- Process mechanics: we study in detail
- Hole drilling and bar preprocessing
- Cutting in several passes
- One-pass method
- Tips and hints
Is threading one of the core skills in plumbing? Of course, every master can learn how to make internal or cut external threads. We will tell you how to make high quality threads, both internal and external, without nerves and broken taps..
Process mechanics: we study in detail
Try to imagine what happens to the metal of the workpiece at the start of the threading tool movement. Its first few turns converge in a cone, forming a lead-in part. A small force is enough for the sharp and hard teeth of a die or tap to push small grooves in the metal and firmly anchor in it.
That’s it, now the tool, as they say, “took a step” and will follow the spiral of the thread exactly, fixing the stronger the more turns. But the fact is that metal cannot deform infinitely. If the teeth of the tool cut too deep, they will squeeze out excess mass, forming chips. If you cut a hole with a regular hardened bolt, the shavings will clog the newly cut threads and the tool will have to be constantly unscrewed to clean the hole. Tap and die have special flutes for chip evacuation.
It is very important to understand that the teeth of the working part do not cut grooves in the metal. They push them, squeezing the metal on either side of them. The excess is removed by a recess between adjacent teeth: it gives the plastic metal a shape, and the rest is thrown into the chip groove.
Hole drilling and bar preprocessing
From the last observation, it can be concluded that the outer diameter of the thread is slightly larger than the original diameter of the bar on which it is cut. In the same way, the axial distance between the tops of the internal thread will be slightly less than the hole.
If you look at any drawing with metric threads, a number of key dimensions can be noted:
- Inner and outer diameter. These values change their names depending on whether the thread is female or male..
- Thread pitch – the distance between the tops of adjacent teeth.
- Shape and dimensions of nominal profile, angles of inclination.
So: the diameter of a rod or a hole for a thread is not equal to either the outer or inner diameter of the thread. The easiest way to determine the size of the hole for the internal thread is to subtract the pitch distance from the outside diameter. For external threads, the same value must be added to the diameter of the bar.
However, real professionals always use tables of standard metric threads, where the values of the recommended diameters also take into account the type of thread, the characteristics of metals and their alloys. So the main problem is finding the right bar or drill..
You set the basis for high-quality cutting at the stage of drilling or rod preparation. The hole must be drilled strictly perpendicularly; from the methods of controlling the right angle, one can suggest combining the drill with the reflection in the placed mirror or a credit card placed next to it.
Rust should be removed from the bar and the side surface should be checked for evenness with a metal ruler. The best way to prepare the bar is to clamp it in a drill chuck and wipe the edge well with a file. When rounding, it is allowed to grind off the rod by a couple of tenths, if it is necessary for alignment, which in practice is more important than completeness of cutting.
This is of little use when working with fixed rods. It is necessary to select the thread diameter according to the diameter of the rod, choosing a value less than the recommended one. For a more convenient entry at the end, you need to remove the chamfer and perform the cutting especially carefully and accurately. Do not forget to liberally apply machine oil to the treated area of the part..
Cutting in several passes
Threads are usually made in several passes using taps with different profile completeness. The main difficulty lies in the starting, defining pass. It is made by a tap with one thin flute on the shank. The tool must be freely inserted into the hole and, pressing it down with little force, turn it a couple of turns. At the same time, the perpendicularity of the introduction is controlled by a credit card, small deviations of 5-7 ° are quite acceptable.
For 5-6 turns, the lead-in part is fully inserted into the hole and the tap confidently becomes on the move. The tool must now be turned without pressure. It will be a mistake to correct minor deviations from perpendicularity at this stage – the tap is not to blame for this, it goes strictly along the hole. After every 1.5–2 feed turns, unscrew the tool half a turn.
After the starting cut, when the thread profile is 50-60% complete, form it with medium (# 2) and finishing (# 3) taps with the appropriate number of flutes on the shank. Here it is only important to check the correctness of the tap on the existing thread, the rest is a matter of technology.
There are practically no peculiarities of working with a die, the greatest difficulty is working on the lead-in part. It is short at the die, only 2.5–2 turns, so it is recommended to hold the tool with both hands.
Single-start threading is typical for mechanized devices. A single cut tap has one wide or three thin flutes on the shank, or may not have them at all. Other differences in machine taps: short start, shank thinner than nominal diameter, full thread profile.
Through one pass, you can cut through threads in thin (2–4 mm) sheet materials. Machine taps are also quite common for tapping blind holes. More precisely, they expand the trace from the lead-in of taps # 1 and # 2, adding another 1.5-2 turns to the thread. If the third number has a long lead-in, it can be completely cut off and use this tap only for full tapping in blind holes..
Single-start taps are no more difficult to work with than dies. There is a difficulty in setting the correct position, but the technique is the same. By the way, most of the dies are also designed for one-pass work..
Tips and hints
When threading, never spare grease. The presence of oil on the tool helps prevent overheating and associated thread breakage. In addition, the friction force is greatly reduced and the tap is perfectly felt with your hands. Add a couple of drops of oil every 4-5 turns.
Do not apply excessive pressure, threads up to 6 mm can only be cut with finger force, up to 14 mm with one hand. The tool can get stuck due to poor chip evacuation. In this case, it must be turned in both directions, gradually expanding the range of the free wheel..
Don’t work with cheap, low-quality tools. Most cheap taps are very fragile. It’s not scary to break one like this, much more trouble is caused by a fragment in a hole that cannot be re-drilled. In this case, the tap must be slightly “warmed” with a hammer through a center punch, and then pulled back with thin-nose pliers or tweezers and gradually unscrew. In sheet materials, it is easier to knock out a chip and cut a thread of a larger diameter, having previously drilled a hole.