ADVANTAGES AND DISADVANTAGES WHEN INSPECTING
THREADS WITH THREAD RING AND PLUG GAUGES
AdvantagesCan be used by almost all with little or no machining experience. Smaller gauges for standard threads are relatively inexpensive and are often in stock. Determines if the inspected thread is the correct pitch and within the correct pitch diameter tolerance.
|
DisadvantagesA thread ring gauge does not reveal if the external major thread diameter is below tolerance. A thread plug gauge does not reveal if the internal thread minor diameter is above tolerance. Thread gauges (Go NoGo) are made for specific diameters and tolerances. The larger the thread the heavier and more expensive the thread gauge. Thread gauges for non-standard and/or unusual threads are expensive and almost always have lengthy delivery times. Gauge wear can become a factor depending on use and the threaded material. Calibration can be both expensive and inconvenient. |
ADVANTAGES AND DISADVANTAGES WHEN MEASURING
THREADS WITH FMS THREAD INSERTS
Advantages |
For measuring threads with flank angles between 55º and 80º one pair of FMS thread inserts can measure several pitches.
e.g. 5 pairs of FMS inserts can measure pitches (both external and internal) from 0.5mm/48 TPI to 8mm/3 TPI.
The only limit to thread diameter measurement is the digital caliper length.
Even although a reference (a FMS calibration plate) is necessary for measuring internal thread pitch diameter the calibration plate can also be used for caliper calibration.
For those requiring certification then, as there is no practical wear on a calibration plate, recertification is probably not necessary.
As the measuring instrument is a digital caliper the change from mm to inches can almost always be done at the press of a button.
When measuring pitch diameter while setting up a machine, knowing where you are can save both time and less inspection if you know you are not near the pitch diameter tolerance limits.
FMS thread measuring inserts can measure both RH and LH threads with the same inserts.
With training and use (plus using a FMS caliper pressure device) a measurement accuracy of ±0.01mm/±0.0005” can be achievable.
Disadvantages |
Getting to know and using FMS to measure thread pitch diameter can for beginners, take a few minutes. Easiest for those that are used to measuring.
To inspect thread pitch a thread pitch gauge should be sufficient.
Measuring internal thread pitch diameter requires a reference. A FMS calibration plate is a reference.
Thread Measurement Alternatives |
There are of course many thread pitch diameter measurement alternatives (thread micrometers, indicator dial instruments, profile projectors etc.) but what they almost all have in common is that they are expensive and most have a limited measurement range.
When thread wires are used to measure pitch diameter it might be worth remembering that the constant to be subtracted is based on the flank angle being exactly the theoretically correct value.
The measurement result might not be as accurate as thought if the flank angle is not exactly what it should be. Ground thread gauges have a much smaller flank angle tolerance than machined threaded components.
Measuring with FMS thread inserts gives the correct pitch diameter even if the flank angle is not the exact theoretical value. It is up to the user as to which method and measurement result is preferred. Measuring (calibrating) thread gauges requires of course more accuracy than machine threaded items.
Thread Inspection & Measurement |
As mentioned the most common inspection of threads is by using manufactured, ground thread gauges and preferably both Go ad No-Go. This becomes expensive with larger thread diameters and long delivery times for “unusual” and non-standard threads.
Measuring external threads with loose thread wires isn’t always easy and require a calculation. Even when buying the thread wires that attach to a micrometer for external pitch diameter remember that a micrometer can have 3 different standard spindle diameters (¼”, 5/16” and 6.5mm) so buying the correct wires is essential. Measuring internal thread pitch diameter with an indicator dial attachment instrument also requires a reference (expensive) and have relatively short measurement span.
Thread wires for external and thread balls for internal are used by calibration facilities and often to measure thread gauges rather than threaded items. Tolerances on manufactured, ground gauges are much less than actual threaded items. Different tolerances also include the flank angle.
Companies that make their own thread gauges almost never make a No-Go gauge so as to whether the item or items are actually within pitch diameter tolerance is an open question.
FMS is certainly not the only company to manufactured components for measuring both external and internal thread pitch diameter but does have several advantages over most. As a digital caliper is the instrument base then just by clicking on a button most digital calipers can change between mm and inches. The only limit to how large a thread diameter hat can be is caliper length. As the pitch diameter is being measure it can be seen where it is within pitch diameter tolerance. Also important is that if the threaded item is to be coated or plated then the threaded item must not be machined to the actual pitch diameter tolerance. A 60º degree thread will have the pitch diameter measurement increased by 4 times the coating thickness and a 29º/30º thread 8 times the coating thickness. If stainless steel is electro polished then the pitch diameter will decrease and not increase.
As elsewhere mentioned in this website there is no method that ensures a perfect machined thread but knowing where within tolerance the pitch diameter is a major advantage. Measuring an external thread’s major diameter and an internal thread’s minor diameter is easy and should always be done.
If you have a “difficult” customer then giving thread measurement results usually gives a good impression. With the supplier and customer both using thread gauges then even with the same gauges (dimensions and tolerance) thread gauges are not identical, they have manufacturing tolerances.
Those that use gauges probably don’t often think how large pitch diameter tolerances are on threaded items.
|
Standard M24x3-6g/6H |
Standard 1-8 UNC 2A/2B |
|
External |
External |
| Internal (6H) 22.051 -0.000 +0.265 (tolerance 0.265mm) |
Internal (2B) min 0.9188” max 0.9276” (tolerance 0.0088”) |
The standard to which most calipers are manufactured is DIN 862/ISO 2012. The specified accuracy for external jaw measurement with a digital caliper is 0.002mm/0.0008” up to 100mm/4”, 0.03mm/0.001” up to 600mm/24” and 0.04mm/0.0015” up to 1,000mm/40”. There is an extra allowance of 0.02mm/0.0008” for measurement with the internal measurement jaws and the depth rod. This accuracy requires a constant measurement pressure but no standard specifies what the pressure should be. A FMS pressure device ensures a constant measurement pressure.
If you can’t measure a thread pitch diameter to within ±0.02/0.001” and to ±0.01mm/0.0005” with FMS after frequent use then you’re probably not doing it correctly. Use of a pressure device makes all the difference and using a FMS calibration plate as reference also calibrates the digital caliper.
If your company manufactures threaded items regularly then FMS is certainly worth looking into and possibly also money and time to be saved. A good start is requesting a quote and then practical and economic decisions can be made. Today there are probably more alternatives for thread measurement than ever before and each having their advantages and disadvantages.