Alexei Volkov
Tapping a hole in plastic can be done by hand with a standard tap wrench or a hand drill. Before tapping a hole in plastic, a hole with the correct diameter for the tap must first be drilled. When tapping a hole, the chips of plastic have a way of exiting the hole via the slots in the tap as you cut, rather than just compressing the plastic with the threads. For holes smaller than 1/4, a bottoming tap should be used, and for holes larger than 1/4, a tap wrench is preferable.
| Characteristics | Values |
|---|---|
| Tools | Tap wrench, hand drill, 3-stage tap set, thread-forming tool, electric driver, Hitachi/Metabo driver, dremel-type device |
| Techniques | Using distilled water as a coolant, drilling a pilot hole, using a bottoming tap, using self-tapping screws, heating with a torch, using thread sealant |
| Materials | ABS, acrylic, Delrin, brass, PVC, Teflon tape, silicone |
| Hole Diameter | Typically smaller than for metals; for plastic, recommended sizes vary from 3mm to 1/2" threaded hole |
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What You'll Learn
Using a tap wrench or hand drill
Tapping a hole in plastic can be done by hand with a standard tap wrench or a hand drill. If you are using a tap wrench, you will need to buy a 3/8" square drive adapter with a 1/4" hex shank and a set of adjustable tap chucks with a 3/8" square drive hole. For plastic, use lots of distilled water as a coolant. If you are tapping a blind hole, fill it up with water.
When using a tap wrench, it is recommended to use chrome-plated taps as the finish will not rust, and the chrome has a lower friction coefficient than bare high-speed steel. For sizes larger than 1/4", a tap wrench is preferable because they take a lot of torque, and it's difficult to keep the drill straight and deliver enough torque to spin the tap.
If you are using a hand drill, you can follow a similar process but be aware that this method works best for a certain range of sizes. In ABS, you can use the drill for any size #2 through 1/4". In acrylic or Delrin, you would probably only use the drill for sizes up to 1/4".
Another option is to use a thread-forming tool, which does not remove any material but deforms it. This method can be useful if the plastic is really tough.
Additionally, it is important to note that when tapping a hole in any material, you first need to drill a hole with the correct diameter for the tap you plan on using. You can find tap drill diameters for each thread size online or in metric and inch thread tables. If there is no exact match, select a thread with a tap drill diameter slightly smaller than the existing hole.
Finally, self-tapping screws can be used as an alternative to tapping a hole. For larger diameters, 3D print the thread and clean it up with a tap afterward.
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Choosing the right tap
Tapping plastic is different from tapping metal, so it's important to use instruments specifically designed for plastic. When choosing the right tap, there are several factors to consider. Firstly, the tap needs to be very sharp to cut through plastic. It is also essential to use the correct tap size for your project. There are typically three types of taps: plug, second, and taper (also known as bottoming, plug, and taper). For sizes larger than 1/4", a tap wrench is recommended due to the amount of torque required. In contrast, for sizes smaller than 1/4", a bottoming tap is preferable as it produces cleaner cuts and less friction and heat compared to plug or taper taps.
Another factor to consider is the material of the tap. High-speed steel (HSS) is more robust and suitable for a broader range of materials, including plastics, but it is more expensive than hardened carbon steel. To prevent the plastic from melting and ensure a clean cut, it is recommended to use a coolant during the tapping process. Distilled water can be used as a coolant and is advantageous as it leaves no residue. Additionally, it may be beneficial to put the plastic pipe in the freezer before threading to stiffen it and reduce smearing.
For manual tapping, a standard tap wrench or a hand drill can be used. However, for larger volumes, CNC machine tapping is more efficient and faster. This process involves using thread-cutting or thread-forming screws for self-tapping. Thread-cutting creates a thread path, while thread-forming generates a permanent thread by deforming the plastic.
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Drilling the hole
It is important to note that drilled hole sizes for to-be-tapped holes are slightly smaller for plastics than for metals. This is because, as the plastic is laid down, it will cool and shrink. When it is laid down in a circle, it has space to shrink into, making the hole smaller.
When drilling into plastic, it is recommended to use lots of distilled water as a coolant. If you are drilling a blind hole, you can fill it up with water. The advantage of using distilled water is that it leaves no residue after being blown clean with compressed air.
For larger holes, a tap wrench is recommended as they can deliver more torque, which is necessary for larger holes. For smaller holes, a bottoming tap is recommended as it makes a cleaner cut and produces less friction and heat.
It is also possible to use a thread-forming tool, which does not remove any material but deforms it. This method requires less force as it is tapered at the tip.
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Using self-tapping screws
Self-tapping screws are a great option for tapping a hole in plastic. They are available in two versions: thread-forming screws and thread-cutting screws. Thread-forming screws, also known as "plastic" screws, are ideal for plastics because they push the plastic aside instead of cutting through it, minimising the chance of cracks or damage. They are suitable for soft materials like plastic, nylon, or other thermoplastics. Thread-cutting screws, on the other hand, are designed for harder materials such as metal or hardwood. They cut threads into the material as they are driven in, removing material in the process.
When using self-tapping screws, it is important to first identify the type of material you are working with and choose the appropriate screw type. For plastic, thread-forming screws are generally recommended. You will also need a power drill with a screwdriver bit that fits the head of your screw. The pre-drilled hole should be slightly smaller than the diameter of your screw to ensure proper thread engagement. Make sure the screw is perpendicular to the material to ensure smooth insertion and reduce the risk of stripping the screw head.
During installation, apply steady pressure and maintain a consistent speed to prevent damaging the material or the screw. Be cautious about the drilling speed and pressure to avoid overheating or over-tightening, which can deform the plastic. The screw should be flush with the material surface, and there should be no movement in the joint.
For thicker or stiffer plastics, thread-cutting screws may be more suitable. These screws have a sharp cutting edge that cuts into the plastic, forming a threaded joint. This type of screw produces reduced internal stresses compared to thread-forming screws. However, it is important to ensure the correct pilot hole diameter and depth to accommodate the removed material.
Self-tapping screws provide an economical and versatile method for joining plastics or metals to plastics. They can create a secure connection without the need for nuts or pre-formed threads, making assembly more efficient. Additionally, they allow for easy disassembly and reassembly, making them a convenient choice for a range of applications.
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Avoiding overheating
Tapping a hole in plastic can be done in several ways, but one must be careful not to overheat the plastic, which can lead to undesirable results. Here are some tips to avoid overheating:
When using an electric driver, be mindful of the speed, clutch, and screw size. Higher speeds and smaller screws can generate more heat, increasing the risk of overheating and deforming the plastic. It is advisable to use a driver with a real clutch, such as the Hitachi/Metabo, instead of an electronic clutch, as it offers better control.
For cordless screwdrivers, using distilled water as a coolant is recommended. Filling the hole with distilled water helps prevent overheating and leaves no residue after cleaning with compressed air. This method is particularly useful for tapping blind holes in plastic.
When drilling holes in plastic, it is essential to use the correct drill speed. A slower speed is generally recommended to prevent damage to the plastic. Drilling at a high speed can generate more friction and heat, increasing the risk of overheating and melting the plastic.
Additionally, it is important to consider the type of drill bit used. Spiral drill bits, commonly used for wood or metal, may not be ideal for plastic as they can crack the material. Step drills or step bit drills are often recommended for plastic as they create a cleaner hole without the risk of cracking.
When tapping a hole, it is crucial to monitor the temperature of the tap. Tapping plastic generates considerable friction, and if the tap overheats, it may fuse with the plastic. Using spiral flute taps designed for soft materials can help manage the heat and reduce the risk of overheating.
By following these guidelines and choosing the appropriate tools and techniques, you can effectively tap holes in plastic while minimizing the risk of overheating.
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Frequently asked questions
You can use a tap wrench or a hand drill. If you are using a hand drill, a 3/8" square drive adapter with a 1/4" hex shank works well.
For sizes larger than 1/4", a tap wrench is recommended. For smaller sizes, a bottoming tap is recommended as it produces less friction and heat.
When designing, undersize the hole so that screws can be screwed in easily. If the walls are thick enough, you can use a tap to create threads.
You can drill a hole and run the tap. To get a clean thread, you can use heat or thread sealant. Another method is to use PVC glue and cement to secure the fitting.
The chips of plastic can exit the hole via the slots in the tap, rather than compressing the plastic with the threads. This results in a cleaner cut.
