Screw Drive Types

[Don Gilmore]

----- Original Message ----- 
From: "Phillip Ford" <fordpiano@earthlink.net>
To: <pianotech@ptg.org>
Sent: Friday, February 06, 2004 2:51 PM
Subject: Re: Screw Drive Types


> I sent scans of the articles to be posted, so you can see for yourself
> later.  The testing was on pan head screws (such as you might see in a
> piano action) - not on heavy duty machine screws, so your mileage may
> vary.  Also, as I understand it, they used a uniform end load on each
> fastener and what determined max torque was drive failure, which I took to
> mean cam-out, but I suppose could mean rounding off the drive.  So cam-out
> on an internal hex of the type used on ordinary pan head screws must not
be that high
> or the material strength is such that they round out.  They didn't specify
what grade
> fasteners they were using.

Now I'm really confused.  As far as I know, there is no such thing as a
pan-head screw with a hex socket...at least in the United States.  There is
a "button head" capscrew that sort of looks like a pan head if you have a
really good imagination, but it's not the same at all--more like a round
head.  Socket head capscrews are usually only available in standard, flat
head and button head, or perhaps some exotic hybrids like "low head".
"Machine screws" are available in all sorts of head styles, e.g. truss head,
fillister head, round head, binder head, etc., but "capscrews" are
different.

I wonder if they are talking about something else.  Surely they aren't
trying to drive a socket head screw with a phillips screwdriver, or anything
silly like that.

> Just a couple of days ago I was tightening the allen head set
> screws which hold the needles on my super-duper Yamaha voicing tool and
> managed to round out one of the internal hex so that I can't get the screw
> out - long before I bent the wrench into a pretzel.  Perhaps this only
> applies on allen head screws large enough for holding bridges together or
on
> high strength socket head screws, like Unbrako or some such.

In my experience only the tiniest capscrews (like with .050 or 1/16 hex)
will ever strip--and even then usually with button or flat head (since they
have shallower sockets than standard SHCS's).  If any screws larger than
that strip out, you probably have a worn out wrench, or are using the wrong
size.

Don A. Gilmore
Mechanical Engineer
Kansas City

> Phil Ford