For an amateur, whose production of optical elements is generally limited, the benefits he can get from a machine is somewhat moderate when compared to the needs of the industrial world. Yet, there are indeed some advantages although their impact may be different according to the stage the project is at.
Using a machine will significantly reduce the time spent over the fine grinding period because it will increase the efficency of abrasive powders by speeding up the movements. This will be of less importance to the amateur who is not so eager to gain time.
During the polishing period though, a machine will offer numerous benefits, for larger mirrors especially :
- Reduced efforts : Because the lap sticks more to the mirror surface the efforts one must make are significantly increased. A machine will relieve the efforts of the maker. This is a major benefit as it will allow longer polishing sequences, without stopping, which is a factor of efficiency.
- Increased efficiency : the speed of the movements (lap moves and mirror rotation) as well as the energy developped by the machines allow a better efficiency than during handwork. Associated to the previous advantage this contributes to the reduction of the time needed to produce a mirror. Considering you cannot expect the pitch to retain its qualities for long, the time factor cannot be overlooked for an amateur who can’t dedicate much time to polishing over a given period.
- Surface quality : Experience tends to prove this. The surface of a machine-polished mirror, under normal working conditions, is often of good quality : absence of microroughness and limited dog-biscuits. This may be due to the smoothness of the movements produced by a machine (regulated speed, regular movements and rounded trajectories).
- Thermic smoothness : The heat generated by the hands on the mirror during polishing creates differential dilatations that may induce shape variations and, as a consequence, difficulties in controling the shape. A machine will therefore eliminate such effects.
- Control of pressures : A machine allows continuous pressure of the lap on the mirror, leading to more homogenous shapes, less liable to the variations of hand-made work, especially when using small laps. It is also possible to increase calculated pressures by adding the weights corresponding to the projected results.
To be perfectly honest, one must also identify some drawbacks linked to machine-polishing :
- Zonal defaults : Due to its very principle a machine will reproduce periodical movements.
Consequently, some zones will be more frequently covered than others, leading to zonal faults. This is a major problem, often cited by those who praise the virtues of handwork. Further on we’ll see how these faults can be “smoothed away” when the designing of the machines and the techniques of using them are adapted. Then these zonal faults may be largely reduced (at least in a proportion comparable to hand-polished mirrors) if not completely eliminated.
- Costs : Although these machines do not generally require expensive parts, building one is evidently a source of extra expense that will be hard to compensate for if you consider grinding only one mirror. One way of limiting the sum will be to use discarded elements collected here and there.
- Extra building time : Building a machine will evidently lead to a delay in the timing of the project. Yet this delay may eventually be compensated by an increased efficiency during the polishing process. Moreover, very acceptable results may be achieved through the use of very simple machines that will be quickly built.
- Need for space : The dimensions of most machines may be an obstacle for many amateurs if they lack space. In order to limit this problem, some parts of the machines may be dismantled.
- Lack of sensitivity : During hand-work one feels the smoothness of the lap-mirror contact (hard spots, slips…) which enables the operator to adapt his action (speed, pressure…) so as not to generate an excess of dog-biscuit. If need be, he can even decide a new lap must be made. When working with a machine, one loses such contact through the senses. And, according to Bernhard Schmidt, it is the main reproach one can make to this method : “If your hand feels a resistance, you must immediately stop work until the temperature drops back to normal, but your machine cannot react this way : it will go on polishing, increasing the temperature at the point of friction ; with a consequent magnification of the fault.” Thus, even when using a machine, it is advisable to do some hand strokes, now and then, in order to verify the good running of the work.
Employing machines to give the final shape of a mirror (figuring and correcting) is less popular. Yet this seems to have remarkable advantages, if we are to believe the testimonies of some amateurs or mirror makers :
- Controlled effects : The very fact that machines reproduce the same movements easily (length, speed…) make it possible to introduce a certain degree of control during the actions of polishing and figuring. A certain setting of the machine, associated to a lap of a certain size will be expected to produce a certain action on the mirror surface (similar causes leading to similar effects).Yet, one must insist on the fact that this is only a tendency as other factors may influence the work, almost accidentally (the temperature, the moisture, the quality of the pitch used…). In spite of all this, this predictability in the actions is more easily controlled than during handwork when regularity and repetitivity are harder to master.
- Longer time of work : It takes a longer time to modify a mirror surface with a machine than by hand. This could be considered as a drawback. In fact such progressive transformations of shape allow immediate corrections and help avoid defects that would soon become impossible to correct once they are well inbedded (turned-down edge, central depression).
- Reducing local corrections : Using a machine may limit, or even eliminate, the need for local corrections through increased pressure as this technique is tricky to control when trying to reach a smooth shape. For this a machine will allow a nicer blend of zones, and lesser dog-biscuit.Yet, local corrrections may be necessary for the correction of a small defect, in particular in the case of large mirrors with short focal length.
Such advantages shouldn’t make an amateur optician think that using a machine is a miracle solution to obtaining a high-quality mirror, with an ideal shape. This method indeed has advantages but nevertheless it requires a mastery that only practice will guarantee. The best guide for a beginner is personal experience through a variety of practises and the use of the tools used for many hours. In spite of this, we are going to try and give some general principles on how to deal with a polishing machine, derived from our own experience and the advice we got from confirmed mirror makers.