Thermal damage or grinding burn is a fundamental issue of all grinding processes. It is usually the limiting factor to grinding operations, hence, controlling heat is important to good grinding practices. The effects of excess heat in a workpiece appear in many different ways ranging from obvious workpiece burn, to changes in hardness, to changes in compressive stresses, to changes in metallurgical structure and composition. Controlling thermal damage can include changes in process parameters such as feed rate, wheel characteristics such as bond structure and type of abrasive grain, dressing techniques, fluid types or fluid delivery. There are many variables that affect the flow of energy that creates undesirable metallurgical changes in the workpiece.
Losses due to thermal damage are very high and have the potential to damage the reputation of grinding as an efficient manufacturing process. It is often caused by cold welding of chips to the surface of the grinding wheel. Grinding wheel makers are therefore called upon to counter this phenomenon. Increase in performance and quality may no longer come from ever more aggressive and harder abrasives, but from methods, which help prevent this cold welding of metal chips to the wheel.
Nature and technology show us various examples of how to prevent dirt and debris adhering to, and building up on surfaces by a controlled alteration of the nano- surface structures. Plant leaves, plate glass on buildings or even toilet bowls, which feature surfaces that prevent any build-up of dirt serve as our guidelines and inspiration for how to modify the surface structures of abrasives, and create partially recrystallised bond systems that stop the cold welding of chips and therefore avoid thermal damage in our processes.
It will be shown that cold welding of metal chips is more damaging to diamond dressing tools than the grinding wheels themselves, and that these newly developed grinding wheels will cause less wear on the diamonds, and therefore reduce the overall costs.
Glass-bonds with nano-surface structures
Bond is the glue that holds a grinding wheel together. A basic rule of grinding states that bonding material does not grind. This means that the more bond in a wheel specification, the higher the danger of grinding burn or micro cracking. The wheel manufacturer must therefore reduce the amount of bonding material without losing sight of edge retention properties, and of course safety. Preventing the cold welding of chips to wheels surface by a controlled modification of the surface structure, takes us much closer to realising the dream of a burn-free grinding wheel.
The 602W bond system consists of partially recrystallised glasses, which are induced by special particles, and also by tightly controlled cooling-off phase in the firing kilns. In comparison to conventional bond systems, the 602W bonds feature higher inherent bond strengths. The fully synthetic composition ensures consistent hardness and structures from one batch to the next. This new bond system allows for a reduction in overall bond material and a simultaneous increase in porosity, without any loss of the wheels strength and without compromising wheel safety.
Additionally, the surface of the grain-bond-system has been modified with self- organizing nano-surface structures, which prevent cold welding chips and prevent any loading with other debris. Consequently, the grinding wheel remains free- cutting, requires less machine spindle power and achieves constant surface finishes without causing any thermal damage to the workpiece.
NanoWin abrasive grains
Not only the bond system but also the abrasive grains must prevent the cold welding of chips. Winterthur Schleiftechnik has developed NanoWin wheels, which integrate these features. A special surface structure prevents liquid steel from sticking to the vitreous ceramic material. Liquid ball bearing steel, which was applied to standard white aluminium oxide, showed a high degree of wettability. In other words, the liquid steel spread across and tried to connect to the base aluminium oxide material. The NanoWin material, on the other hand, acted almost like a non-stick frying pan and rejected any spreading or sticking of the liquid steel.
In order to highlight the differences between NanoWin and white and ceramic aluminium oxides in the grinding process itself, single layer electro-plated grinding wheels were made with these three abrasives. After plunge grinding a given number of workpieces, the surfaces of these grinding wheels were observed under an electronic microscope and checked for metal deposits on the grains. The surface of white aluminium oxide grains became loaded with metal deposits. Under the same grinding conditions, the NanoWin grinding wheel remained free of metal deposits, and the grains cutting edges remained clean and free cutting.
This free-cutting property has been utilized in many applications, and it can be demonstrated that NanoWin uses less grinding energy than an already optimized process with white or ceramic aluminium oxides. Less grinding energy translates into lower risks of thermal damage, and offers the possibility of utilising the available energy reserves by increasing the federates, thereby shortening the cycle times and making the grinding process more economical.
Some case histories from grinding applications have been explained, which are relevant to the argument presented here:
· OD cylindrical grinding
· Creep-feed grinding
· Through-feed centreless grinding
OD cylindrical angle plunge grinding of automotive drive shafts:
The doubling of the dressing cycle was not due to the superior hardness of the NanoWin grinding wheel, but to the fact that no cold welding took place and the grinding wheel remained free cutting. It only had to be dressed because of loss of form and not because of wheel loading with chips.
Creep-feed grinding of fir tree profiles on turbine blades:
Be it gas turbine blades or jet engine blades, thermal damage would lead to fatal consequences. The following case history shows the successful transition from white aluminium oxide (WA) to NanoWin grinding wheels to ensure an even lower risk of thermal damage. Not only was the thermal risk reduced, but also could the feed rates be increased and the in-feeds of diamond rolls reduced.
After switching to NanoWin, no further rates, but also through shorter cycle occurrence of thermal damage was noted. The overall process economy was improved not only due to reduced reject rates, but also through shorter cycle times and higher process stability.
Through-feed centreless grinding of shock absorber shafts:
To counteract severe pricing pressure and to maintain his profit margin, a manufacturer of shock absorbers had to improve his production output significantly. The resin bonded centreless wheel in use had only to be dressed after the first mounting. Thereafter, the process ran in sell sharpening mode without requiring further wheel dressing. Increasing the through-feed rates and have a longer wheel life could save time and money. This case history describes the first rough grinding process. The wheel had to be capable of grinding both soft and hard materials. Wheel dimensions: 610 x 508 x 304.8 mm Material: Carbon steel, soft and induction hardened
Workpiece dimensions: diameter 12 to 23 mm, lengths up to 450 mm
Lifetime of dressing tools
Dressing tools are an important cost factor within the grinding process. It is well known that ceramic abrasives cause more wear on diamond dressing tools than standard abrasives such as WA. On the other hand, ceramic grinding wheels produce more components per dressing cycle, and this has to be factored into the overall economics of the process. However, in the case of NanoWin grinding wheels there was an additional question, which had to be addressed: Is there a difference between dressing clean, unused grinding wheels and grinding wheels loaded with metal chips?
A series of tests was set up, for which clean and loaded grinding wheels were dressed with fixed MCD (mono-crystalline diamond) dressing tools. It was clearly established that loaded grinding wheels caused more wear on the dressing tools that clean grinding wheels. It also demonstrated that NanoWin wheels caused 20 per cent less wear than ceramic abrasives.
From the wheel makers point of view, the use of grinding tools, which significantly prevent the cold welding of chips, reduces grinding costs across the board. These tools make the grinding process more controllable, and they rule out the risk of thermal damage or grinding burn. In the future- the grinding process will continue to play a dominant and interesting role.