Surface roughness of bolts 2

[MintJulep]

I'm looking at a vendor drawing of a special size bolt that otherwise meets the requirements of an SAE J429 Grade 8 bolt.

The drawing specifies a surface roughness requirement of 250 micro-inches.

This seems excessively rough to me.

SAE J1061 limits the roughness on the underhead bearing surface to 110 micro-inches, and ASTM F788 limits underhead roughness to 125 micro-inches.

But I can't find any spec that addresses the surface roughness of the unthreaded shank, or head.

Could anyone point me to a spec or standard that I can use to shoot this drawing down?

 

[dinjin]

I would think it was dependent on the class of the bolt
and size. I have seen it in Machineries Handbook for
Aluminum and Magnesium Alloy studs but do not recall anywhere else.

 

[MiketheEngineer]

If you aren't going to kiss it - who cares??

 

[MintJulep]

Initiation sites for corrosion and fatigue cracks.

 

[CoryPad]

Mint,

Here is one. Section 3.18 of MIL-B-6812E Bolts, Aircraft allows a maximum surface roughness of 0.63 microinches for close tolerance bolts and 125 microinches for all others. This has been deemed acceptable for aerospace and spacecraft applications, so compare your application and see if it needs to be "better" than this.

Another one is MIL-B-85604 Bolt, Nickel Alloy 718, Tension, High Strength, (125 KSI FSU and 220 KSI FTU) High Temperature, Spline Drive, General Specification for, which has the following values:

Shank and underside of head: 32
Head-to-shank fillet: 32
Flanks of thread and root area: 32
Other surfaces: 125

 

[MintJulep]

Thanks Cory,

I think the application should require something "as good" as aerospace quality.

I'm curious as to why SAE, ASME and ASTM don't seem to address this in their standards.

I'm thinking it's a cart before the horse situation. The normal fabrication methods for bolts would typically be expected to result in a surface finish around 125 - 63 microinches anyway, except for possibly the cold-heading operation. So they address the important underhead bearing surface explicitly and leave the other surfaces unspecified since the way it's likely to be made will be good enough.

 

[hydtools]

I would interpret the vendor specification as the maximum permitted roughness leaving you the latitude to provide a finer finish without violating the specification.

Ted

 

[TVP]

Mint,

Typical surface roughness of fasteners after cold forming:

- shank 16 to 63 microinches (essentially wire diameter with little subsequent deformation)
- extruded shank 8 to 32
- contained upset 32 to 63 (square neck, sides of hexagon, etc.)
- underhead/bearing surface 16 to 63
- extruded point/chamfer 8 to 32
- top surface of head 8 to 32 (surfaces that undergo large deformation while contained by the tools)
- trimmed edges 63 to 125 (sides of hexagon, etc.)

Source: "Cold Heading" article in ASM Handbook Volume 14 Forming and Forging, 1988

 

[MintJulep]

Great stuff. Thanks TVP, exactly what I needed.

 

[Duwe6]

250 micro-inches feels like a phonograph record when you scrape your fingernail over it. Give your bolting materials a fingernail test and certify them as " <250 micro-inches".

Its like certifying that "no mercury or other heavy metals were used in the production of these parts". Paperwork exercise.

 

[racookpe1978]

Are you not going to use this bolt over a washer? If so, why care about the roughness of the bolt head under the head?

 

[MintJulep]

Because excessive roughness under the head = embeddment, relaxation, loss of preload and joint failure.

 

[racookpe1978]

?

Measure the difference in height between the min-man roughness value, then compare that to the distance between threads on the bolt.

The two are not comparable. Further, the min-max roughness value actually only shows a different height measured for a very small total area compared to the area of the bottom of the bolt.

 

[MintJulep]

The thread pitch is irrelevant. Bolt elongation due to preload is what matters, and it is a very small length, especially on short bolts. It takes very little embeddment and relaxation to loose a substantial amount of preload.

 

[KENAT]

Um, Duwe6 & hydtools - this is a vendor drawing, not a customer drawing, I don't think Mint is looking to make these bolts, he's looking to use them.

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