Chaque projet a besoin de filets solides et fiables. Pourtant, de nombreuses équipes luttent pour conserver la précision et la cohérence qu'exige l'usinage de précision. Si vous voulez créer des filetages parfaits à chaque fois, vous devez comprendre les principes de base du taraudage. Vous devez également connaître les outils et les meilleures méthodes à suivre pour obtenir des résultats propres et précis.
Vous voulez cesser de commettre des erreurs de taraudage courantes ? Continuez à lire pour apprendre les étapes et les conseils qui vous aideront à réussir.
Qu'est-ce que le taraudage ?
Le taraudage consiste à couper des filets à l'intérieur d'un trou à l'aide d'un taraud. Le taraud possède des arêtes de coupe adaptées au profil du filet. Lorsqu'il tourne dans un trou percé, il enlève les rainures pour former une trajectoire filetée.
Le taraudage est utilisé lorsqu'un composant doit recevoir un élément de fixation, tel qu'une vis ou un boulon. Le résultat est une connexion sûre et réutilisable. Le taraudage peut être effectué à la main ou à la machine. Le taraudage manuel est plus lent mais idéal pour les petites séries. Taraudage CNC est rapide, précis et adapté à la production en grande quantité.
Filets internes et externes
Il existe deux types de filets : les filets internes et les filets externes.
- Filets internes sont coupés à l'intérieur d'un trou. Ils sont réalisés à l'aide d'un taraud. On les trouve dans des pièces telles que les supports, les boîtiers et les carters d'engrenage.
- Filets externes sont coupés autour de l'extérieur d'un cylindre. Ils sont réalisés à l'aide d'une filière ou d'un outil de filetage sur un tour. Les boulons, les vis et les goujons ont tous un filetage extérieur.
Normes relatives au fil conducteur
Les normes de filetage permettent de s'assurer que les pièces fabriquées dans différents ateliers ou pays s'emboîtent les unes dans les autres. Voici les normes les plus courantes :
- Métrique (ISO) : Mesuré en millimètres. Commun dans le monde entier. Une taille typique peut être M6 × 1,0.
- unifié (UNC/UNF) : Utilisé principalement aux États-Unis et mesuré en pouces. UNC correspond à un filetage grossier ; UNF à un filetage fin. Exemple : ¼"-20 UNC.
- BSP (British Standard Pipe) : Utilisé pour l'étanchéité des filetages de tuyaux. Courant dans la plomberie et les systèmes de fluides.
- NPT (National Pipe Thread) : Filets coniques pour une étanchéité parfaite dans les systèmes de tuyauterie. Largement utilisé aux Etats-Unis.
Terminologie et géométrie des fils
Connaître les termes du fil permet de travailler avec plus de précision. Voici quelques notions de base :
- Diamètre principal : Le diamètre extérieur du filet.
- Diamètre mineur : Le diamètre à la base de la rainure du filet.
- Pas: La distance d'un fil à l'autre. Un pas plus petit = un fil plus fin.
- Diriger : La distance à laquelle un fil avance en un tour. Identique au pas dans les filetages à départ simple.
- Angle du fil : L'angle entre les côtés du filetage. L'angle le plus courant est de 60° pour les filetages métriques et unifiés.
Types de méthodes de taraudage
Le choix de la bonne méthode de taraudage dépend de vos besoins en termes de configuration, de matériau et de volume. Vous trouverez ci-dessous les types de taraudage les plus utilisés, chacun présentant des avantages et des inconvénients différents.
Taraudage manuel
Le taraudage manuel est la méthode la plus élémentaire. Elle utilise une clé à tarauder et un jeu de tarauds, généralement en trois étapes : conique, bouchon et fond.
Cette méthode est lente mais très flexible. Elle est idéale pour les petits travaux, les réparations ou lorsque les outils électriques ne peuvent pas atteindre la pièce. Elle fonctionne bien sur les matériaux plus tendres comme l'aluminium ou l'acier doux.
Cependant, il est facile de désaligner le taraud et il y a un risque de rupture si vous ne faites pas attention à la force ou à l'élimination des copeaux. Il est également moins précis en ce qui concerne la profondeur et la régularité du filetage.
Taraudage à la machine
Le taraudage mécanique utilise un équipement motorisé tel qu'une perceuse à colonne, une tête de taraudage ou une machine à commande numérique. Le taraud tourne et s'introduit automatiquement dans le trou.
Cette méthode est plus rapide, plus cohérente et meilleure pour la production. Il est également plus facile de contrôler la vitesse d'avance et la profondeur. Le taraudage à la machine réduit les erreurs de l'opérateur et permet des tolérances plus étroites.
La plupart des ateliers utilisent cette méthode pour les travaux de volume moyen à élevé. Elle permet également de traiter des matériaux plus résistants avec une lubrification et une vitesse de coupe adéquates.
Taraudage de forme (taraudage par roulage) et taraudage par coupe
Il existe deux méthodes principales pour créer des fils : la coupe et le formage.
- Cut tapping removes material to create threads. It works on most metals and is the standard for many jobs. It makes chips, so good chip removal is key.
- Form tapping (roll tapping) displaces material rather than cutting it. It needs ductile metals like aluminum or soft steel. It makes stronger threads because the grain flow isn’t broken.
Form tapping creates no chips. It also reduces tap wear and improves surface finish. But it requires an accurate hole size and higher torque.
Rigid vs Floating Tapping
These refer to how the tap is held and moved during machine tapping.
- Rigid tapping locks the tap holder to the spindle. Feed and spindle speed are synced. This gives high precision and is often used in CNC machines.
- Floating tapping uses a tap holder with axial or radial movement. It allows for slight misalignment or feed variation. This is better for older machines or manual setups.
Rigid tapping is faster and more accurate, but it needs perfect alignment. Floating tapping is more forgiving and helps protect the tap from breakage.
Outils et équipements de taraudage
Tapping success depends on using the right tools. Different taps, materials, and holders suit different jobs. This section breaks down what to use and why it matters.
Types de robinets
Taps come in three common types, each with a different tip and chip path. These types are often used in stages for better control and cleaner threads:
- Taper Tap: Has a long lead-in with about 8 to 10 threads gradually cutting. It starts easily and centers well. Best for starting holes by hand or for soft materials.
- Plug Tap: Has a medium-length chamfer with 3 to 5 threads cutting. It’s the most common type and balances ease of starting with full-depth threading.
- Bottoming Tap: Has almost no lead-in, just 1 to 1.5 threads. It’s made for threading to the bottom of trous borgnes. Usually used last after a taper or plug tap starts the thread.
Tap Materials and Coatings for Different Workpieces
The tap’s base material and coating affect performance, wear, and durability. Here’s a look at common choices:
- Acier rapide (HSS) : Affordable and good for general use on soft to medium metals like aluminum, brass, or mild steel.
- Cobalt Steel: Tougher than HSS. Better for harder materials like stainless steel or cast iron. Lasts longer under heat and stress.
- Carbure: Very hard and wear-resistant. Best for rigid setups and high-speed tapping of abrasive or tough materials. Also, it is brittle, so it is not ideal for hand tapping.
Common Coatings:
- TiN (nitrure de titane) : Reduces wear, lowers friction, and increases tap life.
- TiCN (carbonitrure de titane) : Stronger and more heat-resistant than TiN.
- TiAlN (nitrure de titane et d'aluminium) : Great for high-speed and dry tapping of tough alloys.
Tap Holders, Chucks, and Adapters
Tap holders keep the tap aligned and allow it to rotate smoothly. The type of holder affects accuracy, tool life, and safety.
- Rigid Tap Holders: Used in machines with synchronized feed. Ideal for CNC tapping. Provide tight control over depth and alignment.
- Floating Tap Holders: Allow slight movement. Help avoid tap breakage if there’s misalignment. Useful in manual or older machines.
- Quick-Change Tap Holders: Save time during setup or multi-part jobs. Reduce downtime in production.
Préparation du taraudage
Good tapping starts before the tap even touches the part. Proper planning and prep make tapping easier, safer, and more accurate. This section covers picking the right sizes and setting things up correctly.
Selecting the Correct Tap Size and Drill Size
Before tapping, you must match the tap size to the desired thread. Then, pick the correct drill bit size to make the hole.
The tap size is based on the screw or bolt that will go into the hole. For example, a ¼ “-20 UNC tap is used for a ¼” screw with 20 threads per inch.
Each tap needs a specific tap drill size. If the hole is too small, the tap might break. If it’s too big, the threads won’t grip well.
Charts are widely available, but here are two common examples:
- ¼ “-20 UNC needs a #7 drill bit (0.201″)
- M6 × 1.0 needs a 5.0 mm drill.
Always check the tap type and the material before finalizing the drill size.
Calculating Tap Drill Size Based on Thread Specification
You can also calculate the tap drill size if a chart isn’t handy. Here’s how:
For metric threads:
- Drill size = Major diameter – Thread pitch
- Example: M10 × 1.5 → 10 – 1.5 = 8.5 mm drill
For inch threads:
- Drill size = Major diameter – (1 ÷ Threads per inch)
- Example: ½”-13 → 0.5 – (1 ÷ 13) = 0.423″ drill
This gives about 75% thread engagement, which is good for most applications.
Go slightly larger for hard materials or tough taps. Go smaller only if maximum strength is needed and tapping torque is not a concern.
Workpiece Preparation and Hole Alignment
Clean, accurate holes make tapping smoother and safer.
- Deburr the hole to remove sharp edges.
- Chamfer the opening slightly. This helps the tap start straight.
- Check hole depth for blind holes. Leave enough room for the tap tip and chips.
- Align the tap with the hole center. Misalignment can break the tap or strip the threads.
Start the tap with gentle pressure and turn slowly if you’re tapping by hand. Keep it square to the surface. For machine tapping, confirm spindle alignment before running.
Processus de taraudage étape par étape
Each step in the tapping process matters. Rushing or skipping steps can lead to broken taps, bad threads, or scrapped parts. Here’s how to do it right, from start to finish.
Marking and Drilling the Hole
Start by marking the hole location clearly. Use a center punch to help guide the drill bit. This keeps the drill from wandering, especially on hard or smooth surfaces.
Choose the right drill bit for your tap. Use a drill press or CNC machine for best accuracy. If drilling by hand, keep the bit straight and apply steady pressure.
Drill to the proper depth. For blind holes, ensure extra room at the bottom for chips and the tap tip. After drilling, clean the hole. Remove any burrs or loose chips that could interfere with the tap.
Aligning and Starting the Tap
Place the tap into the hole carefully. Ensure it’s perfectly vertical (or in line with the hole if angled).
For hand tapping, use a tap guide or square to check alignment. Apply light pressure and turn the tap slowly at first. It should start cutting on its own.
Stop if the tap feels like it’s forcing too hard right away. Check the hole size or try a taper tap for easier starting.
Advancing, Clearing Chips, and Maintaining Accuracy
Once the tap is engaged, continue turning slowly and evenly. For hand tapping, turn about a half turn forward, then a quarter turn back. This helps break and clear chips.
Use cutting fluid to reduce friction and extend tool life. It also helps keep threads smooth and clean. Keep checking that the tap stays aligned. Even a slight tilt can cause crooked threads or tap breakage.
In machine tapping, make sure the feed rate matches the thread pitch. Use proper tool holders to maintain straightness.
Breaking the Chip and Avoiding Jamming
Use the reverse-turn method (half-turn forward, quarter-turn back) to break chips when tapping by hand.
Peck tapping can help with machine tapping. This means retracting the tap after a few turns to clear chips before continuing.
Chip buildup isn’t a problem for form taps, but high torque is. Use enough lubricant and the right machine settings to avoid stalling.
Facteurs affectant la qualité du fil
Good threads rely on more than just a sharp tap. Many variables affect how clean, strong, and consistent the threads turn out. Let’s look at the key ones.
Material Properties and Hardness
Some metals are easier to tap than others.
- Matériaux souples like aluminum and brass tap easily but can tear if the tap is dull.
- Harder materials like stainless steel, titanium, or tool steels require more torque and wear down taps faster.
- Brittle materials like cast iron produce fine chips but may crack under stress.
High hardness increases tool wear. Low ductility increases the chance of chipping or poor thread form. Always match the tap type and coating to the material being tapped.
Also, note that heat-treated parts are harder to tap. Pre-drilling before heat treatment may be a better choice for those.
Tap Geometry and Cutting Edge Condition
The shape of the tap and the condition of its cutting edges matter a lot.
- Taps with spiral flutes help lift chips out of blind holes.
- Straight flutes work well for through holes and short cuts.
- Spiral point taps push chips forward, great for machine tapping through holes.
Worn taps lead to poor threads, high torque, and risk of breakage. Dull cutting edges smear the material instead of shearing it cleanly.
Inspect taps often, especially in production runs. Replace them at the first sign of wear or rough thread finish.
Coolant, Lubrication, and Chip Evacuation
Lubrication helps control heat and reduce friction. It also makes threads cleaner and improves tap life.
- Use cutting oil for hand tapping or small batches.
- Use synthetic or water-soluble coolant for CNC or high-speed tapping.
- Apply enough fluid to coat the tap and flush out chips.
Chip removal is just as critical. Chips left in the hole can clog the flutes, jam the tap, or damage the threads. Use compressed air or a tap with chip control features if needed.
For deep or blind holes, back the tap out periodically or use a spiral-flute tap designed for chip evacuation.
Problèmes courants dans le taraudage
Even with planning, tapping can go wrong. Problems with the tool, setup, or material often appear as broken taps or bad threads. Here’s how to recognize the issues and what causes them.
Tap Breakage
This is one of the most frustrating failures in tapping.
Common causes:
- Using the wrong drill size (hole too small)
- Tapping too fast or with too much torque
- Dull or worn-out tap
- Poor chip removal
- Misalignment during hand tapping
- Hard material without proper lubrication
To avoid this, always check your tap condition, use the right feed rate, and break chips often when tapping by hand. Use floating holders or tap guides to reduce side pressure.
Oversized or Undersized Threads
Too loose or tight threads can cause fit issues and weaken the assembly.
Oversized threads often result from:
- Using the wrong drill bit (too large)
- Worn-out tap
- Excessive tap wear at the cutting edges
Undersized threads may happen due to:
- Drill bit being too small
- A form tap is used without a proper hole size
- Incorrect thread pitch or wrong tap
Always verify hole size with a gauge and confirm tap specs before starting.
Cross-Threading and Misalignment
Cross-threading happens when the tap enters the hole at an angle. This leads to damaged threads and weak joints.
Causes :
- Tapping without proper alignment
- Starting the tap without a chamfer
- Not using a tap guide or square
Use a block or guide to hold the tap straight during the first few turns in hand tapping. In machine tapping, ensure the spindle is square to the hole surface.
Poor Surface Finish or Burr Formation
Rough threads or excess burrs reduce thread strength and make assembly harder.
This can happen due to:
- Worn tap or chipped cutting edge
- Wrong tap coating or material for the job
- Lack of lubrication
- High-speed tapping without proper chip evacuation
Use sharp taps, apply enough coolant, and clean and inspect threads after tapping. If needed, a quick deburr with a countersink tool can clean up the hole.
Solutions et meilleures pratiques
To reduce tapping problems and improve thread quality, follow proven methods. These best practices help avoid tool wear, boost consistency, and reduce scrap.
Correct Tap and Drill Size Selection
Always match the drill size to the tap and thread standard. Use tap drill charts or do a quick calculation based on thread pitch.
- Too small a hole leads to broken taps.
- Too large a hole leads to weak threads.
If using a form tap, be even more careful with hole size. Form tapping depends on material displacement, not chip removal.
Check tool markings and confirm with calipers or gauges when unsure. Don’t guess.
Proper Speed, Feed, and Depth Control
Use the right cutting speed for the material. Too fast an increase in heat. Too slow causes rough threads.
Par exemple :
- Aluminum: high speed, low torque
- Stainless steel: lower speed, more torque
In CNC machines, set the correct RPM and feed to match the thread pitch. For rigid tapping, the spindle and feed must stay synced.
Avoid overfeeding. Go just deep enough for full thread length and then stop. For blind holes, allow for chip space at the bottom.
Lubrication Techniques for Different Materials
Use cutting fluid to reduce friction, cool the tool, and clear chips.
- Aluminium : Use light cutting oil or mist coolant
- Steel and stainless: Use sulfur-based or high-pressure cutting oils
- Brass or cast iron: Usually dry, but light oil can help extend tap life
Apply fluid directly to the tap, not just the hole. Reapply for each hole in manual work. For CNC setups, use flood or through-tap coolant if available.
Periodic Tool Inspection and Maintenance
Check taps regularly for signs of wear, chipping, or dullness. Worn taps make rough threads and increase the chance of breakage. Clean taps after use. Remove chips stuck in flutes. Store them in labeled holders to prevent mix-ups and protect the cutting edges.
Replace taps based on thread count, not just feel. In production, track tool life to avoid unexpected failures. Always inspect threaded parts with gauges to confirm quality. Catch issues early before they affect more parts.
Conclusion
Thread tapping is a key step in precision machining. From choosing the right tap and drill size to controlling speed, feed, and lubrication—every detail affects the final thread quality. Good prep, clean technique, and tools help avoid breakage, misalignment, and poor threads.
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Hey, je suis Kevin Lee
Au cours des dix dernières années, j'ai été immergé dans diverses formes de fabrication de tôles, partageant ici des idées intéressantes tirées de mes expériences dans divers ateliers.
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Kevin Lee
J'ai plus de dix ans d'expérience professionnelle dans la fabrication de tôles, avec une spécialisation dans la découpe au laser, le pliage, le soudage et les techniques de traitement de surface. En tant que directeur technique chez Shengen, je m'engage à résoudre des problèmes de fabrication complexes et à favoriser l'innovation et la qualité dans chaque projet.
