Dec. 02, 2024
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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.
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 profile inserts, sometimes referred to as &#;non topping&#; inserts, cut the thread groove without topping or cresting the thread. 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. 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.
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. Few recognize how much impact the choice among these methods can have on the effectiveness of the threading operation.
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.
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. 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.
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.
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.
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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. 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.
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. 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.
Screws and screw threads hold millions of things together. Nearly as many types and forms of threads exist as there are products that use threaded fasteners and connections. Equally, there is much confusion and misuse of threads in general among those who aren&#;t &#;gearheads.&#;
From the machinist&#;s point of view, cutting threads is a satisfying experience. When you&#;re done, hopefully, you have two parts that mate together with a level of precision and smoothness not found in run-of-the-mill, hardware-grade fasteners. I have always enjoyed cutting threads on the manual lathe and have learned a few tricks over the years.
Courtesy of All images: T. Lipton
Align your threading tool against a freshly faced end or against the side of the chuck.
&#; Align your threading tool against a freshly faced end or against the side of the chuck. The little arrow-shaped alignment tools you see are a pain and are only good for gaging hand-ground tool bits.
&#; If you do a lot of threading on a manual lathe, invest in a tool that accepts inserts. The inserts are precisely ground and easily changed. One insert cuts dozens of thread pitches.
&#; I learned how to thread on the lathe using the compound infeed method. Contrary to popular belief, the compound set doesn&#;t have to be at half the thread angle. By using what&#;s called &#;modified-flank infeed&#; and changing this angle, you help alleviate threading problems in difficult-to-cut materials.
&#; Another advantage to threading with the compound is you don&#;t have to keep track of the dial position. The cross-feed dial is always zeroed after each pass, so you have less to remember, such as whether the last pass was at 0.030 " or 0.050 ". The main disadvantage is your Z-axis position changes as you feed in. This is usually not a problem on external threads, but it can be on internal threads that end against a shoulder.
&#; Try the following strategies when you are ending threads and the part designer has not specified a thread. When I want to do something with the groove that gets cut at the end of the thread, I usually use the threading tool and traverse a small relief at the end. It saves a tool change and looks OK. If I want a nicer look, I switch to a radius tool. Just be sure the relief is a little smaller than the thread&#;s minor diameter so the mating part will thread all the way to the shoulder.
Keep a complete set of nuts on rings, with one ring for coarse threads and the other for fine threads.
&#; Use a large DOC on your first pass during threading. The point is small; in the first couple of passes, the area of the tool tip engagement is also small. Taper your DOC as you get deeper. On the last pass, feed straight in with the cross-feed at a light 0.001 " spring cut. This cuts on both tool flanks and removes chatter and tool marks from the thread.
&#; I can never remember which line on the threading dial to use with which thread pitch. If you&#;re lucky, it will be marked. When in doubt, just use the same number or line each time. Always use the same number when cutting multiple-start threads.
Do internal threading from the inside out with left-hand tools. You will get less chatter and see what&#;s happening down the bore. You will need left-hand threading tools, running the lathe in reverse. Remember, it&#;s easy to pull a rope; it&#;s really hard to push one.
&#; When you have a choice, fine threads are easier to cut and need fewer passes than coarse ones. The shallower depth on difficult-to-cut materials might save your bacon.
&#; For quick and easy day-to-day threading gages, I keep a complete set of nuts on rings in my toolbox for fitting threads. One ring holds coarse threads and the other holds fine ones. When you thread, be sure to run the nut the full length of the threads. When left to their own devices, machinists tend to cut threads tighter than necessary.
&#; Mating materials in threaded connections are important. If you must use the same material for male and female threads, do yourself a favor and put a few molecules of thread lubricant or antiseize on them before you crank them together.
A thread file is ideal for straightening the annoying half thread fade at the beginning and end of an external thread.
&#; If you do happen to get male and female threads wedged together in an intimate embrace, a simple trick to separate them is to quickly warm the female portion to 100° F or so, using a propane torch. A quick shot of penetrating lube before you twist might save the work.
&#; When measuring threads, a dedicated thread micrometer is handy and quick to use at the machine. But for the highest accuracy, use the three-wire thread measuring method. This method is more accurate because the wires present a true parallel surface for measuring. If it&#;s good enough for the gage makers, it&#;s good enough for me.
&#; A piece of modeling clay or window glazing putty can help hold pesky thread measuring wires. Better yet, buy a set of the plastic holders that fit the micrometer spindle.
&#; Thread files actually work. They are great for straightening the annoying half thread fade at the beginning and end of an external thread. CTE
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