|
Posted by Tony Jeffree on February 14, 2007, 4:11 am
Please log in for more thread options
On Wed, 14 Feb 2007 08:58:29 GMT, John Stevenson
>The advised tapping drill for materials is selected to give about an 80 % thread
>engagement ratio.
>this is chosen for ease of tapping and to relieve tap breakage but still give a
decent
>thread.
>Quarter Whit tables tell us tat the effective diameter is 0.218 but minor
diameter is
>0.186 which matches the fact that the depth of thread [ one side ] is 0.032
>
>Now a No 20 drill is 0.160, way below the minor diameter.
>So it could be that the taper part of your tap is having to ream out material
first and so
>'stretching' the pitch because of the work it has to do, leading to a loose
thread?
>
>Try using a 3/16" drill as this is bang on size but in soft material will
probably cut a
>tad big.
Tubal Cain's tables recommend 5.3mm for 1/4 Whit, to give 65% thread
engagement.
Regards,
Tony
| 
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML Sitemap
>engagement ratio.
>this is chosen for ease of tapping and to relieve tap breakage but still give a