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|
- 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.