When making threads on the lathe, you need a way to correctly size the threads. Threads are a lot more complicated than you’d think at first and they have many critical dimensions. I don’t pretend to be an expert on them but some of the most basic of these are the threads per inch, major diameter, minor diameter, and pitch diameter. Threads per inch (TPI) is the number of threads within an inch of distance along the screw. The major diameter is the diameter measured at the top of the threads and the minor diameter is the diameter measured at the bottom of the threads. The pitch diameter is a little more complicated. It is a diameter that is between the major and minor diameter usually closer to the minor diameter. It’s exact location, compared to the major and minor diameter, depends on the shape of the threads. What is important is that if the pitch diameter of the screw and bolt are identical and there is clearance between the tops and bottoms of the opposing threads they will both fit together exactly. In the real world there is a tolerance between screw and bolts. So, there are max and mins on all of these values for screws and nuts. As always, Wikipedia has a good page on the topic.
With all that in mind, you need to be able to measure these dimensions when making a screw. Threads per inch is set by the lathe and can be checked with gauges. Major diameter can be measured with a regular micrometer. Measuring the pitch diameter requires another tool such as thread wires, thread triangles, or thread micrometers. Thread wires and thread triangles requires you to place wires or triangles on the top and bottom of the threads. Next, you measure the thickness of the stack with a micrometer and subtract a constant found in the instructions that came with the thread wires or thread triangles. This can be tricky and may require three hands to keep from dropping a wire or triangle. Another option is to use thread micrometers like those pictured below that I snagged from Ebay.
Thread micrometers allow you to take a direct reading of the pitch diameter but are limited in the threads per inch and diameter that they can measure. Thus, you’d require and array of thread micrometer to cover every conceivable thread. In the picture above, one micrometer covers 14-20 TPI and the other 22-30 TPI. Thread wires and thread triangles are cheaper and cover a wider range of threads per inch and diameters. As with many things, there’s a trade off between the methods.
Thread micrometers measure pitch diameter by using a special set of anvils which are the parts of the micrometer that press against the screw. These anvils are shown below. The fixed anvil has a rotating W shape and the moving anvil has a conical shape.
To take a measurement a screw or bolt is placed inside the micrometer with the fixed anvil fitting over the top of a thread and the moving anvil fitting in between two threads. The pitch diameter can now be read off the micrometer.
To cut an external thread requires consulting a chart to find the the appropriate dimensions. Note that there are different classes of threads with different tolerance ranges but I’ll be using the nominal values from the Machinery’s Handbook. For the example in my post, I want to cut the standard 3/8″-16 thread.
The chart tells me I need to make the diameter of my screw 0.375″ which I’ve done below using a normal micrometer to measure.
Next, I consult the chart to find the minor diameter and undercut the end of threads to the diameter given which is 0.298″. This undercut area gives me a place for the tool to stop after making a threading cut.
From here, I verify my compound is set at the correct angle measured from the face of the chuck. (People debate what this angle is but it is usually said to be 29, 29.5 or 30 degrees. I’ve had luck with 30 degrees.) Next, the tool is moved in to lightly touch the surface and the dials set to zero on the compound and cross feed. To verify the threads per inch is what you expect it to be, a very light pass is taken and compared against a thread gauge. With the threads per inch verified, I can now proceed to feed the compound in and make a pass using the longitudinal feed. The feed is disengaged when the tool enters the undercut area, retracted and moved back to before the beginning of the thread. The tool is then fed back in to zero on the cross feed, the compound fed in a little more, and another pass taken. At the end of every pass a measurement of the pitch diameter can be taken with the thread diameter but doesn’t need to be until the threads get closer to completion. You can roughly determine the progress of the threads by looking at the width of the tops of the threads.
Below is a picture of the thread micrometer in use. Just to be clear, as with any measurement, the lathe should be turned off.
Once you get closer to the finished thread you can start using the thread micrometer to aid in determining how far to advance the compound in. At 30 degrees, the value advanced with the compound moves the tool bit in half the value. So, if the compound is advanced in 0.01″ the tool bit has moved 0.005″ closer to the longitudinal axis of the lathe. My lathe measurees depth of cut which means I can use the difference of the measured pitch dimater and the required pitch diameter directly to guide me in advancing the compound. I did this until I measured the pitch diameter found in the chart above of 0.334″.
To finish the threads, I passed a file over the tops of the threads and bevelled the beginning of the threads slightly. To check that my thread actually works, I grabbed an appropriate nut and found that it threaded well by hand with very little play.
Alternatively, if you lack wires, triangles, or a thread micrometer and you just need something that screws together you can make thread cutting passes and check with a nut. This tends to be slow as it requires removing the tail stock each time you want to check with your test nut. I used this technique successfully when making the T nut for my lathe though.