Threading On A Lathe
The right choices in tooling and technique can optimize the thread turning process.
Tools for turning threads have benefited from the same improvements in coatings and material grades that have improved turning tools overall. In addition, there have been design improvements in thread turning inserts resulting in better chip control. In spite of these changes, however, manufacturing engineers tend to spend little time optimizing their threading operations, seeing the thread machining process as a “black box” that doesn’t lend itself to incremental improvement.
In fact, the thread machining process can be engineered for better efficiency. The first step is to understand some basic topics in thread machining.
Why Thread Turning Is Demanding
Thread turning is more demanding than normal turning operations. Cutting forces are generally higher, and the cutting nose radius of the threading insert is smaller and therefore weaker.
In threading, the feed rate must correspond precisely to the pitch of the thread. In the case of a pitch of 8 threads per inch (tpi), the tool has to travel at a feed rate of 8 revolutions per inch, or 0.125 ipr. Compare that to a conventional turning application, which may have a typical feed rate of around 0.012 ipr. The feed rate in thread turning is 10 times greater. And the corresponding cutting forces at the tip of the threading insert can range from 100 to 1,000 times greater.
The nose radius that sees this force is typically 0.015 inch, compared to 0.032 inch for a regular turning insert. For the threading insert, this radius is strictly limited by the allowable radius at the root of the thread form as defined by the relevant thread standard. It’s also limited by the cutting action required, because material can’t be sheared the way it can be in conventional turning or else thread distortion will occur.
The result of both the high cutting force and the more narrow concentration of force is that threading inserts see much more stress than what is typical for a turning insert.
Partial Versus Full Profile Inserts
Partial profile inserts, sometimes referred to as “non topping” inserts, cut the thread groove without topping or cresting the thread. (See Figure 1.) One insert can produce a range of threads, down to the coarsest pitch—that is, the smallest number of threads per inch—that is permitted by the strength of the nose radius of the insert.
This nose radius is designed to be small enough that the insert can machine various pitches. For small pitches, the nose radius will be undersize. This means the insert will have to penetrate deeper. For example, a partial profile insert machining an 8-tpi thread requires a thread depth of 0.108 inch, while the same thread produced with a full profile insert requires only the specified depth of 0.081 inch. The full profile insert therefore produces a stronger thread. What’s more, the full profile insert may produce the thread in up to four fewer machining passes.
Multi-tooth inserts feature multiple teeth in series, with a given tooth cutting deeper into the thread groove than the tooth that went before it. (See Figure 2.) With one of these inserts, the number of passes required to produce a thread can be reduced by up to 80 percent. Tool life is considerably longer than that of single-point inserts because the final tooth machines away only one half or one third of the metal in a given thread.
However, because of their high cutting forces, these inserts are not recommended for thin-wall parts—chatter can result. Also, the design of a workpiece machined with one of these inserts needs to have a sufficient amount of thread relief to allow all of the teeth to exit the cut.
Infeed Per Pass
The depth of cut per pass, or infeed per pass, is critical in threading. Each successive pass engages a larger portion of the cutting edge of the insert. If the infeed per pass is constant (which is not recommended), then the cutting force and metal removal rate can increase too dramatically from one pass to the next.
For example, when producing a 60-degree thread form using a constant 0.010-inch infeed per pass, the second pass removes three times the amount of metal as the first pass. And with each subsequent pass, the amount of metal removed continues to grow exponentially.
To avoid this increase and maintain more realistic cutting forces, the depth of cut should be reduced with each pass.
At least four infeed methods are possible. (See Figure 3.) Few recognize how much impact the choice among these methods can have on the effectiveness of the threading operation.
A. Radial infeed
While this is probably the most common method of producing threads, it is also the least recommended. Since the tool is fed radially (perpendicular to the workpiece centerline), metal is removed from both sides of the thread flanks, resulting in a V-shaped chip. This form of chip is difficult to break, so chip flow can be a problem. Also, because both sides of the insert nose are subjected to high heat and pressure, tool life will generally be shorter with this method than with other infeed methods.
B. Flank infeed
In this method, the infeed direction is parallel to one of the thread flanks, which normally means the tool feeds in along a 30-degree line. The chip is similar to what is produced in conventional turning. (See Figure 4.) Compared to radial infeed, the chip here is easier to form and guide away from the cutting edge, providing better heat dissipation. However, with this infeed, the trailing edge of the insert rubs along the flank instead of cutting. This burnishes the thread, resulting in poor surface finish and perhaps chatter.
C. Modified flank infeed (recommended)
This method is similar to flank infeed except that the infeed angle is less than the angle of the thread—that is, less than 30 degrees. This method preserves the advantages of the flank infeed method while eliminating the problems associated with the insert’s trailing edge. A 29½-degree infeed angle will normally produce the best results, but in practice any infeed angle between 25 and 29½ degrees is probably acceptable.
D. Alternating flank infeed
This method alternately feeds the insert along both thread flanks, and therefore it uses both flanks of the insert to form the thread. The method delivers longer tool life because both sides of the insert nose are used. However, the method also can result in chip flow problems that can affect surface finish and tool life. This method is usually only used for very large pitches and for such thread forms as Acme and Trapeze.
Clearance Angle Compensation
Some threading insert and toolholder systems provide the ability to precisely tilt the insert in the direction of the cut by changing the helix angle. This feature provides a higher quality thread because it tends to prevent the insert from rubbing against the flank of the thread form. It also provides a longer tool life because the cutting forces are evenly distributed over the full length of the cutting edge.
An insert that is not tilted in this way—one that holds the cutting edge parallel to the centerline of the workpiece—creates unequal clearance angles under the leading and trailing edges of the insert. (See Figure 5.) Particularly with coarser pitches, this inequality can cause the flank to rub.
Adjustable systems permit the angle of the insert to be tilted by changing the orientation of the toolholder’s head, generally using shims. Precise adjustment results in leading and trailing edge angles that are equivalent, ensuring that edge wear will develop uniformly.
Miniaturization And Specialization
Inserted tools are available to permit internal thread turning of bores down to about 0.3 inch in diameter. Producing the threads for these small bores through turning offers many advantages. The quality of the thread formed is usually higher, the insert design allows chips to flow out of the bore with little damage to the thread, and the ability to index the tooling results in a lower cost for tooling.
The carbide used for these applications is generally a grade that permits machining at low surface speeds. For an internal threading application in a small hole, machine tool limitations generally leave anything other than a low surface speed out of the question.
Technology improvements have expanded the application range of thread turning tools, and the move to internal thread turning of small bores is one example of this. In spite of the expanded range of standard tools, however, manufacturers continue to encounter special problems that justify custom tooling. (See Figure 6.) Special tooling developed in cooperation with the tool supplier is an option that shouldn’t be overlooked when searching for the right threading tool for a particular job.
About the authors: Stuart Palmer is a marketing consultant to cutting tool maker Vargus Ltd. of Nahariya, Israel. Mike Kanagowski is the general manager of VNE Corp., a sister company of Vargus in Janesville, Wisconsin.
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