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Center Drills & Spotting Drills

Center drills and spotting drills both make a starting point, but they do different jobs. A center drill makes a 60° seat for a lathe center; a spot drill locates a hole for a twist drill on a CNC or drill press.

Ref CT-0013ISO 866ANSI B94.11M

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A center drill (combined drill and countersink) has a short pilot drill and a 60° countersink on one body. It makes a center hole in the end of a part so the part can be held between lathe centers. Plain center drills cut a single 60° cone. Bell type (also called protected or radius type) add a 120° chamfer outside the 60° seat so a dinged edge does not damage the seat.

A spotting drill (NC spot drill) is a short, stiff drill with no pilot. It cuts a shallow cone that guides a twist drill so it starts on location without walking. Common point angles are 90° and 120°, with others made for special uses.

How to identify

Center drills are double-ended, with a short pilot sticking out of a cone at each end. They are sold by number, from #00 up to #8. Spot drills are single-ended, look like a stubby two-flute drill and are marked with diameter and point angle. Many are carbide.

Center drill vs spot drill

Feature	Center drill	Spot drill
Main job	60° seat for lathe centers	Locate and start a drilled hole
Pilot	Yes, small and fragile	No
Point angle	60° countersink (plus 120° on bell type)	90°, 120° and others
Ends	Double-ended	Single-ended
Sold by	Number (#00 to #8)	Diameter and angle
Can also chamfer	No	Yes, a 90° spot can chamfer the hole

Common center drill numbers

Number	Body diameter	Pilot diameter
#1	1/8 in	3/64 in
#2	3/16 in	5/64 in
#3	1/4 in	7/64 in
#4	5/16 in	1/8 in
#5	7/16 in	3/16 in
#6	1/2 in	7/32 in

Key points

Many makers recommend a spot angle equal to or slightly wider than the drill point so the center of the drill contacts first. A 90° spot is often chosen because it also leaves a chamfer.
Spot only deep enough to guide the drill. Spotting to the full drill diameter is common when a chamfer is wanted.
The pilot on a center drill breaks easily. Use slow feed and high enough speed for the pilot size.
Metric center drills follow ISO 866 and the forms in DIN 333. Sizes differ from the inch series.

Common mistakes

Using a center drill as a spot drill on CNC work. The pilot snaps and the tip runs off.
Using a 90° spot with a lathe center. The center needs a 60° seat.
Feeding a center drill too deep so the countersink is oversized.

Governing standards: ANSI B94.11M (twist drills, including combined drills and countersinks); ISO 866 (center drills); DIN 333 (center drill forms).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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