Drilling vs Milling: Key Differences

Drilling makes round holes: a twist drill feeds straight into the part along its own axis, and the hole takes the diameter of the drill. Milling removes material in every direction: a rotating end mill moves in X, Y and Z to cut flat faces, pockets, slots, contours and holes of any size. On a CNC milling machine both are done in the same setup.

Below: both processes compared in one table, the difference between a drill bit and an end mill, when to drill and when to mill a hole, and how drilling and milling machines differ.

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cnc milling
Comparison

Drilling and Milling in One Table

Factor Drilling Milling
Tool motion Feeds along its own axis (Z) only Moves in X, Y and Z while it rotates
Cutting tool Twist drill: two cutting lips and a pointed tip (118° to 140°) End mill or face mill: 2 to 6 flutes that cut on the side and the end
What it makes Round holes Flat faces, pockets, slots, contours, 3D shapes and holes
Hole diameter Fixed by the drill size Any diameter larger than the cutter
Hole bottom Cone-shaped (drill point) Flat
Typical diameter accuracy About ±0.05 to ±0.1 mm About ±0.01 to ±0.02 mm
Surface finish in the bore Ra 3.2 to 6.3 µm Ra 0.8 to 3.2 µm
Time per standard hole Seconds Longer: the cutter follows a circular path
Cost per hole Lowest Higher
Machine Drill press, CNC mill or CNC lathe CNC milling machine or machining center

Typical values for CNC work in aluminum and steel. For holes tighter than a drill can hold, the hole is drilled undersize and then reamed or bored; see reaming to H7.

drilling
Tools

Drill Bit vs End Mill

The tools look alike but cut in a different way. A drill bit only cuts at its tip, so it can only go straight down. An end mill cuts on its sides as well as its end, so it can move sideways through the material.

drill bits

Drill bit

Two helical flutes and a pointed tip. Fast and cheap for round holes at standard sizes, and the flutes carry the chips out of deep holes. It cannot cut sideways and leaves a cone-shaped bottom.

end mills for CNC milling

End mill

Two to six flutes that cut on the side and the end. Mills faces, pockets, slots and contours, and makes holes of any diameter larger than itself by circular interpolation, with a flat bottom.

thread milling cutter

Thread mill

Mills internal or external threads in a drilled hole by following a helical path. One tool covers several hole sizes, and a broken thread mill is easier to remove than a broken tap.

For all tool types, tool materials and coatings, see our guide to CNC cutting tools.

Hole making

When to Drill and When to Mill a Hole

Drill the hole when

  • The diameter is a standard drill size, for example for bolts, screws and dowel pins.
  • A cone-shaped bottom is acceptable, or the hole goes all the way through.
  • The part has many identical holes: drilling is the fastest and cheapest way to make them.
  • The hole is deep: with peck drilling, holes of about 10 times the diameter are routine.

Mill the hole when

  • The diameter has no standard drill, or is large enough that a drill would need high cutting force.
  • The hole needs a flat bottom, a counterbore or a pocket around it.
  • The hole must be accurate in size and position, or breaks through a thin wall or an existing cavity.
  • One tool should make several hole sizes, to save tool changes on small batches.
Precision aluminum machining
One setup

How a CNC Mill Does Both

A CNC milling machine drills with programmed drilling cycles: G81 for standard holes, G83 for deep holes with pecking, and G84 for tapping. A typical sequence is spot drill, drill, ream if the hole needs H7, tap or thread mill, and then mill the faces, pockets and contours with an end mill.

Because everything is done in one clamping, the holes stay in the right position relative to the milled features, and the part needs no second machine or setup. At Davantech this is how we make holes in milled parts, on our 3- and 4-axis machining centers. For the hole-finishing steps in detail, see drilling, reaming and tapping compared.

Machines

Drilling Machine vs Milling Machine

A drilling machine (drill press) has a spindle that only moves up and down; the operator positions the part by hand. It is simple and cheap, and suited to holes in parts that need no other machining. A combined mill-drill adds a cross table that moves, for light milling.

A milling machine moves the part in X and Y under a spindle that moves in Z. A CNC machining center adds an automatic tool changer, so it drills, reams, taps and mills in one program. That is why production shops make holes in milled parts on the machining center rather than on a drill press. For parts made on our machines, see CNC milling services.

LV-850 cnc milling machine
FAQ

Drilling vs Milling Questions

What is the main difference between drilling and milling?
Drilling makes round holes by feeding a rotating drill along its own axis, so the hole takes the diameter of the drill. Milling removes material in any direction with a rotating end mill that moves in X, Y and Z, so it can make flat faces, pockets, slots, contours and holes of any size.
Can a milling machine also drill holes?
Yes. A CNC milling machine drills with a drill in the spindle and programmed drilling cycles, such as G81 for standard holes and G83 for deep holes. Most milled parts are drilled, tapped and milled in the same setup.
What is the difference between a drill bit and an end mill?
A drill bit has a pointed tip and only cuts at the tip, so it can only feed straight down. An end mill cuts on its sides and its end, so it can move sideways, make flat bottoms and cut holes larger than itself.
Which is more accurate, drilling or milling?
Milling. A drilled hole is typically within about ±0.05 to ±0.1 mm on diameter. A milled or bored hole can hold about ±0.01 to ±0.02 mm, and reaming after drilling gives H7 fits.
When should I use drilling instead of milling?
For holes at standard drill sizes, through holes or holes where a cone-shaped bottom is acceptable, many identical holes, and deep holes. Drilling is the fastest and cheapest way to make them.
What is the difference between a drilling machine and a milling machine?
A drilling machine has a spindle that only moves up and down, and the part is positioned by hand. A milling machine moves the part in X and Y under the spindle; a CNC machining center also changes tools automatically, so it drills, taps and mills in one program.
Which process removes material faster?
For a single hole at a standard size, drilling is much faster. For removing volume, such as faces and pockets, milling is faster because the cutter can take wide, deep passes.
Can I mill a hole instead of drilling it?
Yes. An end mill can cut a hole by circular or helical interpolation. It makes any diameter larger than the cutter and a flat bottom, but it takes longer per hole than drilling.
How do drilling, reaming and tapping work together?
The hole is first drilled slightly undersize. Reaming then finishes it to an exact diameter, typically H7, and tapping cuts a thread in a drilled hole of the correct tap drill size. All three are usually done in the same setup on a CNC mill.
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