Characteristics of the polishing process
Following the grinding process, a rubber covered roller is sometimes, but not necessarily always, polished.
It would appear that polishing is a roller treatment used to enhance the product’s quality in certain circumstances or due to specific demands.
In an ideal world it would be possible to check if the desired surface quality has been achieved after grinding and then take a decision on whether or not to polish.
Unfortunately manufacturing processes have to schedule work and plan in advance, which rollers require polishing. The production planning people know both the possibilities and restrictions of the grinding process and can anticipate the need for polishing. But – and here we come close to the central issue of polishing – polishing can be used for different purposes: natural ones and unnatural ones.
For the sake of clarity, we’ve taken the liberty of judging what is ‘natural’ and what is ‘unnatural’.
Grinding – no matter how good or bad it has been done – provides a defined surface; rough or shining, smooth, even, homogenous etc.. This surface may have been achieved by one grinding process, by pre-grinding and final grinding or by a more thorough three-step process. Whatever the process the outcome is a ground roller and additional grinding could not enhance the surface quality.
This does not mean that a better result couldn’t be achieved, just that under the given circumstances the best result has been achieved.
When a roller, ground in a factory with the highest accuracy possible, is not good enough for the customer requirements, people sometimes have the feeling that better grinding would have been possible elsewhere and look for a means to improve the grinding result – i.e. by polishing!
With this thinking we expect to compensate our own lack of grinding professionalism by a surrogate treatment. If you are supposed to be able to achieve 80% of the theoretically attainable result with grinding, these people hope to come to 90% of it by “fake-grinding” e.g. polishing.
To polish a rubber roller with the intention to improve the result of a previous grinding; to add an additional ‘special-grinding process step’ as compensation of a lack of grinding skills and poor machinery is, what we define to be ‘unnatural polishing’.
In carrying out such practices, you suppress the truth about your grinding facilities and you will never take steps to improve them. It’s worth bearing in mind at this point that polishing can be more expensive than grinding.
Firstly polishing as a medicine or cure for second rate grinding takes much more time than improved grinding and thus is normally more expensive. Secondly polishing takes away only a little of the material and cannot correct the mistakes of the previous grinding. Thirdly polishing changes the characteristics of the surface and this is often not expected.
The inversion of the above explanation says that ‘natural polishing’ starts where grinding has ended. It never tries to improve the grinding result, no matter how poor it is, polishing is not a grinding repair service.
In our view, ‘natural polishing’ is a surface treatment that takes off no substantial material but changes the characteristics of the surface, for example, its capability to transport certain liquids.
This changing of the surface is the basic element of polishing and it expresses the difference to grinding. Any amount of additional grinding will not change the surface’s characteristics, but polishing will – immediately.
So in the rubber roller manufacturing process polishing is required to provide certain features to the roller that cannot be obtained by grinding.
In our experience the cases where polishing inevitably is needed are the exception to the rule. Dampening rollers for offset presses, coating rollers in the steel industry and others, glue applying rollers in the furniture industry and more.
We do not deny the need for polishing in the right circumstances; but would like to make you aware that most rubber covered rollers do not need the characteristics provided by polishing and that many rollers are polished so that they ‘look better’.
Please try to polish rollers for natural reasons only and have a sharp and unbiased look at your grinding potential.
Remember, always be aware of the outcome of polishing and try to reduce it to the cases where it is really needed.
Sometimes it seems to be a question of religion: hand-polishing or automatic belt-polishing.
Apparently the conviction for a certain method is related to the skills of the working people; the more well trained and experienced people are available, the more grows the conviction that only hand-polishing provides the required results.
On the other hand, polishing is a physically demanding job and workers often get pain in their shoulders and backs. Polishing also takes a lot of time and so it is quite natural to look for mechanical alternatives like polishing belts on conventional lathes.
It is not easy to work with mechanically driven polishing belts but it is possible. The polishing device gets fixed on a support of the grinding machine, the tension of the belt is mostly adjustable pneumatically and the traveling speed variable. The technical result attainable with automatic polishing can be predicted – to a certain degree.
And here we have the crucial problem of any mechanisation : a machine cannot usually react to individual situations and product demands. As long as the act of polishing is performed on an evenly ground roller, the result will be sufficient. But whenever die conditions at the start of the polishing process vary, the mechanical polishing quickly reaches its limits.
Unfortunately there are lots of varying influences: the compound, the curing process, the grinding process, the time between curing and grinding, rubber hardness, air humidity and others. Thus the predictable result of polishing is rarely related to an exact working pattern. The result varies according to the roller conditions when the method of polishing applied is kept constant.
We know that polishing is a sophisticated process and that the worker must react on the individual situation to obtain a superior result. An automatic polishing process cannot apply this thought and can therefore only provide average results.
Whether or not they are sufficient, from the point of view of the application of the roller, is up to the roller manufacturing manager.
Assuming that sometimes it is sufficient and sometimes it is not, we can say that every high quality roller manufacturer can utilise a mechanical polishing work station, which is supplemented by manual polishing.
After the grinding process, the roller normally remains in the machine for inspection purposes and sometimes for cleaning the ends of the
cores, colouring the cores, re-greasing the bearing seats and so on. It would make sense therefore, to polish the roller in this machine.
But this behaviour shows a typical attitude of rubber roller manufacturers: capacity is not an issue, capacity is always available. To occupy an expensive machine with cheap hand work (like polishing) does not bother the average manager in this industry. Although we would argue that it should.
Machine time is expensive and will be still more expensive in the future when rubber rollers get produced more and more on numerically controlled machines and on machines able to keep tight tolerances.
But it is not only the costs of the machines. Looking closely at the grinding and polishing process you will sometimes discover a relatively unprofessional work flow.
The worker puts the roller into the machine for grinding and then watches the machine working – or operates a second one in the meantime. When the roller has been ground to the end the operator returns to the machine, takes a polishing paper and finishes the roller – or he starts the polishing unit that is fixed to the same machine.
By the way this part of the production process often consumes more personal time than the grinding process itself. Serving two machines necessarily ends in a situation where either the man or the machine is waiting for the other; so productivity declines remarkably with every multi-machine regime.
We can take it for granted that mostly the operator will be waiting for the machine(s) to finish their job. Here we meet a situation that is far from acceptable. But what is the solution?
When we analyse the operator’s time consumption and the machine runtime, we often find an interesting fact: putting an average roller into a machine is perfected within one or two minutes. The grinding time for a roller of 1000 mm width is about 10 to 15 minutes. The manual polishing including cleaning and other finishing work takes another 10 minutes.
Mechanical polishing of a 1000 mm wide roller is also achieved within approximately 10 to 15 minutes.
The first reaction on the fact that two succeeding working steps have the same time consumption but differing skills and characteristics should be to disconnect them and organise parallel production. An additional argument to this decision could be the fact that only part of the rollers have to be polished.
As soon as we mentally cut off polishing from grinding we become aware of a whole host of alternative options.
Firstly we gain flexibility: we are no longer obliged to polish the roller immediately after grinding: now it can be polished whenever we have the time. Secondly, we see the possibility to use a specialised machine for polishing that is far simpler than the expensive grinding machine.
Thirdly, we save time by setting in place an optimised work order for the operators: while the grinding of a roller takes place, the previous one can be polished easily without additional worker’s time consumption.
Fourthly, polishing is perceived as a specific production step and is no longer the final grinding or ‘unnatural polishing’. Finally the use of the grinding machines can be remarkably increased and thus both capacity and cost situation improve.
It’s worth looking at polishing for several reasons. At first we saw the necessity to define polishing compared to grinding and to acknowledge that polishing is not ‘power grinding’ or a remedy for poor grinding facilities. Rather it is a means to change the characteristics of the roller surface according to specific requirements.
It’s important to look at polishing in this particular way to recognise this as a unique process step and not one that is automatically added to the grinding of the rubber roller.
Polishing can be effected manually or automatically. The latter will always be able to provide an average result and if this is sufficient, automatic polishing is a suitable and cost effective procedure to achieve the surface required.
If your expectations regarding the roller surface are above average and if you need a polishing which reacts sensibly to the given situation, then you must return to manual polishing. Only a skilled worker with sensible fingers, trained eyes and ‘a feeling for rollers’ will be able to provide a supreme result with any roller.
Both ways of polishing should be practised in a specific polishing machine. The abuse of expensive grinding equipment for the completely different process of polishing is neither cost effective nor efficient regarding the work flow.
By using a simple polishing lathe besides your grinding machine, you reduce costs, grow the grinding capacity, gain flexibility and even improve the quality of the polishing process itself.
Our discussion on polishing is true under the assumption that the rollers in question are small or medium sized. Once your rollers are wider than 3000mm and above 400mm in diameter, polishing has to be done in the grinding machine with all the restrictions mentioned above. But these rollers are not the norm and most rubber roller manufacturers will benefit from the advantages of a separate polishing process.
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