Twist drills
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Frequently asked questions
How do DIN 338, DIN 340 and DIN 1897 differ?
These are three lengths of straight shank drills: DIN 1897 stub, the most rigid; DIN 338 jobber, the most common; DIN 340 long series for deep holes. Longer drills wander and break more easily, so choose the shortest length that reaches the depth.
Which materials suit 118 and 135 degree point angles?
118 degrees is the general angle for mild steel and non-ferrous metals. 135 degrees, usually with a split point, self-centers better and reduces thrust, suiting alloy steel and stainless. Carbide drills commonly use 140 degrees.
How do HSS, HSS-Co and HSS-E differ?
HSS-Co, also labeled HSS-E, contains about 5 to 8 percent cobalt and retains hardness at high temperature better than plain HSS, suiting stainless and alloy steel. TiN or TiAlN coatings reduce friction and extend life but do not replace the right substrate choice.
Why do drills burn out in stainless steel?
Stainless work-hardens when rubbed, so high speed with too light a feed hardens the surface and wears the drill quickly. Lower the speed, keep a steady, adequate feed, use coolant, and do not dwell in the hole.
How is the tap drill size calculated?
For metric coarse threads, the drill diameter is about the nominal diameter minus the pitch, for example M8 x 1.25 uses 6.8 mm and M10 x 1.5 uses 8.5 mm. Forming (chipless) taps need a larger hole, about diameter minus half the pitch.
Can twist drills be reground?
Both HSS and solid carbide drills can be reground if the edges are not deeply chipped, but the point angle, lip clearance and equal lip lengths must be kept. Unequal lips produce oversize holes and wear one side quickly. Coated carbide drills need recoating after regrinding to recover their original life.
Should I spot drill or center punch before drilling?
With long drills or uneven surfaces, spot with a center drill whose angle is equal to or larger than the drill point angle so the drill does not walk. A spot angle smaller than the drill point makes the drill contact the outer corners first and chip. A 135 degree split point drill usually needs no spotting on a flat surface.
When is peck drilling needed?
When hole depth exceeds about 3 to 4 times diameter without through-coolant, chips pack in the flutes and can break the drill. A peck cycle drilling in stages clears chips and lets fluid reach the bottom of the hole. Drills with internal coolant holes allow deeper continuous drilling.
