THREADING ON THE LATHE
For more information, please visit Threading Pipe On a Lathe Manufacturer.
Introduction
This document presents some of the more common techniques for threading on the manual engine lathe.
Tap Handle
Using a tap handle is the most common method of tapping on the lathe. The workpiece is clamped in the lathe chuck, a spring loaded center (for smaller taps) or a dead center (for larger taps) is clamped in the tailstock, and the tap is held and rotated using a tap handle, as we do with the assigned parts in lab.
Figure 1a: Examples of using standard tap wrenches and spring loaded tap guide (left) or dead center (right) to tap holes on the lathe.
Figure 1b: Example of using an adjustable wrench and a live center to tap holes on the lathe.
Figure 1c: Examples of various tap handles.
Die Handle
Using a die handle is a common method of external thread cutting on the lathe. The workpiece is clamped in the lathe chuck, and the threading die is held and rotated using a die handle.
In general, round-shaped dies are for cutting threads onto a workpiece and hex-shaped dies are for chasing (cleaning up / repairing) existing threads.
Before using a threading die it’s important to make sure the major diameter of the shaft to be threaded matches the range listed in the Machinery Handbook. For example, a ½-20 UNF 2A thread must have a major diameter between 0.4906 and 0.4987”. The smaller the major diameter, the easier the die will cut. In general, undersize the shaft diameter by 2% of the major thread diameter.
When using a threading die on the lathe it’s important to start the thread die collinear to the axis of the part, so use the body of the drill chuck for alignment and guidance. It’s also important to cut a generous chamfer on the end of the part to help the threading die start cutting.
Figure 2a: Examples of various threading die handles.
Figure 2b: Using the drill chuck body to align the threading die axis with the workpiece axis when starting the thread.
Rigid Tapping
Rigid tapping is the second most common method of thread cutting on the lathe. With this technique the tap or die is clamped in the tailstock using a variety of methods and threaded into or onto the workpiece under spindle power. Smaller taps up to 3/8” can be clamped in a keyed Jacob’s style chuck (NEVER a keyless chuck!). Larger taps should be clamped using a split sleeve or heavy duty tap driver, as shown in figure 3b.
Figure 3a: Rigid tapping on manual lathe. Click the image on the right for a video showing the process.
Figure 3b: Example of split sleeve tap driver for lathe tailstock (left) and heavy duty tap driver for lathe tailstock (right).
Figure 3c: Rigid die cutting on manual lathe.
Die cutting video
Single Point Threading
Single point threading involves mounting a threading tool with the proper thread profile to the toolpost and cutting the thread using multiple synchronized passes.
In general there are two types of cutting tool geometries which can be used: partial and full form profiles. Partial profile cutting geometries only cut the minor (or root) diameter of the thread, whereas full profile cutting geometries cut both the minor and major diameters of the thread profile to size. The advantage of partial profile cutting geometries is that one tool can cut a variety of thread pitches, whereas a full profile cutting geometry is only good for one particular thread pitch. The advantage of full profile cutting geometries is that the entire thread is finished in one operation, saving significant thread finishing and deburring time.
Figure 4a: Partial vs full form profile threading geometries.
The following videos explain the process in good detail. Fast forward through the parts which are not interesting to you J.
Figure 4b: Good single point threading video (left; threading starts at 18:08 time stamp) and shorter clip of thread cutting (right).
Figure 4c: Examples of properly designed thread reliefs.
The process for single point turning threads in the design lab is as follows:
1. Clamp the part in the lathe using a live center if necessary.
2. Turn the OD to the target major diameter and include a chamfer on the end at least 0.020” smaller than the minor diameter of the thread profile to be cut.
3. If permissible, cut a thread relief using a grooving tool (as shown in Figure 10 and the two video thumbnail images above). The thread relief should be slightly less than the minor thread diameter.
4. Adjust the gearbox levers on the front of the headstock to cut the proper thread pitch.
5. Adjust the threading tool so it is aligned parallel to the X-axis.
6. Touch off on the part and zero the X-axis.
7. Cut a light (0.001 - 0.002”) scratch pass across the surface of the part to be checked with a thread gage for accuracy.
8. If the pitch of the scratch pass measures correctly, begin cutting the thread to depth; start with deeper depths of cut (.010” in aluminum, 0.005” in steel) and make progressively shallower cuts as the thread gets deeper and the threading tool begins to leave a worse finish)
9. As you approach final thread size, use a fine file to carefully debur the rough edges of the major diameter (unless using a full profile insert, which deburrs the major diameters automatically, as discussed above). The major diameter should end up a few thousandths of an inch under the nominal size, according to the tolerances listed in the Machinery Handbook (e.g. 0.4906-0.4987” for a ½-20 UNF 2A thread). You will know when you are close to the final size by keeping track of your X-infeed value, which will end up smaller than the equivalent internal thread’s minimum minor diameter by the noted allowance (e.g. 0.446” – 0.0013” = 0.4447” for the same ½-20 UNF 2A thread).
10. The procedure for making an actual cut is:
a. Check the direction of the threading direction by engaging the half-nut with the tool a safe distance from the part; for this example, we will thread toward the chuck
b. Adjust the spindle speed to a low setting (100-300 rpm) depending on how brave you are J
c. Position the tool in a safe starting location to begin cutting the thread
d. Advance the tool toward the part the distance (depth of cut) you wish to cut
e. Engage the half-nut for threading (it’s safest to leave this engaged for the duration of the threading session)
f. Turn the spindle ON in the FWD direction and allow the tool to make a cut
g. Turn the spindle OFF before the tool reaches a shoulder (if not exists); you can use the foot brake to stop it quickly if needed; if you stop too early, simply bump the power switch to continue the cut or rotate the chuck by hand
h. Retract the tool a safe distance from the part in the X-direction
i. Turn the spindle ON in the REV direction to allow the tool to return to a safe starting location
j. Repeat steps d. thru i. until the desired minor or pitch diameter is reached.
Figure 4d: Insert comprehensive single point threading video here?
Thread Measurement
Threads can be measured at least three different ways: by checking with a mating nut or thread gage, by using a dedicated thread micrometer, or by using the three wire method.
Mating Nut or Thread Gage
Checking with a mating quality nut or thread gage is the easiest method to determine when the thread is cut deep enough. Nuts are much cheaper than calibrated thread gages, but work fine for most prototyping applications.
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Figure 5a: Checking thread size using an existing, quality nut. It’s convenient to keep a complete set of quality nuts on rings for thread measurement (right).
Figure 5b: Checking thread size using a thread gage. Click the thumbnail for the video.
Thread Micrometer
Using a thread micrometer is the easiest way to accurately measure thread pitch diameter for comparison to the thread data listed in the Machinery Handbook or this link. However, thread mics are fairly expensive.
Figure 6a: Example of a thread micrometer (notice the v-shaped anvils) and its use measuring thread pitch diameter.
Figure 6b: (Poorly made) video on how to use a thread micrometer. Click thumbnail for video.
3 Wire Method
The three wire method uses basic geometry and three identically sized wire rods to allow the pitch diameter to be calculated using any standard micrometer measurement. The three wire method is very accurate and the cheapest method of measuring a variety of thread pitch diameters. The downside to this method is that it requires a lot of dexterity to make an accurate measurement without dropping the precision thread wires.
Figure 7a: Checking thread size using 3 wire method. Click the thumbnails for the videos.
Figure 7b: Thread wire formulas for converting between actual measurement and pitch diameter.
Figure 7c: Three wire measurement method explained. Click image for .pdf file.
Miscellaneous Points
Always use cutting oil when threading on the lathe. WD40 works well for aluminum. Oatey dark threading oil works well for steel. Chlorinated Moly-D works best for materials which are tougher to machine, like stainless and alloy steels.
Photo Credit: Getty Images
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.
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.
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