Drawing tapped holes, clearance holes, counterbores and countersinks correctly

Holes are the most common feature on part drawings — and the most often drawn wrong. From the drawing alone the machinist must know whether a hole is tapped or plain, its size, its depth and how many there are. The rule: draw the hole by the standard, then write a complete note on one line, such as 4× M6 depth 12 / drill Ø5 depth 15.

How to draw threads (ISO 6410)

Drawings do not show individual thread turns; they use a simple convention:

• Internal thread, seen end-on: a thick circle at the tap drill size (thread root) and a thin circle of about three quarters at the thread size. M6 = thick Ø5 circle + thin ¾ Ø6 arc.
• External thread (screw, shaft) seen from the end: a thick circle at the thread size and a thin ¾ circle at the root diameter.
• Tapped hole in section: thick lines at the drill size, thin lines at the thread size, hatching up to the thick lines, and a 120° cone at the bottom of the drilled hole.
• The end of the thread length is a thick line across the hole.

Tap drill and clearance hole table (metric coarse threads)

Tap drill ≈ thread size − pitch. Clearance holes follow ISO 273: fine where position must be accurate, medium for general use with more room for position error. Laser-cutting shops often add a little more on thin sheet; ask them first.

Writing complete hole notes

The counterbore and countersink sizes are common values; always check them against the actual screw head from its datasheet.

Drawing and annotating holes in DXCAD2D Online

1. Draw an M6 tapped hole in the front view: press C, click the centre, type 2.5 (radius of Ø5) and Enter, on the thick-line layer.
2. Draw the thread arc: a ¾ arc of radius 3 on the thin-line layer. The easy way is a radius-3 circle trimmed (X) by the centre lines to remove one quarter, or a 3-point arc (A).
3. Add crossing centre lines on a layer set to the centre line type.
4. Group the finished hole (G), then copy it (CO) to other positions, or use Polar Array (PO) for holes on a circle.
5. Write the note with a leader (LE): click the hole edge, click where the text goes, then type, e.g. 4× M6 THRU.
6. Dimension the hole positions from the datum edges with D. Do not dimension the diameter again — it is in the note.

Common mistakes

• Calling a tapped hole Ø6 instead of M6 — you will get a plain Ø6 hole.
• No depth for a blind hole, or the thread depth without the drill depth.
• No quantity, so someone has to count — and may miscount.
• Clearance holes the same size as the screw (Ø6 for M6): assembly fails as soon as positions are slightly off.

Frequently asked questions

What tap drill size do you use for an M6 thread

A Ø5.0 mm drill (roughly thread size − pitch = 6 − 1.0). M8 uses Ø6.8 and M10 uses Ø8.5.

What size clearance hole for an M6 screw

Ø6.4 mm for a fine (accurate position) fit or Ø6.6 mm for the general medium fit, per ISO 273. Do not use a plain Ø6 — assembly fails as soon as positions drift.

Do you call a tapped hole M or Ø on a drawing

Use M, e.g. M6. Writing Ø6 gets you a plain Ø6 hole, not a thread. State the pitch for fine threads, e.g. M10×1.

What is the difference between a counterbore and a countersink

A counterbore is a flat-bottomed recess that sinks a socket head cap screw flush; a countersink is a 90° cone for a flat (countersunk) head screw.

Can a laser cutter make threads

No. In the DXF, draw a plain circle at the tap drill size (Ø5 for M6) and tap it afterwards, or note on the PDF that tapping is required.

More on dimension layout in ISO dimensioning basics, and on tolerances in ISO 2768 tolerances and fits.