To minimize tapping problems and lengthen tool life, use the largest drill possible to produce a minor diameter that will result in the lowest percentage of full thread consistent with adequate strength. A minor diameter that provides a 55% to 65% thread is sufficient for most requirements, but in some cases a higher percentage of thread may be necessary to conform with the minor diameter limits of the thread class specified.
Drills generally cut holes larger than their diameters. In the form bellow, the probable percentages of full thread were determined by the average amount oversize the various drills are expected to cut. Reaming becomes necessary when close control of the hole size is required.
*Generally, deeper than 1 1/2 times the hole diameter.
Suggested Percentage of Full Thread in Tapped Holes
Material
*Deep Hole
Tapping
Average
Commercial
Work
Thin Sheet
Stock or
Stamping
Free
Cutting
Aluminum
Brass
Bronze
Cast Iron
Copper
Mild Steel
Tool Steel
60%-70%
65%-70%
75%-85%
Hard
or
Tough
Cast Steel
Drop Forging
Monel Metal
Nickel Steel
Stainless Steel
55%-65%
60%-70%

1. Hole Size for Inch Cutting Taps
Major Diameter TPI (threads/inch) % of Thread
 
HOLE SIZE
2. Hole Size for Metric Cutting Taps
Major Diameter Pitch % of Thread
 
HOLE SIZE
3. Hole Size for Inch Forming Taps
Major Diameter TPI (threads/inch) % of Thread
 

HOLE SIZE
4. Hole Size for Metric Forming Taps
Major Diameter Pitch % of Thread
 
HOLE SIZE
Suggested Pipe Tap Drill sizes
Tap sizes 1/16 1/8 1/4 3/8 1/2 3/4 1 1-1/4 1-1/2 2 2-1/2 3 3-1/2 4
Drill
sizes
Taper
Pipe
Tap*
D R 7/
16
37/
64
45/
64
59/
64
1-5
/32
1-1/2 1-47/64 2-7/32 2-5/8 3-1/4 3-3/4 4-1/4
Straight
Pipe
Tap**
1/
14
11/
32
7/
16
37/
64

23/
32

59/
64
1-5
/32
1-1/2 1-3/4 2-7/32 2-21/32      

*Sizes given permit direct tapping without reaming the hole, but only give a full thread for the first two or three threads.
**For Dryseal Straight Pipe Threads suggested drill sizes are as shown, expect; 1/4" pipe, use .444 drill size.

 
NEWS
Technical Sections

Taps --
Technical information on tapping
Drills --
Technical information on drilling
End Mills --
Technical information on milling
Surface Treatments --
Surface treatment infomation
MSDS --
Material Safety Data Sheet
Thread Mills --
Technical information on Thread Mills