Small tutorial on using "minimum" software POLSIM for the machining of a parabolic mirror for telescope, "Assisted" by the use of a simple rotating plate, even in the digging operation of the "arrow" of the initial spherical concavity.
The program was written in 2007 from Martin Cibulski, and it can be downloaded as a polsim.exe (download ) , as other interesting articles from its 2005 in the place
It is a simulator of a simple machine operation consists of a rotating plate, with or without oscillating arm for reciprocating movement of the tool, that with the only turntable motor, however the tool causes entrainment in differential rotation induced.
The’ main utility of this simulation program is to allow novice users to learn to appreciate eye, the type of excavation which occurs on the mirror surface being machined, simulated by a rectangular graph located in a position below the mirror disk image, which represents the thickness, and divided into many histograms as there are areas in which you want to divide the mirror radius, indicated in box "n. zone”.
Obviously this is only a visual indication, and only quantified by visual assessment of the positioning taking histograms that in the graph represent the trend of the profile of the excavation, in accordance with input parameters.
The POLSIM software included a text that no longer works Support. nè (as stated in international forums) unfortunately be more available his programmer Martin Cibulski, in order to understand some features denounced the presence of boxes for data entry, but today without explanation.
(I refer to the boxes: Extension 1 ed extension 2; strokes in advance e working strokes; as the "fast simulation" function).
Once you launch the program, you can click the RUN box of REAL TIME SIMULATION window, which at the moment is superimposed on the image of the turntable (window that is draggable with the mouse sideways to avoid disturbing the vision).
It will start immediately the rotation and the simulation with the default data, which can be replaced during operation, accessing the SETUP PARAMETERS menu →, which will make the new data entry mask appear on screen, which will be applied to the simulation only after having pressed the OK key in the same parameter page ("Settings").
To shorten the description, , I indicate below the data with which the program starts ( (default data); ); and next to each of them I indicate the new data to be inserted, , as an example, , to realize the excavation of the initial sphere in a 500mm , applying the operating modes of the machine named, “FIXED POST”, also called "Spinny grinding"::
THE INSERIBLE DATA ARE :
- Mirror diameter; (default 460mm) – enter 500
- Tool diameter; (default 150mm) – enter 375 (as the fixed post machine requires a tool with a diameter of about 75% of the one of the mirror being worked) )
- Number of zones (default 30) optionally you can enter 60, only for a (perhaps) better “rendering” of the excavation through the smaller width of the histograms. .
- Optionally, put the check in the box “With equal areas” which makes the histograms different in width according to the width of the areas affected by the same abrasion rate (in the sense: : wide histograms = areas of the same low abrasion, ; Narrow histograms = high abrasion).).
- Rotation speed of the mirror; ; (default 4.8): : As a fixed post, , depending on the type of processing, , rotates between 3 RPM and the 60 RPM; enter 35, which is the speed appropriate for digging the arrow..
- Number of strokes per minute of alternating right – left displacement of the tool; ; (default 28.5) leave this data because apparently it serves only as a time for the integration of the internal calculations for the presentation of the histograms..
- Length of the right-to-left – alternate strokes, if any, established for the tool; ; (default 175mm) enter 0mm, ie no left and right movement of the tool, , who, which, simulating a “Fixed Post”, remains fixed in one point. .
- Offset (stroke offset) on the X axis of the tool position; (default 0) – enter 88; (see note *)
- Offset (stroke offset) on the Y axis of the tool position; (default 0) – leave 0.
- Click on RUN
To recap:: SETUP -> PARAMETERS (as well as it is visible in the following short moovie):
Mirror diameter 500 – Tool diameter 375 – optional n. of zones 60 – optional check “With equal areas”. – Turntable, 35 rpm – stroke per minute, leave 28,5 – stroke lenghts, 0 – stroke offset X, enter 88 (see note *) – press OK
You will see the condition (“probably”) better for the excavation of the initial arrow with coarse abrasive grit 60 or vice versa 80, of a mirror diameter 500mm, , with tool diameter 75% of that of the mirror = 375mm – rotation at 35 revolutions per minute - tool not oscillating - tool offset always calculable as follows in *:
Note *: Offset (stroke offset) to make the excavation of a spherical curve in a Fixed Post machine:
Stroke offset = (mirror radius – tool radius) + from 25 to 50mm.
In the specific case from (250-187,5 + 25) = 87,5mm
until (250-187,5 + 50) = 112,5mm.
The adverb of doubt "probably", , indicates that you can easily set the machine with those characteristics, , which may differ a few millimeters from the actual optimal geometric centering of what would have been a tiresome and deep manual excavation, , tiresome , because always correctable with little hand work of the same tool, , with runs having classic offset 1/3 Diameter c.o.c (note**). which, as is known, , always lead to the correction towards the attainment of a good sphere, , if they are used with tools having a diameter equal to that of the mirror, , or not much less..
(Note**: where c.o.c. means “center over center” cioè ad indicare lo spostamento del centro di un disco (mirror or tool) compared superimposed on each other),
By varying the X offset values from 88 until 113mm, and pressing OK, you will see that the shape of the spherical concave curve remains apparently the same, , but slightly deeper in the mirror, , because the peripheral speed of the tool ((which goes hand in hand with its weight to affect the abrasive efficiency), ), it is longer along the edge of contact with respect to the center.. And moving the tool more to the edge of the mirror, , it will work more intensively with the longer edge length exposed to the contact, , with the same greater digging efficiency. . But beware that by exaggerating the offset, , the excavation of the sphere will worsen abruptly changing from concave to convex, , as you can see that happens at the end of the next movie, , inserting a misalignment of 150mm..
Conversely, if by curiosity, , you put the tool with its center coinciding with that of the mirror, , the tendency will be to dig into the glass a convex sort of inverted U, , with almost zero removal at the center, , due to the low rotational peripheral velocity in that area; and with maximum material removal along the tool edge, where is the maximum peripheral speed.