Counterbores, countersinks and teardrop holes in FDM parts

Flush fastener heads, angled countersinks and self-supporting hole shapes each solve a specific manufacturing problem, and each one has its own printability rules that differ from a plain cylindrical hole. This guide covers the four shapes we see most often in customer files: counterbores, countersinks, teardrop holes and captive washer recesses.

Counterbores that print cleanly

A counterbore is a flat-bottomed, larger-diameter recess sitting on top of a smaller through-hole, used to sink a socket head screw flush or below the surface. Printed vertically, with the counterbore facing up, it is one of the easier secondary features to get right, since the flat bottom of the counterbore is just another horizontal surface and the walls are ordinary vertical perimeters. The main risk is bridging at the transition between the counterbore floor and the smaller hole beneath it if the step is treated as a sharp overhang rather than a flat floor with a hole in it.

Keep the counterbore floor at least 0.8 mm thick if it needs to bridge a hole below it cleanly, and always model the counterbore facing up in the intended print orientation. A counterbore printed facing down, into an overhang, will not have a clean flat floor and the screw head will sit crooked.

Countersink angles and layer stepping

A countersink is a conical recess for a flat head screw, and cones are one of the trickier shapes to print cleanly because every layer is a slightly smaller circle stacked on the one below, which naturally creates visible stepping on the angled surface. The standard 90 degree included angle countersink (45 degrees from the hole axis) sits right at the edge of what prints without support, since the overhang angle from vertical is close to the general 45 degree self-supporting guideline.

In practice a 90 degree countersink prints without support but shows visible layer stepping that may need light sanding if the screw head has to sit perfectly flush for cosmetic reasons. If the countersink faces down in the chosen orientation, treat it the same as any other overhang and check it against the part's overall overhang strategy, or reorient the part so it faces up or sideways instead.

Teardrop shapes for horizontal holes

As covered in the horizontal versus vertical holes guide, a plain circular hole printed with its axis horizontal sags at the top because the last layers bridge open air. A teardrop hole solves this by replacing the top of the circle with two angled faces meeting at a point, similar to an upside-down V sitting on top of the circular lower section. Each angled face is steep enough, typically 45 degrees or steeper from vertical, to self-support without bridging, so the hole keeps its shape all the way to the top.

The tradeoff is that a teardrop hole is not round, so it only works where the mating part tolerates the slightly pointed top, for example a clearance hole for a bolt shank, a cable pass-through, or a hole that will be reamed round afterward anyway. It is not suitable where a bearing or precision shaft needs a true circular bore all the way around, in which case reorienting the part to print that hole vertically is the better answer.

Slots and clearance holes

Slots, elongated holes used for adjustable fastening, face the same orientation issues as round holes but add a wrinkle: the two straight sides print cleanly regardless of orientation, but the semicircular ends behave exactly like small round holes and sag if the slot's long axis is horizontal and the part is oriented so the slot faces sideways. Keep slots either fully vertical in their long axis, or accept some rounding at the ends and oversize the slot width slightly to compensate, generally 0.2 to 0.3 mm on each side.

Plain clearance holes for a bolt shank, as opposed to a tapped or press-fit hole, are the most forgiving hole type in the whole cluster. Generous compensation, as covered in the compensation guide, is almost always the right answer since the only requirement is that the bolt passes through freely.

Captive washer recesses

A captive washer recess is a shallow, larger-diameter pocket around a clearance hole, sized so a standard washer sits inside it and cannot fall out or spin, useful for panels and enclosures that get assembled and disassembled repeatedly in the field. Model the pocket diameter about 0.3 to 0.5 mm larger than the washer outer diameter, and the pocket depth close to but not exceeding the washer thickness, so the washer sits nearly flush and the fastener still clamps properly.

Like counterbores, these pockets print best facing up. If a design needs the recess facing down, add a small chamfer at the pocket mouth to reduce the overhang span, or accept a light bridging artefact on the pocket floor, which is cosmetic in most enclosure applications and does not affect function.