The Nuts And Bolts Of Thread Gaging

The Nuts And Bolts Of Thread Gaging
George Schuetz, Mahr Federal Inc.

 No pun intended, but thread gaging has always seemed a rather convoluted subject to me.  So, when I was questioned on the topic recently, I asked my friend, Lowell Johnson, a recognized authority and president of the Johnson Gage Company, to explain some of the basics and offer some tips on the process.  His advice:  Don’t trust Go/No Go thread gaging.  The reason is that this process is still widely used and, despite more than 40 years of evidence to the contrary, is so inaccurate that it will literally allow square screws to fit into round holes.  Lowell even demonstrated this in testimony before Congress.

 The simple fact of the matter is that tests that only demonstrate whether or not two parts will assemble show nothing about the quality or integrity of that assembly.  Just because they go together doesn’t mean they are going to stay together.  And, in the world of threaded fasteners and components, this can be a critical issue.  But why this is so, and why you should be using a system of instrumented dimensional gaging for screw threads, requires some explanation and an understanding of how threaded components function.
 Screw threads are so common we often take them for granted.  But in reality, they are geometrically rather complex, as you can see in Figure 1.  In addition to material and heat treatment, what gives a threaded joint its strength and integrity is the amount or percent of engagement along the flanks of the mating threads.  And because their geometry is so complex, lots of things can happen to limit this contact.  Changing the lead angle in either direction will change the pitch and result in mismatched threads.  A wobble, or “drunkenness” in the thread helix will limit contact, as will a change in diameter, roundness or taper.  Even a change in flank angle will yield a thread which assembles line, but has very little flank contact and almost/zero strength.  No matter how hard you try to tighten it, the nut will vibrate loose almost immediately.

 To understand the geometrical relationships involved and to allow process control, threads are commonly defined as having two diameters:  pitch and functional.  Functional diameter is a measure of fit, or the ability of the threaded product to be assembled.  This is the only dimension checked by Go/No Go gages. 

 But as we noted, this may or may not reflect the real “dimensions” of the product, due to the nature of the helix, roundness, lead and taper.  Think of a set of gears whose teeth are meshed or butting.

 Pitch diameter, on the other hand, is a much more accurate reflection of size.  It is defined as “the diameter of an imaginary cylinder passing through the thread profile at such points as to make equal the width of the ridge and the width of the groove. “This number is significant for a couple of reasons.  One, the measured value for pitch diameter at any point along the axis of the screw thread reflects the actual amount of thread material.  This is called the Minimum Material Size.  Second, for design purposes, pitch diameter is the dimensional factor which governs the shear and tensile strength of the thread assembly.  It establishes the datum from which variations from perfect thread geometry can be referenced.


 This gets much more complicated than we have room to explain, but the important thing to remember is that in a perfect screw thread--and only in a perfect screw thread--the values for functional diameter and pitch diameter are equal.  In all other threads, differences in these two values reflect variations in taper, roundness, straightness, lead and thread angle, including helical path.  In short, they reflect a smaller percentage of flank area engagement, and hence, a reduction in the performance of the threaded assembly.  Likely problems include: galling during installation:  joint loosening (due to vibration): leakage: fatigue and relaxation: slip page: and ultimately, failure.            

 Bottom line, then, is that if you are going to accurately gage screw threads.  You need a dual instrument system that measures both functional and pitch diameters.  You need to know that both are within specified tolerances and you need to know the value of the difference between them, because it is this number which provides a basis for process control (see illustration).