Grinding a Newtonian
Hollowing out the mirror.
First of all, the tool need to be attached on the workbench or grinding table. We do this in a simple matter like you see on the picture. In stead of sawing a wooden wedge, we can use a wash pin. After every number of carborundum, we just throw it away and use another one. If you like to make a wedge on your own, make use of hard wood. Possibly we can make use of a little eccentric piece of wood like on the picture.
The first goal in rough grinding is to remove glass from the center of the mirror and the edge of the tool as quickly as possible. To obtain this goal, you wet the tool with a little bit of water and strew a half of a tablespoon of carborundum no 80 on the tool. Now you put the mirror on the tool and grip the mirror blank firmly with your hands and push it back and forward applying long strokes. Don't be frightened if you'll hear a loud, grinding sound because this is normal. There are two ways to hollow the mirror.
The first way is to use a 1/2 center over center stroke or use a gentle W stroke. In this grinding phase you may use moderate pressure on the mirror. In this phase we have the intention to remove the glass as quickly as possible. But beware you don't push the mirror too far. If the blank overhang the tool too much it could chip the tool or ,even worse, damage the mirror blank. If you increase the mirror stroke little by little, nothing can happen to you.
A second and faster way to hollow the mirror blank is by using the chordal stroke or grinding on the edge of the tool. These method of grinding must only be used by experienced amateurs. If you don't have full control over your blank, you risk to damage it quickly.
The next picture shows quite accurate what happens between two mirror disks when you push them back and forth with great pressure and with carborundum and water between them. Simultaneously with the pushing back and forward of the mirror blank we have to include two other movements, because if we only grind in one direction, the result should be a cylindrical mirror, quite unusable for an astronomical instrument. If we have done eight strokes, we turn the mirror approximately ten degrees clockwise. At the same time we turn approximately ten degrees counterclockwise. This is the famous 'walk around the barrel'. The word approximately is well chosen because it is important that you don't grind too much at the same place. It is important that the grinding is distributed evenly around the mirror and the tool. If you don't respect this random movements, the surfaces of mirror and tool will not be smooth, but will show zonal defects. It will not be possible to correct these defects while polishing.
If we don't have the place to work around a table or a barrel, we use another technique to combine these movements. For this method we do not need much room. Atop a workbench or stable kitchen table, which can be placed against a wall, we fix three plastic round pieces. Take care that the tool still can turn. But yet you don't fix the tool with a wedge. The grinding is done the same way as described above, but the walk around the barrel is replaced by turning the tool approximately ten degrees clockwise. Also here pay attention at the word approximately.
We have to work with the same carborundum powder as long as we hear a creaking sound. Then we take away the mirror blank and we strew a fresh amount of carborundum atop the tool. After that we put some water above it and we start grinding all over again. Don't use too much water, otherwise all of the carborundum will be disappeared after two or three strokes. This whole cycle we call a 'wet'.
While continuing the procedure of grinding with carborundum powder number 80, you don't need to pulverise the powder completely. With a little experience one learns quickly when to start a new wet. When you see after a while that only a thin mud is left atop the tool, then you know that you worked too long with the same powder. When you don't hear the sound of rough grinding any more, then you know its time to start a new wet. If you keep on grinding with 'old' powder, you will only lose a lot of time. If the movements of the mirror get
a little bit tricky you have to sprinkle on a little bit of water with the sprayer. Try not to work too fast, try about 60 strokes every minute. Don't forget to apply heavy pressure and a full stroke. It will take an experienced mirror maker about 2 hours to hollow out a 150 mm f/8 mirror completely to the desired depth. At this time the mirror surface isn't spherical yet.
After rough grinding for about 1 or 2 hours it is time to measure the depth of our mirror surface.
We already know that the upper blank, the future mirror, has become concave and the bottom blank , the tool, has become convex. Take the upper blank and rinse the mirror thoroughly in a bucket. Try to avoid to rinse the mirror in the sink straight away. The large amount of carborundum will stop up the sink.
To measure the depth of the mirror we use the spherometer.
First of all we need to calibrate the spherometer. We place the spherometer on a reference flat and turn the indication on the meter to the zero sign.
After the meter is calibrated we place it very carefully on the mirror blank and we read the depth of curvature
(in our case the measurement is done with a scale of 1/100 of a millimeter).
The target value in our case will not be 0,08 mm but 0,09 mm or 0,10 mm instead. We hollow the mirror a little bit deeper than necessary because the mirror isn't a sphere yet. With the spherometer you can check the deviation from a sphere. You can see that the central part of the mirror is deeper than the outer part.
Later on, if we are grinding with carborundum number 500, we can check the curvature of the mirror with a very simple method, called the 'pencil-test'.
Projection of the sun.
If you don't posses a spherometer, then you have to make use of the sun to measure the depth of your mirror. Because the mirror doesn't have any reflective coating yet, you have to make the mirror wet. (add some detergent to the water) Project the image of the sun on a shady wall. If the image is as small as possible, then you can measure the distance from the mirror to the wall. In this way you can determine the focal length of every objective. Take care that the angle between the incoming ray and the reflected ray is as small as possible. The drawback of this method is that you need another person to help you with the measurements.
It isn't so easy to use this method when you only grind with carborundum number 80. How finer the powder you are grinding with, how easier to project the sun on a wall and define the right focal length.
You even can try to measure the depth of the mirror inside your house with a lamp. But don't forget that the distance between the lamp (or the projection of the lamp on the wall or ceiling)and the mirror is the radius of curvature and not the focal length.
If we reached the desired focal length, we must switch to a more gentle grinding method to achieve the desired spherical surface curvature (the curvature is now hyperbolic with a too deep center and a too flat edge).
To eliminate this error we have to continue grinding with carborundum nr. 80 but with a 1/3 stroke in stead of with the ½ diameter stroke. We also have to slow down (approximately 40 strokes a minute) and reduce the pressure between mirror and tool. A minimum of 5 to 6 wets will be necessary to complete this task.
While making the blank spherical we have to take care that the focal length don't change any more. If our stroke is a little bit longer than 1/3 the diameter, the mirror depth will increase. If our stroke is too short, the focal length of the mirror will increase and the result is could be a mirror with a turned down edge. A turned down edge is very difficult to get rid of with finer carborundum grades. A simple trick to know if the blank is spherical is to observe the little air-bubbles between the two glass disks. The air-bubbles are seen at places where the two disks don't match. If bubbles get stucked in the middle of the mirror and they don't disappear, you have to use a shorter stroke. If you notice that your mirror is too deep, then the focal length will be too short. You can fix that by changing the position of mirror and tool. If you place the mirror below and the tool above, the curvature is reduced. Eventually, use a halve diameter stroke for several wets.
If the mirror is spherical with the right focal length, then it's time to switch to a finer carborundum powder.
Before we work with the finer abrasive, the important step is to clean scrupulously everything on the workbench. The mirror blank, the tool, the table ,the water sprayer, the water-tap, the spherometer and even the glass of wine you were drinking. Some people even wash their hair after every grade session. Notice that a single coarse carborundum grain may ruin several hours labor.
From now on, we grind the mirror with successive carborundum grades. The work now continues with 120 carborundum. We always use the normal 1/3 stroke. Mirror above, tool below. Check the saggitta and the curvature of the mirror on a regular base. After a dozen wets, it's time to inspect the mirror surface. You have to know that before you can switch to a finer abrasive, every coarse pit produced by the preceding carbo has to be gone. To inspect the mirror surface we make use of a little microscope or a good magnifying glass. We have to look for little scattering points against the granular background. Especially check the edges for pits left by the preceding carborundum grade. Use a microscope with a power of 30 and an indirect lightining. A microscope with two legs, one leg with a bulb and a battery and the other leg with the magnifying lens and eyepiece, works very well.
The 'pencil test'.
The microscope is used to check the condition of the surface, but there is a very simple method to check if the figure is spherical. This method can only be used after grinding with carborundum grades 400 or 500.
Before applying this test clean the mirror and tool scrupulously. The tool is placed on a table and a cross is drawn with a greasy pencil. Now we place the mirror above the tool very carefully. We push the mirror 1 or 2 centimeters back and forwards. Turn around the mirror for 120 degrees and do the same thing. Finally step around the mirror for another 120 degrees and push the mirror back and forwards for the last time.
Now we check the mirror surface and look for traces left by the pencil cross. By rubbing the mirror against the tool, the little graphite particles are spread across the hole mirror surface. If we look to the mirror closely, holding the disk against a strong light, we can see the remains of the pencil. The places on the mirror where we can see the pencil remains are places where there is good contact between the mirror and the tool. These zones are higher than the places where we can't see any graphite remains. If you can't see any remains at all, make a cross on the mirror with a wet finger. Your finger removed the graphite powder locally. Hold the mirror at an angle of 30° to 45° and look for the cross. If you see the cross uniformly from one edge to the other edge, the mirror is spherical.
One last warning. Be very carefully when doing this test. Be sure the mirror and tool is very well cleaned.
The test may be a cause of scratches. The rubbing of glass upon glass is always a risky operation.
De volgende poeders.
Het oppervlak van een
glasschijf behandeld met opeenvolgende poeders.
Carborundum nr. 80
Carborundum nr. 120
Carborundum nr. 320
Carborundum nr. 500
Carborundum nr. 800
Carborundum nr. 1200
Als U een tiental wets gedaan heeft met poeder nr. 120 kan U voor de eerste keer de structuur van het oppervlak bestuderen, want alvorens U overstapt naar het volgende poeder (nr.180) moeten alle sporen van het vorige poeder (nr.80) volledig verdwenen zijn. De enige manier om dit goed te kunnen zien is met behulp van een kleine microscoop of een sterke loep. U moet tussen de korrelige structuur van het bewerkte oppervlak zoeken naar putjes of krasjes die duidelijk niets te maken hebben met de korrelgrootte van het poeder waar U mee bezig bent.(m.a.w. grotere putjes of krassen) Er moet vooral aan de randen van de glasplaten gekeken worden omdat daar altijd de laatste putjes te vinden zijn.Zolang we met de groffe poeders bezig zijn kunnen we ons behelpen met een goed vergrootglas of een sterk vergrotend lensje, maar vanaf poeder 240 zijn we absoluut aangewezen op een klein zakmicroscoopje. In de handel zijn kleine tweebenige openklapbare microscoopjes te verkrijgen met een vergroting van ongeveer 30x. In het ene been zit het optisch gedeelte, in het andere de twee batterijen voor de ingebouwde verlichting. Liefst gebruiken we geen al te sterke vergroting, 50x is maximum. Zo gaan we door met slijpen, en zullen we alle poeders gebruiken, altijd oplettend dat :- het brandpunt niet afwijkt (sferometer).- de sfericiteit intact blijft (potloodtest).- we de structuur in de gaten houden (loupe).- de schuine kant van spiegel en tool intact blijven.- we de schuine kant telkenmale met eenzelfde gradatie schuurpapier behandelen als het poeder waar we op dat ogenblik mee slijpen.- U bij het overstappen naar een volgend poeder alles grondig reinigt.
It may happen that you hear a indescribable noise while grinding. It could be the noise of a scratch. Inspect the surface with the microscope and try to estimate the damage. Very tiny scratches only visible with the microscope may be neglected. But if the scratches are deeper, they ought to be removed by grinding with a coarser powder. Which carborundum you will need to remove the scratches entirely, is not always easy to judge. First try to use a one or two sizes coarser powder for 10 minutes and see what happens. If you are lucky, the scratches will disappear completely, if not you will be forced to use a coarser carborundum.
Tool and mirror may, occasionally, lock together.
If the mirror and tool are not spherical while fine grinding and the amount of abrasive used is reduced to a minimum or the abrasive is used too long, mirror and tool will sometimes stick to one another. This should be avoided by all means. Change the carborundum frequently and use a little bit more carborundum if you feel that the movement of mirror and tool becomes difficult.
Separating the two locked disks may be extremely difficult. Heat the mirror and tool slowly warm water until you reach 50 degrees Celsius. Put both on a table, place a log of wood next to the mirror and knock with a hammer at one side of the mirror. Ask someone else to help you and catch the mirror if necessary..