TAP & DRILL SIZE

[askimball (Doc)]

QUESTION..

For the Star Die/s Thread Size >  11/16 x 24 TPI  

Is the Drill Size for the 11/16x24 Tap  >  21/32..

[maurilew]

my book says 41/64 alum    21/32 steel

[askimball (Doc)]

10-4

[rbwillnj]

I use 41/64 even for Steel (Cast Iron?).  According to the book, that yields a 90% thread.

[askimball (Doc)]

Bruce
The Chart I have Shows 75% Thread  

http://www.shender4.com/thread_chart.htm

[fc60]

Greetings,

The 11/16-24 tapped hole in the tool head is precisely located. Simply drilling may not leave the hole in the correct location. To be done properly, it should be drilled undersize then bored to 0.650".

The 0.650" is also a 16.5mm drill.

You need a 0.650" hole minimum to clear the 9/16" lock nut on the die. Otherwise, you will not be able to insert the seating die assembly with the nut attached.

Cheers,

Dave

[askimball (Doc)]

Thanks to rbwillnj  &  fc60  

[rbwillnj]

Dave is correct about making sure the hole is located correctly.  It should be fully indexed to determine the center of the hole.   Holding the tool head in the proper position during drilling to assure the hole is perpendicular to the flange of the tool head is also important.  You can't just clamp the tool head in a vice and drill away.  I have a jig to hold the tool head and mount it in the milling machine.  Then you have to find the exact center of the 3/8" hole before drilling and tapping.  This is not a job that can be done properly in a drill press.

[askimball (Doc)]

Thanks Guys for all the 'Heads Up'

I have a Friend that has a HUGE Bridgeport Mill.. His vise is large enough to set the head down flat... He first lined up the hole with
a 3/8 drill bit, then drilled it, not sure what size drill he used...Measeured it with a inside type mike thing, checked it and took a
reamer & took off a little more, tapped it.....Perfect fit.....Charged me $10.00.

[LeupoldMRK4]

D-1/N=Tap drill size
diameter minus.........1 divided by the number of threads.(pitch)
This is the formula I use in the shop.