Countersink Holes

Countersink Diagram
[Countersink Diagram]

Why our hole diameter is usually greater than the body diameter

When punching a hole in sheetmetal in preparation for countersinking, we punch a hole large enough to leave a minimum of .005" at the bottom of the hole so that there is no burr formed. The diagram above demonstrates this .005" minimum.  We have calculated the hole diameter required for a number of popular screw sizes and have listed the dimensions in the tables below. 

Table for 82 Degree Countersinks

Screw Size

#4

#5

#6

#8

#10

#12

1/4-20

Max. Head Dia.

0.225

0.252

0.279

0.332

0.385

0.438

0.507

Min. Head Dia.

0.207

0.232

0.257

0.308

0.359

0.410

0.477

Top Ctsk. Dia.

0.216

0.242

0.268

0.320

0.372

0.424

0.492

Body Dia.

0.112

0.125

0.138

0.164

0.190

0.216

0.250

Screw Size

#4

#5

#6

#8

#10

#12

1/4-20

Thickness

Hole Sizes Before Countersinking for 82 Degree Screws (+/-.005)

0.020

0.1950

0.2220

0.2490

0.3020

0.3550

0.4060

0.4770

0.025

0.1870

0.2130

0.2400

0.2930

0.3460

0.4060

0.4680

0.030

0.1760

0.2010

0.2280

0.2810

0.3340

0.4060

0.4560

0.035

0.1660

0.1960

0.2210

0.2740

0.3280

0.4060

0.4490

0.042

0.1600

0.1870

0.2130

0.2630

0.3170

0.4060

0.4370

0.050

0.1410

0.1700

0.1960

0.2500

0.3030

0.3830

0.4240

0.060

0.1280

0.1520

0.1790

0.2320

0.2850

0.3830

0.4060

0.074

0.1250

0.1280

0.1560

0.2080

0.2630

0.3680

0.3830

0.090

0.1250

0.1280

0.1560

0.1940

0.2500

0.3360

0.3680

0.120

0.1250

0.1280

0.1560

0.1870

0.2130

0.3030

0.3120

0.179

0.1250

0.1280

0.1560

0.1870

0.2130

0.2320

0.3120

0.250

0.1250

0.1280

0.1560

0.1870

0.2130

0.2320

0.3120

Our standard for the diameter for the top of the countersink is to take the middle of the range for the nominal dimension with a tolerance of plus or minus .010. For example for a #8 flat head screw in .042 thick material, we would punch a .263 diameter hole and countersink to .320 (.332 plus .308 divided by 2) plus or minus .010.  Any dimension for the top of the countersink from .310 to .330 would be considered within tolerance.

The hole punched in the sheet metal is designed to leave a minimum .005 wall after the countersink is complete. This will prevent burr formation on the bottom side of the sheet metal. For the example above we would punch a .263 diameter hole, although the body diameter of the screw is .164. Part of the head will extend below the surface of the sheet metal. 

Table for 100 Degree Countersinks

Screw Size

#4

#5

#6

#8

#10

#12

1/4-20

Max. Head Dia.

0.231

0.257

0.283

0.335

0.387

0.439

0.507

Min. Head Dia.

0.201

0.227

0.253

0.305

0.357

0.409

0.477

Top Ctsk. Dia.

0.216

0.242

0.268

0.320

0.372

0.424

0.492

Body Dia.

0.112

0.125

0.138

0.164

0.190

0.216

0.250

Screw Size

#4

#5

#6

#8

#10

#12

1/4-20

Thickness

Hole Sizes Before Countersinking for 100 Degree Screws (+/-.005)

0.020

0.1870

0.2130

0.2490

0.3020

0.3460

0.4060

0.4770

0.025

0.1870

0.2130

0.2320

0.2930

0.3460

0.4060

0.4680

0.030

0.1760

0.1960

0.2210

0.2740

0.3360

0.3830

0.4560

0.035

0.1600

0.1870

0.2130

0.2630

0.3120

0.3680

0.4370

0.042

0.1410

0.1760

0.1960

0.2500

0.3130

0.3680

0.4240

0.050

0.1250

0.1280

0.1760

0.2320

0.2850

0.3360

0.4060

0.060

0.1250

0.1280

0.1560

0.2130

0.2850

0.3830

0.4060

0.074

0.1250

0.1280

0.1560

0.1870

0.2130

0.2320

0.3120

0.090

0.1250

0.1280

0.1560

0.1870

0.2130

0.2320

0.3120

0.120

0.1250

0.1280

0.1560

0.1870

0.2130

0.2320

0.3120

0.179

0.1250

0.1280

0.1560

0.1870

0.2130

0.2320

0.3120

0.250

0.1250

0.1280

0.1560

0.1870

0.2130

0.2320

0.3120

Our standard for the diameter for the top of the countersink is to take the middle of the range for the nominal dimension with a tolerance of plus or minus .015. For example for a #8 flat head screw in .042 thick material, we would punch a .250 diameter hole and countersink to .320 (.335 plus .305 divided by 2) plus or minus .015.  Any dimension for the top of the countersink from .305 to .335 would be considered within tolerance.

The hole punched in the sheet metal is designed to leave a minimum .005 wall after the countersink is complete. This will prevent burr formation on the bottom side of the sheet metal. For the example above we would punch a .250 diameter hole, although the body diameter of the screw is .164. Part of the head will extend below the surface of the sheet metal.