An Odd Screw

[Robert DoddsParticipant @robertdodds43397]

Trying to tidy up and checking the contents of long forgotten boxes of assorted screws I spotted half a dozen of these today.

They are in steel, either 3/16 BSF or 2 BA with a Phillips type head but what is the purpose of the L.H. threaded portion just under the head?

Regards Bob D

[Robert DoddsParticipant @robertdodds43397]

[Swarf, Mostly!Participant @swarfmostly]

Hi there, Robert,

One purpose of screws like that was/is to secure loudspeakers to their baffle board. The coarse left-hand screw engages the plywood baffle board and the machine screw portion takes washers and nuts to secure the loudspeaker. Tightening the machine screw nut also, because of the left-hand thread, tightens the wood-screw part.

Best regards,

Swarf, Mostly!

[Robert DoddsParticipant @robertdodds43397]

Swarf,

I think you've "nailed it" The top bit does look remarkably like a wood screw thread.

Regards Bob D

[Robert Atkinson 2Participant @robertatkinson2]

Not just loudspeakers, but that as a common appliction. Used in all sorts of wooden cabinets with mechanical fixings. The one pictured is a bit long for loudspeakers.
There are similar looking wood screws with a different "thread" on the shank that is supposed to reduce the driving torque when installing but keeping the screw tight when installed..

See also pronged tee nuts, not the type used in machine tables.

Robert G8RPI.

[Bo’sunParticipant @bosun58570]

Definitely looks like left over flatpack hardware. Check your Swedish furniture.

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