--- title: "Countersink or counterbore a screw hole for 3D printing" description: "Sink a screw head flush in a printed part: tap Hole, pick the screw size, choose Counterbore for a socket head or Countersink for a flat head, Apply. M3 is a 3.4 mm hole with a 6.5 × 3.4 mm counterbore or a 6.6 mm countersink; the M2 to M8 table and how each prints." canonical: https://speakcad.com/guides/countersink-counterbore-screw-hole published: 2026-10-05 updated: 2026-10-05 publisher: SpeakCAD --- # Countersink or counterbore a screw hole for 3D printing > Sink a screw head flush in a printed part: tap Hole, pick the screw size, choose Counterbore for a socket head or Countersink for a flat head, Apply. M3 is a 3.4 mm hole with a 6.5 × 3.4 mm counterbore or a 6.6 mm countersink; the M2 to M8 table and how each prints. A screw head that stands proud of a lid catches on everything and stops the lid sitting flat. Sunk into the plastic, it disappears. A socket head sits in a counterbore, a flat head in a countersink, and in SpeakCAD both are a choice on the Hole card, with the sizes for M2 to M8 already filled in. ## Steps 1. **Tap Hole, then the face the head sits on.** Tap Hole in the rail, then the face the screw goes in from: the face the head will sit on, not the one the screw comes out of. The card opens with Diameter, Depth and the Position rows. 2. **Pick the screw size.** Tap the screw's chip, M2 to M8, for its clearance hole (M3 is 3.4 mm) with Through on, so the screw passes through the wall. 3. **Choose the Screw head.** The Screw head choice reads None, Counterbore, Countersink. Counterbore for a socket head cap screw, Countersink for a flat (countersunk) head. With a screw size picked, the head's numbers follow it, in either order: an M3 gets a counterbore 6.5 mm across and 3.4 mm deep, or a countersink 6.6 mm across at the surface. 4. **Adjust the head if your screws differ.** Head diameter (and Head depth, for a counterbore) are numbers like the rest: measure the head with calipers and add 0.2 to 0.4 mm for a printed part. 5. **Place it and Apply.** Type the centre's distance from the face's edges in the Position rows, then press Apply hole. The head stays on the hole's card; Duplicate the hole for the others and every copy has the same head. ## Counterbore or countersink? - **Counterbore**: a flat-bottomed, wider hole on top of the clearance hole, for a **socket head cap screw** (ISO 4762), or any cylindrical head. The head sits just below the surface on a flat ring of plastic, which takes a firm tightening well. - **Countersink**: a 90° cone from the surface down to the clearance hole, for a **flat head** screw (ISO 10642, DIN 7991). The head ends flush with the face and centres itself. The cone wedges the plastic apart as you tighten, so stop when the head is flush. - **Neither**: a button or pan head sits on top of the face; a plain hole is all it needs. ## The sizes the presets use | Screw | Clearance hole (mm) | Counterbore, diameter × depth (mm) | Countersink, diameter at the surface (mm) | |---|---|---|---| | M2 | 2.4 | 4.8 × 2.4 | 4.6 | | M2.5 | 2.9 | 5.5 × 2.9 | 5.6 | | M3 | 3.4 | 6.5 × 3.4 | 6.6 | | M4 | 4.5 | 8.0 × 4.4 | 8.6 | | M5 | 5.5 | 9.5 × 5.4 | 10.6 | | M6 | 6.6 | 11.0 × 6.4 | 12.6 | | M8 | 9.0 | 14.0 × 8.4 | 16.6 | The clearance holes are the medium series (ISO 273) the Hole chips already use. A counterbore is the socket head's diameter plus about 1 mm, and its height plus 0.4 mm, so the head sits a little below the surface. A countersink is the flat head's nominal diameter plus 0.6 mm. All of them are rounded up for printing, because a printed hole comes out 0.2 to 0.4 mm small. Your screws are the reference: when a head measures differently, type its Head diameter. So the answer to "what size countersink for an M3 screw" is 6.6 mm across at the surface, 90°, over a 3.4 mm hole; and for an M3 socket head, a counterbore 6.5 mm across and 3.4 mm deep. ## How each one prints - **Head on the top face.** Both print without supports: the counterbore's floor is a small top surface like any other, and the countersink's cone opens upwards. - **Head on the bed.** A countersink facing the bed is a 45° overhang and prints cleanly. A counterbore facing the bed leaves the ring at the bottom of the bore hanging over air; at M2 to M4 most printers bridge it with a little sag. For a clean ring, print the part the other way up, or ask the AI for a sacrificial layer, a 0.2 mm skin across the hole that you push out with the screw. - **On a side wall.** The hole and its head print lying down, like any horizontal hole: slightly flattened at the top. The clearance sizes already allow for it. ## Leave plastic under the head A counterbore needs material below it: an M3 counterbore is 3.4 mm deep, so on a 3 mm plate it would go straight through and leave the screw head nothing to press on. Keep 2 mm or more of plastic under the head. On a thin plate, choose a countersink (an M3 cone is 1.6 mm deep), make the plate thicker with [Extrude](/guides/extrude-a-face), or stand a boss on it with **Add** and put the hole through the boss. ## Many holes, one head A head belongs to its hole, so [Duplicate](/guides/duplicate-a-hole-pattern) copies it: one counterbored M3 hole in a corner, then a 2 × 2 grid, and the four corners are done. Change the Head diameter on the card and every copy follows. ## The other end of the screw The part the screw goes into needs its own hole. For a **heat-set insert**, a plain hole of the insert maker's diameter (around 4.0 mm for a common M3 insert), blind, a millimetre deeper than the insert. For a **screw that cuts its own thread**, the Hole card's **tap** toggle gives the smaller sizes (M3 2.5 mm). A **hex nut trap** is not a hand tool: "cut a 5.5 mm hex pocket 2.5 mm deep around this hole" is an AI request (2 credits). [Add or resize a hole in an STL](/guides/add-a-hole-to-an-stl) has the insert sizes. ## On your own STL A file you downloaded takes a countersunk or counterbored hole like any part: open it with no change typed (the **Upload an STL or 3MF** card in a new model's empty editor, or the [Edit an STL](/edit-stl) box with the change left empty), tap **Hole** on the face, and choose the head. An existing hole in the mesh cannot take a head by hand: tapping inside it lands on its wall. Drill the new one beside it, or ask the AI to countersink that hole. ## What it costs Shaping a part by hand (a hole and its screw head, a cut, an added tab, raised or engraved text, an extruded face, rounded corners, and copies in a row, a grid or a circle) and downloading what you shaped are free. Each AI request spends 2 credits, a download of AI work 4 credits, and G-code 5 more. A screw head is part of a hand edit: free, and a part shaped only by hand downloads free with a free account. ## About SpeakCAD SpeakCAD is the most intuitive CAD for 3D printing: tap a face and type a size to shape a part with simple tools, or describe it to the AI, on your phone or computer, and download a print-ready STL, 3MF or G-code sliced for your printer. SpeakCAD works inside ChatGPT and Claude as a plugin (an MCP server): ask the assistant for a part with its measurements and SpeakCAD's own CAD AI designs it, shows it in 3D in the chat and hands over a print-ready STL, 3MF or G-code sliced for your printer. - Live demo, no account: https://speakcad.com/demo - In ChatGPT, Claude and other MCP clients: https://speakcad.com/chatgpt-plugin, https://speakcad.com/claude-connector, https://speakcad.com/mcp-server - Pricing: https://speakcad.com/pricing - FAQ: https://speakcad.com/faq - Site index for AI assistants: https://speakcad.com/llms.txt