Screw Types and How to Choose Them

Bolts, screws, self-tapping screws and nuts: what's the difference

People often call them all "screws", but each type does a different job.

Type	What it looks like	When to use it
Machine screw / bolt	Uniform thread, flat tip; goes into a nut or a threaded hole	Joints you take apart and reassemble often
Self-tapping screw	Pointed tip, coarse sharp thread that cuts its own thread	Driving straight into plastic or wood; rarely removed
Nut	Hex block with an internal thread, paired with a bolt	Clamping two parts, or embedding in a print
Washer	Thin flat ring	Spreading clamping force so the head doesn't sink into plastic
Grub screw (set screw)	Headless, sits fully inside the hole	Locking a pulley or gear onto a motor shaft

Head types and drive types

For 3D printing, the socket head cap screw is the default choice. It tightens firmly, rarely strips, and comes in every size.

Head type	Common standard	Pros	Design notes
Socket head cap screw	DIN 912 / ISO 4762	Strong, the most common	Leave a counterbore as deep as the head height
Button head	ISO 7380	Low profile, clean look	Small drive, strips easily if overtightened
Countersunk / flat head	DIN 7991	Sits flush with the surface	Needs a 90° countersink; hard to print if the hole faces the bed
Hex head	DIN 933	Driven with a wrench, handles high torque	Needs clearance around the head for the wrench

Drive types: hex socket (Allen) and Torx strip far less than Phillips or slotted drives. If a part will be taken apart often, use hex socket screws.

Reading specs: what M3x10 means

M3x10 has three parts: M means metric thread, 3 is the outer thread diameter (3 mm), and 10 is the shank length (10 mm). For cap and button heads, length is measured from under the head. For countersunk screws, it includes the head.

Thread pitch is the distance between two adjacent thread crests. If nothing else is written, it is the standard coarse pitch. A marking like M8x1 means a fine thread, which needs a nut with the same pitch.

Size	Standard pitch (mm)	Nut wrench size (mm)	Cap screw hex key (mm)
M2	0.4	4	1.5
M2.5	0.45	5	2
M3	0.5	5.5	2.5
M4	0.7	7	3
M5	0.8	8	4
M6	1.0	10	5
M8	1.25	13	6

Choosing length: add up the thickness of the parts being clamped, then add thread engagement. Into a nut, the screw should stick out by 1 to 2 threads. Straight into plastic, engagement should be at least twice the screw diameter.

Strength class: steel screws are often marked 8.8, 10.9 or 12.9 on the head. Higher numbers are stronger. With plastic parts, the plastic almost always fails before the screw, so strength class rarely matters.

Materials and coatings

Indoors, plated or black-oxide steel is enough. Outdoors or in humid places, go stainless.

Material	Characteristics	Good for
Black-oxide steel (usually 12.9)	Hard and cheap, but rusts if it gets damp	3D printers, robots, indoor items
Zinc- or nickel-plated steel	Mild rust resistance, bright finish	Household items, visible parts
Stainless 304 (A2)	Good rust resistance, softer than 12.9 steel	Outdoor, kitchen, humid places
Stainless 316 (A4)	Better rust resistance than 304	Near the sea, chemical environments
Brass	Doesn't rust, conducts electricity	Heat-set inserts, electrical work
Nylon	Light, insulating	Mounting circuit boards, low-load parts

Fastening into 3D printed parts

If a part will be taken apart often, use heat-set inserts or captive nuts. Screwing straight into plastic only suits parts you assemble once and leave alone.

Method	How it works	Durability over repeated assembly	Suggested size for M3
Through hole + nut	Screw passes through the part, nut tightened on the back	Very good	3.2–3.4 mm through hole
Captive nut pocket	Print a hex pocket and drop the nut in (you can pause the print to embed it)	Very good	5.7–5.9 mm across flats, 2.5 mm deep
Screw straight into plastic	Print a smaller hole; the screw cuts its own thread	Poor, strips after a few cycles	2.5–2.8 mm pilot hole
Heat-set insert	Heat a brass insert with a soldering iron and press it in; molten plastic grips it	Very good, clean and tidy	Per seller's spec, usually 4.0–4.2 mm

These numbers are only a starting point. Every printer has different tolerances, so print a test coupon with holes 0.1 mm apart and pick the best fit.

Extra tips:

• Put a washer under the screw head to spread the force and avoid cracking the plastic.
• Increase walls around screw holes to 3–4 perimeters for a stronger hole.
• Horizontal holes often sag at the top. A teardrop-shaped hole prints cleanly without supports.

Starter kit and buying tips

If you buy only one kit, get a box of M3 socket head cap screws. It's the most common size on 3D printers and small projects.

• M3: 6, 8, 10, 12, 16 and 20 mm lengths, plus M3 nuts, washers and heat-set inserts.
• M2 and M2.5: for circuit boards, small servos and displays.
• M4 and M5: for load-bearing frames, aluminum extrusion and larger motors.
• M3 and M4 grub screws: for locking pulleys and gears onto shafts.
• An L-shaped hex key set from 1.5 to 5 mm, plus calipers for measuring screws.

Where to buy: an assorted screw box from an online marketplace (in Vietnam, Shopee or Lazada) is usually the cheapest way to start. For bulk or unusual sizes, go to a hardware or fastener shop. When buying online, check whether the screws are black steel or stainless, and that the head standard is the one you need.

Summary

The quick pick: socket head cap screws, size M3, just long enough for good thread engagement, with heat-set inserts wherever you'll assemble and disassemble often. Before printing the real part, print a screw-hole test on your own printer.

In the next post, I'll walk through designing tolerances so 3D printed parts fit together snugly.