Rough and fine grinding continued

III) Now onwards the following process will be there till the Primary Mirror is fully polished

1) You have to change the side-only or chordal straight forward and back grinding motion to center over center zigzag 'W' shape motion (see the moving picture below).

2) Henceforth uptill the last, total grinding stroke length (up and down) would be 4.5" + 0.75 + 0.75 = 6" i.e. Mirror would go 0.75"out (upside) from the Tool and 0.75" from down side.

3) From side ways i.e. from left and right of the Tool, the Mirror's stroke deviation would be averaging around 0.56" making the total side stroke length 4.5" + .56 + .56. = 5.625".

4) The rotation process would be same that after every 3 Mirror's rotation 120 degrees apart (i.e. stand at 12'0 clock, 4'0 clock and 8'0 clock) one Tool's rotation at 45 degrees and one M's rotation around 75 or 90 degrees will be there. You have to change the side-only motion to center over center 'W' shape motion as mentioned in above para 1. Also observe the moving picture above and below).

(D+1/6D(up) + 1/6D(down) or D+1/3D = 6" and D+1/8D(left)+1/8D(right) = 5.6")

5) Now, you have also to interchange the position of the Tool and Mirror i.e. after a fixed time or fixed number of strokes and rotations, you have to put Mirror beneath the Tool i.e. Tool will come over top of Mirror (ToT - Tool on Top) and the same process of grinding as mentioned in (III) will be repeated.

6) If the saggita increases more than required, keeping Mirror more time downside will decrease the saggitta of Mirror. If saggitta is less, keeping Tool more time downwards and slightly increasing the length of Mirror's stroke (i.e. slightly more than 1/3rd or instead of total 6" up and down stroke it should be 6.1"/6.2") will increase the saggitta.

7) You have to change the grit (grinding powder) after certain intervals when you see that previous grit's all bigger pits have been replaced by new grit's smaller pits and specially at edges/corners.

8) But before changing the grit, you have to invariably and thoroughly clean everything i.e. both the Mirrors M & T, cleats, Bench/Stool etc. and also change the papers. This is absolutely necessary. Even single speckle of previous grit may make scratch on the Mirror.

IV) Since you were grinding with #80 grit, and now having reached the required saggitta, start the grinding process with # 120 grit as mentioned in paras (III)1 to 8.

During the process you have to always gauge the saggitta of the Mirror after every 2-3 circle of rotations or after a fixed time of 5-10 minutes. You will see that when you are grinding Mirror on top (MoT) the saggitta may increase slighly and when Tool on top (ToT), it may decrease. Thus you can control your saggitta as mentioned in above para no. (III) 6) and you have to maintain your saggitta in this manner. But you have to keep in mind that the required saggitta of 0.8mm is reached/maintained uptill grit no. 320.

For changing the positions of Mirror and Tool either you fix 2-3 rotation circles of Mirror or a fixed time say 5-10 minutes.

With this process of changing the disc (M & T) positions and grinding in rotation as mentioned in above paras, gradually both M and T will start taking spherical shape.

While grinding by number 80 you will observe that big pits are formed in both the glasses. Grinding by number 120 you will see that these pits are getting smaller. After grinding for some time say 30-40 minutes or after 4 to 8 rotations and interchanging the position of Mirror and Tool (4 rotations Mirror up and 4 rotations Tool up) when you see that pits are no longer getting smaller and almost all bigger pits of #80 have gone specially from the corners, at that time you have to clean the Mirror and Tool and switch over to another number 220 grit.

With this 220 number also same process of 120 number will be repeated untill the pits no longer get smaller from the corners and now you go over to #320 (or 302 or MA2) and repeat the same process.

After #220 you will see that both the Mirrors are taking spherical shape. Both the Mirrors somewhat start sticking to each other. This is a sign that they are takintg spherical shape and matching each other. You have to be very careful that they are not stuck to each other. To avoid sticking you have to see that grit mixture between the Mirrors is always neither too thin, nor dried. Also you have not to keep the Mirrors idle i.e. without motion even for a few seconds. But if perchance they are stuck with each other. Don't separate them by force. Dip them in water-bucket and inside water itself by a wooden hammer slowly strike at Tool in a standing position. After few strokes inside water they will separate

After #220 also drill or scratch a half or one mm deep hole of same dia in the perfect centre of Mirror M with the help of an optician or a sharp iron nail, minimum 3 times tempered. This perfect centre-point will help us in giving perfect collimation.

After #320 now the saggitta of Mirror should be around 0.8mm. You now don't have to care for saggitta whether it slightly increases or decreases. But you always have to alter the positions of Mirror and Tool.You have to see that both Mirror and Tool become perfect sphere.

After #320 both M and T must reduce into a perfect spherical shape and after the process for #320 is over you have to perform Sharpie test for seeing whether both M & T are spherical and matching with each other or not.

Sharpie Test : Clean both M & T thoroughly. Draw a cross (x or +) on the Mirror and Tool by pencil lead's very light pressure. Very softly give grinding motion MoT and ToT without putting anything in between the plates i.e. with bare dry thoroughly cleaned disks over each other.

If the pencil line drawing of 'x or +' is erased after few 'W' strokes that means both the glasses are spherical and perfectly matching. If the drawing is not erased, it is left either on periphery or center that means M & T are not spherical and perfectly matching. Your grinding strokes may be slightly longer, side-ways or erratic.

So adhere strictly to 1/3 and ¼ center over center "W' strokes (as mentioned in para (III) and also change the MoT and ToT frequently. After some time again perform the Sharpie Test and now you will see that the pencil marks are erased quickly. One more time, you can perform this test after #500 with utmost care so that no scratches come to ensure that both or spheres.

This test is very necessary.

In the same manner you will go for #500, 800 (MA 2.5) and 1,000 (MA 3).

After every fine number of grit you will notice that surfaces of both Mirror and Tool are getting smooth.

After #800 (MA2.5) to 1000 number a silky smoothness will come and you can also see a faint reflection of a bulb filament at a grazing angle (between 120 to 180 degrees).

After # 800 or 1000 now the rough grinding is over and you may switch over to polishing process. (Some persons go upto 1300(M4) and 1800 (M5) also to cut polishing time. But it somewhat risky also as you may get scratches. So go very carefully.

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