I can tell you from very expensive personal experience that 82 and 90 deg screws are both common for #6 and #10, and that a good ME will select the proper angle based on the thickness of the material being held, but that getting them mixed up on a production line can cause no end of grief.
Drywall screws probably aren't a good choice for much more than wood or their intended use (although they do self-drill thru fiberglas nicely). The bad feature of most DW screws is that they're brittle, and just snap.
I prefer to use decking screws (for wood projects), which will typically bend before breaking, but testing a chosen brand? (likely to be imported) is probably a good idea.
The way the heads are transitioned from the shank isn't very consistent for either type.. there may be a radius as mentioned, or a combination of a countersink that blends into a tapered section.
You've neglected to mention what materials are to be fastened, but quality countersunk-head machine, sheetmetal or wood screws will likely have more accurate/better matching head geometry for a machine-cut countersunk hole.
If this fastening task is related to an earlier situation of applying a thin, hard skin over a soft substrate, I'd expect DW screws to be about the worst choice.
And rust like crazy. The coarse thread has an especially nice "bite", as the root diam to total diam is much smaller. Yeah, these things have a good Rockwell, I think. I occasionally use them (and concrete nails) as scribers.
Decking screws appear to me to be higher quality drywall screws. I have a bunch of SS DeckFast deck screws: square drive, "auger point" (slit like in tapping chassis screws), and *two* included angles (one a transition to the neck) as you mentioned, and raised lines under the head, presumably an anti-loosening strategy. Starborn Industries, Avenel, NJ. Not cheap.
Attaching metal apparatus to wood. From a c-sink pov, I should proly go to sheetmetal, but from a drilling/grabbing pov, I really like coarse drywall screws, whose threads are deeper, sharper.
Oh, the wood filler/epoxy thread.... Why are drywall screws bad for that, besides rusting like crazy? What is a good choice?
I don't recall the details about the earlier discussion wrt metal-over-substrate, but it wasn't about wood-epoxy filler products, not that I recall.
A while ago you were asking about adding a skin over foam or MDF or maybe some other substrate(s).
Then there was an even earlier topic of screws with deep/aggressive threads, such as those used for plastics.. but you were contemplating using them in metal (mounting a device to metal door frames, IIRC).
I assume that you're aware that countersinking thin materials/sheetmetal can be a little troublesome since common cutting tools will tend to lift the material, making consistent countersink dimensions problematic.
Using a multi-flute countersink cutter which doesn't have any, or little cutting edge relief angle (like dressing a drill points' relief angles for drilling brass) so the cutting forces are more like that of a reamer - scraping away the metal.. should eliminate lifting of the thin materials.
Sheetmetal screw threads are very reliable in wood, although depending upon which particular wood and the screw size (diameter), predrilling may be a good idea. Driving sheetmetal screws into dry hardwoods will likely cause a lot of pressure, and possibly splitting when not predrilled (definite maybe).
Predrilling doesn't necessarily mean a HSS twist drill, when doing carpentry-type stuff.. a nail with the head removed, mounted in a drill (or flutedYankee drill) will generally suffice.
The cutting points of decking screws (at a more premium price, but probably a better quality fastener) can eliminate the need for drilling in most S-P-F woods, but maybe not in dry hardwoods YMMV.
Good memory!! Apropos of that excellent discussion, I came up with an elegant solution: No glue at all!! I just had delivered 1/32" sheet alum, and 1/2" ply, and I'm just going to let the alum "float" or slide on the ply substrate, as it will be very well constrained along all 4 edges, by 3/4 x 1.5 x 1.5 (1/8") channel.
Eventually I WILL use glues of sorts, as I think that altho mechanical engineering insights in that thread were valid, that I'll still be able to generate add'l stiffness without the aluminum shearing the top layer of ply off the wood. It will be an ongoing experiment.
But, as it is, the ply itself is stiff enough, just more would be better.
You have a scary memory!! LOL Indeed, and I found that the coarse drywall screw was actually pretty well suited for this, esp. with being so hard -- but indeed a bit brittle.. I found some other type screws as well, but I'll have to dig those up -- sumpn with "mixed pitch threads", or sumpn.
Yes, those SS deck screws I have seem very high quality, and "built to drill".
The FH screw Q was not for sheetmetal, tho, but for that apparatus in a doorway, with sufficient alum "meat" to c-sink. I myself would not even contemplate c-sinking sheetmetal -- there's nuthin to c-sink!!
Same problem exists for FH machine screws in material that's too thin, eg,
1/4-20 FH in 1/8 alum. You wind up enlarging the hole itself.
I solved my problem with the MSC 82 deg spot drill for any #10 FH screw, and drywall screws, altho not 82 deg, nevertheless "fit". I most likely won't stick with drywall screws, I'll do sumpn with deck screws, or trad'l sheetmetal screws, or sleuth down some type of custom/specialty screw, of which there are an absolutely dizzying array.
Another tool which is nice for thin metals is the tool made for aircraft sheet metal use. The Microstop countersink cage, such as this one on eBay:
Auction # 280731984065
It chucks in a drill press or a hand drill. It uses special piloted countersinks which screw into the spindle. It will push down with the cage until the preset depth is reached (adjustable in 0.001" steps), at which point the ball bearing will stop the advance with the cage in contact with the workpiece. IIRC, it has a certain amount of spring travel before it hits the stop, but I may be mis-remembering that part.
It would be nice if the eBay auction came with some sample countersink bits, but this one does not have them.
O.K. This one has two cages, and four bits of different sizes:
Auction # 190569970965
Some bits, such as this one, are drilled for a pilot but you put your choice of size pilot in and secure it by a setscrew in the body of the countersink.
Auction # 230633535081
The search term to see several styles and groupings is:
Industrial type contact cement would be an easy way to bond the alumininum to the plywood and might work for you.
My choice in screws for this application would be what I call "cabinet screws" They have a conventional countersink head, a straight shank, and fairly deep threads. The screws are meant for use with wood or particle board and thus have threads deeper than a wood screw, but a bit less than a true particle board screw. Unlike sheetrock screws they aren't brittle.