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GOOD Stefano, It is a beautiful sphere , it is fine!
not insist over with past long, you're going to have a change in trend, from raised edge to edge retorted.
the edge is back and see small imperfections that can only be corrected by digging evenly across the sphere.
If you really want to do it ( but at this point it is not necessary ) continues with some session 1/3 COC, using the same tool that is now well adapted, but only to improve the connection between zones, in other words, for even more straight lines.When you go to parabolizzare, these small imperfections on the edge will almost certainly “absorbed” by the action of past W on the peripheral part of which is now in 99% correct, the defect is too little extended to influence the results.
It is this correct, It is barely perceptible but the lines at the periphery tend to “flood” ( in as much affected by the machining with long strokes ) or , if you prefer, The center tends to remain “more closed”.
Precisely for this reason I suggested long strokes TOT and then 1/3 COC to reconcile.
Is’ clear that the long runs will not long retain the ball “stable” and they must be accompanied by the classic ones for the ball, but you need to note is the reduction of the step to the edge, therefore I insist a little’ with the two combined actions.I often read around the net a lot of confusion about this aspect. let's clear:
With the tool of pitch, It is always the center that determines which zone will be most affected by processing, unless you exert a localized pressure.
Stretching races with mirror under, the tool center also reaches the side of the mirror device and consequently the edge most affected by the abrasive action. Not surprisingly, the risk is to generate the TDE that both TOT With MOT.
The image you posted ( thanks for the quote
) It is this aspect. The wider W stroke in the transition to the mirror center, It allows to work more the peripheral zone.In this case we can think of the central elongated stroke TOT as a simplification of that mentioned in W, the result is analogous, It characterized by a less aggressive and more balanced action between the various sectors but still in line with the results obtained with the W.
In fact the last image, I see no overcorrection in the middle but I see an almost completely retracted edge in the spherical shape, definitely improved over the previous Ronchi.
However, it is improved! Now it is a beautiful ball even though there are still those few mm to the edge of the field-
To break down the board with TOT races must be long, if you make them too short regenerate the raised edge.
You can try to do some’ racing W-TOT and then align with the usual 1 / 3D, so now that edge has past counted…
Hi Stefano, with the premise that you're doing a great job, definitely at a higher level than that normally obtained with the first mirror, It might say that the flaws you see are small and do not create any problem for the ball, but create them for the parable, I'll explain:
For parabolizzare you will necessarily bring down the board and connect better sectors, at the moment, I am a bit’ “corrugated”, the lines are not completely straight.
And if the Ronchi seems a small thing, the foucault will reveal their exact measurement.
So you still need to come to terms, and it is certainly easier to reconcile a sphere that connect a parable !So in my opinion, better to do now more easily than they otherwise will have to be done then more difficult way.
More good is the ball starting the greater the chance of getting an excellent dish and, anyhow, parabolizzare for good, it is necessary that you understand how to fillet the sectors with continuity and how to intervene on the local defects, otherwise it is mostly luck ( which in any case it is always handy ! )
well Stefano, you're almost there ! the border is still a bit’ high while the peripheral part is almost correct. Always consider that there are two ways to achieve your goal:
1- break down the raised edge and bring it into line with the center
2- dig the center and bring it into line with the edge ( you will have a ball in this case more “closed” )in your first case ( AGAINST 1/2 ) get the solution 2 e, the other way around, the 1.
If the grating is used around 10 lines / mm, then the defect is really very small and recoverable in the short term, two or three sessions at most ( once you have found the cause ). I think the difference between the sector “indicted” and the sphere does not exceed lambda.
Mah… hard to say, the only certainty is that shown by the Ronchi namely, a more machined area compared to the other with the races 1 / 3D, This indicates a non-uniform contact. The contributing factors may be all and contrary to all, why I think it is important to get to know and optimize your setup prior to the application of specific techniques.
Finding the cause at this time prevents reaching the ball is more instructive than a parabolizzazione performed without any problem… but I want to repeat, the work was done very well up to this point, we are talking about very minor defects that constitute the norm in optical processing manuals, it would have been unusual and unforeseeable circumstances otherwise, ie reach the ball without any problem of processing.
Same thing will happen to the parable, therefore each grattavetro sooner or later realize that the essence of the process is precisely this, find the causes of manufacturing defects and act accordingly each time finding the right fit, It is seen that each processing is a story in itself, different from previous.
Yes, right, with longer strokes Could it work better the outdoor area, but only up to a certain moment, namely, until the tool retains the same “defect”.
In the moment in which the tool will change again form ( because forced to work more their central ) then even long runs will not be the most effective to reach the ball, but it appears a new abnormality in operation of the new tool shape. Then he will return to face the problem of adaptation.Keep in mind that in reality the defect is minor, the fit is all in all very close to the optimal one, to the point that the difference between the contact areas could also be explained by an abnormal pressure in some points, due for example to the position of the hands during the races, or to a non-uniform temperature that develops during the tool / mirror racing.
Surely the thickness of only 3 mm does not help in this sense, then redo the tool is a good idea.
You Stefano, your last reasoning is correct:
Look at the lines of Ronchi carefully: we intrafocale in suburban lines are wider, or more open with respect to the center. This means that in the suburbs we are closer to the fire than the center.
In other words, the periphery of the curvature radius is shorter than the center, namely, the peripheral profile is is more curved than in the center. So the profile of your mirror at the moment is elliptical with K> 0.
In fact, a variation can be seen compared to the previous Ronchi, but unfortunately it is not for the better, the tendency to “to open” in the periphery is increased, the center remains spherical but the periphery is further decreasing the radius of curvature.
Although it may seem strange ( and I understand why I had the same problem with a 300 f3.8 ) the only way to obtain this type of defect is that the edge is reached by processing less than the peripheral area. In this way you create a profile in “bowl”, where the peripheral curvature is more accentuated with respect to the center. So there remains the problem of adaptation.At this point, even if the pressure does not get anything, try to stretch their rides for a session, until 1/2 D ( but you can stretch further if it does not work ). This elongation can give a measure of how to be extended compared to the diameter of the actual contact area.
Please note that this is a very effective method to get a nice edge retorted, that when you do not because the edge is exactly the contrary, that is “raised”.
I agree with Giulio, I had missed this and I can only confirm what he says ” the teacher”
This “hoax” of cerium “I superdiluito” It runs the network for some years, I do not know who was the author but was definitely somebody that pointed more to the savings that the effectiveness… and how it's wrong, given the prices of these products !
Regarding the aluminum oxide, the high dilution is made necessary by poor tenacity of this material, in practice after a short time on the grains I am “beveled”, the edges of the granules, which originally are angular and sharp, After a few strokes become rounded and end their abrasive action.
If you exceed with the concentration of aluminum oxide, I realized when I used it, you create a “bearing effect”, where the grains glide over one another and the mirror seems to be moving on the tool as it would on the ice, He warns the lack of any friction also increasing the pressure.Absolutely yes, You are working well, there are no big mistakes and figure to Ronchi is very good. This is about a few hundred nanometers glass more or less, too few to be evaluated with a film…
The signal which is an adaptation problem comes directly from the Ronchi, as central rides like you're doing, They must lead to the ball, each anomaly detected gives you the measure of where and how much must be optimized setup.
You are very close to having an optimal setup for parabolizzare, but you need to understand and solve this little problem to point directly to excellent results !One thing you can try is to increase the pressure at the center during the races 1/3 COC, in this way “forces” the tool to a full contact and , since you're sottocorretto, do nothing but push the processing in the desired direction, that is, increasing the depth at the center compared to the edge.
If the technique is effective, you'll have the added uneven adaptation confirmation.
But be careful not “to abuse” of this variant, because the “side effects” could be worse than the problem you want to solve.And, as you pointed out yourself, there is no uniform contact coating / mirror. The adaptation for a tool to be 30 cm is not simple, also heating it is not said that it reaches. And the more “squares” of that size they are able to support a lot’ weight without deforming ( cold ), the 10 kg may not be enough.
Once I remember I watched the first half of a game on TV “comfortably” sitting above the tool / mirror , and other instead, I have risen above with both feet ( weight 87 kG ) for about ten minutes. At the end of the adaptation was perfect. but do not think that the film quickly gave way under the weight, it was necessary to insist and repeat the operation several times…