Zinc-Plated fasteners and ASME B18.3 1

[ED-209]

Hello, new to the forum.

I'm new to the profession and am trying to learn what I can to better my knowledge base. We use a combination of zinc-plated and black oxide alloy steel fasteners at my workplace. I noticed when browsing the fasteners on McMaster-Carr that there are far more fasteners in Black Oxide that meet ASME B18.3 than fasteners that have a Zinc-Plated finish. Is there something about the zinc-plating process that prevents fasteners from meeting ASME B18.3?

Additionally, we use a lot of Socket Flat Head Cap Screws in our designs over regular Socket Head Cap Screws. Is there an inherent benefit to c-sink's vs. c'bore's?

Thanks for reading.

 

[Jboggs]

"Additionally, we use a lot of Socket Flat Head Cap Screws in our designs over regular Socket Head Cap Screws. Is there an inherent benefit to c-sink's vs. c'bore's?"

I can't answer your first question, but the second one hits a nerve. I have learned the hard way over the years to avoid flat head socket head screws, especially in any critical situation, if at all possible. They will inevitably become a pain in the ass and almost impossible to remove. "Self-locking" is a good thing, but "impossible to remove without stripping the socket or the wrench" is not a good thing.

Here's why that happens. Screws are almost always used in "patterns" of multiple screws through a common surface. Threads are by their very nature imperfect. It is impossible to create tapped holes with the same precision as drilled or reamed holes. That's why "screw clearance holes" are always a little larger than the screw OD.

As for the screws themselves, no matter how it's created a thread will have its own centerline. And that centerline likely will not be exactly coincident with the centerlines of other features of the screw, like the head of a flat head screw or the body of a shoulder screw.

So you have a pattern of tapped holes in one body, an imprecise pattern at best. And you have a "matching" pattern of clearance holes in the mating body. If the fasteners are standard socket head cap screws then the inexact nature of the "matching" patterns is no issue because the bottoms of the screw heads are all flat, and can "move around" a little with no effect. But a cone shape is self-centering. You mate it with a female cone and it will go to its center. Flat head screws lock up because the centers of the threads and the centers of the cone bottoms are ALWAYS slightly offset.

Then you throw in the fact that the size of the socket itself in a flat head screw is smaller than the socket in a standard screw, and you have even less ability to withstand the increased torque required to remove it.

Bottom line: I avoid flat head screws like the plague. There are usually other better options like counterbores, button head screws, or a type most engineers are completely unaware of: low head socket head screws. I have used low head screws in many applications. And they can be fitted in relatively shallow counterbores very easily to create a completely flat surface if needed.

 

[MintJulep]

Is there something about the zinc-plating process that prevents fasteners from meeting ASME B18.3?

No.

 

[drawoh]

ED-209,

I think Jboggs stated this, but it is worth repeating.

You need to build clearance into your bolted/screwed interface. Countersinks and tapped holes are self centering, i.e. there is no clearance to account for mis-aligned holes. Centre-bores are a better way to access tapped holes. A countersink and a clearance hole for a nut have clearance, but not as much as a centre-bore and a clearance hole for a nut.

It sounds like Jboggs has had the same bad experience I did. Small flat-head and button head cap screws have small, easily stripped sockets. A stripped flat-head screw is very difficult to remove. I don't trust anything smaller than #10 or M5.

--
JHG

 

[JohnRBaker]

Are those Zinc or Cadmium plated? Back when I was a machine designer for a company that manufactured food machinery, we were not allowed to use Cadmium plated fasteners. We had to use Zinc plated or stainless steel. Not sure what it's like today, but back then (the 70's) we had to warn our customers that they couldn't just go down to their local hardware store and buy some nuts and bolts since most of the common fasteners sold in hardware stores were Cadmium plated and therefore not suitable for use where food was being processed. We often had to supply extra loose Zinc plated fasteners just in case.

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[MintJulep]

Having now had the chance to get to my office and re-read B18.3 I will retract my prior post in this thread.

1.7 Finish
Because of the high hardness of these products, it is recommended that they not be electroplated.

I don't have ready access to a more current version.