When buyers compare drill bit quotations, the point geometry is often the first thing to be compromised. Beyond the familiar 118 or 135 degree point angle there is a second, quieter specification that decides how a drill starts a hole, how it clears chips and how long it survives in hardened material: the web thinning. This guide explains the four DIN 1412 thinning forms and when each one is the right choice.
What the web and the chisel edge do
The web is the steel column at the center of the drill that connects the two flutes. At the tip it forms the chisel edge - the part that extrudes rather than cuts, and the reason a standard point walks on the surface before it bites. Web thinning grinds away material behind the chisel edge to shorten or split it. Thicker webs give a stronger, more rigid drill; thinning recovers the centering and chip flow that a thick web would otherwise sacrifice.
Form A - the easiest grind, the workhorse
DIN 1412 form A thins the chisel edge symmetrically and is the easiest form to grind on production equipment. It keeps a narrow web, good chip removal and favorable centering, and it preserves rigidity in drills with narrow webs and wide flutes. If a supplier quotes form A on a general purpose HSS drill, that is a sound, honest specification.
Form B - for low cutting resistance materials
Form B suits work materials with low cutting resistance and good chip evacuation, such as cast iron, aluminium and plastics. It is also applied when drills for hardened steels are designed, because the grind decreases the rake angle at the cutting lips and avoids chipping of the cutting edges.
Form C - the split point
Form C is the split point you see quoted as 135 degree split point. Grinding away the web at the tip splits the chisel edge into two cutting corners, so the drill centers itself on contact, needs noticeably less thrust and breaks the chip at the center. It is the correct choice for hardened and tough materials: heat treated steel, titanium alloys, stainless steel, Inconel and Nimonic. On premium sets every bit should be split point ground - including the small wire-gauge sizes, which is exactly where budget manufacturers economize.
Type R - helical thinning
Helical thinning grinds the web with a helical motion so that chips curl, break and evacuate easily through the flutes. It opens the chip room up to the center, removes the chisel edge effect and keeps good centering. It is common on premium parabolic-flute and deep-hole drills where frequent chip breaking matters more than grinding cost.
What to ask before you order
1. Which DIN 1412 form is quoted - A, B, C or R?
2. Is the split point (form C) ground on every diameter, or only on the large ones?
3. Is the thinning type marked on the packing and checked during QC?
FANXI Tools produces HSS, masonry and SDS drill bits in all four thinning forms, roll forged, milled and fully ground, in M2, M35, M42 and 9341 grades. Tell us the material and the machine, and we will quote the right point geometry - not just the right price.