High stock removal from metal plates using wide belt grinders
There are a number of well-established techniques for material removal from plane surfaces. Each has it's own merits, depending on the required productivity, surface finish, amount of material to be removed, the material in question, etc. This paper looks at recent developments in the technique of wide belt grinding, which has opened up new possibilities for high stock removal from hard materials such as titanium, nickel alloys, molybdenum, etc.

High Stock Remover

Background

Within the field of engineering there are already a number of well-established technologies for the processing of material from plane surfaces. Such techniques include milling, broaching, honong, lapping, wide belt grinding, horizontal spindle grinding and the use of abbrasive jets. With each technique offering it's own merits, the final selection will depend on the nature of the material to be processed as well as customer demands such as the required surface integrity, the material tolerances, the cost per part, productivity, etc. This research focuses on the technologies available for removing a substantial amount of material from plane surfaces. The research was initiated after the identification and analysis of a crucial problem in stock removal applications, i.e. heating of the work material. In the field of stock removal, increased temperature of the work material creates many problems and with that, come limitations. Conventional methods like milling and conventional wheel grinding generate too much heat (locally). These technologies especially find it difficult to keep up with the evolution in modern ('new generation') materials, and therefore often called 'difficult to machine' materials. These materials require a cool cutting process.

Following a specific request from a client, research work was conducted to determine how the wide belt grinding process could be further optimised for high stock removal from plates made of titanium alloys and other special metals. The research work focussed on a comparison of the three most commonly used processes for high stock removal: milling, vertical spindle grinding with grinding segments and belt grinding. Close co-operation was also sought with clients and belt supplier Hermes Schliefmittel of Germany. Then five separate areas where identified for attention: surface quality, tolerances, productivity, specific energy and costs. These will be addressed below.

Surface quality

Surface integrities were evaluated for roughness, metallurgical damage, micro crack and residual stresses and finally given a visual appraisal. In this area belt grinding gave the most wide-ranging results on both surface roughness and cosmetic finishes.

Tolerances

Key issues here include flatness, thickness and parallelism. In this area, wheel grinding can be said to give the best results, followed by belt grinding. Milling is the least suitable process when rated according to tolerances.

Productivity

Productivity was found to be of crucial importance in the market of stock removal. Productivity was measured as the volume of work material removed in a minute time (Q) (cm3/min). Research showed that both mechanical and physical properties are important. In general terms, the machinability of aluminium and cast iron is limited by the clearance of chips whilst the machinability of stainless steel, nickel alloys and titianium is limited by heat generation. The belt grinding process was seen to have on average three times higher productivity than wheel grinding. The belt grinding stock removal rates compete directly with the removal rates of milling.

Costs

Costs were seen as major driver in the decision process of the user of stock removal technologies. An evaluation was performed according to cost factors such as the tool costs, labour costs and energy concumption, as well as machinery deprecation costs. To compare in a realistic way, the costs were calculated per removed cubic centimetre. Whilst milling came out as having the lowest costs per cubic centimetre, belt grinding closely followed it. The costs of segment wheel grinding where explored to be at least 20% higher than milling and belt grinding.

fig 2

Specific energy

Specific energy is the energy that is required to cut a unit volume of work material (e). Normally, a low specific energy level is preferred, as this means there will be less heating of the material (heating can cause unwanted effects such as metallurgical damage and residual stresses) and also lower cutting forces. That equates to lower energy consumption, lower tool wear rates and superior tolerances.
Although much literature was available for milling and wheel grinding no references could be made to the specific energy of belt grinding. To ensure a fair comparison was made, teste were therefore compared on all three types of machine equipment, using materials of standard size (1 metre wide by 2 metres long). For all materials, it was seen that the milling process has the lowest specific energy, but is closely followed by the belt grinding process - see figure 1. Wheel grinding, however, shows a much higher specific energy - in fact, almost six times higher than belt grinding for materials such as stainless steel, nickel alloys and titanium.
Although, as has been said, each process has it's own merits, an overall analysis (see figure 2) indicates that belt grinding gives superior performance for hard materials such as titanium and nickel alloys.

Technological processes

To explore further, attention was paid to the technological processes underpinning belt grinding. Special attention was given to specific energy. Specific energy was seen to be the most crucial factor in the comparison between the technologies. Tolerances, productivity, surface integrity and costs are all directly related to specific energy. The key variables are therefore the machine tool and the grinding belt.

Grinding belts

Early on, it was observed that the belt grinding process is both efficient and a 'cool' technique. In other words, the work material does not become as hot as when using certain other machine systems. Attention turned to the abrasive grains on the belt itself, which were seen to be very regularly aligned. The belt manufacturer is able to use an electrostatic charge; it is possible to control both the orientation and the distribution of the grains. this means the sharp edges are perpendicular to the belt.
Moreover, the microstructure of the grains means that when they do break, only a small portion is lost and the preferred cutting angle is maintained for superior, long-lasting cutting action. In contrast it was observed that the grains on a grinding wheel are randomly oriented, which contributes to the generation of heat in the work material. The energy required to cut a unit volume material is theorerically determined by the material properties and the machine process. The actual amount of energy used by a technique is lergely consumed by friction of the machining process. When the friction energy is low, the specific energy is low. The amount of friction energy ofbelt grinding is lower than wheel grinding.

Whilst considering the specific energy (cutting energy), the following four issues were seen to influence the amount of friction energy:

• Cutting angle - As the grain alignment can be controlled, the cutting angle can be optimised to ensure material is removed at lower consumption
• Chip thickness - Larger chips require less energy for removal [Vieregg]. Thanks to the high possitive angles milling machines remove the largest chips, whilst wheel grinders remove much smaller chips [factor 10.000 difference] and therefore consumes more power. As belt grinders tend to remove medium sized chips they benefit from modest power consumption.
• Chip chamber - Refers to the 'empty space' surrounding each chip, and is an indication of how easily machined chips can be removed (chips which become stuck in the work area will lead to an increase in specific energy and a decrease in surface integrity). The chip chamber is a crucial for high stock removal. Thanks to longitudinal alignmenr plus the spatial arrangement of the grains, belt grinders have optimum chip chambers, helping to avoid heat build-up in the workpiece.
• Contact length - Defines how long each grain remains in contact with the surface to be machined. Small contact lengths are preferred, partly to ensure efficient chip removal and partly to facilitate the efficient use of cooling emulsions. The belt grinding process has very low contact lengths, of the order of one millimetre. The contact lengths in for example vertical spindle stone grinding are in the order of metres.

fig 3

The machine tool

The high stock removal process has also been enhanced thanks to innovations made on the machinery itself. For example, by modifying the contact drum dimension and materials, contact lengths have again been minimised. Clamping methods for folding the work piece to provide proper transport. Thorough changes have also been made to the feed system

Conclusion

Following advances in the design of belt grinding machinery as well as in the production of the belts themselves, belt grinders are now a viable, economical alternative for the high stock removal of material from for example titanium, nickel alloy and molybdenum plates. The technology has already been installed at a number of leading suppliers and is understood to exceed all expectations.