Prior to utilizing the TT2″, a pre-hole must be punched. For best results, always tap the holes in the same sequence as the pre-holes are punched. This will eliminate any backlash that is in the punch press.
The necessary pre-hole punch sizes are listed on the chart below. This chart is based on 65%-70% thread. For every .001 difference in hole size the % of thread changes by 5%.
The pre-hole punch sizes listed on these charts are for both extruded and thru-holes. For extruded hole size use flat material pre-hole. If multiple pre-hole diameters are listed, use 0 – .080.
WARNING – Do not exceed .002 smaller pre-hole size than required. Serious tool damage may result.
UN Thread / American Punches
| Tap Size | Material Thickness | Punch Size |
| 12-24 | 0 – .090 | .199 |
| 12-24 | .100 – .250 | .201-.205 |
| 1/4-20 | 0 – .090 | .228 |
| 1/4-20 | .100 – .250 | .23-.232 |
| 1/4-28 | 0 – .090 | .234 |
| 1/4-28 | .100 – .250 | .236-.238 |
| 5/16-18 | 0 – .125 | .288 |
| 5/16-18 | .135 – .250 | .292-.294 |
| 5/16-24 | 0 – .125 | .294 |
| 5/16-24 | .135 – .250 | .296-.298 |
Please note:
- When tapping stainless steel increase the hole size by .001 -.002 on material .07 (1.75mm).
- Please note that the pre-hole size changes when tapping certain threads on material .09 (2.5mm) or thicker.
- A smaller H or D factor tap may be needed if tapped hole is too loose. Please see Tap Factor page for further explanation.
- If tapping extrusions pre-hole is determined by material thickness not thickness of extruded hole.
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