The finish on a fastener is an important and integral part of the entire joint attachment. Just as hardness, shape, roughness, diameter and number of threads per inch all have a major effect on the final security of the attachment, the finish thickness, type and other characteristics have a tremendous effect on the final bolted joint. One of the last steps in the manufacture of a bolt or nut is the finish. Historically the finishing source is left uninformed as to the parts function and how the plating or coating will interact with the rest of the joint components. In the following series of articles the various facets of a good joint attachment will be discussed with emphasis on the factors of finishing, which affect how the parts fit together.
The finishing source is often the last operation in a long manufacturing process and usually is the one blamed for any hiccup in timing. Usually corrosion test results are delayed because of the amount of time necessary to run the actual test. With today's long-life coatings, 400 hours of salt spray is still 16-2/3 days, any way you count it. To avoid costly rejects and charge-backs, the coater overplates the part as a matter of course. If 0.0002 inch is the minimum specification, then running the part with 0.0005 inch will certainly pass the requirements. And there is little fear that the parts will become a deficit because of a customer B117 reject. What does happen is that the design of the threads of the part only allows a certain amount of space for plating increase (allowance). When this amount is exceeded, the mating parts go together with difficulty, jamming and using up torque. Since the installation torque has been calculated to correctly tighten the bolted joint (to be explained in the second of this series), the final attachment is not as clamped together as required. This leads to loosening of the joint, followed by fatigue of the joint, and WHAM, your wheel comes off (in the worst case). All because the finisher wanted to do a better than required job.
This month we review the requirements for thickness and gauging. Thread dimensional specifications (ANSI B1.13 and B 1.13M for metric threads) allow a certain amount of space to accommodate various processes, one of which is the plating. Since it is much easier to figure one variable than two, the specifications only allow the increase on external threads. The reasoning is that the internal thread dimensional increases should be accommodated for on the external threaded mating part. This requires manufacturers who specify a thicker coating to make the threads undersized to accommodate the additional space. They do not! For reasons economical, technical and just laziness, it is simpler to make all the parts with a thread size and pitch to the same dimensions than it is to run separate setups to allow for dimensional variations. The maximum thickness that can be plated/coated on a standard tolerance external thread without interference with mating part is:
|Thread Pitch/ Per Inch||Maximum Thickness (inch)|
|32 or less||0.00015|
|30 to 13||0.00020|
|10 to 5||0.00030|
|Greater than 5||0.00050|
As can be seen by these numbers, most coatings in use today exceed this. What allows the system to work at all is the fact that there is some give and take in the manufacture. The installation process pushes some of the coating aside (especially the soft dip/spin organic based ones), and that the interference is overcome with the sheer capacity of the installation tooling. Unfortunately, today many of the popular coatings are organics with only a typical thickness specified (usually 0.001 inch). Also, the nut may be coated with a similar compound (with no allowance) and the coater does not want a corrosion reject so he runs the thickness to 0.0015 inch, and a problem occurs.
Plating/coating thickness on a fastener is figured in the following manner. Since the thickness listed in the specification is that of a flat surface, a fastener thread, being a cylindrical shaped object, shows the plating thickness on the diameter to be enlarged by addition of the thickness to each side of the part (north, south, east and west). This makes the increase in thread dimension 4X the typical thickness. Since some process allowance is necessary to insure that the coating is above the minimum thickness every where, some factor is added to the specification. Standard practice and experience have shown that 50 pct of the minimum listed value is a good rule. This adds 2X to the requirements. Hence the maximum amount of plating/coating that can be added to the threads is only 1/6 of the specified plating allowance in the dimensional standards. And we did not even think about the nut!
When a customer comes in with a fastener, it may be a good practice to gauge the threads first to insure that they meet dimensional standards. Fasteners that are "garbage in" are "garbage out" even when it comes to plating. It's usually charged as the platers fault when the fasteners do not mate. Bolts and screws correctly gauged and coated with a correct amount of plating will cause no problems (even when they are assembled into the attachment down the line). Bolts that are too tight should be put aside, and the customer informed as to the concern and disposition. Be sure to get directions in writing. With the information above you can easily figure what thickness exceeds fit allowances. Undersized fasteners will be necessary if the amounts are out beyond any insignificant value.