Hole Making Operations & Drill Size Chart

Hole-making is a class of machining operations that are specifically used to cut a hole into a workpiece. Machining, a material removal process, creates features on a part by cutting away the unwanted material and requires a machine, workpiece, fixture, and cutting tool. Hole-making can be performed on a variety of machines, including general machining equipment such as CNC milling machines or CNC turning machines. Specialized equipment also exists for hole-making, such as drill presses or tapping machines. The workpiece is a piece of pre-shaped material that is secured to the fixture, which itself is attached to a platform inside the machine. The cutting tool is a cylindrical tool with sharp teeth that is secured inside a piece called a collet, which is then attached to the spindle, which rotates the tool at high speeds. By feeding the rotating tool into the workpiece, material is cut away in the form of small chips to create the desired feature.

Hole-making operations are typically performed amongst many other operations in the machining of a part. However, hole-making may be performed as a secondary machining process for an existing part, such as a casting or forging. This can be done to add features that were too costly to form during the primary process or to improve the tolerance or surface finish of existing holes.

Source:https://www.custompartnet.com/wu/hole-making

Machined holes

In machining, a hole is a cylindrical feature that is cut from the workpiece by a rotating cutting tool that enters the workpiece axially. The hole will have the same diameter of the cutting tool and match the geometry (which may include a pointed end). Non-cylindrical features, or pockets, can also be machined, but they require end milling operations not hole-making operations. While all machined holes have the same basic form they can still differ in many ways to best suit a given application. A machined hole can be characterized by several different parameters or features which will determine the hole-making operation and tool that is required.

Diameter – Holes can be machined in a wide variety of diameters, determined by the selected tool. The cutting tools used for hole-making are available in standard sizes that can be as small as 0.0019 inches and as large as 3 inches. Several standards exist including fractional sizes, letter sizes, number sizes, and metric sizes. A custom tool can be created to machine a non-standard diameter, but it is more cost effective to use the closest standard sized tool.
Tolerance – In any machining operation, the precision of a cut can be affected by several factors, including the sharpness of the tool, any vibration of the tool, or the build up of chips of material. The specified tolerance of a hole will determine the method of hole-making used, as some methods are suited for tight-tolerance holes.
Depth – A machined hole may extend to a point within the workpiece, known as a blind hole, or it may extend completely through the workpiece, known as a through hole. A blind hole may have a flat bottom, but typically ends in a point due to the pointed end of the tool. When specifying the depth of a hole, one may reference the depth to the point or the depth to the end of the full diameter portion of the hole. The total depth of the hole is limited by the length of the cutting tool.
Recessed top – A common feature of machined holes is to recess the top of the hole into the workpiece. This is typically done to accommodate the head of a fastener and allow it to sit flush with the workpiece surface. Two types of recessed holes are a counterbore, which has a cylindrical recess, and a countersink, which has a cone-shaped recess.
Threads – Threaded holes are machined to accommodate a threaded fastener and are typically specified by their outer diameter and pitch. The pitch is a measure of the spacing between threads and may be expressed in the English standard, as the number of threads per inch (TPI), or in the metric standard, as the distance in millimeters (mm) between threads.

Hole making operations

Several hole-making operations exist, each using a different type of cutting tool and forming a different type of hole.

Drilling – A drill bit enters the workpiece axially and cuts a blind hole or a through hole with a diameter equal to that of the tool. A drill bit is a multi-point tool and typically has a pointed end. A twist drill is the most commonly used, but other types of drill bits, such as a center drill, spot drill, or tap drill can be used to start a hole that will be completed by another operation
Reaming – A reamer enters the workpiece axially and enlarges an existing hole to the diameter of the tool. A reamer is a multi-point tool that has many flutes, which may be straight or in a helix. Reaming removes a minimal amount of material and is often performed after drilling to obtain both a more accurate diameter and a smoother internal finish.
Tapping – A tap enters the workpiece axially and cuts internal threads into an existing hole. The existing hole is typically drilled by the required tap drill size that will accommodate the desired tap. The tap is selected based on the major diameter and pitch of the threaded hole. Threads may be cut to a specified depth inside the hole (bottom tap) or the complete depth of a through hole (through tap).
Boring – A boring tool enters the workpiece axially and cuts along the internal surface of an existing hole to enlarge the diameter or obtain more precise dimensions. The boring tool is a single-point cutting tool, which can be set to cut the desired diameter by using an adjustable boring head.
Counterboring – A counterbore tool enters the workpiece axially and enlarges the top portion of an existing hole to the diameter of the tool. Counterboring is often performed after drilling to provide space for the head of a fastener, such as a bolt, to sit flush with the workpiece surface. The counterboring tool has a pilot on the end to guide it straight into the existing hole.
Countersinking – A countersink tool enters the workpiece axially and enlarges the top portion of an existing hole to a cone-shaped opening. Countersinking is often performed after drilling to provide space for the head of a fastener, such as a screw, to sit flush with the workpiece surface. Common included angles for a countersink include 60, 82, 90, 100, 118, and 120 degrees.

Drill Size Chart

Drill size	Diameter (in)	Diameter (mm)

#107	0.0019	0.0483
0.05 mm	0.0020	0.0500
#106	0.0023	0.0584
#105	0.0027	0.0686
#104	0.0031	0.0787
#103	0.0035	0.0889
#102	0.0039	0.0991
0.1 mm	0.0039	0.1000
#101	0.0043	0.1092
#100	0.0047	0.1194
#99	0.0051	0.1295
#98	0.0055	0.1397
#97	0.0059	0.1499
#96	0.0063	0.1600
#95	0.0067	0.1702
#94	0.0071	0.1803
#93	0.0075	0.1905
0.2 mm	0.0079	0.2000
#92	0.0079	0.2007
#91	0.0083	0.2108
#90	0.0087	0.2210
#89	0.0091	0.2311
#88	0.0095	0.2413
#87	0.0100	0.2540
#86	0.0105	0.2667
#85	0.0110	0.2794
#84	0.0115	0.2921
0.3 mm	0.0118	0.3000
#83	0.0120	0.3048
#82	0.0125	0.3175
#81	0.0130	0.3302
#80	0.0135	0.3429
#79	0.0145	0.3680
1/64 in	0.0156	0.3969
0.4 mm	0.0158	0.4000
#78	0.0160	0.4064
#77	0.0180	0.4572
0.5 mm	0.0197	0.5000
#76	0.0200	0.5080
#75	0.0210	0.5334
#74	0.0225	0.5715
0.6 mm	0.0236	0.6000
#73	0.0240	0.6096
#72	0.0250	0.6350
#71	0.0260	0.6604
0.7 mm	0.0276	0.7000
#70	0.0280	0.7112
#69	0.0292	0.7417
#68	0.0310	0.7874
1/32 in	0.0313	0.7938
0.8 mm	0.0315	0.8000
#67	0.0320	0.8128
#66	0.0330	0.8382
#65	0.0350	0.8890
0.9 mm	0.0354	0.9000
#64	0.0360	0.9144
#63	0.0370	0.9398
#62	0.0380	0.9652
#61	0.0390	0.9906
1 mm	0.0394	1.0000
#60	0.0400	1.0160
#59	0.0410	1.0414
#58	0.0420	1.0668
#57	0.0430	1.0922
1.1 mm	0.0433	1.1000
#56	0.0465	1.1811
3/64 in	0.0469	1.1906
1.2 mm	0.0472	1.2000
1.3 mm	0.0512	1.3000
#55	0.0520	1.3208
#54	0.0550	1.3970
1.4 mm	0.0551	1.4000
1.5 mm	0.0591	1.5000
#53	0.0595	1.5113
1/16 in	0.0625	1.5875
1.6 mm	0.0630	1.6000
#52	0.0635	1.6129
1.7 mm	0.0669	1.7000
#51	0.0670	1.7018
#50	0.0700	1.7780
1.8 mm	0.0709	1.8000
#49	0.0730	1.8542
1.9 mm	0.0748	1.9000
#48	0.0760	1.9304
5/64 in	0.0781	1.9844
#47	0.0785	1.9939
2 mm	0.0787	2.0000
#46	0.0810	2.0574
#45	0.0820	2.0828
2.1 mm	0.0827	2.1000
#44	0.0860	2.1844
2.2 mm	0.0866	2.2000
#43	0.0890	2.2606
2.3 mm	0.0906	2.3000
#42	0.0935	2.3749
3/32 in	0.0938	2.3813
2.4 mm	0.0945	2.4000
#41	0.0960	2.4384
#40	0.0980	2.4892
2.5 mm	0.0984	2.5000
#39	0.0995	2.5273
#38	0.1015	2.5781
2.6 mm	0.1024	2.6000
#37	0.1040	2.6416
2.7 mm	0.1063	2.7000
#36	0.1065	2.7051
7/64 in	0.1094	2.7781
#35	0.1100	2.7940
2.8 mm	0.1102	2.8000
#34	0.1110	2.8194
#33	0.1130	2.8702
2.9 mm	0.1142	2.9000
#32	0.1160	2.9464
3 mm	0.1181	3.0000
#31	0.1200	3.0480
3.1 mm	0.1221	3.1000
1/8 in	0.1250	3.1750
3.2 mm	0.1260	3.2000
#30	0.1285	3.2639
3.3 mm	0.1299	3.3000
3.4 mm	0.1339	3.4000
#29	0.1360	3.4544
3.5 mm	0.1378	3.5000
#28	0.1405	3.5687
9/64 in	0.1406	3.5719
3.6 mm	0.1417	3.6000
#27	0.1440	3.6576
3.7 mm	0.1457	3.7000
#26	0.1470	3.7338
#25	0.1495	3.7973
3.8 mm	0.1496	3.8000
#24	0.1520	3.8608
3.9 mm	0.1535	3.9000
#23	0.1540	3.9116
5/32 in	0.1563	3.9688
#22	0.1570	3.9878
4 mm	0.1575	4.0000
#21	0.1590	4.0386
#20	0.1610	4.0894
4.1 mm	0.1614	4.1000
4.2 mm	0.1654	4.2000
#19	0.1660	4.2164
4.3 mm	0.1693	4.3000
#18	0.1695	4.3053
11/64 in	0.1719	4.3656
#17	0.1730	4.3942
4.4 mm	0.1732	4.4000
#16	0.1770	4.4958
4.5 mm	0.1772	4.5000
#15	0.1800	4.5720
4.6 mm	0.1811	4.6000
#14	0.1820	4.6228
#13	0.1850	4.6990
4.7 mm	0.1850	4.7000
3/16 in	0.1875	4.7625
4.8 mm	0.1890	4.8000
#12	0.1890	4.8006
#11	0.1910	4.8514
4.9 mm	0.1929	4.9000
#10	0.1935	4.9149
#9	0.1960	4.9784
5 mm	0.1969	5.0000
#8	0.1990	5.0546
5.1 mm	0.2008	5.1000
#7	0.2010	5.1054
13/64 in	0.2031	5.1594
#6	0.2040	5.1816
5.2 mm	0.2047	5.2000
#5	0.2055	5.2197
5.3 mm	0.2087	5.3000
#4	0.2090	5.3086
5.4 mm	0.2126	5.4000
#3	0.2130	5.4102
5.5 mm	0.2165	5.5000
7/32 in	0.2188	5.5563
5.6 mm	0.2205	5.6000
#2	0.2210	5.6134
5.7 mm	0.2244	5.7000
#1	0.2280	5.7912
5.8 mm	0.2284	5.8000
5.9 mm	0.2323	5.9000
A	0.2340	5.9436
15/64 in	0.2344	5.9531
6 mm	0.2362	6.0000
B	0.2380	6.0452
6.1 mm	0.2402	6.1000
C	0.2420	6.1468
6.2 mm	0.2441	6.2000
D	0.2460	6.2484
6.3 mm	0.2480	6.3000
1/4 in	0.2500	6.3500
E	0.2500	6.3500
6.4 mm	0.2520	6.4000
6.5 mm	0.2559	6.5000
F	0.2570	6.5278
6.6 mm	0.2598	6.6000
G	0.2610	6.6294
6.7 mm	0.2638	6.7000
17/64 in	0.2656	6.7469
H	0.2660	6.7564
6.8 mm	0.2677	6.8000
6.9 mm	0.2717	6.9000
I	0.2720	6.9088
7 mm	0.2756	7.0000
J	0.2770	7.0358
7.1 mm	0.2795	7.1000
K	0.2810	7.1374
9/32 in	0.2813	7.1438
7.2 mm	0.2835	7.2000
7.3 mm	0.2874	7.3000
L	0.2900	7.3660
7.4 mm	0.2913	7.4000
M	0.2950	7.4930
7.5 mm	0.2953	7.5000
19/64 in	0.2969	7.5406
7.6 mm	0.2992	7.6000
N	0.3020	7.6708
7.7 mm	0.3032	7.7000
7.8 mm	0.3071	7.8000
7.9 mm	0.3110	7.9000
5/16 in	0.3125	7.9375
8 mm	0.3150	8.0000
O	0.3160	8.0264
8.1 mm	0.3189	8.1000
8.2 mm	0.3228	8.2000
P	0.3230	8.2042
8.3 mm	0.3268	8.3000
21/64 in	0.3281	8.3344
8.4 mm	0.3307	8.4000
Q	0.3320	8.4328
8.5 mm	0.3347	8.5000
8.6 mm	0.3386	8.6000
R	0.3390	8.6106
8.7 mm	0.3425	8.7000
11/32 in	0.3438	8.7313
8.8 mm	0.3465	8.8000
S	0.3480	8.8392
8.9 mm	0.3504	8.9000
9 mm	0.3543	9.0000
T	0.3580	9.0932
9.1 mm	0.3583	9.1000
23/64 in	0.3594	9.1281
9.2 mm	0.3622	9.2000
9.3 mm	0.3661	9.3000
U	0.3680	9.3472
9.4 mm	0.3701	9.4000
9.5 mm	0.3740	9.5000
3/8 in	0.3750	9.5250
V	0.3770	9.5758
9.6 mm	0.3780	9.6000
9.7 mm	0.3819	9.7000
9.8 mm	0.3858	9.8000
W	0.3860	9.8044
9.9 mm	0.3898	9.9000
25/64 in	0.3906	9.9219
10 mm	0.3937	10.0000
X	0.3970	10.0838
Y	0.4040	10.2616
13/32 in	0.4063	10.3188
Z	0.4130	10.4902
10.5 mm	0.4134	10.5000
27/64 in	0.4219	10.7156
11 mm	0.4331	11.0000
7/16 in	0.4375	11.1125
11.5 mm	0.4528	11.5000
29/64 in	0.4531	11.5094
15/32 in	0.4688	11.9063
12 mm	0.4724	12.0000
31/64 in	0.4844	12.3031
12.5 mm	0.4921	12.5000
1/2 in	0.5000	12.7000
13 mm	0.5118	13.0000
33/64 in	0.5156	13.0969
17/32 in	0.5313	13.4938
13.5 mm	0.5315	13.5000
35/64 in	0.5469	13.8906
14 mm	0.5512	14.0000
9/16 in	0.5625	14.2875
14.5 mm	0.5709	14.5000
37/64 in	0.5781	14.6844
15 mm	0.5906	15.0000
19/32 in	0.5938	15.0813
39/64 in	0.6094	15.4781
15.5 mm	0.6102	15.5000
5/8 in	0.6250	15.8750
16 mm	0.6299	16.0000
41/64 in	0.6406	16.2719
16.5 mm	0.6496	16.5000
17 mm	0.6693	17.0000
43/64 in	0.6719	17.0656
11/16 in	0.6875	17.4625
17.5 mm	0.6890	17.5000
45/64 in	0.7031	17.8594
18 mm	0.7087	18.0000
23/32 in	0.7188	18.2563
18.5 mm	0.7284	18.5000
47/64 in	0.7344	18.6531
19 mm	0.7480	19.0000
3/4 in	0.7500	19.0500
49/64 in	0.7656	19.4469
19.5 mm	0.7677	19.5000
25/32 in	0.7813	19.8438
20 mm	0.7874	20.0000
51/64 in	0.7969	20.2406
20.5 mm	0.8071	20.5000
13/16 in	0.8125	20.6375
21 mm	0.8268	21.0000
53/64 in	0.8281	21.0344
27/32 in	0.8438	21.4313
21.5 mm	0.8465	21.5000
55/64 in	0.8594	21.8281
22 mm	0.8661	22.0000
7/8 in	0.8750	22.2250
22.5 mm	0.8858	22.5000
57/64 in	0.8906	22.6219
23 mm	0.9055	23.0000
29/32 in	0.9063	23.0188
21/23 in	0.9130	23.1913
59/64 in	0.9219	23.4156
23.5 mm	0.9252	23.5000
15/16 in	0.9375	23.8125
24 mm	0.9449	24.0000
61/64 in	0.9531	24.2094
24.5 mm	0.9646	24.5000
31/32 in	0.9688	24.6063
25 mm	0.9843	25.0000
63/64 in	0.9844	25.0031
1 in	1.0000	25.4000
25.5 mm	1.0039	25.5000
1 1/64 in	1.0156	25.7969
26 mm	1.0236	26.0000
1 1/32 in	1.0313	26.1938
26.5 mm	1.0433	26.5000
1 3/64 in	1.0469	26.5906
1 1/16 in	1.0625	26.9875
27 mm	1.0630	27.0000
1 5/64 in	1.0781	27.3844
27.5 mm	1.0827	27.5000
1 3/32 in	1.0938	27.7813
28 mm	1.1024	28.0000
1 7/64 in	1.1094	28.1781
28.5 mm	1.1221	28.5000
1 1/8 in	1.1250	28.5750
1 9/64 in	1.1406	28.9719
29 mm	1.1417	29.0000
1 5/32 in	1.1563	29.3688
29.5 mm	1.1614	29.5000
1 11/64 in	1.1719	29.7656
30 mm	1.1811	30.0000
1 3/16 in	1.1875	30.1625
30.5 mm	1.2008	30.5000
1 13/64 in	1.2031	30.5594
1 7/32 in	1.2188	30.9563
31 mm	1.2205	31.0000
1 15/64 in	1.2344	31.3531
31.5 mm	1.2402	31.5000
1 1/4 in	1.2500	31.7500
32 mm	1.2598	32.0000
1 17/64 in	1.2656	32.1469
32.5 mm	1.2795	32.5000
1 9/32 in	1.2813	32.5438
1 19/64 in	1.2969	32.9406
33 mm	1.2992	33.0000
1 5/16 in	1.3125	33.3375
33.5 mm	1.3189	33.5000
1 21/64 in	1.3281	33.7344
34 mm	1.3386	34.0000
1 11/32 in	1.3438	34.1313
34.5 mm	1.3583	34.5000
1 23/64 in	1.3594	34.5281
1 3/8 in	1.3750	34.9250
35 mm	1.3780	35.0000
1 25/64 in	1.3906	35.3219
35.5 mm	1.3976	35.5000
1 13/32 in	1.4063	35.7188
36 mm	1.4173	36.0000
1 27/64 in	1.4219	36.1156
36.5 mm	1.4370	36.5000
1 7/16 in	1.4375	36.5125
1 29/64 in	1.4531	36.9094
37 mm	1.4567	37.0000
1 15/32 in	1.4688	37.3063
37.5 mm	1.4764	37.5000
1 31/64 in	1.4844	37.7031
38 mm	1.4961	38.0000
1 1/2 in	1.5000	38.1000