John wrote:
> There is, or was, a standard for straight screw slots which was based on wire
> gauges. If anyone can come up with a reference on this, I would greatly
> appreciate it.
>
> The intended scope of the standard is unclear; wood screws, machine screws, or
> both. British or US?
Machinery's handbook has a fair bit of information.
The 8th ed (1931), for example, shows the ASME standard slot to be 0.173d+0.015"
(d=body diameter)for all head types and screw sizes up to #30 (0.450"). A 1/8"
dia (0.125", or #5) would have a 0.037" wide slot. This applies to machine
screws.
Then you get all of the others:
1/8" dia Button head from Nat'n'l Acme, for example, had 0.025" wide slot, while
the same screw from Hartford Machine screw Co. was 0.028".
Other manufacturers used different widths, as well.
Stepping back to the 5th edition, we find the same rule for machine screws,
except for all of the examples that are different. For example, fillister head
screws (no reference on the chart: page 771, if you want to look) in 1/8" dia
have a slot 0.025" wide, though two pages prior, the size was 0.037 for other
1/8" (#5) screws. Wood screws (page 777) are the same as in the 8th edition.
For wood screws, there is less information, but the slot sizes are similar to
the ASME standard, but not the same (#5 is 0.036", for example... a #5 wood
screw is 0.0014" smaller diameter than a #5 machine screw, BTW), at leas in the
Asa Cook Co standard. Other makes differed, of course.
More modern sources (machinery's 26th, for example) give diameters for wood
screws that are of the same sequence as machine screws (0.013"*n+0.060"), rather
than the older form (0.01325"*n+0.056 or 0.01316"*n+0.05784", depending on the
time frame &c). The slot width is given a decidedly sloppy range here, a #5
being max 0.043 to min 0.035" (ANSI B18.6.1-1981, Rev1997), allowing even
standard compliant screw slots to be a hazard to life, limb, and product.
Looking through other sources I have on hand (Engineers Handbook, Kent, Marks,
&c), I find similar information, that that I can summarize as there are several
standards, manufactures did what they wanted, and, in the end, the ANSI standard
(US-ian equivalent of ISO, but it doesn't always play ball with the ISO despite
official policy that it does when possible) was designed to compromize and
pretty much bracket, in a non-specific and un-useful way, the various prior art.
If there were other standards in the past, I do not have ready reference to them
in the 60 minutes I am willing to devote to the search right now while I
procrastinate before doing a final shovel pass on my and two disabled
neighbors' walkways before dark.
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