The Motor Spindle

A motor spindle is a direct-driven precisely moun­ted shaft with inte­gra­ted tool inter­face. The motor spindle is an important compo­nent of many modern machine tools.

Clas­sic motor spindle

Motor spind­les are usually moun­ted on roller bearings and are electri­cally driven. The direct coupling of the drive with the precisely moun­ted shaft allows very precise machi­ning of the work­piece with high rota­tion speed is possi­ble. For machi­ning a work­piece by means of a tool through rota­tion, it is irrele­vant which one rota­tes. There­fore, motor spind­les as well as spind­les in gene­ral are divi­ded into tool carry­ing and work­piece carry­ing spind­les. Typi­cally, the tool bearing motor spind­les are used in dril­ling, grin­ding and milling machi­nes, the work­piece bearing motor spind­les are more likely to be found in lathes. In addi­tion to the drive and the storage of the shaft, the tool inter­face is one of the most important compon­ents of a modern motor spindle. The tool is auto­ma­ti­cally chan­ged and fixed by a clam­ping system. This is why the motor spindle is nowa­days the central compo­nent of a machine tool and is largely respon­si­ble for its perfor­mance and accu­racy. Due to the complex struc­ture and inter­ac­tion of the indi­vi­dual compon­ents, motor spind­les are not stan­dard products but are deve­lo­ped and produ­ced accord­ing to indi­vi­dual requi­re­ments. Motor spind­les are mainly used in the following areas: machi­ning centers and CNC machine tools from the areas of lathes, grin­ding and milling machi­nes, HSC, HPC, tool and mold making as well as auto­mo­tive produc­tion and aviation.

History

The spindle (also called main or working spindle) is a histo­ri­cally evol­ved term which, in rela­tion to machine tools, refers to a shaft with an inte­gra­ted inter­face for holding a work­piece or a tool. This shaft performs a rota­tio­nal move­ment to machine the work­piece through the tool rota­ting either the tool (e.g. milling machine) or the work­piece (e.g. lathe) depen­ding on the appli­ca­tion. The incre­a­sed demands placed on machine tools, prima­rily in the area of high-speed chip­ping in the 80’s, the tool carry­ing spind­les had to be directly driven to enable more precise machi­ning at higher rota­tio­nal speeds. The rapid deve­lo­p­ments in the areas of tool geome­tries and mate­ri­als that began at that time allo­wed higher cutting speeds in machi­ning which resul­ted in higher rota­tio­nal speeds. With conven­tio­nal drive tech­no­lo­gies, which usually coupled the drive and the working spindle by means of a gear and other trans­mis­sion elements, these high speeds could hardly be achie­ved at all or only with consi­der­ably grea­ter effort. At the same time, the first speed varia­bles electric motors which, toge­ther with the progress achie­ved with contact ball bearings and frequency conver­ters, led to the deve­lo­p­ment of the motor spindle. In order to reach the necessary speeds, a gear­box was no longer requi­red and instead, the spindle was directly coupled with the drive. In the 90s, many machine tool manu­fac­tu­rers star­ted to outsource entire depart­ments. The motor spindle was very well suita­ble as an inde­pen­dent assem­bly group for outsour­cing which resul­ted in suppliers specia­li­zing in the deve­lo­p­ment of motor spind­les. The compact design, simple and safe main­ten­ance, low noise emis­sion and readi­ness to supply from specia­list compa­nies are addi­tio­nal advan­ta­ges of motor spind­les, as a result of which their range of appli­ca­ti­ons has expan­ded and is no longer limi­ted to the field of high-speed machining.

Struc­ture

The basic struc­ture of motor spind­les is often the same regard­less of the manu­fac­tu­rer. Serious diffe­ren­ces can be found depen­ding on the appli­ca­tion which can be divi­ded into the func­tio­nal clas­ses of milling spind­les, work­piece spind­les and inter­nal grin­ding spind­les. Parti­cu­larly work­piece carry­ing motor spind­les often have diffe­rent requirements.

Housing and cooling

The exter­nal shape of the spindle is deter­mi­ned by the instal­la­tion dimen­si­ons in the machine tool. An essen­tial feature of motor spind­les is their compact design which has a posi­tive effect on the space requi­red in the machine construc­tion area. As a conse­quence, both air and water need to be used for suffi­ci­ent cooling of the motor. The most common type, which is used, is a water cooling of Stators inte­gra­ted in the housing.

Shaft

The key element of the motor spindle is the work spindle which is a shaft with inte­gra­ted tool inter­face. The shaft must be stiff enough to be able to with­stand the impact of more radial forces so it does not bend. The aim is to achieve the highest possi­ble stiff­ness which basi­cally depends on the diame­ter of the shaft and the mate­rial. Howe­ver, a larger diame­ter will in turn result in a higher moment of iner­tia which incre­a­ses the energy requi­red for acce­le­ra­tion. The dyna­mic beha­vior of the shaft also plays an important role. The rota­ting shaft with drive and bearings repres­ents a vibra­tory system, which can lead to serious dama­ges when reaching it reso­nance frequency. In addi­tion, an inter­nal coolant supply is requi­red with more and more machine tools. The coolant is then routed to the tool via a rotary trans­mis­sion leadthrough into an axial bore hole. The tool itself must contain small bore holes through which the coolant dischar­ges and can ther­eby cool the tool. For addi­tio­nal lubri­ca­tion, cooling lubri­cants can also be used during machi­ning. In addi­tion, a supply of clea­ning air is incre­a­singly requi­red, which can be used to blow away any machi­ning resi­dues, e.g. chips. This is either achie­ved through a sepa­rate bore in the shaft or by using the coolant bore requi­ring the remai­ning coolant to be blown out beforehand.

Tool or work­piece interface

A tool-bearing work spindle on a machine tool only makes sense if the tool can also be chan­ged. Modern machine tools should work as auto­ma­ti­cally as possi­ble and there­fore also be able to change the tool auto­ma­ti­cally. The requi­re­ment is there­fore a so-called tool inter­face which allows a very high repeat accu­racy, i.e. the same tool, which is clam­ped twice in a row, should run with exactly the same accu­racy. This running accu­racy has a direct effect on the machi­ning accu­racy. On the other hand, an inac­cu­racy leads to an unba­lance which influ­en­ces the entire process and can have serious conse­quen­ces at high rota­tio­nal speeds.

The main tool holder that has become stan­dard is the steep taper and the hollow shank taper. The hollow shank taper has some advan­ta­ges espe­cially at high speeds but steep taper tools are still widely used by users which is why the steep taper is still used. At high rota­tio­nal speeds, as is the case, for example, with machine tools for the auto­mo­tive indus­try or HSC appli­ca­ti­ons, you will find hollow shank tapers, almost without exception.

In addi­tion to the tool holder, the tool inter­face of a power­ful motor spindle consists of an auto­ma­tic tool clamp which is suppo­sed to fix the tool to the spindle. The choice is between hydro­me­cha­ni­cal or mecha­ni­cal systems, i.e. systems based on spring force. The robust design of the diaphragm spring tensio­ner is still by far the most frequently used system. The release of the tool is carried out via a hydrau­lic or pneu­ma­tic unclamp unit that pres­ses against the spring force during standstill and thus unclamps the tool. Tool clamps with a gas spring are new to the market but are curr­ently still in the testing stage.

Simi­lar to tool carry­ing spind­les, work­piece carry­ing spind­les also have an inter­face but which is refer­red to as a clam­ping chuck. Howe­ver, the work­piece is rarely chan­ged auto­ma­ti­cally, since the work­pie­ces to be machined usually differ in their outer shape and the asso­cia­ted fixing.

Drive

Acces­si­ble motor spindle for trai­ning purpo­ses. Rotor and stator are clearly visible.

Anot­her essen­tial compo­nent of the motor spindle is the drive in the form of an electric motor. Here, the essence of the motor spindle as drive emer­ges as there is no gear between drive (motor) and output (spindle) for the trans­mis­sion present as it is the case with spind­les with exter­nal drive. The motor design with respect to rota­tion speed and torque must there­fore directly corre­spond to the desi­red requi­re­ments of the spindle.

The maxi­mum power of a motor is directly propor­tio­nal to the stator volume. At the same time, the motor is an inte­gra­ted compo­nent and must there­fore be adap­ted to the spatial, usually very compact, dimen­si­ons of the head­stock. In addi­tion, there is the problem of waste heat which incre­a­ses with the conti­nuous output of the motor and must be dissi­pa­ted by adequate cooling. For these reasons it is very diffi­cult to incre­ase the motor power under the given spatial condi­ti­ons and quickly reaches its limits. Conver­sely, an opti­mum of motor perfor­mance must be deman­ded from the given spatial condi­ti­ons which repres­ents the essen­tial task in the design of the motor. One approach is to incre­ase the quality of the current signal for which the frequency conver­ters is respon­si­ble. By opti­mi­zing the frequency conver­ter, the current should become a sinu­so­idal signal as ideal as possi­ble which incre­a­ses the power dissi­pa­tion and incre­a­ses the conti­nuous power of the motor. Which type of motor is ulti­mately used enti­rely depends on the appli­ca­tion. Synchro­nous motors are mainly suita­ble for spind­les that have to convert high torques at low speeds. A signi­fi­cantly higher torque can be provi­ded with the motor volume and the same current. Anot­her appli­ca­tion for synchro­nous motors can be seen with highly dyna­mic, fast running spind­les, which have to deli­ver low conti­nuous power. The advan­ta­ges of asyn­chro­nous motors espe­cially lie in the area of “stan­dard motor spind­les”, i.e. spind­les for univer­sal centers with speeds of up to 20,000 rpm where rela­tively high torques have to be worked with in the lower area and nevertheless suffi­ci­ent power is also requi­red at high speeds.

Storage

The storage of the shaft also has a signi­fi­cant influ­ence on the vibra­tion beha­vior of the system and must be adap­ted to the requi­re­ments. In spindle construc­tion, angu­lar contact ball bearings, also known as spindle bearings, have almost exclu­si­vely been used. In addi­tion to radial forces, angu­lar contact ball bearings can also accom­mo­date unevenly applied axial forces which occur due to the feed. The high rota­tio­nal speeds of the shaft ensure high centri­fu­gal force loads in the contact ball bearings which is why hybrid contact ball bearings (cera­mic ball, steel rings) are frequently used. Due to the use of cera­mic (sili­con nitride) for the balls the firm­ness can be incre­a­sed while the density can be redu­ced, int turn redu­cing the centri­fu­gal force loads. The angu­lar contact ball bearings are always amoun­ted in pairs. Depen­ding on speed and mecha­ni­cal load, the bearings are paired differ­ently, in the simp­lest case in an O arran­ge­ment. Because of the simple hand­ling, the majo­rity of the spind­les are still grea­sed for life. Non-toxic synthe­tic grea­ses are mostly used, the base oils of which are conti­nuously supplied to the bearing over a very long period of time. For higher speeds, howe­ver, in the last few years, the oil-air lubri­ca­tion has proven to be more suita­ble. An extre­mely small amount of highly viscous oil is perma­nently added to an air stream which trans­ports the oil directly into the bearing. This requi­res an oil supply bore holes in the spindle and an oil-air unit on the machine. Despite the higher costs, oil-air lubri­ca­tion is curr­ently indis­pensable at very high speeds.

Sensor Tech­no­logy

Since modern motor spind­les are used in highly produc­tive machi­nes, any malfunc­tions that may occur must be detec­ted at an early stage and passed on to the machine control system. In addi­tion to the motor tempe­ra­ture, the posi­tion of the tool clamp is also captu­red. The use of control­led motors makes it necessary to capture the rotor posi­tion. In addi­tion to these stan­dard sensors, there are nume­rous opti­ons, ranging from bearing tempe­ra­ture moni­to­ring to record­ing the vibra­tion condi­tion and captu­ring the exact tool position.

Source: German Wiki­pe­dia