The pre-drill diameter for a roll tap is different from that for a cutting tap.
With roll tapping, the thread is not cut but formed by material deformation.
Because of this chipless process, the pre-drill diameter is larger than that for a regular cutting tap.
Roll tapping pre-drill diameter formula
Since the correct pre-drill diameter for a roll tap differs from that of a cutting tap, the question arises:
How do you determine the correct pre-drill diameter? Here’s a handy calculation for that.
The pre-drill diameter for roll tapping is calculated as follows:
nominal diameter minus (the pitch (P) x 0.45)
So...
• A roll tap M8 has a pre-drill diameter of 8 - (1.25mm x 0.45) = 7.4 mm
• A roll tap M10 has a pre-drill diameter of 10 – (1.5mm x 0.45) = 9.3 mm
• A roll tap M12 has a pre-drill diameter of 12 – (1.75mm x 0.45) = 11.2 mm
No pre-drill table for roll taps at hand? Use this simple pre-drill diameter formula for roll tapping.
5 advantages of roll tapping
Here are the 5 advantages of roll tapping:
- No chips, so the tap won't jam due to leftover chips
- Faster and stronger thread production
- A roll tap can be used for both through and blind holes
- Greater thread depth achievable
- In general, roll taps have a longer service life
Especially with roll taps, the lower friction coefficient of a TiN coating works very positively; this results in higher cutting speeds and longer tool life.
Here you can find roll taps, and for each individual roll tap, you will see the correct pre-drill size in the given drill table.