How to cut a thread in a blind hole
How to restore a thread with a tap while maintaining its diameter and location
If you need to restore the thread while maintaining its diameter, use one of the following methods.
Welding the hole and then cutting a new thread
This method is rarely used due to the fact that the strength of the new thread obtained with this technology is lower. This method is also chosen in the absence of special devices (screws and spiral inserts).
This method of thread restoration involves the following steps.
Removal of old threads by reaming.
Welding of a hole. The choice of technique depends on the material of the parts.
Electric arc or gas shielded welding is used to weld holes in steel products.
When working with cast iron parts, gas or electric arc welding is used when cold or with general/local heating.
As additives use electrodes (MNCh-1, OZCh-1, CCh-1), cast iron bars with high silicon and other materials.
Flush machining of the hole with the base metal.
Thread renewal with a screwdriver
Screws are special cylindrical devices with threads of the desired diameter and pitch on the inside and a coarse thread on the outside. Such products are made of steel, brass, bronze, copper, and other materials. In the final stages of production, the pins are tempered and further hardened.
Picture : Screw-in screws for thread refurbishment
If you want to restore the thread with a screwdriver and retain the diameter, proceed as follows.
Rebore the hole. The drill needs to be selected so that the diameter of the hole can be made so that a thread can be drilled in order to turn the inverted screwdriver.
Cut the thread with a tap. Observe the above rules.
Screw in a thumbscrew. It must be installed flush. If this is not possible, mill the workpiece and remove the protruding part of the fixture.
Make a notch on the boundary of the new thread and the screwdriver with a drilling core. This prevents the tool from unintentional unscrewing.
Restore threads with spiral insert
Spiral (also called wire and spring inserts) are also often used to repair damaged threads.
Picture : spiral inserts for thread restoration
These fixtures have high-precision rhombic thread profiles on the inside. Almost all models are equipped with special driving tongues for screwing fixtures in.
Heavy-duty, high-quality stainless steel is used for these products. This ensures that the reconditioned threads are resistant to distortion and corrosion.
The thread reconditioning with spiral insert involves 4 steps.
Drilling. Reboring the hole. Choose the diameter of the cutting tool according to the table that the manufacturers enclose with the spiral inserts.
Picture : Drilling the hole for the spiral insert
Thread forming. Please note! Use the special taps that are supplied with the spiral insert to cut the thread. Observe all regulations and recommendations.
Picture : forming a thread for the coil insert
Assembling fixture. Put the spiral insert on the special tool supplied and screw it into the hole.
Picture : screwing a hexagon insert into a hole
139 How to cut / make a thread in a blind hole
Removing driving tabs. You can get rid of it with a special tool (also supplied by the manufacturers). To remove the tongues from large diameter inserts, ordinary pliers are suitable.
Using professional spiral threaded inserts has the following advantages.
The insert is tightened where it fits. This completely prevents wrenching. The attachments are placed in the threads of the recipient with practically no gaps. There is no need to use glue for additional fixation of products.
The elasticity of the inserts means that loads and stresses are distributed evenly. This creates ideal conditions for the transmission of forces between the bolts and the receiver threads.
Spiral inserts are universal. They are used not only for restoration of threads but also in case of necessity of joints reinforcement. Spiral inserts are used when working with low- and medium-strength and high-strength metals, plastic and wood.
Method of thread restoration
Damaged threads can be fully restored using the following technique:
Drill the tapped hole to the diameter given in table 2 below. Cut a special profile thread in it with a special tap 3. Screw a spiral repair insert in the resulting thread (a coupling made of durable stainless steel, using a special tool.
Threaded insert
After such operation, the thread becomes stronger than the original thread in good condition. This effect is particularly noticeable when threads are made in a soft material, such as aluminum. There is no “bite” effect when a bolt or stud is torn off in the threaded insert. A stripped bolt (stud) can easily be removed.
See the video on how to use a threaded insert
What options do threaded inserts have
This method is especially suitable for restoring threaded holes in car engine housing parts, such as the spark plug hole. The spark plug thread becomes very strong and secure.
Threaded insert screwed in Threaded insert screwed in
If the thread for a stud is torn off, it is sometimes drilled out and the thread is cut to the next size. In such a case, a stepped stud of unknown origin and dubious quality has to be used. And not every stepped bolt can be found at the right time. If the hole is repaired using a threaded insert, it is still possible to use a regular, branded stud.
Another use of threaded repair inserts is in the brass nuts of the exhaust manifold. It is a nut that combines very strong threads and the property of not “sticking” to the studs.
Owners of some foreign cars should also pay attention to the possibility of replacing the M12 x 1.5 threads by M12 x 1.25 threads. It often happens that a bolt with such thread is damaged, and it is almost impossible to find it in the retail network in your city (we have such bolts in our store). A turner can’t make a decent bolt because.к. The threads on the factory bolts are formed by pressure (knurling), which provides the necessary strength. And on a lathe, the thread is made with a chisel, and the fibers of the steel rod are cut, and the thread turns can not carry the necessary load.
If you replace the thread in the body part for a more common in Russia M12 x 1.25, it is possible to pick up a bolt of the required size and strength in the range of Russian and European manufacturers of automotive fasteners represented in our catalog.
Video. how the threaded insert works, demonstration
What tools are used when restoring threads
| no. n/a | Dimensions of restorable threads | Drill diameter |
| 1 | M5 x 0.8 | 5,2 |
| 2 | M6 x 1 | 6,3 |
| 3 | M8 x 1 | 8,3 |
| 4 | M8 x 1.25 | 8,4 |
| 5 | M10 x 1 | 10,25 |
| 6 | M10 x 1.25 | 10,4 |
| 7 | M10 x 1.5 | 10,5 |
| 8 | M12 x 1.25 | 12,25 |
| 9 | M12 x 1.5 | 12,5 |
| 10 | M12 x 1.75 | 12,5 |
| 11 | M14 x 2 | 14,5 |
| 12 | M14 x 1.5 | 14,5 |
| 13 | M14 x 1.25 (spark plug) | 14,25 |
For spark plug hole repair we recommend stepped taps. It is centered in the old hole by its entry part and immediately cuts a new thread, bypassing the operation of reaming.
The stepped tap greatly reduces the risk of damage to the spark plug hole, but it is considerably more expensive than a conventional tap.
An ordinary twelve-sided socket head 10 with the appropriate tool can be used to extend the tap when repairing a deeply sunk spark plug hole.
3.Spindle (tool for screwing in the insert)
the working part of the spindle resembles the threaded part of a bolt, with a catch at the end.
The insert is a diamond-shaped spiral.
At one end it has a lead, by which the insert is rotated by the spindle. The inserts have different lengths.
Subtleties of tapping with dies and crescent tools
A different thread cutting method is used for the male threads than for the female threads. The part is machined on the outer surface, it is turned until it reaches the required diameter. This value must not be greater than the inside diameter of the die, but must be 0.1-0.4 mm greater than the required final diameter. To make threading easier, the edge of the workpiece must be beveled, so the tool cutters can easily make the first cuts. Different threading tools are used for different cylindrical workpieces.
Tap taps are used on almost any kind of item: fastening studs, towing eyes, pipes, bolts, etc.д. They can be used to thread standard metric pipe threads in diameters from 4 to 20 mm.
A tap resembles a nut in shape, but it has cutting edges on the inside and special holes for chip evacuation. A distinction is made between split dies and solid. The last ones fix the blank more reliably and thread more accurately, so the elements that are made can be used in critical connections.
Clubs are similar to dies, only they have additional elements such as faceplates, pressure rings and pressure bolts. They hold the workpiece very securely and you can get a high-precision thread. These tools are used to cut inch threads on water pipes.
To cut a thread with a strip or clip, you need to fix the tool in a special holder. It is then centered on the workpiece and screwed onto it, the rotation being created by rotating the handle of the holder. the first cutting edges cut the coarse threads, while the others are used to finish them. turning on should be smooth, without jerks and excessive force, then the threads are neat.
How to thread with a tap?
Tap is the basic tool for tapping internal threads. Depending on the type, purpose, and method of application, a distinction is made:
- machine and machine-hand taps;
- universal and complete;
- Pipe, cylindrical and metric;
- For through-hole and single-sided holes;
- with straight, helical, staggered teeth and grooves.
No directional tapping, left-hand threading. It is considered standard. The tools used are the same. Reverse rotation is reversed and the cutting edge is flipped 180°. the slide is driven from the opposite side.
Tooth profile in cut with different shapes. The types of threads used that are made on lathes:
- metric;
- metric-conical;
- tubular cylindrical;
- conical pipe threads;
- inch;
- trapezoidal;
- thrust;
- round.
To use taps and dies, the workpiece is held in a chuck. Threaded cutting tool is pressed by the center of the tailstock. When cutting with a cutter, a long workpiece is clamped with a tailstock; a short workpiece is clamped with a mushroom. Tool is positioned on the slide and aligned in the workpiece axis.
Tapping dies and taps for threading
Preparing for Threading
The hole before threading is prepared in several ways. by sharpening, turning or drilling the surface of a metal workpiece. For male threads, the diameter must be reduced by 0.1 to 0.4 millimeters. At the beginning of the area to be threaded, a chamfer is made by grinding. this is necessary to blunt the thread entry. And if the hole is bored before threading, its diameter is increased to 0.4 mm. The thread hole must be slightly larger in diameter than the thread itself to prevent breakage of the thread-cutting tool.
What is tapped internal thread general information
The device is called a tap. It can be of two varieties. manual and machine, according to the method of furrowing. The material that is processed is metal, but not only metal. There are also lightweight models, they create a thread in plastic or wood. The last option can not be called the most common.
Factories use mechanized technique. metal drills make through holes (or a non-slotted cavity), after the blade is applied several turns with a predetermined pitch. The advantage of such machining is high accuracy. Calculations are made using computer programs for computer-aided design, then the data are entered into the control module. manually or by CNC. The second advantage. it is virtually impossible to skew the angle of the helix, errors.
But at home and in small industries, a simpler but less precise procedure is more commonly used. tapping internal threads by hand. The work can be done on the spot, for this you need to buy the device itself and a drill to make a preliminary hole.
The tool resembles a herringbone, because the working surface is a herringbone blade. Structurally, the product is a rather complex tool steel configuration. This material is used because of its strength and ability to handle most alloys, even cast iron. Only hardened metal is not very good to work with. it has internal stresses, so it is considered brittle and can crumble while cutting.
What tools are suitable for tapping?
You can thread pipes internally and externally with a variety of tools.
- Tap is a tool that is used to thread pipes internally. Visually resembles a rod with threads and grooves on the outside. The working part of the tap usually consists of three sections that make a rough cut, an intermediate cut, and a final cut in sequence. Some manufacturers offer 2 to 5 tap sets that are used in turns. Taps are most often used for tapping metric threads, but there are varieties that are suitable for working with pipes. Details on the process of threading with a tap we told in this article.
- Tap. a tool for threading external threads on pipes and other products. It looks like a large nut with cutting edges inside to remove material from the workpiece and holes to remove chips. A tap is screwed onto a workpiece and simultaneously forms a thread. There are several types of dies: solid, spring-loaded, clip-type. dies come in round, square, prismatic and hexagonal shapes. Round (disk-shaped) variety is the most popular. For those who have encountered threading on pipes for the first time, the use of this tool can be problematic. The slightest misalignment can lead to a defect, due to which the product will be spoiled.
- The cluppe is a tool designed specifically for pipe threading. Design is the same as a die, but placed in a cylindrical metal holder. The cylinder contains high-carbon steel cutters, plus press rings and bolts, for improved clamping to the workpiece. They prevent tilting of the workpiece and increase thread precision. A handle is usually provided for turning the tool. If it is not included in the delivery set, a gas tool can be used as an alternative. Manual tongs should be chosen for pipe diameter. Tools with replaceable cutters that can be used for cutting pipe threads of various diameters are also available.
- An electric clevis is a type of clevis that comes in handy when you need to thread a large number of pipes. Tool is equipped with an electric motor that runs at low speed. Electric tongs are similar to an angle grinder, only instead of a wheel, a die is installed in the spindle.
Threading according to all the rules
To make it easier for you to understand how to work with the tap, let’s talk about the stages of tapping a through hole.
1 stage.
Knowing the diameter of the hole, you need to select drill bits and taps that fit these parameters. For example, you want to get a threaded hole with a diameter of 8 mm, so it is suitable tap with thread M8. Drill a hole with a smaller diameter.
Step 2.
Drill the hole where the tap is to be screwed in. The recommended values for drill bit selection are given in the table:
| Thread diameter, mm | Diameter drilled hole, mm | |
| In parts made of soft metals (brass, steel) | In hard metal parts (cast iron, bronze) | |
| 3 | 2,5 | 2,5 |
| 4 | 3,3 | 3,3 |
| 5 | 4,2 | 4,1 |
| 6 | 4,9 | 5 |
| 7 | 6 | 5,9 |
| 8 | 6,7 | 6,6 |
| 9 | 7,7 | 7,7 |
| 10 | 8,4 | 8,3 |
Step 3.
First screw a rough tap into the prepared hole, then a finish tap. The first has larger cutters on the working part, so when screwing in removes up to 75% of the material, and the second is necessary for the final removal of 25% of the remaining metal and obtain a neat thread. The tap’s shank is fixed in the tap holder, a special device that will create a rotary movement. T-handle shape makes it easier for the tapping tool to turn in the body of the workpiece while the tap itself is securely locked in the seating hole. Taps should be selected based on the type of taps you are using, for example, the STAYER 28039-T4 is suitable for tools of M6 to M12 diameter, and the STAYER 28039-T2 is for M3 to M6.
Do not make any sudden movements when turning the tap handle, but let it run smoothly. Therefore, do not forget to lubricate the working part of the thread tapping tool with oil or kerosene before screwing it into the hole. This will make it easier to penetrate the material and help you get better threads, in addition, the grease will prevent corrosion on the working part. Do not forget to remove the metal shavings that form from the hole, this will achieve a more accurate threading and reduce the chance of a broken tap.
As you can see, there is nothing complicated about tapping. The main thing is to follow the rules and with time the skill will come. But that’s not all there is to know. In practice, you do not only have to thread through holes but also blind holes and work with different shaped workpieces and materials. In order for you to be ready to perform such work, let us give a few tips.