When Rotary Broaching, a pre-drilled pilot hole is required. The typical pilot hole should be pre-drilled 3% larger than the “across the flats” dimension of the Broach for hexagons and 10% larger for square shapes. For example, if you are using a 1/4” Hexagon Rotary Broach, the across the flats dimension is .2525”, therefore your pre-drill dimension would be about .260”. This may vary depending upon your application, material and final product specifications. Here is a calculator to determine the most common pre-drill pilot hole sizes.
|
Choose Rotary Broach Shape: |
|
|
Choose the rotary broach size from drop-down: |
|
|
or enter the across the flats dimensions: |
example: .2525 |
Recommended Pilot Hole Pre-Drill Size is:
Here is a chart to show the most common hex pre-drill pilot hole sizes
| Nominal Size | Finish | Pri-Drill |
|---|---|---|
| .050″ Hex | .0510 | .053 |
| 1/16″ Hex | .0635 | .065 |
| 5/64″ Hex | .0790 | .081 |
| 3/32″ Hex | .0950 | .098 |
| 7/64″ Hex | .1110 | .114 |
| 1/8″ Hex | .1265 | .130 |
| 9/64″ Hex | .1425 | .147 |
| 5/32″ Hex | .1585 | .163 |
| 11/64″ Hex | .1740 | .179 |
| 3/16″ Hex | .1895 | .196 |
| 7/32″ Hex | .2215 | .228 |
| 1/4″ Hex | .2525 | .260 |
| 9/32″ Hex | .2840 | .293 |
| 5/16″ Hex | .3155 | .325 |
| 11/32″ Hex | .3470 | .357 |
| 3/8″ Hex | .3785 | .390 |
| 13/32″ Hex | .4100 | .422 |
| 7/16″ Hex | .4415 | .455 |
| 15/32″ Hex | .4730 | .488 |
| 1/2″ Hex | .5045 | .520 |
| Nominal Size | Finish | Pri-Drill |
|---|---|---|
| 1.3mm Hex | .0515 | .053 |
| 1.5mm Hex | .0610 | .063 |
| 2.0mm Hex | .810 | .083 |
| 2.5mm Hex | .1010 | .104 |
| 3.0mm Hex | .1210 | .125 |
| 4.0mm Hex | .1610 | .166 |
| 5.0mm Hex | .2000 | .206 |
| 6.0mm Hex | .2400 | .247 |
| 7.0mm Hex | .2790 | .287 |
| 8.0mm Hex | .3185 | .328 |
| 9.0mm Hex | .3580 | .369 |
| 10mm Hex | .3985 | .410 |
| 11mm Hex | .4370 | .450 |
| 12mm Hex | .4775 | .492 |
| 13mm Hex | .5170 | .533 |
Squares 10% larger than the “across the flats” dimension. |
||