Skillful threading
The most common mistakes and how to avoid them

Threading occurs at the end of the pro­duction chain when you have already invested a lot of time, energy and mon­ey in the component. If something were to go wrong now, this would result in expensive reworking and, in the worst case, even the loss of the component. We show you typical problems that oc­cur when thread tapping, thread milling and fluteless tapping and how you can prevent them in advance.

Thread Tapping

Thread chips

Swirling chips that wind around the tool shaft are often caused by insufficient cutting speed. By increasing the cutting speed, the chips are pushed away from the tool.

If the spiral shape of the thread tap is too shallow, this can also be problematic for chip flow. Here tools with 45° to 50° spiral shape are recom­mended. In the case of steel materials, it is also possible to use tools with a bright flute or bright correction. This creates shorter chips.

Thread is too small or too big

The thread is smaller than intended? This may be due, for example, to the fact that the thread tap has decreased in circumference due to heavy wear. In this case, the only op­tion is to use a new tool. Lag errors on the machine can also be the cause of threads that are too small, for example if the machine does not run exactly on the helix angle due to rigid clamping setups. This problem can be solved by using a synchro tapping chuck.

A thread can also be enlarged unintentional­ly due to excessive cutting speed or jammed chips. You should also check whether there is an axis offset between the pre-drilled hole and the threading tool. In this case, the hole position may not be correct and the align­ment of the hole may not match the thread­ing tool. In this case, clamping of the workpiece can help.

Tool breakage

The main reason for tool breakage when thread tapping is jammed chips. Here, you should ensure good chip removal by using high spirality or adjusting the cutting speed.

In the case of through holes in particular, it must be ensured that the tool runs completely through the thread and that the final turn is completely cut out so that the chips can be safely discharged downwards. Only then should you switch to counterclockwise ro­tation. In the case of blind hole threads, on the other hand, it is important that the thread tap does not run down to the bot­tom of the hole, as this can also lead to tool breakage. Therefore, check the core hole depth or drill deeper if permitted. Alternative­ly, you can use a thread tap with a shorter chamfer lead to create more space between the hole base and the thread depth.

Fluteless Tapping

Thread is not fully formed

If the thread profile is not fully formed after fluteless tapping,this may be because the pre-drilled hole is too large. In this case, you should select a suitable pre-drill diameter for the next core hole.

Thread is overformed

Conversely, the thread may be overformed if the pre-drill diameter selected was too small, meaning the fluteless tap had to displace too much material. In addition to choosing the correct pre-drill diameter, suf­ficient cool­ing lubri­cation is crucial here, too: If the oil concentration in the emulsion is too low, this leads to a build-up of material in the hole and, as a result, the thread in the core diameter becomes too small.

Thread surface is unclean

Here, too, the cause is often insufficient cool­ing lubrication and material adhesion on the tool. Make sure that the oil concentration in the emulsion is sufficient.

Thread Milling

Thread tapers downwards

It is often the case that the thread gets nar­rower at the bottom or is too narrow as a whole. Tapered threads are mainly creat­ed during climp thread milling. It is therefore worth adjusting the programming to conventional thread milling. Our free programming aid CNC Gühro ThreadMill helps you with reprogram­ming.

Excessive radial load can also be the cause of a tapered thread. You can spread this radi­al pressure using a radial cut index (2/3 to 1/3) and thus relieve the load on the tool. For very deep threads (2.5xD), an axial cut in­dex (50:50) helps. An HPC chuck, which is perfectly suited for thread milling, is also a quick solution. You can also reduce the radial pressure by reducing the feed rate.

Cutting edge chipping

Again, too high a feed rate can cause the cutting edges to chip. The problem can be solved by reducing the feed rate per tooth. Collisions often occur during machining, for example when the tool hits an interfering edge on the workpiece or fixture. Poor chip removal during thread milling can also result in tool breakage. An internal coolant supply is therefore advisable.

Poor thread surfaces

Poor thread surfaces are particularly com­mon when thread milling takes place under unstable conditions. Once again, the tool holder makes the difference here: The HPC chuck ensures a clean thread without chat­ter marks. You can also check whether the tools’ projection length can be reduced fur­ther or whether contamination in the holder or on the shank is interfering with the con­centricity. Even with unstable workpiece clamping, the component tends to vibrate, which is reflected on the thread surface.

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