Befestigungselemente
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.
Welches Befestigungselement benötigen Sie?
Stop here. Match your application to the right fastener type before worrying about grades or torque.
| Application | Use This | Why |
|---|---|---|
| Allgemeines Spannen, zwei verschraubte Teile | Innensechskantschraube (SHCS) | Stärkste Kopfgeometrie pro Größe. Passt in Senkkopfbohrungen. Häufigstes CNC-Befestigungselement. |
| Geringer Kopffreiraum, optische Oberfläche | Linsensenkkopfschraube | Niedriger Domkopf, keine scharfen Kanten. 20–30 % schwächer als SHCS gleicher Größe. |
| Flush-Oberfläche erforderlich (kein Überstand) | Senkkopfschraube | 90°-Flachkopf sitzt bündig. Erfordert Senkloch. Schwächste Kopfarm — bei hoher Last vermeiden. |
| Secure a component to a shaft | Set Screw (Grub Screw) | Headless, drives into a shaft via threads. Point type matters — see below. |
| Große Strukturverbindung, Schlüsselzugang | Sechskantschraube | Externer Sechskantkopf, schlüsselgetrieben. Günstigste für M10+. Standard für Baustahl. |
| Drehpunkt, Scharnier, Gleitgelenk | Bundschraube | Präzision geschliffener Bund wirkt als Lagerfläche. Teuer, aber eliminiert separaten Stift. |
| Precision location between two parts | Dowel Pin | No threads. Press-fit locates parts to ±0.01mm. See dowel pin section below. |
| Abgestreifte Gewinde, weiches Material, häufige Demontage | Helicoil / Gewindeeinsatz | Erzeugt ein starkes Stahlgewinde in Aluminium, Kunststoff oder beschädigten Bohrungen. Siehe Einsatz-Abschnitt unten. |
| Axiale Sicherung auf einer Welle | Splint | Günstig, einmalige Verwendung. Funktioniert mit Kronenmutter oder Durchgangsbohrung in Welle. |
| Lager oder Bund auf einer Welle sichern | Sicherungsring (Seegerring) | Sitzt in einer Nut. Nur axiale Sicherung. Günstig und schnell zu montieren. |
| Tragender Stift (Mutter auf beiden Seiten) | Doppelgewindestift | Beidseitig oder voll gewindet. Verwendet, wenn eine Seite keinen Schlüsselzugang hat (z. B. Motorblock). |
Befestigungselemente im Überblick
| 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 |
| Sechskantschraube | External hex (wrench) | Hex head | Tension + shear | 0.6x | M5–M42 |
| Set Screw | Hex socket | Headless | Axial retention | 0.7x | M2–M12 |
| Bundschraube | Hex socket | Hex head + shoulder | Shear + pivot | 3.0–5.0x | M5–M16 |
| Dowel Pin | N/A (press fit) | Keine | Shear (location) | 0.5x | 3–20mm dia. |
| Splint | N/A (split leg) | Keine | Axial retention | 0.2x | 1–12mm dia. |
| Retaining Ring | Pliers (snap ring) | Keine | Axial retention | 0.3x | 3–200mm dia. |
Innensechskantschrauben (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.
Güteklassen — Welche Klasse?
| 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.
Kopffreiraum-Anforderungen
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.
Stiftschrauben (Madenschrauben)
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).
Spitzenarten — Die richtige wählen
| Point Type | Standard | How It Works | When to Use | Downside |
|---|---|---|---|---|
| Flat Point | ISO 4026 | Flaches Ende stützt gegen Wellenoberfläche | Häufige Demontage. Beschädigt Welle nicht. Mit einem Flach an der Welle für eindeutige Position verwenden. | Geringste Haltekraft. Beruht nur auf Reibung. |
| Cone Point | ISO 4027 | Spitzes Ende bettet sich in Welle ein | Permanente oder halbpermanente Montage. Höchste axiale Haltekraft. | Beschädigt die Welle. Nicht für Teile, die demontiert werden müssen. |
| Dog Point | ISO 4028 | Verlängerte flache Spitze passt in eine gebohrte Öffnung in der Welle | Eindeutiger Eingriff ohne Wellenbeschädigung. Am besten für präzise axiale Position. | Erfordert eine passende Bohrung in der Welle. Addiert eine Bohroperation. |
| Cup Point | ISO 4029 | Gerades Ende gräbt sich leicht in Welle ein | Häufigste Spitzenart. Gute Balance aus Griff und Wiederverwendbarkeit. | Hinterlässt kleine Markierung auf Welle. Kann sich unter Vibration lösen. |
Häufigster Stiftschraubenfehler
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.
Passstifte
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).
Arten
| Type | Standard | Tolerance | Material | Use Case |
|---|---|---|---|---|
| Straight Dowel | ISO 2338 / DIN 6325 | m6 | Gehärteter Stahl (60 HRC min) | Am häufigsten. Presspassung für Präzisionspositionierung. |
| Pull-Out Dowel | DIN 7979 | m6 | Hardened steel | Gewindeende für leichte Demontage mit Schraube. Essential für Sacklöcher. |
| Taper Pin | ISO 2339 / DIN 1 | Selbstsichernde Konus | Stahl | Scherlasten. Konuspassung eliminiert Presse. Wiederverwendbar. |
Presspassung — 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.
Gewindeeinsätze (Helicoil)
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.
Wann Einsätze verwenden
| Situation | Reason | Without Insert |
|---|---|---|
| Aluminum / magnesium parts | Soft material strips under repeated load | Thread pulls out after a few assembly cycles |
| Häufige Montage / Demontage | Stahleinsatz verschleißt besser als Aluminiumgewinde | Gewinde fressen und lockern nach 5–10 Zyklen |
| Abgestreiftes Gewinde reparieren | Stellt eine beschädigte Bohrung auf ursprüngliche Gewindegröße wieder her | Teil ist Ausschuss oder erfordert übermaß-Befestigungselement |
| Kunststoffteile | Kunststoff hat keine nennenswerte Gewindefestigkeit | Befestigungselement reißt beim ersten Drehmoment aus |
| 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.
Befestigungselement-Güten
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 | Niedriggekohlter Stahl | Keine | Nicht-kritisch, dekorativ, geringe Last |
| 8.8 | Grade 5 | 800 | 640 | Medium carbon, quenched & tempered | 8.8 | General purpose, cost-sensitive |
| 10.9 | Grade 8 | 1000 | 900 | Legierter Stahl, vergütet | 10.9 | Standard für CNC. Strukturverbindungen. |
| 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.
Drehmomentwerte
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).
Metrische Befestigungselemente (Nm) — Trocken, verzinkt
| 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 |
Zöllige Befestigungselemente (ft-lbs) — Trocken, verzinkt
| 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 |
Anziehsequenz
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 |
| Von innen nach außen | Konzentrische Schraubenbilder | Zylinderköpfe, Mehrschrauben-Flansche |
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.
Sicherungsmethoden
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 |
|---|---|---|---|---|---|
| Schraubensicherung (Loctite) | Anaerober Klebstoff füllt Gewindespalten, härtet zu Feststoff aus | Allgemeine Vibration. Häufigste Methode. | Mittel (242/262) oder permanent (271) | $0.05–0.15/bolt | Oberfläche muss sauber sein. Temperaturgrenzen (150 °C für blau, 200 °C für rot). |
| 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. |
| Stoppmutter (Nyloc) | Nylon-Einsatz greift Schraubengewinde | Mittlere Vibration. 5–10x wiederverwendbar. | Ja (bis Temp-Grenze) | $0.10–0.50 | Nylon baut über 120 °C ab. Nicht für Hochtemperatureinsatz. |
| 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.
Häufige Anwendungsfehler
| 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. |