FTM 113: The Effect of Plating on Thead Fit

An excessively thick coating does have a big effect on thread fit, more so than any other part of the fastener.

This edition of the Fastener Training Minute with Carmen Vertullo was originally published February 9, 2017 as “The Effect of Plating on Thread Fit” during episode 113 of Fully Threaded Radio.
Well hello everyone this is Carmen Vertullo with the Fastener Training Minute coming to you from the Carver FACT Center in El Cajon, California and with the Fastener Training Institute.
I recently had an inquiry from one of my clients and their concern was that one of their customers asked them to put an excessively thick coating of electroless nickel (in this case on a screw thread) and they were concerned about the thread fit. Well coating does have a big effect on thread fit, more so than any other part of the fastener. When we return with the Fastener Training Minute, we’re going to tell you what that effect is and why it’s really important that you understand it.
Now if you happen to have a copy of the Fastener Technology Handbook from the IFI (the black book) and you turn to page 62, there’s a very good illustration of what I’m about to explain to you. It helps to have an illustration in front of you because it’s a little bit complicated. There’s some math involved. I’m not going to explain the math but the illustration does, so here’s what happens.
Imagine that you are just going to put a coating down on just the surface of a flat piece of metal and that coating is going to be one ten thousands of an inch thick. If you measure that surface before and after, or the thickness of that plate before and after, and the coating is only on one side, then obviously the thickness is going to change by the amount of that coating, or by one ten thousandth of an inch.
If we plated both sides of that part, then it’s going to change by two ten thousands of an inch. And that’s the effect that plating would have on the diameter of a screw or a screw head or something where we’re measuring both sides. It’s going to be double the coating thickness. So if you just have a rod or a pin and you put a ten thousands of thickness coating on it, it’s going to increase the diameter by two 10,000 of an inch.
Threads are different though because threads have a 60 degree angle and we’re measuring of course both sides of the thread. We end up with an exaggerated effect on what’s called the pitch diameter or the primary measurement that we take on threads for functional fit.
And the way to think about this is imagine that you are my guests as I take you through a theater of the mind, because we don’t have a screen for you to look at. But if you have page 62 of the Fastener Technology Handbook hoping you’ll be able to see this.
Imagine that you are measuring the thickness of a layer of snow that’s just fallen. So you go to the sidewalk and stick a ruler in the snow and it comes up to two inches, you think two inches of snow has fallen. So assume that your roof has a 60 degree pitch which is similar to a 60 degree thread, and climb up on your roof and measure the snow thickness again, putting the ruler perpendicular to the earth (that is not perpendicular to the roof, but straight up and down relative to the earth). Now assuming that you haven’t fallen off the roof yet, you’re piercing that layer of snow in such a way that it’s much thicker. It is in fact two times as thick going straight down as it would be going perpendicular to the angle of the roof.
And that is the reason why we end up with an effect on pitch diameter of four times the plating thickness. Anytime we put plating on screw threads its effect is twice the thickness because of the fact that we’re going on this 60 degree diagonal, plus it’s on two sides of the thread. So that’s why thread fit is an important issue when plating fasteners. and why we must be very careful not overplate and gauge, and it’s also why for the most part coating thickness on screw threads is restricted to a relatively thin coating of one 10,000 to a few ten thousands of an inch thick.
Well, that’s our Fastener Training Minute and once again if you do you want the details on that the Fastener Technology Handbook page 62 has a very nice illustration, or you can just email me or contact me anyway, and I will send it to you.
This has been Carmen Vertullo with the Fastener Training Minute for the Fastener Training Institute. Thanks for listening.

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