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Articles on Spindle

Article 1: Preference of multi spindles over single spinde
There is always greater flexibility and productivity when machining centers use multi spindles in their machine tools instead of a single spindle. Depending on the machine design, multiple spindles perform in different ways.

Machining centers have today emerged as more fast, more accurate and less expensive. There is the need of high volume production in a short span of time. Under such circumstances, the need of multi spindles for performing multi tasks in the same tool arise.

The greatest advantage for this is flexibility. A machining center can easily move from running one part number to running another related part in the same family. It is also easy to reconfigure for running a different work altogether. In recent years, many machine tool companies catering to high-volume applications have expanded their services to include machining centers that have two parallel or many spindles able to machine identical workpieces at once.

Reasons for popularty
There is an increasing demand for multi spindles in machining centers today. There are many reasons for this such as:

More work in little space: One main reason for popularity is the floor space. A two-spindle or a multi spindle machine has the potential to deliver almost double the productivity, within a space only slightly larger than that of a single-spindle machine.
Sharing of burdens: The multi spindles consolidate not only floor space, but also the number of operators required to achieve a given production rate. They also reduce a manufacturer's or shop's maintenance burden, because the multi spindles machining two or more workpieces share the same fluid systems and other machine resources and accessories that would otherwise require separate maintenance on separate machine tools.
Accuracy: It has also beem notices that multi spindles can perform with must greater efficiency than a single spindle. For twin or multi spindles, there is a sort of insurance policy, that even when one breaks down, there will still be 50 percent capacity available in the other spindle.
Independent motion: Multi spindles or twin spindles are capable of imdependent motion. For even tighter accuracy, there is the option of leaving the tool out of one spindle while the other spindle is used alone to cut.
One workpiece, multi spindles: There is another option for using two spindles on a machining center-a spindle that is involved in alternating cuts rather than simultaneous machining. Instead of producing two identical pieces at a time, a machine tool might use multi spindles to attack a single piece in the same cycle. While one spindle is cutting, the other spindle can go for a tool change and prepares for the next cut. Thus the multi-spindle, one-workpiece strategy reduces the impact of tool change time.

Article 2: Importance of fluid/water bearings in high speed milling spindles
Bearings used in spindles have undergone various incremental improvements. The hybrid ball bearings using ceramic balls are very popular compared to the steel bearings. But nowadays, manufacturers are opting for other options in high speed milling spindles, besides the traditional bearings. Manufacturers are opting for fundamental change in pindle bearings used in milling. One such option is to replace mechanical ball bearings with hydrostatic bearings or fluid bearings, the bearings that are free of contact between moving surfaces. The fluid used in these bearings is water.

What is fluid bearing?
This is however not a new concept. A fluid bearing or the idea of using water as the bearing medium is not new. What is new, is a heightened demand to apply this technology in milling spindles. A fluid bearing is a type of non-contact bearing where there is no mechanical contact between moving surfaces. Spindle motors are now available to deliver more power, and cutting tools available today can put the increased power to use. The constraint here is the bearing. Hybrid ball bearings used in the milling spindles appear to represent the practical limit of how much power and speed a mechanical bearing can put to use and still maintain a long service life. But using fluid bearings having higher load capacity can overcome this performance limit.

This type of bearing is very suitable to high speed and high power machining. The fluid helps in delivering smooth motion, ensures low runout and damps vibration. Such type of bearings are used in the aerospace industry, in hard steel mold and die milling.

Advantages
In a fluid bearing, there is a complete isolation of the spinning component from the stationary one by the fluid. Even air can create an effective non contact bearing. But there are certain drawbacks. While a magnetic bearing requires an expensive spindle design, an air bearing lacks stiffness. Using a fluid as the bearing medium produces a spindle that can be both cost-effective and effecient for heavy cutting. Moreover, fluid bearing can bear a heavier load. Spindle's low runout is also because of the lack of contact. Fluid bearing doesnot require much maintenance since with no mechanical contact to wear out moving parts, the bearing stands to deliver considerably longer life.

Drawbacks
A drawback to using a fluid as the bearing medium is the inefficiency related to liquid's viscosity and this is liquid friction. The spindle motor has to work harder to overcome this resistance in the bearing. Minimizing the losses of viscosity accounts for why water is used. Though hydrostatic bearings running at low speeds can use a viscous liquid like oil, higher rpm requires an easier-flowing liquid. However, using water is not a simple substitution. Oil is a good option because it is essentially non-reactive and non-conductive but water is not. To use water as the bearing medium, the spindle maker must take into consideration various additional steps during manufacturing like coating internal components, sealing the motor and using deionized water.

Article 3: One spindle drive with both features of high speed and torque
When we talk about metalcutting operations, they are usually divided into two sections-one to remove large quantities of material using high depth of cut with slower feeds and speeds and the other is high speed machining using shallow depth of cut with high rotational speeds and fast feed rates. Both these represent roughing and finishing operations.That means, you get lots of material out of the way with less consideration to dimensional accuracy or surface finish which is roughing. And for finishing, you get the part to print size and surface specification as fast as possible. However, in most cases the need to perform fundamentally two different operations requires the use of two distinct machine tools-one for finishing and the other for roughing .

To overcome this problem, a new type of spindle is coming up in the market comprising two motors in a common drive unit. This spindle uses two concentric shafts, one from each motor which can run independently or in cobination. The using of the two motors in a common drive unit gives the machine tool very high torque and speed capability in a single spindle drive package. Such type of spindles become very beneficial in many situations. For example, in production process where tool stations are limited and tool changes takes lot of time , combination cutting tools can be used to drill and bore in a single phase where each tool can be rotated at its highest speed. The dual drive and tool system represents an interesting development in spindle technology.

Article 4: Spindle error-motion testing tool: Importance
Spindles cannot be blamed always for a lot of machining problems. In most cases, it is seen that when there is any machining problem, the spindles are considered to be the cause of the problem. But in reality, this is not always true. There are other factors which are equally responsible for machining problems.

To find out whether your spindle needs to be changed for any problem, manny manufacturers have come up with the idea of using a "spindle error-motion testing tool". This tool helps in determining if the spindle is the cause of machining problems or if something else is causing unwanted spindle motion.

What is spindle motion testing?
Spindle-motion testing is a comprehensive way to analyze machine tools for possible errors than machining test parts. Excessive errors when machining test parts create only scrap and some errors may become more clearly visible only when future jobs are run. Spindle error testing allows manufacturers of spindles to determine the characteristics of new spindles they produce.

Importance
This testing tool is especially important for shops that want high accuracy. It is alos important for those shops that machine expensive parts for which machining errors that lead to scrap are expensive. Also, shops that purchase new machine tools can find out the machine's capabilities through this tool as a guide for scheduling jobs on the new machine based on the precision that is required for the job.

How it is done?
Shops use spindle error-motion testing tool to measure the characteristics or features of a spindle as it is mounted in the operating machine. In addition to the spindle itself, a test in an operating machine measures the effects of other possible error sources.

Government research laboratories benefitted the most from spindle metrology and spindle error-motion tests. Those laboratories were always shrouded in secrecy, and restricted from disclosing information about their projects, yet they needed to buy and use machine tools. As they could not test the accuracy of the machine tools they purchased by making test parts, they opted for another route. That is they determined the specifications of the spindle motion that they needed. After that test procedures and software were developed to ensure appropriate spindle performance.