|
|
 |
|
|
In Focus - Archivo Diciembre 2007
|
| |
|
|
|
Balance quality – round is better
|
|
01.12.2007 -
Modern machining processes have extremely high requirements for the balancing and concentricity of toolholders. As the world market leader in clamping technology, SCHUNK again plays a pioneering role here. All SCHUNK toolholders are fine-balanced to a first-class balance quality – G 2.5 at 25,000 RPM – a further contribution from SCHUNK to excellent results in machining.
Extremely high balance quality provides for maximum smooth running even at high RPMs and thus ensures optimum machining results. Vibrations are also reduced, thus decreasing wear on the spindle and increasing the tool life.
|
|
Imbalance is caused by rotational forces, which occur in rotating bodies when their masses are distributed asymmetrically around a center point.
In automobiles, imbalance is noticeable in the wheels. It can be caused by additional mass from valves or rim joints, or by radial or lateral out-of-roundness.
Imbalance has serious effects not only on wheels of bicycles or automobiles. It is an important criterion in the construction of machines, chucks and toolholders - and is not to be underestimated.
|
|
What is imbalance?
|
|
When a mass body rotates on a rotary axis, this always produces outwardly exerted centrifugal forces. If the mass in the rotational body is distributed evenly, the centrifugal forces cancel each other out and the mass remains in place during rotation. An example of this would be a top. If the mass is distributed unevenly, on the other hand, additional forces and torques (commonly known as "imbalance") are exerted on the body, thus causing a relative movement of the rotational body to the rotational axis. If the body is permanently mounted, this causes strain on the bearings.
In general, there are two types of imbalance, which can also occur in combination.
|
 |
Static imbalance
|
|
|
Static imbalance
If one imagines the mass body as a narrow disk with the mass Mi, the centers of gravity are all at the distance r from the rotational axis and in the same orientation to the axis. This means that the center of gravity of the rotational body is not on the rotational axis, but that the central principal inertia axis is parallel to the rotational axis at a distance e from the axis. During rotation, therefore, centrifugal forces that cannot be compensated are exerted perpendicularly to the rotational axis, resulting in the imbalance, which is defined as the product of the concentrated mass M and its distance r from the rotational axis. This imbalance can already be measured at standstill, for example with a balancing scale for grinding wheels.
|
 |
|
Dynamic imbalance
|
|
|
Dynamic imbalance
Here, the center of gravity of the rotational body is on the rotational axis, but the principal inertia axis is not parallel to the axis, but rather at a particular angle. The reason for this is that the center of gravity of the individual disks is not on the rotational axis. Therefore, each of the disks exerts centrifugal forces that produce imbalance. Although the imbalance forces add up to zero, i.e. no forces are exerted laterally, they are on parallel lines of influence, which causes a moment load on the rotational body. This results in a so-called bending moment during rotation, which in turn causes an irregular, wobbling rotary motion. Dynamic imbalance can be measured only on rotational bodies that are rotating.
|
 |
|
General dynamic imbalance as a combination of static and dynamic imbalance
|
|
|
General dynamic imbalance as a combination of static and dynamic imbalance
Here, the rotational axis and the mass moment of inertia axis are askew. This is usually the case with technical rotational bodies.
|
|
The trend is obvious. Due to the increasing number of high-speed spindles and high-performance machining centers, fine balancing, especially of toolholders, is becoming increasingly important – because imbalanced toolholders damage the spindle. Balancing of toolholders with the cutting tools enables maximum precision and service life for both the machine and the cutting tool.
|
|
What does imbalance mean?
|
|
Balance quality G
DIN ISO 1940-1 defines the principles for measuring imbalance and balancing. The accuracy of balancing is specified by the balance quality G. The balance quality always applies only to a particular operating speed of the rotor. The permissible residual imbalance is calculated based on the balance quality, the operating speed and weight of the rotor.
|
 |
|
Formula "Permissible residual imbalance"
|
|
|
Example: A milling machine is clamped in a collet-style holder. Total weight: 0.8 kg. The tool is to be operated at a speed of 15,000 RPM. The spindle manufacturer requires a balance quality of G 2.5.
Permissible residual imbalance: Uzul = 1.3 gmm
|
|
Effects
|
|
In machining, imbalance of the overall toolholder/tool system is especially important. Tests by renowned research institutes, such as the Institute for Production Management, Technology and Machining Centers (PTW) in Darmstadt, have shown that toolholders with high imbalance produce several negative effects during machining:
1. Poorer surface quality of the machined workpieces due to vibrations at the toolholder, measurable as centerline average Ra
2. Vibrations at the toolholder
3. Restriction of the possible cutting speeds
4. Reduced production accuracy
5. Reduced service life of the tools due to uneven wear of cutters
6. Bearing damage to the spindle due to uneven and constant forces exerted on the spindle
|
|
Eliminating imbalance
|
|
To prevent these negative effects, the user basically has two different strategies to choose from:
1. Use of very low speeds, e.g. < 100 RPM.
Such speeds are, of course, out of the question for economical operation. A real solution is provided only by the second alternative.
2. Balancing of the toolholder,
preferably of the toolholder with the clamped tool, corresponding to the technically and economically feasible balance quality.
Balancing means compensation of the asymmetrical mass distribution of the toolholder. This can be achieved in three ways:
1. Addition of mass, e.g. by means of balancing screws
2. Removal of mass, e.g. by drilling out material
3. Displacement of mass, e.g. by means of rotary rings
There are two types of balancing:
a) Balancing in one plane
Compensation of the static element of the imbalance, by moving the center of gravity of the rotational body back to the rotational axis (eccentricity e -> 0). In this case, however, the dynamic imbalance remains.Compensation of the static element of the imbalance, by moving the center of gravity of the rotational body back to the rotational axis (eccentricity e 0). In this case, however, the dynamic imbalance remains.
b) Balancing in two planes
This compensates both for the static and the dynamic imbalance. Any two balancing planes can be used here, but the distance between them should be as large as possible.
|
|
Goal of balancing
|
|
The example illustrates how imbalances result in bearing loads, bearing vibrations and shaft deformations. High bearing loads and vibrations cause premature wear of bearings and even serious bearing damage. Shaft deformations can cause bridging of clearances and contact of the rotor with stationary parts.
The goal of balancing is to reduce bearing loads, bearing vibrations and shaft deformations to acceptable values. This requirement applies to the rotor in the assembled machine.
|
|
Solutions from SCHUNK
|
 |
|
Measuring the imbalance with the ACURO balance measuring machine
|
|
|
To eliminate the damaging effects of imbalance to the greatest extent possible, SCHUNK delivers its toolholders with standard fine balancing to a balance quality of G 2.5 at 25,000 RPM. This is based on the balancing recommendations of the balancing task force of the German Association of Industrial Research, of which SCHUNK is a member.
Measuring the imbalance
Before the imbalance can be compensated, it first has to be determined analytically by means of measuring procedures. The ACURO balance measuring machine, for example, is suitable for this purpose. The imbalance measurement is based on the following measuring principle:
1. The toolholder is clamped in the balancing spindle and set rotating. It is important for the toolholder to be clamped in the spindle, because this is the only way to achieve the same conditions as in a milling machine for repeatable results.
2. Measurement of the centrifugal forces by means of force sensors. The result has the form of a sinusoidal measuring signal, since the direction of the centrifugal forces also rotates with the spindle.
3. Calculation of the imbalance based on the measured signal for each balancing plane.
4. Calculation of the required imbalance compensation from the measured imbalance.
|
 |
|
Measuring the imbalance
|
|
|
Three different methods of measuring are available for the measurement:
1. Single measurement
The overall imbalance of the spindle, adapter and toolholder is measured in one measurement. Although this method is very fast, the accuracy suffers. Therefore, the method is suitable only for measuring rough imbalance (U > 20 gmm).
2. Measuring with spindle compensation
In a preliminary measurement the imbalance of the spindle and adapter is measured first. Then single measurements are used to determine the overall imbalance and the previously measured imbalance of the spindle and adapter is compensated.
This measuring method enables high accuracy, but it cannot take into account all interfering factors, such as inclination of the tool.
3. Reversal measurement
Two measurements are used for this method. The toolholder is turned 180° between the first and second measurement. The result is displayed only as the resulting imbalance after comparison of the two measurements. This method is the most accurate, but also the most time-consuming. High balance qualities can be achieved only with reversal measurement.
|
 |
|
Balance drilling
|
|
|
After measuring the imbalance, the ACURO balancing machine displays the exact position of the imbalance and recommendations for eliminating it. For example, the software recommends drilling to a depth of 5 mm with a 3 mm drill at a particular position.
|
|
Limits of balancing
|
|
With every type of toolholder balancing, it is necessary to consider both what is technically and economically feasible. The better the balance quality of the toolholder, the higher the expenditure involved. Also, a feasible balance quality is already exceeded as soon as the concentricity error of the spindle (even new spindles have a concentricity error of up to 5 µm) or the repeat accuracy during tool changing (between 1 µm and 2 µm for good spindles, determined primarily by contamination) ruins the outstanding balance quality of the toolholder.
|
|
Calculation of imbalance – example 1:
|
 |
|
permissible residual imbalance
|
|
|
A toolholder with a mass of 800 g is to be used at a speed of 15,000 RPM. The spindle manufacturer requires a balance quality of G 2.5 according to DIN ISO 1940-1. The permissible residual imbalance of the toolholder must not exceed the following value:
|
 |
|
residual eccentricity
|
|
|
The residual eccentricity of the toolholder is calculated as follows:
This means that the toolholder center of gravity may be offset by a maximum of 1.6 µm from the rotational axis.
|
|
Calculation of imbalance – example 2:
|
|
Balance quality G = 1
Operating speed n = 40,000 RPM
Tool weight M = 0.8 kg
Uzul = 0.2 gmm
ezul = 0.3 µm
This permissible eccentricity cannot be achieved in practice. The repeat accuracy during changing of tools alone is 1-2 µm for good spindles. Minimal contamination causes a significant worsening of the result.
The overall imbalance of a milling spindle is the result of many factors:
 |
|
Imbalance of the spindle as an individual component |
|
|
Imbalance due to concentricity errors in the spindle |
|
|
Concentricity error of spindle attachments |
|
|
Lateral distortion of the clamping system during clamping (spring pack, tension rod) |
|
|
Concentricity error and inclination of the toolholder in the spindle |
|
|
Imbalance of the toolholder as an individual component |
|
|
Concentricity error of the pull-back bolt (offset) |
|
|
Concentricity error of the tool |
|
|
Imbalance of toolholder attachments (e.g. clamping nuts) |
|
|
Conclusion: A permissible residual imbalance of less than 1 gmm is pointless!
|
|
As with products for private use, quality characteristics of toolholders are hardly perceptible at first. In the long-term operation of the machine, however, the wheat is separated from the chaff. Decades of experience and production know-how, pioneering spirit and the continuous development of already advanced products are the reasons why SCHUNK plays the leading role in the tool clamping market. Precision toolholders from SCHUNK are manufactured according to stringent quality guidelines at the plant in Lauffen – Made in Germany.
Large selection of precision tool holding systems
The TOTAL TOOLING product line from SCHUNK offers several precision tool holding systems, all of which have one decisive feature:
standard fine balancing to G 2.5 at 25,000 RPM.
|
|
| |
|
| |
|
|
|
| |
|
|
12.2007
Balance quality
Round is better
 más…
|
| |
|
|
11.2007
Patented and very impressive: the SCHUNK multi-tooth guide
Distribution of the load on many shoulders – that is the principle of multi-tooth guidance, developed and patented by SCHUNK
más…
|
| |
|
|
10.2007
Micro-handling
Handling in miniature -
a gripper that fits in a matchbox
más…
|
| |
|
|
09.2007
Run-out Accuracy of Toolholders
Micron’s best friend – with precision for precision
más…
|
| |
|
|
08.2007
Service Robotics
A Sensitive Touch for Sensitive Tasks – Robots in Service Gain Independence
más…
|
| |
|
|
07.2007
Tool Clamping
A tiny difference can have a big impact
más…
|
| |
|
|
06.2007
Hygienic Design
High-tech creations for culinary delights
más…
|
| |
|
|
05.2007
Reduction of set-up costs with the quick-change pallet systems
Savings potential of up to 90 percent
más…
|
| |
|
|
04.2007
Minimum quantity lubrication - MQS
- For minimum costs and maximum protection of the environment
A possible solution in searching for the optimum lubrication system
más…
|
| |
|
| |
|
|
|
| |
In Focus - 2013
In Focus - Archivo 2012
In Focus - Archivo 2011
In Focus - Archivo 2010
In Focus - Archivo 2009
In Focus - Archivo 2008
In Focus - Archivo 2007
|
|
