LaymarCrafts Woodturning Information

 

Tapping Drill Sizes for various Threads

Many of the Projects covered in the Hints n Tips section of the LaymarCrafts Web Site require fixings that entail Drilling and Tapping for various Set Screws and Grub Screws.

 

I have standardised on Metric Sizes (M5, M6, M8 & M10 Sizes) as this reduces my investment in the necessary Drills and Taps and at the same time makes sourcing Screws and Fixings that much easier.

It is important if you are to achieve a good fixing that the correct size Drill is used at all times to ensure that the Thread Profile is of the right proportions and therefore strength, the table below gives the relevant data for the more common Thread Types.

British Standard Fine [BSF] British Standard Whitworth [BSW]
Thread Size Tapping Drill Clearance Drill
3/16" 5/32" 4.85mm
7/32" 4.65mm 5.70mm
1/4" 5.30mm 6.50mm
5/16" 6.75mm 8.10mm
3/8" 8.25mm 9.70mm
7/16" 9.70mm 11.30mm
1/2" 7/16" 13.00mm
Thread Size Tapping Drill Clearance Drill
3/16" 3.70mm 4.85mm
1/4" 5.10mm 6.50mm
5/16" 6.50mm 8.10mm
3/8" 5/16" 9.70mm
7/16" 9.25mm 11.30mm
1/2" 10.50mm 13.00mm
   
   
Unified Fine [UNF] Unified Course [UNC]
Thread Size Tapping Drill Clearance Drill
1/4" 5.5mm 6.5mm
5/16" 6.9mm 8.1mm
3/8" 8.5mm 9.7mm
7/16" 9.9mm 11.3mm
1/2" 11.4mm 13.0mm
Thread Size Tapping Drill Clearance Drill
1/4" 5.2mm 6.5mm
5/16" 6.6mm 8.1mm
3/8" 8.0mm 9.7mm
7/16" 9.4mm 11.3mm
1/2" 10.8mm 13.0mm

 

British Association Thread [BA] System International [M]
Thread Size Tapping Drill Clearance Drill
0 5.10mm 6.10mm
1 4.50mm 5.50mm
2 4.00mm 4.85mm
3 3.45mm 4.25mm
4 3.00mm 3.75mm
5 2.65mm 3.30mm
Thread Size Tapping Drill Clearance Drill
M4 3.3mm 4.15mm
M5 4.2mm 5.15mm
M6 5.0mm 6.2mm
M8 6.8mm 8.2mm
M10 8.5mm 10.25mm
M12 10.2mm 12.3mm

 

Drills are generally available in 4 Size Ranges:-

The difference in size between Inch Calibrated Drills is too wide for use in Thread Sizing and therefore we have both Gauge Number and Letter Drills which bridge the gaps and we also have Metric Drills calibrated in mm, it will be very rare to achieve an exact match for the above required Drills in every case and therefore the nearest size within reason will always have to be used.

Although Metric Drills are available in steps of 0.10mm they are difficult to obtain from Commercial Outlets and the more common sizes only are included in the table below.

For instance if we take the 2BA Thread above a 4mm Drill is a common Size but the 4.85mm Clearance Drill Size is not, so you will need to use one of the following for Tapping and Clearance Drill.

Thread Drill Inch
  5/32" .1562
  22 .1570
2BA 4.00mm .1575
  21 .1590
     
  12 .1890
2BA 4.85mm .1909
  11 .1910

The Table below gives the most common Drill Sizes available and the equivalent Decimal Dimension in Inches:-

Inch mm Gauge Inch
    37 .1040
    36 .1065
7/64     .1094
    35 .1102
    34 .1100
    33 .1130
    32 .1160
  3   .1181
    31 .1200
1/8     .1250
    30 .1285
    29 .1360
  3.5   .1378
    28 .1405
9/64     .1406
    27 .1440
    26 .1470
    25 .1495
    24 .1520
    23 .1540
5/32     .1562
    22 .1570
  4   .1575
    21 .1590
    20 .1610
    19 .1660
    18 .1695
11/64     .1719
    17 .1730
    16 .1770
  4.5   .1772
    15 .1800
Inch mm Gauge Inch
    14 .1820
    13 .1850
3/16     .1875
    12 .1890
    11 .1910
    10 .1935
    9 .1960
  5   .1968
    8 .1990
    7 .2010
13/64     .2031
    6 .2040
    5 .2055
    4 .2090
    3 .2130
  5.5   .2165
7/32     .2188
    2 .2210
    1 .2280
    A .2340
15/32     .2344
  6   .2362
    B .2380
    C .2420
    D .2460
1/4   E .2500
    F .2570
    G .2610
17/64     .2656
    H .2660
  7   .2756
    J .2770
Inch mm Gauge Inch
    K .2810
9/32     .2812
    L .2900
    M .2950
19/64     .2969
    N .3020
5/16     .3125
  8   .3150
    O .3160
    P .3230
21/64     .3281
    Q .3320
    R .3390
11/32     .3438
    S .3480
  9   .3543
    T .3580
23/64     .3594
    U .3680
3/8     .3750
    V .3770
    W .3860
25/64     .3906
  10   .3937
    X .3970
    Y .4040
13/32     .4062
    Z .4130
27/64     .4219
  11   .4331
7/16     .4375
29/64     .4531


The Table below gives the Decimal Inch Conversion for the various sizes of Metric Drills shown in the Tapping Drill Size tables above, this will enable you to select the nearest equivalent Drill Size for your requirements. Standard Sizes which are readily available are Highlighted.

mm Inch   mm Inch   mm Inch
2.65 .1043   5.15 .2028   8.5 .3346
3 .1181   5.2 .2047   8.5 .3346
3.3 .1299   5.3 .2087   9.25 .3642
3.45 .1358   5.5 .2165   9.4 .3701
3.7 .1457   5.7 .2244   9.7 .3819
3.75 .1476   5.75 .2264   9.9 .3898
4 .1575   6.2 .2441   10.2 .4016
4.15 .1634   6.5 .2559   10.25 .4035
4.2 .1654   6.6 .2598   10.5 .4134
4.25 .1673   6.8 .2677   10.8 .4252
4.5 .1772   6.9 .2717   11.3 .4449
4.65 .1831   8 .3150   11.4 .4488
4.85 .1909   8.1 .3189   12.3 .4843
5 .1969   8.2 .3228   13 .5118
5.1 .2008   8.25 .3248      

 



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