Tapping drill size

[firebird]

Hi

I have a stock of 9/16 & 5/8 round brass bar so I am going to buy 9/16 & 5/8 x 32tpi taps and dies from RDG tools. Heres a link. http://www.rdgtools.co.uk/acatalog/32TPI_TAPS_AND_DIES.html
I have looked through all my charts and searched the internet but can only find info up to 1/2 inch. Can anybody tell me what the tapping size drill is for 9/16 & 5/8.

Cheers

Rich

 

[mklotz]

One of the advantages of the DRILL program on my site is that, unlike the usual charts, it will find tap drill size for any size tap.

As an example, here's a typical output:

Depth of thread desired [75 %] ?

Tap Diameter = 0.6250 in = 15.8750 mm
Tap Pitch = 32.0000 threads/in = 0.0313 in/thread
= 1.2598 threads/mm = 0.7938 mm/thread
Thread form constant used = 1.2990
TAP DRILL DIAMETER FOR 75% DOT = 0.5946 in = 15.1017 mm

Depth of thread desired [75 %] ?

Tap Diameter = 0.5625 in = 14.2875 mm
Tap Pitch = 32.0000 threads/in = 0.0313 in/thread
= 1.2598 threads/mm = 0.7938 mm/thread
Thread form constant used = 1.2990
TAP DRILL DIAMETER FOR 75% DOT = 0.5321 in = 13.5142 mm

 

[Mainer]

Are they Whitworth taps? Then Marv's numbers might need to be adjusted slightly? Probably not enough to matter, though.

 

[bearcar1]

Well I was going to add that most likely 17/32" and 19/32" are probably the closest imperical sizes available, they are listed for those sizes but 27 TPI.

 

[Davyboy]

For "special" threads, the method I use:

Determine the lead of the thread, in this case 1/32", or .031
Then subtract .625-.031=.594
Drill the hole 19/32 = .5938"

For metric, it's the same. Major diameter minus thread lead = drill size

Works pretty close to 75% thread depth. Can be used for single point threading on the lathe, too.

DB

 

[mklotz]

The problem with the "major diameter - pitch" formula is that it takes no account of the depth-of-thread (DOT).

For sixty degree threads, the full formula is:

TD = MD - 0.013*DOT/P

where:

TD = tap drill diameter
MD = major diameter
DOT = depth-of-thread expressed as percentage
P = pitch of thread (tpi)

In the simpler "MD - 1/P" formula, we've set:

0.013*DOT = 1

Solving this yields DOT = 77%

which is reasonable for much threading. However, with tough materials, e.g. bronze, you'll want to use a smaller DOT (i.e., a larger tap drill) to prevent tap breakage. Below are some suggested values.


Recommended Depth of Thread:
MATERIAL % DOT

MILD AND UNTREATED STEELS 60-65
HIGH CARBON STEEL 50
HIGH SPEED STEEL 55
STAINLESS STEEL 50
FREE CUTTING STAINLESS STEEL 60
CAST IRON 70-75
WROUGHT ALUMINUM 65
CAST ALUMINUM 75
WROUGHT COPPER 60
FREE CUTTING YELLOW BRASS 70
DRAWN BRASS 65
MANGANESE BRONZE 55
MONEL METAL 55-60
NICKEL SILVER (GERMAN SILVER) 50-60

 

[Jasonb]

The ME Taps I've had from RDG (Apex brand) in the past have had the tapping size engraved on them.

If you want to make sure you have the right bit, it may be worth ringing RDG to get the size so you can order in one go if you don't have it.

Jason

 

[firebird]

Hi

Thankyou for the info gentlemen. Jason, I e-mailed RDG with the question before I posted it here but did not recieve a reply from them.

Cheers

Rich