Subject: Re: hole size for metric taps

From: [email-masked] (Richard Griffin)
Date: 25 Apr 1996 12:40:45 +0100

Steve Austin followed up Honore de Groneckel with some very useful information
on "manipulating" thread sizes to suit different metals but one comment he made
worried me: 

>Most of the tap drill charts that are still around were done for older
>thread systems.  Since the early 70's, we in the U.S. have been using
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>the UN thread form and all of the standard bolts and machine screws
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>that you buy are made to this form (metric included).  
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So all the equipment made in the US for export to Europe has UN threadform to
mate up with the Metric screws we have standardised upon, all manufactured
rigorously with correct Metric threadform. Isn't this taking National Identity
too far, do you want to get a bad reputation for "standard screws don't fit in
American equipment". You've got away with funny size gallons for years but 
you'll want 99 centimetres in a metre next :^)

If the English can give up measuring distances in poles and furlongs why can't
you give up your funny thread sizes?

>Thats why its
>good practice to figure out the tap drill size for yourself.

Point taken, in future I'll be very careful which threads I wrap my nuts around

Richard Griffin.

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From: "John R. Johnson" <[email-masked]>
Date: Thu, 25 Apr 1996 13:40:30 -0500

The UN threadform is essentially the same as the standard metric threadform.
It calls for a 60 degree thread with a rounded or flat top of the threadform
to allow clearance in the sharp point of the V.  This allows threads made
with "less than perfect" tools to work.  It has NOTHING to do with diameter
of the screw or bolt and NOTHING to do with the thread pitch.  It DOES have
to do with the threadform or "shape" of the thread itself.  For example
ACME is a threadform,  etc.
John