Thread cutting - instructions for cutting threads

Thread cutting - instructions for cutting threads: instructions

Building your own metal construction, repairing defective components, unscrewed screws or rusty threaded holes - The re-cutting and cutting of threads in metal may be required for many reasons. Even if this requires some basic knowledge, anyone can cut a thread that has the necessary and important tools.

In addition to the required tools, some basic knowledge is also needed. Taps are very slim and made of a hard material, so they can break quickly when mishandled. This should be avoided with our instructions, because a broken tap is difficult to remove from the threaded hole.

Step by step, we explain how to identify the core hole and the matching tap, how to cut the thread, and what advantages and disadvantages are inherent in tapping or tapping by hand.

Required tools and machines for thread cutting

Depending if that inner thread By hand or machine cut, the tools needed differ. In the first place will be one matching core hole for thread cutting and of course a drill needed.

  • Core hole drill (see table)
  • Bench drill (drill, cordless screwdriver...)
  • sinker

To cut an internal thread, a matching tap or tap set is needed. Which tap is the right one explains the paragraph "How does a tap work?"

  • Machine taps (machine tapping, thread cutting in blind holes)
  • Taps and turning tools (manual tapping)
  • Lubricants (cutting oil, grease, petrolatum...)

For cutting a external thread In addition to a die and die holder, the right round material is needed, which is available in many DIY stores. However, there are also finished threaded rods in the hardware store or on the Internet for little money to buy, so it is rarely profitable or useful to make an external thread yourself.

Safety instructions for thread cutting

Thread cutting - instructions for cutting threads: threads

When tapping, pay attention to the safety glasses!

Even if manual tapping can not be compared to working on electric machine tools, some safety instructions must also be observed here.

  • As you work with manual thread cutting, but also with automatic thread cutting at a very low speed and speed, Fewest think of wearing safety goggles, However, taps and dies are made of a very hard material, which can break and in the same small corners can burst. These can greatly accelerate due to the internal material tension and cause eye injuries. The same applies when blowing threaded holes.
  • Taps produce due to the special bevel for long sharp-edged flow chips, Do not try to remove them during the cutting process. On the one hand, they are useful for chip removal, on the other hand, they can cause cuts.
  • If the core hole of the thread is drilled, the same applies - as with all rotating tools - without gloves. Moving tools or chips can do that Grasp glove and wrap around the tool, A cut injury without a glove is easier to treat than an arm wrapped around the tool.

How does a tap or which tap work for which hole?

How a tap cuts and which cutting angle generates which chips is of little importance to the user. However, it is crucial to know which one Taps for which hole suitable is. Because the shape of the tap is crucial for the direction in which the chips flow. We would like to introduce some types of taps here. Very important is the difference when tapping in blind holes or through holes.

Tapping set - Hand tap

Thread cutting - instructions for cutting threads: threads

Tapping set with different markings (rings)

Something came in the days is the variant of the tapping set. This consists of three taps with different markings.

  • The pre-cutter has a ring and forms the thread easily.
  • The trimmer is marked with two rings and cuts a clear thread form.
  • Without ring is the finishing cutter, which cuts the tips.

With the mechanical production of threaded holes, the tapping set became unusable, because a CNC machine can not detect the threads of the individual drills and thus can not set the appropriate gear of the next drill. Therefore, be Taps sets also known as hand taps and are becoming increasingly rare in many workshops. Nevertheless, they still have their advantages today. Their very shallow rise makes it easier to set the tap straight.

Even when re-cutting a damaged thread, the long bleed has its advantages. If the first threads of a thread are damaged, it is hardly possible to accurately set a tap for regrooving. However, the long bleed allows the Taps for deeper threads align. For blind threaded holes such drill sets are hardly suitable.

Passage taps

Thread cutting - instructions for cutting threads: thread

Through taps with straight flute

Passage taps carry their name as they are for through holes (Nuts, sheets...) are meant. They are very easy to recognize on their straight flute. This ensures that the chips are discharged down into the hole. This has the advantage that they can not settle in the flute and hinder the cutting process.

For blind holes, so not through holes, through taps are not suitable. Their longer cut does not allow threads in a blind hole to be cut almost to the bottom. In addition, the unfavorable chip removal into the hole, which can lead to blockages and to break the tap.

Blind hole Taps

Thread cutting - instructions for cutting threads: hole

Blind hole hob / machine tap with spiral groove

Blind holes are holes that do not go through the material - similar to a dead end. A particular problem here is that the resulting chips can not fall down out of the hole. For this blind hole taps have a turned flute, which rolls the chip and discharges upwards out of the hole. The spiral in the chip flute also makes it possible to keep the gate relatively short, so that the thread can be cut almost to the bottom of the blind hole.

Since through-holes mean that chip removal is unimportant, blind-hole taps can be used for blind holes and for through holes. In mechanical use, changing the two drills is too time-consuming and mainly only blind-hole drills are used. Therefore, they are often referred to as machine taps.

The correct core hole to the tap - table

So the Taps is able to form the thread properly, the core hole must match the thread. A tapping tap not only cuts the thread, it also shapes the tips by crimping burrs. This only works if the cutting depth matches the tap.

If the core hole is drilled too large, there is not enough material available for the To form thread tips and the thread becomes very rough. Too small a hole leads to a large depth of cut, poor chip removal and an increased spandr pressure, which can lead to the breakage of the tap.

Often rules of thumb are used to determine the core hole drill, which is not always helpful.

  • Nominal thread diameter x 0.8 = core hole drill: This frequently used rule of thumb is unfortunately too inaccurate and should therefore not be used. As an example, let's say M12: multiplying 12 by 0.8 would result in a core hole of 9.6, but the correct core hole would be 10.2 mm and 10.5 mm for a lighter cut.
  • Nominal thread diameter - pitch = core hole drill: This method of calculating the core hole drill is more suitable, as it always refers to the slope, which has a great influence on the difference between the nominal diameter and core diameter. Example M12: 12 mm - 1.75 mm (thread pitch) = 10.25 mm, which is close to the exact core hole.

For a quick overview of the required core hole drilling our table should provide. It shows on the one hand the finished core diameter, on the other hand, but also the usual core drill, which always fails a bit larger.

Thread (nominal size)Thread pitch in mmCore diameter in mmused core drill bit in mm

From the table can also be the Core drills for fine threads derived. For example, if the thread M10x1 is to be cut, the thread M6 is used with a pitch of 1 mm. The core hole drill is here 1 mm smaller than the nominal diameter. The same applies to the fine thread M10x1, which requires a core hole drill of 9 mm.

Tapping with the bench drill

Thread cutting - instructions for cutting threads: cutting

Tapping chuck or thread equalizing chuck

The use of thread cutting machines often has the advantage absolutely exact drill holes and thread, However, it also carries the risk of breaking the Gwiindeschneider because the thread can not be cut emotional. However, drilling should always be done on a bench drill or drill stand, if possible. This is the only way to guarantee that the hole is exactly vertical. Also, the subsequent subsidence (part not relax) becomes even and central to the hole.

Anyone who regularly cuts threads on the drill should think about buying a tapping chuck (thread-matching chuck). This replaces the quick chuck in the drill and is able to accommodate different taps. An adjustable friction clutch ensures that the tap remains in the blind hole or at an increased resistance when reaching the drilling depth and a break is prevented. If the table drill has a counterclockwise rotation, the tap can simply be unscrewed again. Without counterclockwise rotation, the tapping chuck is opened and the tap is manually removed with a reversible iron. Tapping chucks are not cheap and the investment should pay off with frequent use.

But even without tapping chuck, the bench drill can certainly be used sensibly for internal threads and should at least be used for pre-drilling for thread cutting. This is the only way to guarantee that the hole runs exactly vertically.

Tapping by hand using a bench drill

  1. Clamp the workpiece securely in a vise and pay attention to a lateral clamping direction. A clamp, which only presses on the workpiece from above, can not reliably prevent a rotation of the workpiece.
  2. According to our table, drill the required core hole and lower the hole so that the Outer diameter of the chamfer slightly larger than the nominal diameter of the thread, Example: Threaded hole M12 - core drill 10.2 mm - chamfer outside diameter 12.5 mm.
  3. Continue to clamp the part and Change the countersink to the appropriate tap out. Do not forget to apply a suitable lubricant to the tap and to the drill hole. Suitable for this thread cutting paste or cutting oil. If a rare thread is cut, even simple oil or grease is enough.
  4. Now move the drill chuck down far enough so that the tap lies in the edge of the drill hole. Although the drill should touch the hole or chamfer, no great pressure should be exertedso as not to damage the blades.
  5. If your drill has a very slow speed, it can be switched on and off briefly under some pressure on the handwheel. The tap should grip the hole and cut the first threads. If the drill does not have a slow speed or is difficult to turn, the chuck can also be turned clockwise by hand.
  6. Once the tap has been gripped and the thread cut, this can be done Open the chuck and move the drill head upwards. The tap remains in the workpiece.
  7. Depending on the available space, the workpiece can remain on the clamping table of the drill or must be re-clamped elsewhere become.
  8. On the screwed taps is in last step a turning iron (also called wind iron or reversible iron) plugged and cut the thread in a corridor.

Tapping by hand

Although it is recommended to use a bench drill to make the threaded hole, but if necessary even without possible. Here, however, should be worked very carefully and concentrated.

If the threaded hole already exists or if there is no possibility to machine the workpiece using a bench drill, the tap can be set by hand using a reversible iron. It is particularly important here that the core hole has been sufficiently and centrally countersunk and the countersink is larger than the outside diameter of the thread.

After the application of a Lubricant, the tap is rotated with some pressure in the hole and checked again and again the vertical seat to the hole. If the cutting process is oblique to the bore, the thread is executed unevenly and produces a higher cutting pressurewhich can lead to the breakage of the tap. Therefore, should to Cutting always used a reversible iron become. If the tap has enough guide, a razor can be used for a quicker cut to a tap. As with a reversible ratchet with socket, the ratchets can be moved back and forth, which makes settling unnecessary and requires less space. However, if the thread is difficult to cut, the one-sided load increases the risk of breakage. Ratchets for threading are therefore only conditionally recommendable.

Machine threading without chips

Thread cutting - instructions for cutting threads: hole

Machine thread forming

In addition to the manual cutting of a thread, threads can of course also be produced in higher quality. In mass production, however, come here less often machining processes and instead tapping for use.

External threads are often used on special machines Thread rolling respectively thread rolling manufactured. The big advantage lies in the fast and non-cutting production. The thread form is not cut with a die into the round material, but pressed by rollers. This cold forming ensures that the material surface is pressed and solidified. The structure in the metal is not destroyed and rolled threads have a special quality.

In a similar way, the Production of internal thread with a thread former, These have no cutting and press the thread in the outer wall of the core hole. Since no material is removed, the core hole does not correspond to the usual core holes during tapping. Thread forming is used when it comes to producing no chips or difficult to machine the material. just softer Matrialien, such as aluminum obtained by cold forming firmer threads, with a better dimensional stability.

Without the removal of chips, it is of course particularly important that the keunloch is maintained exactly. If this is only slightly larger, the thread can not be formed. On the other hand, too small a core hole quickly leads to overloading of the thread former, since excess material can only be further pressed and not removed. In the private sector, thread forming is therefore rarely used.

Re-cut or repair threads

Thread cutting - instructions for cutting threads: threads

Often ridiculed, they actually exist: the thread parts

Just as a new thread can be cut, even damaged threads can be reworked. Possible causes of this may be rust or deformation of the thread crests. However, if there is a lack of material on a thread, the reworked thread will never reach the same form and adhesion again.

Of course, the simplest form of reworking is the use of taps. This is especially recommended when the threads are added by rust and contaminants. Especially with impurities, however, care should be taken to ensure that there is no sand in the threads. Sand grains are very hard and would damage the cutting edges of the tap immediately. It is advisable to wash out the damaged thread with benzine or similar beforehand.

To re-thread the thread, the tap must be placed exactly in the existing gear and guided by it. If misaligned, the thread crests would be cut so that the thread subsequently becomes completely unusable. To avoid that, should be Recutting a thread always uses a hand tap become. This has a very long bleed and makes it easier to find the thread.

Deformed external threads can also be reworked with a thread. Often laughed at as a joke, they actually exist. It has different pitches, which correspond to the different threads and re-formed by the file movement the thread. Care must always be taken that the thread pitch of the screw matches the pitch of the file.

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Video Board: External Threading - A Beginners Guide