Threading is a part modification process that involves using a die tool or other appropriate tools to create a threaded hole on a part.
These holes functions in connecting two parts. Hence, threaded components and parts are important in industries such as the automotive and medical part manufacturing industry.
Threading a hole requires understanding the process, its requirement, machines, etc. As a result, the process can be challenging.
Therefore, this article will help people who want to thread a hole as it extensively discusses hole threading, how to thread a hole, and other related things.
What Are Threaded Holes?
Threaded holes are circular holes with internal threads. A threaded hole can have threads made by tapping or threading since there are several ways of making threads in a hole.
Machinists often use a cutting tap to make these threads in the internal diameter of the drilled hole.
These holes with threads function as non-permanent joints, providing a position for secure fastener installation in mechanical assemblies.
Tapped or threaded components accommodate a threaded shaft like metal and machine screws, providing a reliable connection.
The threads create resistance, preventing the fastener from loosening over time. It also allows the easy removal of the fastener when necessary.
Types of Threaded Holes
Threaded holes fall into two main categories: blind holes and through holes. Each type requires specific considerations to ensure easy machining.
Blind Hole
A blind hole has a specified depth and does not pass through the entire workpiece. The depth of the hole can end on a flat surface or taper into a cone.
When creating a blind hole using an end mill, it will end on a flat surface. However, if a traditional mill is used, it may end with a cone shape. If it is required to extend the full thread to the bottom of the blind hole, a bottoming tap is used.
Dimensioning of a blind hole involves specifying the hole diameter and controlling the depth. The remaining material or the specified hole depth helps control the depth accurately.
Through Hole or Clearance Hole
A through hole extends completely through a part. Threaded holes are present on both sides of the part or workpiece. The thickness of the part determines whether the cutting tool or tap can thread the entire depth of the hole.
How To Make Threaded Holes?
With the right tools and knowledge, threading can be a very simple process. With the steps below, you can easily cut internal threads into your parts:
Step #1: Create A Cored Hole
The first step in making a threaded hole is to cut a hole for a thread using a twist drill with eyes toward achieving the desired hole diameter.
Here, you should ensure you use the right drill to achieve not only the diameter by the required depth.
Note: You can also improve the hole surface finish by applying a cutting spray to the drilling tool before making the hole for the thread.
Step #2: Chamfer The Hole
Chamfering is a process that involves using a drill bit that moves in the chuck slightly until it touches the edge of the hole.
This process helps align the bolt and achieve a smooth threading process. As a result, chamfering can improve the tool’s lifespan and prevent the formation of raised burr.
Step #3: Straighten The Hole By Drilling
This involves using a drill and a motor to straighten the created hole. There are a few things to take note of under this step:
- Bolt size vs. Hole Size: The bolt size will determine the hole size before tapping. Typically, the bolt’s diameter is bigger than the drilled hole because tapping will increase the hole size later. Also, note that a standard table matches the drilling tool size to the bolt size, which can help you avoid mistakes.
- Going too deep: If you don’t want to create a thorough threaded hole, you must be careful of the hole depth. As a result, you should watch out for the type of tap you use as it will influence the hole depth. For instance, a taper tap does not produce full threads. As a result, when using one, the hole needs to be deep.
Step #4: Tap The Drilled Hole
Tapping helps create internal threads in the hole so that a fastener can stay firm. It involves turning the tap bit in a clockwise direction.
However, for every 360° clockwise rotation, make a 180° anticlockwise rotation to prevent the accumulation of chips and make room for cutting teeth.
Depending on the chamfer size, three taps are used for tapping holes in part manufacturing.
Taper Tap
A taper tap is suitable for working with hard materials due to its strength and cutting pressure. It is the most coming tapping tool characterized by six to seven cutting teeth that taper from the tip.
Taper taps are also suitable for working on blind holes. However, using this tap to finish threading is not advisable because the first ten threads might not fully form.
Plug Tap
The plug tap is more suitable for a deep and thorough threaded hole. Its mechanism involves a progressive cutting motion that cuts the internal threads gradually. Hence it uses as by machinists after the taper tap.
Note: it is not advisable to use plug taps when the drilled hole is near the workpiece edge. This can lead to breakage when the cutting teeth reach the edge. Furthermore, the taps are unsuitable for very small holes.
Bottoming Tap
Bottoming tap has one or two cutting teeth at the beginning of the tap. You use them when the hole needs to be very deep. Using the bottoming tap depends on the hole’s desired length.
Machinists usually start with a taper or plug tap and end with a bottoming tap to achieve good threading.
Types of Taps Used in Hole Threading
Machinists often use three main types of tap for making threads in holes of machined parts. These include
#1. Taper Taps.
A taper tap is ideal for making threads in harder materials because of its high strength and cutting pressure. It has six to seven cutting teeth cutting teeth that begin taping from the tip.
The taper tap possesses a conical tip with a slight angle to the threads. More so, the tapered angle of its threads helps start hole threading easier than with the bottom tap.
However, the first ten threads made with taper taps may not form completely. Hence, finishing threading with this type of tap is not advisable. Also, taper taps are unsuitable for working very hard material or holes with a large diameter.
#2. Plug Tap.
The plug tap is well-suited for making deep and thorough threaded holes. It gradually cuts the internal thread in a workpiece using a progressive cutting motion.
Machinists often use it after the taper tap; hence, it is often regarded as the “second tap.” These taps are suitable for correcting threads that need minor adjustments.
However, experts advise against using plug taps when the drilled hole is closer to the edge of the workpiece, as it can cause breakage when the cutting teeth get to the edge. More so, plug taps are unsuitable for working very small holes because the tap can break.
#3. Bottoming Tap.
This type of tap possesses one or two cutting teeth at its beginning. The bottom tap is ideal for situations when you need to thread deeper holes.
The desired length of the hole often determines the use of the bottoming tap. The bottom tap threads holes from the bottom of the hole in cases where there is insufficient space to start the tap from the top.
Using a bottom tap, you can easily cut threads down to the bottom of a through hole. It is also suitable for threading a shallow blind hole.
Moreover, machinists often begin threading a hole with a taper or plug tap and complete it with a bottoming tap to derive accurate threading. However, the bottom tap is unavailable in all sizes since it is a specialized tool.
Threaded Holes Vs. Tapped Holes: Their Differences
A tapped hole is similar to a threaded hole, although they use different tools. On the one hand, tapping a hole is achievable using a tapping tool.
On the other hand, you need a die to create threads in a hole. Below is a comparison of both holes:
Speed
In terms of the speed of operation, tapped holes take relatively less time to cut threads. However, tapping may require different tap types for just a single hole.
Therefore, such holes that require switching taps will have a longer production time.
Flexibility
On the one hand, tapping has lesser flexibility because it is impossible to alter the thread fit after the process ends.
On the other hand, Threading is more flexible as you can modify the thread size. This means the tapped hole has a fixed location and size after threading.
Cost
The process of making threads on a surface helps to save costs and time. One can make holes with varying diameters and depths with a single thread milling.
On the other hand, using different tap tools for a single hole will increase tooling costs. Furthermore, the tooling cost may increase due to damage.
Aside from the cost, tool damage can also lead to broken taps, although there are now ways to remove broken taps and continue threading.
Material
Though you can create threaded and tapped holes on many engineering materials, a tapping tool has an edge in very hard ones. You can make tap holes on even hardened steel with the right tool.
Tips to Consider When Making Threaded Holes
Several factors, such as the properties of the intended material, hole characteristics, and others, can influence the threading of holes in parts. Below are considerations for effective hole threading:
Material’s Hardness
A material’s hardness determines the force required to drill and tap a hole. The harder the material, the more force you need to make a hole in it and tap it.
For instance, it would be best to use a tap made of carbide to thread a hole in hardened steel because it has high heat and wear resistance.
Besides, applying lubricants to the tap tool helps to ease the hole threading and prevent tool and material deformation.
Similarly, reduce the cutting speed of the tool when machining hard materials to minimize tool wear and ensure better surface quality.
Ensure Optimal Hole Depth
It can be difficult to achieve the preferred depth when threading holes in a part, especially when dealing with a blind tap hole. Unlike a through hole, a blind hole possesses high restrictions.
Hence, to avoid going too deep in the hole or not deep enough, you must reduce the feed rate or cutting speed to achieve optimal depth.
Maintain Standard Thread Size
The chosen thread size may impact the whole threading process. Standard thread sizes ensure that the thread fits in the part perfectly.
The standard thread size you can use includes Metric thread (ISO), the British standard, or the National (American) standard.
The most popular of all three is the metric thread standard. Its thread sizes have a corresponding diameter and pitch.
For instance, M6 1.00 has 6 mm as the bolt diameter and 1.00 as the diameter between the threads. M12 1.75 and M10 * 1.50 are other typical metric sizes.
Pick Suitable Machinery
Using the right tools is crucial to guarantee the success of any machining process. You can use cutting tools (single lip cutting tool or cutting taps) or forming taps to create a tapped or threaded hole.
Although both tools can make internal threads, their mechanisms differ. As such, the material texture and bolt diameter will determine your choice.
Cutting tap tools remove materials to form internal threads designating a space for the screw thread to fit in.
On the other hand, forming a tap is a highly efficient tool that rolls material to produce threads. Hence, it doesn’t create metal shavings.
Besides, it is ideal for threading products made from less rigid materials such as brass or aluminum.
Proper Positioning
Threading has to occur in the proper position to ensure a successful and effective process. The thread’s position can be anywhere, i.e., close to the edge or middle.
However, threading close to the edge requires more care as any mistakes can affect the surface finish of the part and damage the tapping tool.
Angled Surfaces
When handling a workpiece with an angled surface, experts advise that you slide the tapping tool down the surface carefully, as it can break due to bending stress.
For instance, when working parts with angled surfaces, you can mill a pocket to create a flat surface for the tapping tool.
Techniques for Creating Threaded Hole
There are two main categories for creating threaded holes: machining processes and non-machining methods. Each offers advantages depending on the application, material, and desired outcome.
Machining Processes for Threaded Hole
Thread Tapping
This is the most common and versatile method. A tap, a fluted tool with a gradually decreasing diameter and matching thread profile, is used to cut the threads into a pre-drilled hole.
Tapping is relatively simple but requires careful selection of tap size and lubricant to ensure smooth thread formation and prevent tool breakage.
Thread Milling
This CNC-controlled process utilizes a rotating tool with a specific thread profile to mill the threads directly into the workpiece.
It offers high precision and is suitable for creating various thread types, including internal and external threads on complex shapes. However, it requires specialized equipment and may be less cost-effective for simple applications.
Thread Rolling
This method uses a forming tool, often a pair of rolling dies, to cold-form the threads into the material. The dies press the desired thread profile onto the workpiece, displacing the material rather than removing it.
Thread rolling is faster than tapping, strengthens the material around the hole, and creates work-hardened threads with superior wear resistance. However, it’s limited to specific materials and thread types.
Non-Machining Methods for Threaded Hole
Welded Inserts
Threaded inserts are pre-manufactured metal rings with external threads. These inserts are welded into a pre-drilled hole, creating a strong and reliable internal thread. This method is beneficial for weak or thin materials where tapping might not be suitable.
Press-Fit Inserts
Similar to welded inserts, press-fit inserts are pre-threaded components. However, they are mechanically pressed into a slightly undersized hole, creating a frictional force that holds them in place.
This method is faster than welding and suitable for applications where heat is undesirable.
Maintenance and Repair of Threaded Holes
Common Issues and Challenges
Even the most carefully created threaded holes can encounter problems over time. Wear, tear, and improper use can lead to damaged threads, compromising the integrity of the connection.
- Stripped Threads: This occurs when the thread profile is worn down or damaged, preventing the fastener from gripping properly. It can happen due to over-tightening, cross-threading (forcing a screw in at an angle), using the wrong size fastener, or corrosion.
- Seized Threads: Threads can become fused together due to rust, corrosion, or excessive use of thread-locking compounds. This makes it difficult or impossible to remove the fastener.
- Broken Studs or Bolts: Occasionally, the threaded fastener itself can break within the hole, leaving a fragment stuck inside.
Techniques for Repairing Damaged Threads
Re-Tapping
If the damage is minor, re-tapping the hole using a tap with the same thread size can often restore functionality. However, this is only suitable for limited damage and may not be possible if the material is too thin.
Using Threaded Inserts
For more severe damage or when re-tapping is not an option, threaded inserts offer a reliable solution. These are pre-threaded metal sleeves that are inserted into the damaged hole, creating a new internal thread. There are different types of inserts, including:
- Heli-coils: These wire-like inserts are wound into the damaged hole, creating a strong and permanent repair.
- Solid Threaded Inserts: These are solid metal sleeves that are pressed or glued into the hole. They offer a high strength solution for larger holes.
Preventive Maintenance Strategies
Following these strategies can extend the life of threaded holes and ensure reliable connections for the long term.
- Use the Correct Fastener: Ensure the fastener matches the thread type and size of the hole perfectly.
- Proper Torque: Tighten fasteners to the recommended torque specifications to avoid over-tightening and damage.
- Lubrication: Apply a light coat of lubricant to threads before assembly to reduce friction and wear.
- Corrosion Protection: Use corrosion-resistant materials or coatings for both the hole and fastener, especially in exposed environments.
- Regular Inspection: Regularly check threaded connections for signs of wear, damage, or looseness. Address any issues promptly to prevent further deterioration.
Applications of Threaded Hole
Manufacturing
- Automotive: Engine components, body panels, chassis elements, and countless other parts rely on threaded holes for assembly and attachment.
- Machinery: From industrial equipment to machine tools, threaded holes secure gears, bearings, shafts, and various components for smooth operation and power transmission.
- Electronics: Circuit boards, heat sinks, and enclosures utilize threaded holes to mount electronic components and ensure proper heat dissipation.
- Furniture: Knock-down furniture relies heavily on threaded inserts, screws, and bolts for easy assembly, disassembly, and structural integrity.
Construction
- Buildings: Structural beams, columns, and other framing elements are often secured using large bolts and threaded connections for immense load-bearing capacity.
- Bridges: Threaded connections play a vital role in assembling bridge components, ensuring the structural integrity of these critical infrastructure elements.
- Plumbing and HVAC Systems: Pipes, valves, and fittings heavily rely on threaded connections to create leak-proof assemblies for water, gas, and air circulation systems.
Aerospace
- Aircraft Assembly: Threaded holes of various sizes and high strength are crucial for securely fastening critical aircraft components, demanding exceptional reliability and safety standards.
- Engine Construction: Internal engine components like pistons, cylinders, and crankcases utilize threaded holes for precise assembly and high-pressure operation.
- Spacecraft Assembly: Similar to aircraft, threaded connections are essential for assembling spacecraft structures and ensuring they withstand the extreme conditions of space.
Consumer Goods
- Appliances: From washing machines to refrigerators, threaded holes secure various internal components and enable the attachment of external parts like knobs and handles.
- Electronics: Threaded holes on the casing of electronics allow for the secure attachment of covers, batteries, and other user-serviceable components.
- Toys: Threaded connections are often used in complex toys to enable assembly, adjustability, and durability for playtime.
Medical Devices
- Prosthetic Limbs: Threaded connections are used to attach prosthetic limbs to patients, requiring biocompatible materials and precise tolerances.
- Surgical Instruments: Many surgical instruments utilize threaded connections for secure assembly and functionality during delicate procedures.
- Medical Implants: Certain types of medical implants, like bone screws, rely on threads for secure placement and functionality within the body.