surface finish and tool longevity. This method is generally reserved for large pitches and specific thread forms like Acme or Trapeze.
Certain threading insert systems allow precise tilting of the insert in the direction of the cut by modifying the helix angle, resulting in superior thread quality by preventing flank rubbing. This tilting also enhances tool life by evenly distributing cutting forces.
When the cutting edge of the insert remains parallel to the workpiece’s centerline, it leads to uneven clearance angles at the leading and trailing edges, which can cause the flank to rub, especially with coarser pitches.
Adjustable systems facilitate better angle control by altering the toolholder's head orientation, leading to uniform edge wear.
Inserted tools now allow for internal thread turning in bores as small as 0.3 inches in diameter. Thread forming in these small bores provides multiple benefits such as better thread quality, effective chip removal, and lower tooling costs due to indexable designs.
The carbide utilized for these tasks typically allows low surface-speed operations, which is essential for internal threading in confined spaces.
Ongoing technological advancements have broadened thread turning tool applications; however, unique issues may still arise, necessitating tailored tooling solutions created in collaboration with the tool supplier.
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Screws and their threads are essential in countless applications. The variety of thread types is nearly as extensive as the number of products utilizing threaded fasteners. Unfortunately, confusion regarding threads prevails among individuals who aren't deeply familiar with mechanics.
From a machinist's standpoint, threading can be an incredibly rewarding task. The end result is two parts that fit together with a precision and smoothness that outmatches conventional fasteners. Having spent years threading on a manual lathe, I’ve picked up a few strategies worth sharing.
Photo courtesy: All images: T. Lipton
Align your threading tool against a freshly faced end or against the side of the chuck.
Instead of using alignment tools, which often complicate the process, consider investing in a threading tool that accommodates inserts for precision and ease of replacement.
I learned threading on a lathe by employing the compound infeed method. Contrary to common belief, the compound does not have to be set at half the thread angle. Using modified-flank infeed and adjusting the angle helps mitigate challenges with difficult-to-machine materials.
The compound enables you to avoid needing to remember the dial position after each pass, as the cross-feed dial resets after each operation, simplifying the process.
When concluding threads without specified ends by a designer, a threading tool can be used to make a small relief which prevents additional tool changes. Alternatively, utilizing a radius tool can enhance visual appeal, ensuring the relief remains below the thread's minor diameter for full mating capability.
Keep a complete set of nuts on rings, one for coarse and another for fine threads.
Initially, employ a larger depth of cut (DOC) on your first threading pass for efficiency, then taper your DOC as you proceed deeper. For the last pass, a light 0.001" spring cut can remove chatter and smooth out any tool marks.
Consistency is crucial; always refer to the same line or number on the threading dial, especially for multiple-start threads.
For internal threading, begin with left-hand tools from within the bore to minimize chatter and maintain visibility down the hole. Ensure that these tools can run the lathe in reverse.
Fine threads are generally less demanding and require fewer passes than coarse threads, which can save you significant time when dealing with hard-to-machine materials.
To effectively fit threads, a complete set of nuts on rings is essential. Be diligent about running the nut the full length of the threads, as machinists often unconsciously produce tighter threads than necessary.
Mating materials can significantly influence threaded connections. If using the same materials for both male and female threads, applying a thread lubricant or anti-seize before tightening can greatly enhance performance.
A thread file is an excellent tool for correcting minor flaws at the beginning and end of an external thread.
For stuck male and female threads, gently heating the female section with a propane torch to around 100°F can help release them, especially with a quick application of penetrating lube.
When measuring threads, a dedicated thread micrometer is convenient and quick, but for precision, the three-wire measuring technique is preferred, offering a true parallel surface for accurate measurement. It's a reliable method favored by gauge makers.
To hold measuring wires, a bit of modeling clay or window glazing putty can be effective, or investing in holders specifically designed for micrometer spindles is advisable.
Thread files can be quite effective, especially for smoothing out the common issue of thread fade at the start and end of external threads. CTE
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