The techniques sub diameter tool, try to solve some of the problems that arise with the use of the full-diameter at the time when the size or the curvatures of the optical surface of the work, assume limit values ​​in which standard techniques can be ineffective or difficult to apply or even, in some cases, counterproductive.

We can summarize the main cases in which it is preferable to use a sub-diameter:

• machining of large diameters ( beyond 50 cm.)
• optical configurations of large focal opening ( f2-f4)
• hyperbolic and elliptic curves
• small interventions of correction zonal optical surface.

Generally we speak of sub-diameter in the moment in which the tool has a smaller diameter than the mirror in machining of at least 40 %.

A tool to 70-80 % the diameter-mirror, It has indeed, a not very different action from that of the full diameter and, in most cases, It can also replace throughout the full diameter with similar results in the application of techniques.

In reality, for a general overview, it would be more correct to speak of several tools at the sub-diameter over a same optical production, chosen with sizes ranging from 10% to a 60 % depending on the type of work that we are going to make, or if we want to, depending on the type of action that we intend to apply our mirror.

Before going into the details of the action exerted by the sub-diameter, we highlight some of the main differences immediately apparent from an initial analysis of the two types of tools.

We said what are the processes in which it is preferable to the use of sub-diameter, However, nothing forbids us to use a tool also reduced for the workings “classical”.

For example, suppose you want to perform the polishing of a mirror 300 f5 with a tool at the sub-diameter and make some considerations:

• During a stroke with a sub-diameter, mirror surface on which the abrasive action is exerted, It decreases quadratically with respect to the linear decrease of the diameter. It is easy to realize that in order to remove or polish the same amount of materiare with a tool to 50% will take time 4 times larger than the full diameter, for a sub-tool to 30% it will take time 9 times greater, and so on.
• A polishing, or even a spherical surface made with a sub-diameter will inevitably lead to elongation of processing times, not only, also the consumption of the patina, and thus its maintenance, increase accordingly in the same proportion .
• To make matters worse, that we work “at the same instant” only a portion of the surface, It makes it more difficult to achieve a smooth shape as the sphere, as it is more difficult to achieve a processing “symmetric” in which the tool exerts its action the same number of times and with the same intensity on the entire surface of the mirror in a uniform manner.
• Though “the law of large numbers” sooner or later he is able to take over and to standardize the processing, it is not unlikely that the use of sub-ports in diameter correction phase, which do not require large amounts of sessions, the appearance of zonal errors as a result of a non-uniform processing.

We summarize thus also disadvantages to sub-diameter tool.

• lengthening processing times
• difficulty in achieving regular figures
• appearance of zonal errors
• increased pitch consumption/deterioration .

 

BUT THEN, WHY’ USE A SUB-DIAMETER ?

Certainly not to solve the processing steps where the full diameter demonstrates its effectiveness , to see in action the sub-diameter must leave the field of play”classic” and go away, on multiple fields “hostile” where the normal techniques go “breathlessness” and fail to fulfill their potential.

In other words, with the sub-diameter, we must change the prerequisite, the approach to work is fundamentally different: We should not try to create a smooth surface ( as with the full diameter) relying on the regularity of the process but we must create “controlled errors” on the surface that once they return uniformed figure sought.

To realize better than these features try to see how it acts the sub-diameter.

1. whatever it is the diameter of the sub-tool, processing with longitudinal-tangential races generates a circular crown , a low pressure area whose extension is slightly less than the diameter of the tool, while the radial races are generally ineffective.
2. The diameter tool does not change the result of the processing but only the extension of the treated area.
3. The application of pressure It is very effective because the weight is concentrated on a small portion of the mirror surface and then, although in general the abrasion and the regularization of the total area requires much more time with the sub-diameter, the “excavation” small circular sectors takes place very quickly, in other words , to dig a real “chasm” concentrated in a mirror area with the sub-diameter just only a little’ of … carelessness !
4. the “localized pressure” , or the application of the weight on the tool at a point which is not necessarily the central, It allows to address the greater intensity of the excavation with great precision, exactly where we want it to go.

try to clarify with the graphic examples.

TOOL 30%, PRESSURE CENTER, TANGENTIAL STROKE ON OUTDOOR AREA:

In this case, the abrasive action is mainly localized in the tool center, as a result we have a circular crown depressed compared to the rest of the surface whose extension is slightly lower than the sub-diameter.

Being the center of the area “excavated” relative to the adjacent zones, as a result we will have an increase of the curvature in that sector .

The areas in red on the graph represent the place where the major action tool, areas in yellow are partly covered and constitute the “junction” green areas in which instead are not affected by the action of the races.

TOOL 30%, PRESSURE LOCATED AT THE EDGE , TANGENTIAL STROKE ON OUTDOOR AREA

in this case the abrasive action is more pronounced toward the edge, the resulting circular sector will have a depth, and then a non-symmetrical curvature . The greater depth will be in the vicinity of the edge, the remaining area will be fitting with the remaining surface. It should be noted that the outermost part of the edge remains little affected by processing.

AN EXAMPLE OF PRACTICAL APPLICATION

Suppose we have divided our mirror 5 areas according to the criteria of Couder or Texereau, and having to intervene on the zone No. 3, which turns out to be “high” compared to the other which, still have “measure” and should not be modified.

In this case we will use a sub-diameter whose extension is of the order of magnitude from that of the work area, the trajectory on which to apply the races will be the one delimited by the extension of the area 3 and we apply the pressure at the center according to the procedures of races previously described.

The result you will get should not be very different from the graphical representation:

If we want to make sure that the processing does not interfere with one of the adjacent areas , we can take advantage of the localized pressure to more accurately define the radius of action of the races:
in the example below using the same sub-diameter to 20% with pressure to the edge so as not to interfere with the outer zones.

These early examples of use of the sub-diameter serve to highlight the different approaches to the construction of an optical surface. Numerous techniques and different types of use of the same have been developed and can be applied in addition to those described.

Even races “classical” COC or radial races for the parabolizzazione “W” They are commonly used depending on the circumstances in which they can prove to be profitable.

What unites the different techniques with sub-diameter is that it generates a depressed area compared to the rest of the surface, which can serve to increase or decrease the performance of the curvature of our figure in processing. The application of different races and pressure can cause us to determine where and how we are going to create the depressed sector.