FAQs on damped machining tool with Self-Tuning Mass Dampers

What is tool chatter? Why does the machining tool vibrate in turning and milling operations?

All machine tools are vibrating ever since the turning or milling inserts engage with the work piece. The difference depends on whether the vibration in the machining tool will accumulate and grow in amplitude by itself.

tool chatter machine tool vibration

We can think of the machining (turning/milling) tool vibrating on a certain frequency, when it is cutting a work piece at high speed. The machine tool vibration will leave bumping surface marks on the work piece, and these marks will create a bumping force on the machining tool at the same frequency if the cutting speed remains the same. Most often, the “certain frequency” contains a wide range of frequency band and it also contains the natural frequency of the machining tool. The machining tool is “weak” at its natural frequency as a small force will make the tool vibrating at a large amplitude.

With time goes by, the vibration at the tool’s natural frequency dominates and the surface bumps texture start to follow the machining tool’s natural frequency. Indeed, the machining tool will vibrate more and more over time because:

1. the surface bump texture will make the machining tool vibrate at its natural frequency, where the machining tool is very “weak”
2. the weaker the machining tool, the higher the amplitude (peak-valley) of the surface bump texture, as the tool will jump in and out on the work piece surface
3. the higher of the amplitude of the surface bump texture, the higher is the force amplitude that makes the machining tool vibrating

In short, a machining tool is prone to vibrate at its natural frequency, at which the machine tool structure is “weak”.

The so called “tool chatter” refers to the situation where the machining process is chaotic/violent in vibration and the vibration makes it difficult or impossible to do machining. The “regenerative tool chatter” theory was first explained by Tobias et al in 1969, and it is more complex then the simplified explanation provided here.

What is a damped / de-vibe machining tool holder?

A damped or de-vibe machining tool holder refers to the tools with damping mechanism designed in the holder for operations such as turning, milling and drilling / hole-making. The typical damping mechanism includes tuned mass damper, piezoelectric actuator damper, and joint interface damper. Of all the available damping mechanisms on machining tool holders, the mass dampers are the most effective solution in terms of performance over cost.

A tungsten carbide or tungsten alloy bar is not a damped machining tool and the carbide material does not provide higher damping comparing to steel. The tungsten carbide material, however, has higher rigidity than steel and has less vibration problem comparing to steel holders. It is also important to know that tungsten is a conflict material in turbulent regions.

A tungsten carbide or tungsten alloy bar with additional damping mechanism shall be called a “damped tool holder”, and the typical examples are the tungsten carbide reinforced damped tool holders having mass damper installed.

How does the mass damper work on the damped / de-vibe machining tools?

The mass damper works by directing the vibration / mechanical energy flow inside a machining tool structure. The mass damper will direct the vibration energy from the cutting point to flow the mass damper, and leave the machining tool body remain steady.

Think of it as listening on a radio, you will only hear the music and people talking when you have the same frequency, because there is energy flow between the radio and the radio station. For mass dampers to work effectively, the mass dampers are normally tuned to the same frequency as the machining tool’s natural frequency. Because the mass damper has the same frequency with the machining tool, the vibration energy on the machining tool will flow towards the mass damper, making the mass vibrate and make the machining tool stable. The energy on the mass damper will be transferred to heat by the damping function of the mass damper.

When do we need a damped machining tool holder?

The rule of thumb goes that you will need a damped machining tool when your L/D (length to diameter) ratio is above 4. The length is measured from the cutting point to the clamping end of the machining tool. The following parameters will determine if a machining tool will vibrate: 1. the rigidity of the machine, 2. the clamping method of the tool, 3. the insert geometry and rake angle, 4. the entrance angle of the insert, 4. the work piece material, and 5. other parameters in machining set up.

A machining tool is likely to vibrate when the rigidity of the machine is low, the bending stiffness of the clamping method is low, the insert geometry is negative, then entrance angle is not close to 90 degrees and the work piece material is tough to machine.

How to reduce vibrations in machining?

To reduce vibrations in machining, the following actions are commonly suggested:

1. select a rigid machine if there is one available, an old machine might as well be rigid
2. choose tool holder clamping modules with high bending stiffness, the tool vibration is often in bending motion
3. use positive inserts, negative inserts tend to increase the cutting force
4. start with small nose radius, higher nose radius means higher vibration
5. choose larger diameter tools when the space in the component allow, typical selections are 10 mm, 12 mm, 16 mm, 20 mm, 25 mm, 32 mm, 40 mm, 50 mm, and 60 mm.
6. use medium-to-high feed to find a starting point where the tool is stable and then gradually change to feed to map out the operation window
7. maintain the speed at which the cutting insert suggests for different types of work piece materials, reduce the speed does not necessarily reduce vibrations
8. use sharper edge inserts, a PVD coated insert normally has a sharper edge comparing to CVD inserts

These common practices can be applied for both conventional tool holders and damped machining tool holders.

Which tolerance and run-out can a damped tool holder achieve?

The run-out of the machined bore / hole by the MAQ damped tool holder can be compared to a conventional tool holder in the same machine at low L/D ratio without vibrations. As an example, the MAQ STMD M25-330 product with SDUCR-25 cutter head and DCMT-11T304 insert can achieve a run out of less than 5 µm at 11xD (275 mm over hang). Normally, the smaller the diameter or rigidity of the tool holder, the higher is the run-out of the machined bore / hole. The run-out of the work piece prior machining will also affect the run-out of the machined surface. It is preferred that the run-out of the work-piece in the set-up stage should be as small as possible.

It is normally safe to plan for IT8 or H8 tolerance of the machined hole with MAQ damped tool holder. The achievable dimensional tolerance of the machined bore / hole with MAQ damped tool holder will also depend on:

• the accuracy of the guide-way of the machine
• the work piece material hardness variation between batches
• tool wear and cutting force variation
• the specific cutting resistance of the work piece material
• other machining process parameters.

How do you select a damped machining tool holder holder?

Damped machining tool holder selection depends upon sizes of the components you’re going to machine. Large-diameter tool holders are most preferable because they offer higher rigidity. If your work piece has an internal ID of 20 mm (0.787 inches), for example, it can only accommodate a 16 mm (0.629 inches) bar to avoid colliding with the work piece. It also means that a 16 mm damped machining tool holder will be preferred over a 12 mm damped machining tool, because the 16 mm damped machining tool holder has a higher rigidity/stiffness.

After selecting the correct-diameter damped machining tool holder, you need to consider the damped machining tool’s overhang length, or the overhang length to diameter ratio (L/D ratio). Less overhang length means higher rigidity, and you need: a) enough tool holder length to process the part and b) enough tool holder length to connect effectively to the spindle (the tool turret in turning machines).

The length needed to connect a tool holder to the spindle depends on the type of coupling method available, such as mechanical, hydraulic or shrink fit.

A standard MAQ vibration damped tool holder can cover a length-to-diameter (L/D) ratio of 8 or 9. In the case of longer overhang length, an additional version of damped machining tool with an L/D ratio of 10 or -11 is available, and further versions of even up to 14 or 15, is required.

How do you install a damped machining tool holder on your machine?

To install a MAQ damped machining tool holder on your machine, it is best to use an adapter for mounting the tool, via means of reduction sleeve, all around clamping, hydraulic clamping or shrink-fit clamping. Generally, when installing an damped tool holder with tuned mass damper inside, avoid using a direct screw mount screw to engage with the tool holder surface with screws because direct contact may damage the dampening system and the direct screw mount has a low bending stiffness/rigidity.

 

How do you align a damped machining tool holder on your machine?

During installation, make sure the cutting point is aligned with the center line of the machine. Normally, the cutting insert surface (rake face) has a angle in reference to the center line, and it can’t be be used as a reference to align the damped machining tool holder.

Either measure the cutting point manually or use a level that can be installed on cutter head to check the cutting point’s horizontal alignment. MAQ provides two alignment methods to align the tool holder,

a. use the central groove on the holder surface and align with a plunger screw on the reduction sleeve (faster)

b. use a digital spirit level that can be calibrated to 0° of the machine guide ways and then measure the tilting of the reference flat surface on the cutter heads. (fast)

C. use an indicator to measure the flatness of the reference flat surface on cutter heads (slow)

The digital spirit level can also be found under the product page for turning tools. It is also a best practice to use in-machine or stand-alone measuring and aligning devices where available.

How do you troubleshoot a problem with your vibration damped machining tool holder?

Vibration in machining can come from the either tool holder or the machine itself, and it is often difficult to pinpoint the source. Every vibration damped machining tool has a preferred operating condition, which normally refers to the L/D ratio range for which the tool is best suited. This is because the machining tool’s vibration frequency is depending on the L/D ratio setup. For mass dampers to function well, it is necessary to match the mass damper’s frequency with the machining tool’s vibration frequency. The problem arises when the machine tool’s vibration frequency becomes un-predictable, as it depends not only on L/D ratios, but also on the machine itself and other parameters.

Normally, reducing the L/D ratio will increase tool rigidity, thereby reducing vibration. However, there are also cases when reducing L/D ratio increases cutting tool vibration because a lower L/D setup may make the cutting tool vibrate at a frequency outside the frequency range where the mass damper is optimized. We often observe this situation when using tools with a diameter smaller than 16 mm, where the change of L/D ratio dramatically changes vibration frequency. As an example, a 16mm tool can have a vibration frequency of about 700 Hz at 5XD and 500 Hz at 6XD (a change of 200 Hz per unit change of L/D ratio) whereas, a 25 mm tool has a vibration frequency of about 500 Hz at 5XD and 450 Hz at 6XD (a change of 50Hz per unit change of L/D ratio).

Because many variables determine a tool holder’s effectiveness for any given application, it’s best to

consult the MAQ team if you have questions about the operation of your tool holder.

I have a vibration damped turning tool that claims to work at 10XD, but it only works at 9XD for me, what can be the problem?

The MAQ vibration damped turning tool holders are designed by assuming that the end-users have a machine with reasonable rigidity. The issue that the tool holder claims to work at 10XD, but only works at 9XD, mostly refer to the tools with diameter above 50 mm and overhang length above 500 mm. Tools with diameter above 50 mm and overhang length above 500 mm have a weight above 10 kg and it becomes a weight that will affect the overall rigidity of the tool turrets. A nominal cutting force of 200 N will apply a bending moment of 100 Nm on the screws used for clamping the tool seat. This will affect the clamping rigidity of the tool turret as it may exceed the critical limit and the tool turret will loose its rigidity at a high bending moment.

If It is the first time for you to use a vibration damped turning or milling tool, we would recommend you to try different machining parameters such as speed, feed and depth of cut, to find out the critical limit of the operations.

If you are facing the problem even though you have a very rigid machine and the tool had worked well before, we will then recommend a service of the tool turret and make sure that the tool seat is rigid after a long time usage.

How do you keep your vibration damped tool holder in top working condition?

To maximize the effectiveness of your tool holder, it should never operate at a temperature above 100°C (212°F) because overheating reduces effectiveness and accelerates aging of the tool holder’s internal components.

The MAQ vibration damped machining tool holders can be stored either vertically or horizontally, and there is no leakage issue with the tool holder.

How do you know when your vibration damped tool holder needs to be replaced?

Tool holder life depends of frequency of use, environment, and many other factors. Although MAQ tool holders do not have limited tool life or shelf life by design, when your tool holder begins to malfunction under conditions in which it has successfully performed in the past, it’s time for a replacement.