Cutting ToolsConsumable
Twist Drill Bits
Helical-fluted cutters for making holes in metal, wood and plastic. The size on the shank is the hole, and the point geometry decides what the hole does at the far side.
What it does
- Drilling pilot and clearance holes
- Opening a hole to a tap drill size
- Starting a hole for a jigsaw or a hole saw
Materialswhat it is used on
All materialsWhat to look forthe judgement calls, before the spec sheet
- The tap drill for a metric thread is the nominal diameter minus the pitch — 5 mm for M6, 8.5 mm for M10. That figure is listed for every thread in this database, so it does not need to be worked out each time.
- For a clearance hole the drill is larger than the nominal thread size, not equal to it. The clearance diameters in a standard's table are the ones to use when the joint has to close.
- Coatings change the tool's life rather than its geometry. For stainless and for anything abrasive, a cobalt or carbide grade is the difference between a hole and a polished spot.
Misuse and safetythe ways this tool causes rework
- Sizing a hole with a drill that has been sharpened by hand is a guess. A reground bit cuts oversize, and the error shows up as a loose fit or a stripped thread.
- Run the correct speed for the material. Too fast in steel work-hardens the surface, and the hole becomes harder than the bit.
Projects that call for itand what it is doing on each one
All projectsFrequently asked questions
What size hole do I drill for an M8 bolt?
8.5 mm for a clearance hole in the metric coarse series, or 6.8 mm as the tap drill if the hole is going to be tapped. The two figures are different jobs and both are in the thread tables.
Why does a drilled hole come out oversize?
A blunt or hand-sharpened bit wanders, and a bit smaller than the chuck's minimum tends to run out. Measure the hole before trusting the size printed on the shank.