Fastener Selection

Most assembly failures trace back to the wrong fastener, wrong grade, or wrong torque — not a bad design. This guide covers what you actually use on CNC machined parts: socket head cap screws, set screws, dowel pins, thread inserts, and the grades and torque values that matter.

Which Fastener Do You Need?

Stop here. Match your application to the right fastener type before worrying about grades or torque.

Application	Use This	Why
General clamping, two parts bolted together	Socket Head Cap Screw (SHCS)	Strongest head geometry per size. Fits in counterbored holes. Most common CNC fastener.
Low head clearance, cosmetic surface	Button Head Cap Screw	Low-profile dome head, no sharp edges. 20–30% weaker than SHCS of same size.
Flush surface required (no protrusion)	Countersunk Screw	90-degree flat head sits flush. Requires countersunk hole. Weakest head type — avoid for high load.
Secure a component to a shaft	Set Screw (Grub Screw)	Headless, drives into a shaft via threads. Point type matters — see below.
Large structural joint, wrench access	Hex Bolt	External hex head, wrench-driven. Cheapest for M10+. Standard for structural steel.
Pivot point, hinge, sliding joint	Shoulder Bolt	Precision ground shoulder acts as a bearing surface. Expensive but eliminates a separate pin.
Precision location between two parts	Dowel Pin	No threads. Press-fit locates parts to ±0.01mm. See dowel pin section below.
Stripped threads, soft material, frequent disassembly	Helicoil / Thread Insert	Creates a strong steel thread in aluminum, plastic, or damaged holes. See insert section below.
Axial retention on a shaft	Cotter Pin	Cheap, one-time use. Works with castle nut or through-hole in shaft.
Retain a bearing or collar on a shaft	Retaining Ring (Snap Ring)	Sits in a groove. Axial retention only. Cheap and fast to install.
Load-bearing stud (nut on both sides)	Stud	Threaded both ends or full length. Used when one side has no wrench access (e.g., engine block).

Fastener Types at a Glance

Type	Drive	Head Style	Load Type	Relative Cost	Common Sizes
SHCS	Hex socket (Allen)	Flat-top cylinder	Tension + shear	1.0x	M2–M30
Button Head	Hex socket	Low dome	Tension + shear	1.1x	M3–M16
Countersunk	Hex socket / Phillips	90° flat	Tension only	0.8x	M3–M12
Hex Bolt	External hex (wrench)	Hex head	Tension + shear	0.6x	M5–M42
Set Screw	Hex socket	Headless	Axial retention	0.7x	M2–M12
Shoulder Bolt	Hex socket	Hex head + shoulder	Shear + pivot	3.0–5.0x	M5–M16
Dowel Pin	N/A (press fit)	None	Shear (location)	0.5x	3–20mm dia.
Cotter Pin	N/A (split leg)	None	Axial retention	0.2x	1–12mm dia.
Retaining Ring	Pliers (snap ring)	None	Axial retention	0.3x	3–200mm dia.

Socket Head Cap Screws (SHCS)

The workhorse of CNC assemblies. ISO 4762 (metric) / ASME B18.3 (inch). Strong head geometry means you can use a smaller SHCS than a hex bolt for the same clamp load. The hex socket drive keeps the head compact — fits in counterbores where a wrench can't reach.

Grades — Which Class?

Class	Tensile (MPa)	Yield (MPa)	Hardness	When to Use
8.8	800	640	22–32 HRC	Non-critical, cost-sensitive. Low-stress assemblies.
10.9	1000	900	32–39 HRC	Default for CNC assemblies. Good strength, not brittle. Covers 90% of cases.
12.9	1200	1080	39–44 HRC	High-stress, space-constrained. Engine mounts, mold clamps. More brittle — avoid in impact/vibration.

Default recommendation Use Class 10.9 SHCS as the standard for all CNC assemblies. The cost difference vs 8.8 is negligible (5–10%), and you avoid the brittleness risk of 12.9. Only go to 12.9 when you've done the math and confirmed 10.9 isn't enough.

Head Clearance Requirements

The counterbore for a SHCS must be deep enough for the head to sit fully below the surface, plus a small clearance. For the hex socket drive, you need enough depth for the Allen key to engage — typically the key inserts at least 70% of the socket depth.

Size	Head Dia. (mm)	Head Height (mm)	Socket Size (mm)	Min. Counterbore Depth (mm)
M3	5.5	3.0	2.5	3.5
M4	7.0	4.0	3.0	4.5
M5	8.5	5.0	4.0	5.5
M6	10.0	6.0	5.0	6.5
M8	13.0	8.0	6.0	8.5
M10	16.0	10.0	8.0	10.5
M12	18.0	12.0	10.0	12.5

Common mistake Undersizing the counterbore diameter. The head diameter must have clearance around it (H7/g6 fit is typical) or the screw head binds in the bore before seating. Add 0.2–0.5mm to head diameter for clearance depending on size.

Set Screws (Grub Screws)

Headless screws threaded through a hub to bear against a shaft. They transmit torque by friction or by the point digging into the shaft. ISO 4026 (flat point) / ISO 4027 (cone point) / ISO 4028 (dog point) / ISO 4029 (cup point).

Point Types — Choose the Right One

Point Type	Standard	How It Works	When to Use	Downside
Flat Point	ISO 4026	Flat end bears against shaft surface	Frequent disassembly. Doesn't damage shaft. Use with a flat on the shaft for positive location.	Lowest holding power. Relies on friction only.
Cone Point	ISO 4027	Pointed end embeds into shaft	Permanent or semi-permanent assembly. Highest axial holding power.	Damages the shaft. Not for parts that need removal.
Dog Point	ISO 4028	Extended flat tip fits into a drilled hole in the shaft	Positive engagement without shaft damage. Best for precise axial location.	Requires a matching hole in the shaft. Adds a drilling operation.
Cup Point	ISO 4029	Dished end digs into shaft slightly	Most common point type. Good balance of grip and reusability.	Leaves a small mark on shaft. Can work loose under vibration.

Most common set screw mistake Using a cup point on a hardened shaft. The cup can't dig in, so it slips. Use a flat point with a flat machined on the shaft, or a dog point with a drilled hole. On hardened shafts (HRC 45+), set screws have very limited holding power regardless of point type — use a keyway or clamp instead.

Dowel Pins

Precision ground cylinders pressed into holes to locate two parts relative to each other. They don't clamp — they position. Use in conjunction with fasteners (which provide the clamping force).

Types

Type	Standard	Tolerance	Material	Use Case
Straight Dowel	ISO 2338 / DIN 6325	m6	Hardened steel (60 HRC min)	Most common. Press-fit for precision location.
Pull-Out Dowel	DIN 7979	m6	Hardened steel	Threaded end for easy removal with a screw. Essential for blind holes.
Taper Pin	ISO 2339 / DIN 1	Self-locking taper	Steel	Shear loads. Taper fit eliminates need for press. Reusable.

Press Fit — H7/m6

The standard fit for dowel pins is H7 hole / m6 pin. This is a light press fit (transition fit technically, but behaves as press fit at small diameters). The hole should be reamed, not drilled, to achieve H7 tolerance.

Dowel Dia. (mm)	Hole (H7) Range (mm)	Pin (m6) Range (mm)	Press Fit (approx.)
3	3.000–3.010	3.004–3.012	0.004–0.012mm interference
5	5.000–5.012	5.008–5.018	0.008–0.018mm interference
6	6.000–6.012	6.008–6.019	0.008–0.019mm interference
8	8.000–8.015	8.010–8.023	0.010–0.023mm interference
10	10.000–10.015	10.012–10.027	0.012–0.027mm interference

Layout rule Use two dowel pins at maximum spacing for best positional accuracy. Three pins is over-constrained and causes fitting problems. Two pins + fasteners for clamping is the standard pattern. Pin diameter is typically 6mm or 8mm for most CNC assemblies — larger only for heavy machinery.

Material note Dowel pins must be hardened steel (60 HRC minimum). Unhardened pins deform during installation and don't hold position. Stainless dowel pins exist (A2/A4) but are softer — use only when corrosion resistance is required and accept reduced positional accuracy.

Helicoil / Thread Inserts

A helical coil of diamond-cross-section stainless steel wire installed into a tapped hole. The insert itself becomes the threaded surface. The parent material only sees the OD threads of the insert, which distribute load over a much larger area.

When to Use Inserts

Situation	Reason	Without Insert
Aluminum / magnesium parts	Soft material strips under repeated load	Thread pulls out after a few assembly cycles
Frequent assembly / disassembly	Steel insert wears better than aluminum threads	Threads gall and loosen after 5–10 cycles
Stripped thread repair	Restores a damaged hole to original thread size	Part is scrap or requires oversize fastener
Plastic parts	Plastic has no meaningful thread strength	Fastener pulls out on first torque
High vibration	Insert + Loctite is more reliable than threads in soft material	Fasteners work loose over time

Installation

1. Drill the hole to the Helicoil tap drill size (larger than standard tap drill for that thread).

2. Tap with the special STI (Screw Thread Insert) tap — NOT a standard tap. The STI tap is oversized because the insert will occupy space inside the hole.

3. Install the insert with the Helicoil installation tool. The tool drives the coil into the tapped hole. The insert has a tang on the end — break it off after installation with a punch or tang break tool.

Cost impact Helicoil inserts add roughly $0.30–1.50 per hole depending on size and quantity. The tap, installation tool, and inserts are cheap in volume. For a run of 100+ aluminum parts with frequent assembly, inserts are almost always cheaper than warranty returns from stripped threads.

Common mistake Tapping the hole with a standard tap instead of the STI tap. The thread will be too small, the insert won't fit, and you'll either force it in (ruining the part) or have to drill and start over. Always use the STI tap specified by the insert manufacturer.

Fastener Grades

The grade (ISO class or SAE grade) tells you the fastener's strength. Higher grade = stronger but more brittle. The marking on the fastener head identifies the grade.

ISO Class	SAE Grade	Tensile (MPa)	Yield (MPa)	Material	Head Marking	When to Use
4.8	Grade 2	400	320	Low carbon steel	None	Non-critical, decorative, low load
8.8	Grade 5	800	640	Medium carbon, quenched & tempered	8.8	General purpose, cost-sensitive
10.9	Grade 8	1000	900	Alloy steel, quenched & tempered	10.9	Default for CNC. Structural joints.
12.9	—	1200	1080	Alloy steel, Q&T (higher carbon)	12.9	High stress, space-limited. Avoid under impact.
A2-70	—	700	450	304 stainless	A2-70	Corrosion resistance needed. 70% strength of 8.8.
A4-80	—	800	600	316 stainless	A4-80	Marine / chemical. 80% strength of 8.8.

Hydrogen embrittlement warning Grade 10.9 and 12.9 fasteners with zinc plating are susceptible to hydrogen embrittlement if the plating process isn't properly controlled. This causes sudden, unexpected failure under load. For safety-critical applications, use plain (black oxide) or phosphate-coated fasteners, or specify baking after plating.

Torque Values

Correct torque is critical. Under-torque means the joint loosens. Over-torque strips threads or breaks the fastener. These values are for dry, zinc-plated or plain steel fasteners. Reduce by approximately 25% for lubricated fasteners (with oil, anti-seize, or Loctite).

Metric Fasteners (Nm) — Dry, Zinc-Plated

Size	Thread Pitch	Class 8.8	Class 10.9	Class 12.9	A2-70 SS
M3	0.5	1.1	1.5	1.8	0.9
M4	0.7	2.5	3.5	4.1	2.1
M5	0.8	5.0	7.0	8.3	4.1
M6	1.0	8.5	12.0	14.0	7.0
M8	1.25	21.0	29.0	34.0	17.0
M10	1.5	41.0	58.0	68.0	33.0
M12	1.75	71.0	100.0	118.0	58.0
M14	2.0	110.0	155.0	182.0	90.0
M16	2.0	175.0	245.0	288.0	143.0

Inch Fasteners (ft-lbs) — Dry, Zinc-Plated

Size	Threads/Inch	Grade 5	Grade 8
#10	24/32	2.5–3.0	4.0–4.5
1/4"	20	6–7	9–10
5/16"	18	12–13	18–19
3/8"	16	20–22	30–33
1/2"	13	45–50	65–75
5/8"	11	85–95	125–140
3/4"	10	140–155	200–225

Tightening Sequence

For joints with multiple fasteners (flanges, covers, plates), the tightening sequence matters. Always tighten in a star or crisscross pattern, working from the center outward. This ensures even clamping pressure and prevents warping.

Pattern	Use When	Example
Star pattern	Circular flange (4+ bolts)	Pipe flanges, motor end bells
Crisscross (X)	Rectangular cover (4 bolts)	Gearbox covers, manifold caps
Spiral outward	Large plate (8+ bolts)	Machine bases, large flanges
Inside-out	Concentric bolt patterns	Cylinder heads, multi-bolt flanges

Two-pass method For critical joints, tighten all fasteners to 30–50% torque in sequence, then do a second pass to full torque in the same sequence. This settles the joint evenly and prevents uneven loading that can distort the parts.

Locking Methods

Fasteners loosen under vibration, thermal cycling, and dynamic loads. The right locking method depends on the environment, reusability requirement, and cost.

Method	How It Works	Best For	Reusable?	Relative Cost	Limitations
Loctite (threadlocker)	Anaerobic adhesive fills thread gaps, cures to solid	General vibration. Most common method.	Medium (242/262) or permanent (271)	$0.05–0.15/bolt	Surface must be clean. Temperature limits (150°C for blue, 200°C for red).
Split Lock Washer	Sprung washer bites into surfaces under tension	Low-to-medium vibration. Quick, cheap.	Yes (replace washer)	$0.02–0.08	Least effective method under severe vibration. Can damage soft surfaces.
Nyloc Nut	Nylon insert grips bolt threads	Medium vibration. Reusable 5–10x.	Yes (up to temp limit)	$0.10–0.50	Nylon degrades above 120°C. Not for high-temperature use.
Castle Nut + Cotter Pin	Cotter pin through nut and bolt prevents rotation	Critical safety joints. Wheel bearings, steering.	Yes (replace cotter pin)	$0.15–0.40	Requires drilled hole in bolt. Not for confined spaces.
Nord-Lock Wedge	Wedge-locking washer pair — tension, not friction	Severe vibration. Heavy machinery.	Yes	$0.30–1.00	More expensive. Requires access to both sides.
Double Nut	Two nuts locked against each other (jam nut)	Adjustable tension, no special parts needed.	Yes	2x nut cost	Bulky. Requires wrench access for both nuts.

Practical advice For most CNC assemblies, Loctite 243 (medium strength, oil-tolerant) is the go-to. It works on slightly oily threads (unlike 242), holds against moderate vibration, and you can remove the fastener with hand tools. Use Loctite 271 (high strength, red) only for permanent assemblies. Never use Loctite on nylon-insert nuts — the chemical degrades the nylon.

Common Mistakes

Mistake	What Happens	Correct Approach
Using 12.9 grade everywhere "for safety"	More brittle — snaps under impact or fatigue. Hydrogen embrittlement risk with plating.	Use 10.9 as default. 12.9 only when stress calculation confirms it's needed.
Over-torquing with an impact driver	Stripped threads, broken fasteners, distorted parts. Especially common on M5 and smaller.	Use a torque wrench. Impact drivers are for construction, not precision assemblies.
No thread engagement in tapped hole	Fastener pulls out under load. Minimum is 1x diameter in steel, 1.5x in aluminum, 2x in plastic.	Design the hole depth for adequate engagement. Use inserts in soft materials.
Using cup-point set screw on hardened shaft	Point can't dig in. Set screw slips under load.	Use flat point with shaft flat, or dog point with drilled hole. Consider a keyway.
Tapping Helicoil hole with standard tap	Insert won't fit. Hole is too small.	Always use the STI tap specified by the insert manufacturer.
Single dowel pin for location	Parts rotate around the pin. No angular constraint.	Always use two dowel pins at maximum spacing. Never three (over-constrained).
Using stainless fasteners with carbon steel plates	Galvanic corrosion at the interface. Stainless is cathodic — the steel plate corrodes faster.	Use same material family throughout, or add an insulating washer.
Not accounting for Loctite on torque	Over-torqueing. Loctite acts as a lubricant during installation, then locks when cured.	Reduce torque by 25% when using Loctite. Follow Loctite's torque chart.
Ignoring thread pitch	Coarse thread (default) vs fine thread specified on drawing but coarse supplied.	Always specify pitch: M8x1.25 (coarse) vs M8x1.0 (fine). Don't rely on "standard."
Reusing prevailing-torque nuts	Locking capability degrades with each use. Nut eventually spins freely.	Replace nyloc nuts after 5–10 cycles. Inspect prevailing-torque nuts each use.

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