Thread Inspection: An Overview of the Most Popular Methods

May 6, 2019 11:27 am

The goal of thread inspection is to ensure that a machined thread meets design requirements.

There’s a multitude of methods and tools that can help with the process, some more accurate than others. In this article, we’ll give you an overview of the advantages and disadvantages of the most popular methods used to inspect external threads together with the most recommended tools.

 

Anatomy of a thread

 

A thread is a uniform helical groove cut inside of a cylindrical workpiece, or on the outside of a tube or shaft.

Cutting threads by using a lathe requires a thorough knowledge of the different principles of threads and procedures of cutting. Hand coordination, lathe mechanisms, and cutting tool angles are all interrelated during the thread cutting process. Before attempting to cut threads on a lathe a machine operator must fully understand all the principles, terminology, and uses of threads.

 

Thread section

Image credit: smithy.com

 

The goal of inspecting (or measuring) a thread is making sure that the thread is cut at the right depth and that the pitch diameter matches the design specifications.

The pitch diameter is the diameter of an imaginary cylinder formed where the width of the groove is equal to one-half of the pitch.

Basic thread dimensions can be found in thread charts, which are generally present in most machining manuals and guide books. If you don’t have one, we recommend getting this.

Reading these charts is fairly simple, all you need to know is the major diameter of the part, something that can be easily measured with a caliper, and the number of pitches per inch, something that you can easily determine by using a thread pitch gauge like this. Once you have all this information you are ready to inspect your thread. Here are the most popular methods.

 

Go/No Go Gauges

G3/4" thread ring gage GO NOGO

G3/4″ thread ring gage GO/NOGO

Go/No Go ring gauges are the most commonly used tool for thread inspection. This method, however, provides limited results as thread gauges “only” say if the part is acceptable or not acceptable, but doesn’t provide a specific measurement. As the name suggests, ring gauges come in couples (the Go gage and the No Go gage) and use of them is very simple: if the thread is within the specifications, the Go gage will fit the part and the No Go gage won’t fit.

 

Advantages

  • The easiest way to inspect an external thread.
  • Doesn’t require any additional training or practice.

 

Disadvantages

  • This method only reveals if the thread is ”good” or ”bad”, but doesn’t provide any specific measurement.
  • Ring gauges need to be recalibrated from time to time, and the process is difficult and expensive.
  • Ring gauges are not universal. They can only be used for the specific thread size stated on the gauge.

 

Thread Wires

Thread wire set

Fowler Thread Measuring Wire Set

 

Thread wires are the most used method by those that want to obtain a more accurate result. These tools measure the pitch diameter on external threads.

Thread wires are definitely cheaper than ring gauges, but they come with a catch: they require the inspector to have a good level of dexterity as they are not easy to position and hold in place.

Wire sets come with charts that look like this:

 

thread chart

Image credit: machineshoptalk

 

These charts are used to determine the diameter of the wires you’ll need to use to measure the thread and the constant that you’ll need to subtract from the initial measurement to calculate the pitch diameter. To measure the thread using this method you’ll need 3 wires and a micrometer. Here are the steps to follow:

1. Place and hold two wires in two adjacent pitches on the top of the part

2. Place and hold the third wire in the pitch located across the part from where the two top wires are positioned

3. Use the micrometer to measure the distance between the top wires and the bottom wire

4. Subtract the constant that refers to the wires from the measurement to obtain the pitch diameter size.

 

This video will make it easier to understand the process.

 

Advantages

  • This is a very accurate method (assuming that the flank angle is correct).
  • It can be applied to all external threads.

 

Disadvantages

  • This method requires a calculation to find the correct measurement result.
  • It requires practice.
  • It only measures thread pitch diameter.

 

 

Thread Micrometers

Thread micrometer

Fowler Digit Micrometer with 60-Degree Point, 0-1″ Measuring Range

Thread micrometers are used to measure the pitch diameter of a thread. They are easy to use but they are not cheap. Unlike regular micrometers, thread micrometers have V-shaped anvils designed to fit into a 60° thread angle.

 

Advantages

  • Takes an accurate measurement (assuming correct flank angle).
  • It can be used on all threads with the same flank angle.

 

Disadvantages

  • More expensive than other methods.
  • Only measures thread pitch diameter.
  • All deviations from the correct flank angle will affect measurement accuracy.

 

Thread Triangles

thread triangle

Fowler Thread Triangle Gages

Thread triangles are little, hardened prisms designed to fit into a 60° thread angle. They are used in a similar way to the three-wire method, but the triangular shape and the rubber harnesses make them easier to place and hold while measuring the distance between them with a micrometer. Just like three wires sets, thread triangles come with a chart indicating the value that needs to be subtracted from the reading on the micrometer to obtain the measurement of the pitch diameter.

 

Advantages

  • It’s a very accurate method (assuming that the flank angle is correct)
  • It can be applied to all external threads

 

Disadvantages

  • It requires a calculation to find the correct measurement result
  • It requires practice
  • Only measures thread pitch diameter

 

What’s the best method?

Although there’s no simple and straightforward answer to this question, the three-wire method has been generally considered to be the most satisfactory and universal method when properly executed.

 

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19 Comments

  • CD says:

    Fit your thread to part whenever possible

  • Mark says:

    You have to be very careful with the thead wires!!! I have found inexpensive thead wires to variate in the diameter and roundness. If the wires have two or three thousands variation imagine how much it would affect the measurment. The best thread wires are van kurens, hardened and ground to one or two tenths on the diameter and roundness

  • Joseph says:

    If you are doing a repair on a part that is right in front of you, and you will have control over that part forever, go ahead and make it fit the part. Otherwise NEVER use another part as a gage. Its a fast and sure fire way to get in trouble. Thread specs and standards exist for assured interchangeability across town, or across the world. You can also cut and inspect threads with 1, 2 or 3 of the wires in any set. I have a youtube channel dedicated to machining and have proven this as fact.

  • Marty says:

    Then there is cutting and measuring a LONG , – ACME- thread . I actually cut a 60″ inch Acme for a new lead screw in an early 1970’s IKEGAI Engine Lathe back in 2979 or ’80 . I was advised by perhaps the ” Greatest Machinist who ever lived ” on how to do this. I started with the ground finished metric stock they had ordered. I made this thread with ONE PASS ! I set up a Follower Rest ,with 3 Cobalt MoMax Steel Form Bits I had ground in a giant Aloris Block ,one to rough ,one to form and one to finish. I set it up on a shadow graph and then squared and verified the positions with dial indicators. Set my speed and gearing then confirmed on a chunk of scrap aluminium. I then layed on the sulpher oil and let her RIP. The American Pacemaker Lathe HOWLED during the pretty quick cut ,and I followed the tool and wound the feed out at the end of the cut. The NEW screw sat there smoking and GLEAMING ! My lead man and bigger BOSS all came out hearing the noise. Most of the guys in the large shop (about 40+) shut off their machines and were looking about. My supervisors were rather suspicious and ready to be contentious and scolding with my 19 year old self. With everybody staring at my machine and the new screw….the rather unliked “Greatest Machinist” came over and after a two second view he declared ” It is PER-FECT , I guarantee it ” !!! It actually WAS , I measured it with bearings , ran indicators everywhere and it was a great result. I cleaned and polished the razor sharp burrs and then drilled some shear pin holes. Installed on the splendid engine lathe ,it was better than NEW ! Per my suggestion , we had Re-Engineered the original design . There was no drive screw shear pins originally and when an inattentive operator let the carriage drive too far OUT , it bottomed out on the screw housing of the lathe bed. And the threading screw just KEPT TURNING and twisting ! The machine was operating at it´s travel limits with the tailstock removed . Quite a job after quite a little disaster making Middle parts way back then . Lots of threading of all kinds and sizes back in that shop.

  • Robert says:

    Thread ring gages are a quick way to determine the go, no go, condition of a threaded part. However, as the ring gages wear and are reset to a thread setting gage, which has a cleared minor diameter, the minor diameter of the thread ring gage can go undersize, cutting into the manufacturing tolerance of the part. If your threaded product is being rejected, be sure to check the ring gage effective minor diameter to see if it is undersize. Federal Standards H-28 books are a good source of information.

  • Dave says:

    Do not fit your thread to part. Always use wires. If cutting on an engine lathe always pass the rear flank angle with the compound by 1/2 degree. This will force the tool to cut 95% with the leading edge while always maintaining the proper included angle.Always use industry standards. The other matching vendor part be damned.

  • piobair says:

    I am best described as a self-taught apprentice machinist. I am still on the steep end of the learning curve.
    Making my own 3/4″ ID hydraulic cylinders, I wanted to use the lathe to cut 16 threads per inch on the exterior of a 3/4 nps schedule 80 pipe. I cut the thread and compared it with a 3/8-16-unc screw. They didn’t match. At 1.0″, there was 1/2 thread difference.
    Checking with a vernier caliper, I found that the threads that I had cut were correct, but the off-the-shelf screw had 15-1/2 threads per inch.
    Digging into the federal specification for screw threads, I found that, up until a few years ago, a male thread was satisfactory if it fit without binding into a nut that meets the federal standard. The problem with that acceptance criteria is that in the case of the screw that I had, not all threads would actually be in contact. The Air Force was discovering that fasteners were failing under these conditions.

  • Mike says:

    Put the two wires on the bottom.The fixed anvil prevents them from moving by the spinning part of the mic on the top.
    I find using two methods of measurement ,like wires or mic AND ring gages,to be best,as wires don’t really verify form,whereas
    rings do.(Like previous comment as to using a mating part).Rings also verify pitch,btw.
    And a word of caution:A broken insert tip can cause a bad root rad,which where I come from(aircraft and or naval manufacture) ,is a scrap part.This is undetectable with most types of mechanical gaging.
    Look at the thread form with a comparator or at least a magnifying glass without fail! I have seen,made,and caught as an inspector bad threads by people,including myself, who relied too much on the gages!
    PS;Look up”Johnson Gage”.They verify pitch AND form very accurately.(But not a bad root rad haha)

  • Joe says:

    How does one MEASURE the lead of the thread. That would be important.

  • Shuttle guy says:

    Good for a hobbyist but completely unacceptable in a commercial or production environment where drawings dictate threads per specifications, which point to measuring standards that don’t permit that practice.

  • Tony says:

    many years, my boss had me cut all the special threads in the shop, I used wires most of the time and heavy grease to aid in keeping them in place, some triangles and a few gages, damn cut so many threads over a 45 year time, whitworth, J, acme, buttress, square, so impressed how far machining of metal parts has come through the years.

  • Robert says:

    One of the best explanations of thread measuring that I’ve seen in 45 years of Machining. Thank you for publishing it

  • Joe Pie says:

    Using thread wires doesn’t have to be as awkward as everyone makes it. I posted a video on my YouTube channel that proves you can use 1,2 or all three for task. And I have to stress, never use the mating part as the gage unless you will have both parts under your control forever. Use thread standards whenever possible to keep interchangeability in play.

  • Artmuscle says:

    When I worked as Quality Assurance at a major aircraft manufacturer, we used a shadow graph comparator for measuring pitch, minor and major diameters, Johnson Thread gages for more accurate measurements on more important threads and for critical threads we used an air gage.

  • Either you don’t know or you left one method out that measures both external and internal pitch diameter.

    https://www.youtube.com/watch?v=rKqp0o2nK54

  • Robert says:

    I retired after 30 years in a world class aerospace metrology laboratory.
    Adjustable thread ring gages, that are set to a master thread setting gage, wear over time and if they are too worn will allow the minor diameter of the thread ring gage to go undersize because most setting gages are cleared at the minor diameter. This condition eats up a lot of the tolerance on the part they are checking, making it difficult, if not impossible, for the machinist to make a part that is within tolerance. You can use Deltronic cylindrical pins to check the minor diameter of smaller thread ring gages.

  • I like how you point out that both thread depth and pitch diameter are important when you’re measuring threads. My brother is thinking of becoming a machinist and needs to learn how to use thread gages. The Go and NOGO thread gages appear to be an indispensable tool for every machinist to have.

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