3D print countersink: sizes for flat head screws and how to print them

Size the countersink from the sharp-corner head diameter in the screw’s standard plus 0.2 mm, at 90° for metric screws and 82° for inch ones. Calipers on a real screw read less than that diameter, and a countersink modeled from them leaves the head proud. Facing up, a countersink prints like any other surface; facing the bed, its wall is a 41 to 45° overhang that prints without support.

What size countersink do you model for a flat head screw?

Model the countersink at the screw’s head angle, 90 degrees for metric and 82 degrees for inch screws, with its top diameter 0.2 mm larger than the head’s sharp-corner diameter. An M3 socket countersunk screw (ISO 10642) gets 6.92 mm over a 3.4 mm clearance hole; a #6 flat head machine screw gets 7.38 mm.

McMaster-Carr gives the same angle rule: 82° countersinks for inch screws, 90° for metric.

The sharp-corner diameter is where the head’s cone would meet its flat top if the edge were a knife edge. Real heads have a rounded or flat edge, so calipers read less. AmesWeb defines the ASME B18.3 figure as the diameter of an 82° countersink in which a maximum-size head sits flush. Model from that figure.

An M3 Phillips head (ISO 7046) has a 6.3 mm sharp diameter but measures at most 5.5 mm. A countersink modeled at 5.5 mm leaves that head 0.4 mm proud.

D     = sharp head diameter + 2 × c
depth = (D - d) ÷ 2 ÷ tan(angle ÷ 2)

90° countersink: tan 45° = 1
82° countersink: tan 41° = 0.869

• D is the countersink’s diameter at the surface.
• c is the clearance per side: 0.1 mm, or 0.2 mm total on the diameter.
• d is the clearance hole underneath, the normal fit from the screw hole size calculator.
• depth runs from the surface down to where the cone meets the clearance hole.

Each 0.1 mm per side lowers the head 0.1 mm in a 90° countersink and 0.115 mm in an 82° one. The 0.2 mm total puts a maximum-size head about 0.1 mm below flush. AIMS Industrial’s countersinking guide calls 0 to 0.2 mm below flush a clean finish.

The 0.1 mm per side is the alignment-peg clearance the University of the Arts London fabrication lab gives for its Bambu H2D Pro printers (0.1 to 0.15 mm), and the snug fit in the tolerance calculator.

If your printer prints holes small, add the same undersize to D that you add to the clearance hole. The hole compensation calculator turns a measured test hole into that number.

Worked example: M3 socket countersunk screw

1. Sharp head diameter: 6.72 mm, the ISO 10642 maximum, which Unbrako’s table marks “to theoretical sharp corner”.
2. D = 6.72 + 0.2 = 6.92 mm.
3. Clearance hole: 3.4 mm (ISO 273 medium series).
4. Depth = (6.92 - 3.4) ÷ 2 ÷ 1 = 1.76 mm.

In CAD, that is a 1.76 mm equal-distance chamfer on the edge of the 3.4 mm hole. In a hole feature, set the countersink to 6.92 mm and 90°.

Worked example: #6-32 flat head machine screw

1. Sharp head diameter: 0.283 in (7.18 mm). ASME B18.6.3 checks the head in an 82° gauge hole 0.220 in across and lets it stand up to 0.036 in above the gauge. A maximum head therefore sits flush in a hole 0.220 + 2 × 0.036 × 0.869 = 0.283 in across.
2. D = 7.18 + 0.2 = 7.38 mm (0.291 in).
3. Clearance hole: 0.170 in (4.32 mm), the ASME B18.2.8 normal fit.
4. Depth = (7.38 - 4.32) ÷ 2 ÷ 0.869 = 1.76 mm (0.069 in).

The head itself measures at most 0.262 in (6.65 mm). A countersink modeled to that size leaves the head 0.3 mm proud.

Countersink sizes for metric screws (90°)

In the tables below, the “Countersink to model” column adds 0.2 mm to the diameter (0.1 mm per side). Clearance holes are the ISO 273 medium series.

Socket countersunk screws, ISO 10642:

Screw	Sharp head diameter	Countersink to model	Clearance hole	Depth
M3	6.72 mm	6.92 mm	3.4 mm	1.76 mm
M4	8.96 mm	9.16 mm	4.5 mm	2.33 mm
M5	11.20 mm	11.40 mm	5.5 mm	2.95 mm
M6	13.44 mm	13.64 mm	6.6 mm	3.52 mm
M8	17.92 mm	18.12 mm	9.0 mm	4.56 mm

Böllhoff gives the ISO 10642 head angle as 90 to 92°. Its table and Unbrako’s both start at M3.

Phillips (type H), Pozidriv (type Z) and slotted countersunk screws, ISO 7046 and ISO 2009 (the 2011 editions share one head):

Screw	Sharp head diameter	Head as measured, max	Countersink to model	Clearance hole	Depth
M2	4.4 mm	3.8 mm	4.6 mm	2.4 mm	1.10 mm
M2.5	5.5 mm	4.7 mm	5.7 mm	2.9 mm	1.40 mm
M3	6.3 mm	5.5 mm	6.5 mm	3.4 mm	1.55 mm
M4	9.4 mm	8.4 mm	9.6 mm	4.5 mm	2.55 mm
M5	10.4 mm	9.3 mm	10.6 mm	5.5 mm	2.55 mm
M6	12.6 mm	11.3 mm	12.8 mm	6.6 mm	3.10 mm
M8	17.3 mm	15.8 mm	17.5 mm	9.0 mm	4.25 mm

An M3 socket head is wider than an M3 Phillips head (6.72 against 6.3 mm), and at M4 it is the other way round (8.96 against 9.4 mm). Model for the screw you will use: a countersink sized for the socket M3 sinks a Phillips M3 head 0.31 mm below the surface.

Countersink sizes for inch screws (82°)

Clearance holes are the ASME B18.2.8 normal fit.

Socket flat head screws, ASME B18.3:

Screw	Sharp head diameter	Countersink to model	Clearance hole	Depth
#4	0.255 in (6.48 mm)	0.263 in (6.68 mm)	0.128 in (3.25 mm)	0.078 in (1.97 mm)
#6	0.307 in (7.80 mm)	0.315 in (8.00 mm)	0.170 in (4.32 mm)	0.083 in (2.12 mm)
#8	0.359 in (9.12 mm)	0.367 in (9.32 mm)	0.196 in (4.98 mm)	0.098 in (2.50 mm)
#10	0.411 in (10.44 mm)	0.419 in (10.64 mm)	0.221 in (5.61 mm)	0.114 in (2.89 mm)
1/4 in	0.531 in (13.49 mm)	0.539 in (13.69 mm)	0.281 in (7.14 mm)	0.148 in (3.77 mm)

Unbrako gives the head angle as 82° (+0°, -2°).

Flat head machine screws, slotted or Phillips, ASME B18.6.3:

Screw	Sharp head diameter	Head as measured, max	Countersink to model	Clearance hole	Depth
#2	0.174 in (4.43 mm)	0.162 in (4.11 mm)	0.182 in (4.63 mm)	0.102 in (2.59 mm)	0.046 in (1.17 mm)
#4	0.228 in (5.78 mm)	0.212 in (5.38 mm)	0.236 in (5.98 mm)	0.128 in (3.25 mm)	0.062 in (1.57 mm)
#6	0.283 in (7.18 mm)	0.262 in (6.65 mm)	0.291 in (7.38 mm)	0.170 in (4.32 mm)	0.069 in (1.76 mm)
#8	0.335 in (8.50 mm)	0.312 in (7.92 mm)	0.343 in (8.70 mm)	0.196 in (4.98 mm)	0.084 in (2.14 mm)
#10	0.386 in (9.80 mm)	0.362 in (9.19 mm)	0.394 in (10.00 mm)	0.221 in (5.61 mm)	0.099 in (2.53 mm)
1/4 in	0.511 in (12.98 mm)	0.477 in (12.12 mm)	0.519 in (13.18 mm)	0.281 in (7.14 mm)	0.137 in (3.47 mm)

The 2013 edition of ASME B18.6.3 allows the edge of the head to be rounded or flat and a head angle of 80 to 82°. It gives no sharp diameter. The sharp column is the gauge hole plus the allowed protrusion, as in the #6 example, from the gauge figures AFT Fasteners reproduces.

US flat head wood screws (ANSI B18.6.1) also have an 82° head. Metric chipboard screws no longer have a technical standard, and their head sizes vary by maker, so take the head diameter and angle from the maker’s drawing.

For a screw whose standard you don’t know, work the sharp diameter out from the screw’s nominal diameter and its head height: diameter + 2 × head height for a 90° head, diameter + 1.74 × head height for an 82° head. The standards’ figures follow the same rule (3 + 2 × 1.86 = 6.72 mm for the M3 ISO 10642 head). The point where the cone meets the shank is hard to see, so print a test countersink before the part.

How do you 3D print a countersink?

Print the countersink facing up if the part allows it. The cone then widens as the print rises, so every layer sits on the one below and needs no support.

Countersinks facing the bed

Here the cone narrows as the print rises, so each layer overhangs the one below. A 90° countersink’s wall leans 45° from vertical. An 82° countersink’s wall leans 41° from vertical (49° from the bed), a slightly gentler overhang.

Both sit inside the published limits:

• Hubs (Protolabs) says an overhang can usually be printed up to 45° without losing quality.
• Prusa says printers cleanly print overhangs of 45 to 60°, and less than 45° with small nozzles such as 0.25 mm.
• UltiMaker Cura’s default Support Overhang Angle is 50°, so it adds no support to either countersink.

The overhang is also short: 1.76 mm across on the M3 example.

Elephant’s foot closing the edge

On a countersink facing the bed, the mouth is in the first layer. Hubs calls the first layer’s flare an “elephant’s foot”. The flare narrows the mouth where the head’s widest edge seats, so the head sits proud.

• In the slicer: turn on elephant foot compensation. PrusaSlicer’s setting (Print Settings > Advanced) shrinks the first layer in XY by the value you set and defaults to 0. In Bambu Studio it is under Process > Quality > Precision.
• After printing: clean the mouth with a countersink bit or a deburring tool.

Countersinks on a horizontal hole

A hole with its axis lying flat can need support, and Hubs says rotating the part 90° removes that need. A countersink on such a hole adds a cone to the overhang at the top of the hole, so stand the hole upright where the part allows it.

If the head still sits proud

• Countersink modeled from a caliper reading. Use the sharp diameter from the tables. On an M3 Phillips head the difference is 0.4 mm of height.
• Elephant’s foot at the mouth. On a countersink printed against the bed, turn on elephant foot compensation or deburr the edge.
• Holes printing small. Add your measured hole undersize to the countersink diameter as well as to the clearance hole.
• Wrong angle. An inch head in a 90° countersink stands 0.28 to 0.53 mm proud (next section).

If a flush head is not required, a counterbore is the other option: a flat-bottomed step for a socket head cap screw, with no cone angle to match.

Can you use a 90° countersink for an 82° screw?

No, not if the head must sit flush. With the same mouth diameter, an 82° inch head in a 90° countersink bears only near the shank. It stands proud by about 0.13 times the head height, with a ring-shaped gap at the surface.

Screw in a 90° countersink	Head height	Head stands proud by
1/4-20 socket flat head (ASME B18.3)	0.161 in	0.021 in (0.53 mm)
#6-32 flat head machine screw (ASME B18.6.3)	0.083 in	0.011 in (0.28 mm)

The 0.13 comes from the geometry: 1 - tan 41° = 0.131. Widening the 90° mouth drops the head to flush, but the gap stays and the contact stays at the narrow end. Remodel the countersink at 82°.

The reverse mistake, a 90° metric head in an 82° countersink, bears only on the head’s outer rim. AIMS Industrial warns that this contact at the lip creates a stress concentration, which can snap the head off or loosen the screw under load or vibration.