Abrasive grinding

TECH INSIGHT

Studer

G Venkatesh and Goh Tz’en Long take a look at some of the latest CNC grinding technologies and software driving the development of a key metalworking process.

Getting Into The

26 equipment news September 2004

Rollomatic

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rinding is an important metal-removal process for the finishing of machined parts and subassemblies and is one of the final processes in the metalworking process chain. It imbues the machined part with the desired surface finish and dimensional tolerances. Aberrations and irregularities introduced by flaws in the metal removal sub-processes upstream can be eliminated during grinding. It is a value-adding process that can give the manufacturer an edge over its competitors in the market place.

While the grinding process has been traditionally considered to be more of an art than a science, modern multi-axis CNC grinding machines are giving the machinist an unparalleled control over the grinding process. Innovations in abrasive materials used in grinding wheels and in-process control technology allow very fi ne fi nishes with sub-micron tolerance levels to be achieved with great consistency and minimal variability.

Software is increasingly becoming an important factor in modern CNC grinding machines where the performance of the grinding operation relies less upon

The grinding process can be carried out with great precision by specialised CNC grinding machines.

TECHINSIGHT

Thread grinding operations can also be carried out on a cylindrical grinding machine.

Cylindrical Grinding Cylindrical grinding is a finishing process whereby a workpiece which is symmetrical about its axis, such as a shaft, supported by centres or Studer

The internal grinding spindle and wheel allows internal surfaces to be ground.

The universal workhead is available for universal live grinding operations and grinding between centres.

a chuck, is rotated against a grinding wheel which acts as an abrasive cutting tool. The grinding wheel can be fed in a radial direction, as in plunge grinding or along the axis of the shaft.

The basic cylindrical grinding process can be classified into inner diameter (ID), outer diameter (OD) or centreless grinding. ID grinding is a process whereby the internal surface of a workpiece is being ground while

Studer

in OD grinding, a grinding wheel is brought against the external surfaces of a workpiece in order to achieve the required finish.

Cylindrical grinding carried out on high speed CNC grinding m a c h i n e s , a l l ow s e xce p t io n a l roundness and surface fi nish to be achieved at fast material removal rates. ‘Universal’ CNC grinding machines allows processes such a s I D, OD, tapered a nd thre ad grinding to be performed on a Studer

the dexterity and skillsets of the machinist than on the features, functionalities and user-friendliness of the control software and humanmachine interface. A well designed u se r- i nte r face a nd fe at u re s enable setting-up, programming, process monitoring and control to be easily and efficiently carried out . For e x a mple , S t u de r ha s designed software where grinding processes can be freely defined and easily programmed with icons or pictograms representing CNC micro-functions. The StuderGRIND software allows online and offl ine programming of grinding machines and can be run on PCs and Windowsbased machine control units.

single machine. A n exa mple is the Studer S33 universal grinding machine which has options for a universal workhead or an external workhead. The universal workhead allows for external and internal grinding with an external grinding wheel on the right and an internal grinding spindle while the external wheelhead, solely for OD grinding, can be swivelled on 0˚ and 30˚ end stops.

• Centreless Grinding Centreless grinding is an OD grinding process where the workpiece is not held between centres or chucks and does not need to be driven against the grinding wheel by a motor. The workpiece is instead supported by a blade and constrained between the larger grinding wheel and a smaller regulating wheel. The unique feature of the centreless grinding process is its ability to restore roundness to out-of-round workpiece conditions.

I n t h rou g h - fe e d ce nt re le s s grinding, the regulating wheel is inclined at an angle relative to the axis of the grindng wheel, imparting a horizontal velocity component to the workpiece which drives it in the axial direction. This process is useful for the rapid processing and feeding of parts in large volume production.

• Single Point OD Grinding Single point OD grinding is a n i n n ov a t i v e p ro ce s s b a s e d o n the principle of a CNC turning operation. Junker’s Quickpoint CNC grinding machines uses this process whereby a CBN or diamond superabrasive wheel only a few millimetres wide is tilted to the workpiece axis which effectively reduces the contact between the wheel and the workpiece from a line (as in conventional grinding) to a point. The process is flexible enough to allow operations such as thread grinding, profile grinding a nd plunging operations to be performed on a single machine.

Electrochemical Grinding The mechanism of electrochemical grinding (ECG) can be explained a s follows. The wheel a nd the workpiece (assume that it is made of steel), serve as the electrodes of an electrochemical cell, with an electrolyte separating them by equipment news September 2004 27

to other ECG machines in close vicinity

TECHINSIGHT

DC Supply

GRINDING WHEEL Automated valving arrangement

PUMP NOZ

ZLE

Fe (OH)2 + Fe (OH)3

SUMP OF SODIUM HYDROXIDE

gap of 0.025 mm

SODIUM ATOMS

washed away with Na atoms

STEEL WORKPIECE

FILTER

Figure 1: Illustration of the molecular decomposition process (electrochemical grinding).

• Molecular Decomposition Process T he mole cu la r de comp o sit ion process (MDP) is a variant of the electrochemical grinding process developed by Chevalier of the US. Refer to Figure 1 for an illustration of the MDP process. In MDP, (taking sodium hydroxide as the electrolyte), the used electrolyte, which consists of iron hydroxide (divalent and trivalent), deposited sodium metal ‘pulled out’ of the wheel surface by the electrolyte leaving the wheel28 equipment news September 2004

workpiece interface, water, and a small amount of undissociated sodium hydroxide, is led to a filter, where non-reactive sludge and abraded material is filtered out.

Sodium metal reacts with some Fe(OH) 2 , which is more reactive than the trivalent hydroxide, and forms sodium hydroxide. The iron and the sludge are compressed to a cake, and disposed off, while the sodium hydroxide which has been salvaged is recycled back for use as electrolyte. While disposal is made safer than before, over a period of time, the net consumption of

electrolyte decreases drastically which results in cost savings.

In fact, if there are many ECG units on the same shop floor, a central sludge disposal, filtration a n d p u m p i n g s y s te m c a n b e installed with automated valving arrangements to direct the filtered (and partly topped up) electrolyte only to those machines which are being worked.

Pre-Grinding Setup The pre-grinding set up process can be a time consuming activity and can negatively influence the

Studer

about 0.025 mm. The electrolyte is pumped from a tank and directed through a nozzle to the space between the wheel face and the workpiece sur face, to act a s a medium for the flow of ions and electrons, and carry away the heat and the ground matter from the surface. The electrolyte could be sodium chloride, potassium nitrate, sodium nitrate or sodium hydroxide etc – in a nutshell, salts (or alkalis) which dissociate almost completely, into t he comp one nt ion s, a nd whose pH remains almost constant in solution.

used electrolyte from other ECG machines

Active-lengthpositioning probes are used in the set up process.

TECHINSIGHT overall efficiency of the grinding operation. Grinding machine makers have developed innovative ways to optimise the set-up process and minimise set-up times.

Studer has developed a solution in which intelligent software is used in combination with activelength-positioning probes on the wheelhead to measure the grinding wheels directly on the machine. The operator is aided by a user-friendly interface which simplifies the entire set-up process.

Setting up for an ID grinding operation such a s inter na l threading and bore grinding can be particularly challenging due to lack of visibility on the part of the operator. In this area, acoustic sensors have been used to detect whe el - to - work pie ce contact.

Drake Manufacturing, USA, has

de ve lop e d t he S m a r t S p i nd le which is based on the acoustic emission technology whereby an acoustic sensor mounted in the wheel spindle senses wheel contact to within 0.5 mm. The acoustic gap elimination system minimises time spent grinding air and detects the workpiece faster.

Process Monitoring And Control W heel ba la ncing is integ ra l to a successful grinding operation especially in processes such as OD plunge grinding. An unbalanced g rinding wheel ca n result in chatter, waviness and out-of-round conditions in the workpiece. Wheel balancing systems provide real-time balancing compensation.

M a r p o s s ha s develop e d a n external mounted flange balancing

head as well as a spindle type balancing head, which allows the g r inding whe el to b e cha nge d without removing the balancer. The balancer consists of two masses rotat i n g a lon g t he sa me a x i s, acting as counterweights which compensate for a n unbala nced condition. An acoustic sensor is also integrated into the system wh ic h a l l ow s ga p e l i m i n at io n between the grinding wheel and the workpiece or the dressing wheel as well as crash detection.

A process control device such as the P7 from Marposs allows the workpiece to be continuously me a su re d du r ing t he g r inding p r o c e s s . A d j u s t m e n t s to t h e machine cycle are made based o n t he a mou nt o f mate r ia l to be removed and feed rates are optimised to ensure that surface

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igh-speed grinding is characterised by high material removal rates at high cutting speeds (Vc > 80 m/s) and over-proportional feed rates. A feature of high-speed grinding with CBN ( cubic boron nitride) wheels is the use of grinding oils. High lubricity of the oils reduces friction between the workpiece and the cutting edges of the abrasive particles and this reduces the amount of heat generated. This in turn, eliminates thermal problems such as grinding cracks, tempering and annealing.

As water-miscible cutting fluids are only partially suitable because of foaming problems, particularly at high pressures, new application potential is offered by ester-based products which may also be self-emulsifiable or

the future – more and more based on unconventional fluids like esters and polyalphaolefins will gain increasing importance. These products show in grinding applications superior performance regarding foam behaviour and air release properties. Special selected ester provide for excellent detergency and dispersancy which will permit increasing feed in grinding operations. Higher flashpoint, lower emission ratio and good skin compatibility are additional advantages under work safety considerations.

The use of unconventional cutting fluids, and these include esters, will steadily gain importance in line with progress made in machining technology.

One important characteristic of these products is the wetting property which has been compared for several base

by fully synthetic products containing low foaming antiwear additives and lubricity improvers.

Neat oils formulated with hydrocracked base stocks and – in

fluids in the same viscosit y grade.

Ester shows clearly the best results, followed by solvent-refined base stocks, mineral seal and polyalphaolefins. From this experience, it can be seen that

FUCHS PETROLUB AG

Innovative Coolant Concepts For Grinding

Unconventional fluids like esters will gain increasing importance.

polyalphaolefins should be mixed with a specific amount of ester to provide the necessary wetting characteristics for the final grinding fluid.

The need for these new unc onve ntiona l cut ting f luid s is primarily technical. More favourable characteristics compared to solvent cuts and hydrocracked products allow shorter cycle times, ie: larger material removal rates and thus an increase in productivity.

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TECHINSIGHT finishes and dimensional tolerances are within specification.

Adva nced electrochemica l grinding systems employ closed loop process control software, which monitor electroly te proper ties, spindle performa nce, wheel position, power output, and material decomposition rate, and modulates the flow of current in the circuit to enable precise metal removal. Thus, ‘over grinding’ and ‘under grinding’ are slowly becoming problems of

the past. Further, when single point cutting tools are to be re-sharpened, precision takes precedence over speed, and in this context, control software are indispensable.

CNC Grinding Modern grinding processes are aided in no small measure by CNC grinding machines. Grinding of cylindrica l workpieces such a s shafts, spindles, tool and cutters are greatly simplified and able to realise

the performance efficiencies gained from the software and hardware technology enhancements made to these machines. Grinding has become a highly precise and repeatable metal-removal and finishing operation that is economica lly viable as compared to other finishing processes for tool manufacturing or re g r i nd i n g op e rat ion s, a nd manufacturers are able to realise cost savings by investing in CNC grinding machines. MEN

Abrasive

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K. Jung

Talk

Ted Giese, Director of the Abrasive Engineering Society (USA) talks to MEN about abrasive materials used in grinding wheels.

If a pie chart is drawn to depict the usage of different abrasives for grinding, how would it look?

I would only be able to speak for North America in this regard. Nearly 99 percent of abrasives used in grinding wheels are synthetic, with aluminium oxide forming the largest chunk of the pie followed closely by silicon carbide. Synthetic diamond and cubic boron nitride come in third and fourth respectively. Zirconia, which is used in combination with other abrasives (mostly alumina) accounts for a small portion of the pie. The natural abrasives are generally used for fine finishing and polishing operations. Natural diamonds account for a very small portion of the abrasives pie, with over 95 percent of the diamonds used in grinding wheels being synthetic.

long life. Used in the correct ratio with other abrasive materials and bonds, the abrasive is particularly suited for organic and vitrified grinding wheels when high stock removal rates are required.

Other new abrasives in the market are blends of aluminium oxide and zirconia or aluminium oxide and sol gel (the ceramic abrasive that is a microcrystalline form of aluminium oxide).

Any new abrasives on the anvil in 2004?

There are a number of abrasives that are refinements of existing forms of aluminium oxide, cubic boron nitride and diamond. Typical improvements in these abrasives are new coatings, and surface treatments that improve bonding in wheels. In terms of abrasives that are significantly different, only the product ABRAL marketed by Pechiney Electrometallurgie, Abrasives & Refractories, is a truly new abrasive material, which is being used in Europe. It is priced between ceramic (sol gel) abrasives and cubic boron nitride. This product is a solid solution of alumina and aluminium nitride characterized by high grinding performance and a

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Is there a possibility of a greater demand for the zirconia-alumina blend in the years to come?

Zirconia-alumina blends are still essentially minerals used in coated abrasives. There may be a few companies experimenting with this blend for grinding wheels, but there isn’t any trend towards greater demand for this combination. There could be some technical problems that hinder its use in grinding wheels.

TECHINSIGHT

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FUCHS PETROLUB AG

Tool And Cutter Grinders hen cutting tools with complex geometries are required for special CNC machining applications and cannot be bought ‘off-theshelf’ or found on standard tooling catalogues, they are usually custom designed and manufactured in-house or outsourced to cutting tool manufacturers. In either case, CNC cutter grinding machines are proving to be indispensable in realising the complex tool geometries, which can be machined from blanks, or simply for the regrinding of worn out tools from materials such as tungsten carbide, polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or HSS tools. All types of endmills, reamers, drills and step and form tools can be manufactured with relative ease and efficiency.

Software is an integral part of modern tool and cutter design and fabrication. CNC cutter grinder manufacturers such as ANCA, Schneeberger, Walter and Rollomatic have developed tool design software such as 3D Cimulator, Quinto NT, Helitronic Tool Studio and VirtualGrind

r e s p e c t i v e l y, t o w o r k with their cutter grinding machines. The tool design software enables offline programming and design of the cutting tool geometries base d on finishe d par t r e q u i r e m e n t s . I n m a ny cases, a description of the finished part in a CAD file format such as .dxf or .dwg is all that is required to generate a 3D image of the tool which can be viewed in various perspectives.

The software simulates the grinding process at the tool programming stage and ensures that the visual image of the tool generated is based on NC data, which can be recalculated as the parameters are changed.

Knowledge bases founded on comprehensive data gathered on tool geometries allow even complex tool geometries to be defined with relative ease. Programming of the CNC cutter grinders is carried out within user-friendly, menu-driven, Windows-based interfaces where defining a new cutter from a database of generic cutter types is simply a matter of picking the right icons.

Fast Cycle Times & High Accuracy

Enhancements to the CNC include a read-write CD and read DVD for software updates and backups. A UPS maintains power to the CNC during a power failure and initiates a controlled shut down sequence which avoids the likelihood of file corruption.

The RX7 has a single direct drive multi-directional spindle with dual wheel packs positioned at either end of the spindle which give unrestricted access to the workpiece. The spindle has a peak power of 19 kW and maximum speed of 12,000 rpm. It has more power at low speed with its highest power at 3600 RPM, which is very well suited for carbide grinding.

The RX7 CNC Tool and Cutter Grinder from ANCA is designed for fast cycle times and high accuracy of surface finish. Firstly, all axes on the machine use ANCA direct drive technology.

With both motor and ballscrew coupled directly in line together, backlash and loss of position is eliminated.

Secondly, the new ANCA 5DX CNC and Digital Servo Drives increases the speed of the 5DX resulting The RX7 has a single direct drive in faster positioning multi directional spindle with dual feedback giving wheel packs positioned at either greater positional end of the spindle. accuracy and increasing dynamic performance of the machine.

The ANCA 5DX supports a touch screen display and a USB port has also been added allowing any USB device to be be plugged into it such as a memory stick or disk drive.

Enquiry No. 6104 Linear Motors On All Five Axes The Helitronic Vision CNC grinding machine from Walter is designed for rotationally symmetrical carbide or HSS tools ranging from 3 – 320 mm in diameter – and all with increased levels of accuracy and improved surface qualities. All its five CNC axes – both rotary axes C and A and also linear axes X, Y and Z – are fitted with direct drive systems.

The linear motors permit traversing speeds of up to 50 m/min and high positioning accuracy. A patented gantry design combined with the use of mineral casting for the bed

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