AS9100 aerospace companies often have internal manufacturing process documentation that specifies this type of thing. Twist drilled holes tend to have poor surface quality for many reasons. So a surface roughness limit of 250 microinch would not seem unreasonable.
I don't ever recall seeing a specification requirement for hole surface roughness. Probably near impossible to measure. Holes usually have a (tight) diameter tolerance.
If the surface finish of the hole matters, you ream it. That can get the surface down to 63 mics.
When it does matter, it is likely for a special fastener, and in that case there is often a process specification for its installation.
If used by the aircraft OEM, you can even find the process spec in their manuals.
Interesting question. I just went through some SRM's to see what I could find. We control the type of drill, RPM, and how to pilot step and Ream for various class holes. But I couldn't directly find a surface finish call out.
When I make a print I guess I assume that the global surface finish in the title block applies to hole bores (usually 63 micro inch) ... unless I call out that surface.
Keep one of these handy, for when surface finish questions come up. Use it often enough, and you will get comfortable recognizing different surface quality conditions.
NOTE.
The data in AND10387 was re-stated in USAF T.O. 1-1A-8 ENGINEERING MANUAL SERIES - AIRCRAFT AND MISSILE REPAIR - STRUCTURAL HARDWARE...
Table 21-1 Twist Drill Sizes
Table 21-2 Standard Drilled Hole Tolerances.
2. Unfortunately identifying drilled hole 'quality', IE: machined/mechanical-finish quality is a lot tougher... appears to be specific to corporate processes and/or aircraft SRM.
NOTE.
These comments ONLY relate to drilled-holes for fasteners... NOT applicable to reamed or bored holes with specified dimensions/tolerances for any close-tolerance function.
HOWEVER... I found a fairly universal definition of 'good drilled-hole-quality' and 'unacceptable drilled hole quality'... as I learned it... in [a very old] 707 SRM Part 51, Chapter... HOLE PREPARATION AND STOPDRILLING OF CRACKS...
1. General
A. Hole sizes listed in 51-2-5, Figure 1, and used for the normal repairs
within this manual require the use of properly sized and maintained
drills. When necessary, use a bushing tool as a guide for obtaining a
hole not larger than the drill size and perpendicular to the work surface. Use the proper drill speed (feet per minute) {##} which will leave a smooth,
bright finish throughout the hole surface. A hole which indicates a
rough or galled surface should be properly enlarged by a hand-reaming operation
to obtain a smooth hole surface. Strict adherence to the recommendations
indicated will contribute to the service life of repairs and
preventive modifications. {## 'and feed'}
From my working experience, using white-light [back-side] and ~low magnification front side, it is 'easy to see defects thru a hole... visible scratches [length-wise, spiral, circumferential], chatter, chips burrs, pits, corrosion etc. [Note: this is same as how to inspect a gun-barrel for 'condition'].
Roughness defects in a hole are unacceptable for practical/measurable reasons, thus...
Actual hole dimensions cannot be obtained with accuracy: calipers, barrel mic's, go-no-go gages etc... are ineffective in obtaining reliable/repeatable hole sizes.
and...
Non-destructive inspection of holes using eddy current at appropriate sensitivity levels WILL BE affected by hole surface roughness. ECI of a hole with "...smooth, bright finish throughout the hole surface..." will inevitably have valid defect indications of cracks, pits, etc... HOWEVER even minor surface finish roughness 'defects' and 'irregularities' will NOT pass ECI... it is likely that the hole appears defective in multiple locations/aspects/thru the length in relationship to the visually obvious defects.
CAUTION.
Rough holes have [3] primary mechanisms leading to fastener failures...
(a) Roughness affects fastener fit which inevitably results in fastener looseness.
(b) Fastener looseness 'to irregular/varying degrees' leads to irregular load transfer among fasteners/parts... affecting hole-wall loading in mathematically unpredictable ways.
(c) In addition to [over-laying] (a) and (b)... structural cracks readily initiate in the mechanical surface defects and grow in the stress field present.
Counter-intuitively... tight-fasteners among a field of loose fasteners... in rough holes... are fatigue-cracking [and possibly SCC and EXCO] worries/nightmares.
I will continue to dig for data on this, since drilled-hole [and countersink] quality are part of a similar/parallel question I was posed regarding conventional, and non-conventional [IE: NACA Method], solid riveting practices, related to post-production repairs, modification and major structure replacement [hands-on artisan work].
NOTE.
After looking at +20-documents [SRMs, T.O.s, texts, training materials, etc] relating to solid riveting practices, a concise picture is emerging from of all the bit-n-pieces in each document... of what I knew already, IE: what I learned as a kid helping dad build his all-metal airplane... in factory production environments... in depot service engineering... as a field service engineer shoulder-to-shoulder with highly experienced Techs... in hands-on training classes, etc... and but couldn't find 'all-together'. Oddly it seems that part of the ROOT-problem I'm having is that these skills are typically 'classroom and hands-on trained' by highly experienced instructors. Unfortunately, in the environment that I work in, FORMAL Structural repair technician TRAINING [including refresher training] is almost non-existent... all learn by doing from someone-else who learned the same way... copy-of-a-copy-of-a-copy-of-a-copy-of-a-copy... etc. Exceptions: those who went to tech schools [A&P, MIL] and those who were in FAA-regulated repair shops.
DANG... once-again I digress... gottagobacktowork.
xxxxxxxxxx
FYI... for grins, ONLY.. Twist drill and reamer specifications...
Twist Drills
NAS907 DRILLS, HIGH SPEED STEEL AND COBALT, 1/16 INCH THRU 1/2 INCH
NAS937 DRILLS, DOUBLE MARGIN STEPPED, HIGH SPEED STEEL AND COBALT, 1/8 INCH THRU 1/2 INCH
NAS965 DRILLS, HIGH SPEED STEEL AND COBALT, THREADED SHANK, 1/16 INCH THRU F
NAS988 DRILLS, CARBIDE, DOUBLE-MARGIN, 1/8 INCH THRU 1 INCH
ASME B94.11 TWIST DRILLS
Reamers...
NAS897 REAMERS, CHUCKING, RIGHT-HAND CUT, 0.0938 THROUGH 1.5000 INCH
NAS898 REAMERS, SOLID CARBIDE, RIGHT-HAND CUT, .1250 THROUGH .7500
ASME B94.2 REAMERS
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]
Count on Wil to find the comprehensive answer, with references!
Allow me to seize on this one passage (from the 707 SRM):
"hand-reaming operation to obtain a smooth hole surface."
I want to highlight the "hand" in the operation. Driving a reamer by machine allows it to be done more quickly, but it also overcomes much more chip build-up in the flutes, scoring the surface we're trying to finish. Hand-reaming is much more sensitive to chip accumulation and they will be cleared before they fill the flutes. The same goes for the lubricant in the operation (when done correctly) and hand-reaming will much more often have excess lubricant.
Using a specific process (reaming holes) vs specifying a measured surface roughness is a better policy for developing a hole quality program. Note hole quality is not simply a function of surface roughness. You also have to consider angularity and planar out of roundness. To accomplish this you will need to establish in house NDI/maintenance standards and processes. An example of a good maintenance practice may be to final ream all holes using a fixture/jig and then NDI. A good place to start for NDI is FPI and magnified visual inspections. These techniques will allow you to find flaws to around 0.020 inch (a decent flaw size to start crack growth analysis). A less robust way to handle this would be to do some research and try to quantify the effects of surface roughness on crack initiation life of test specimens...
