A father-son team fine grind using
#220 grit on a bench with a turntable
in the 2001-02 Stellafane Mirror Class.
The Pencil Test - Checking for Good Contact
Use this checklist to make sure you are ready to start fine grinding:
You may click on the hyperlinks in the above checklist to visit the pages where we discuss each of these items. When all the check list items are done, you are ready to start grinding!
Warning!With any glass grinding operation, you must work wet, to prevent glass dust from entering your lungs. Always keep the glass and tool wet to prevent glass dust from becoming airborne. Never grind glass dry! Inhaling glass dust can result in Silicosis, a serious lung disease. Keeping your work wet is a proven technique to prevent inhalation of glass dust. Keep your work and wet and you should not have to worry! |
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Fine grinding refines the basic shape you have ground into the surface of the glass with rough grinding. The sequence of ever finer grits that you use smoothes the rough surface that was made in rough grinding, and prepares you for polishing. You are also refining the shape of the surface to be a section of a sphere; it is likely that rough grinding left you surface as only a crude approximation of this shape.
Throughout fine grinding, you must pay attention to maintaining your desired focal length. You must measure you sagitta and adjust your ratio of tool-on-top and mirror-on-top times to keep at your sagitta target.
GOAL: You have completed fine grinding when you have:
Smoothed the mirror surface until it feels "buttery soft" to the touch with pits approximately 5 microns in size so that it is ready for polishing.
ABRASIVES: You will use a sequence of ever finer grits to smooth your mirror surface and get it ready for polishing. There is no set grit sequence, but typically each step in the sequence is approximately half the size of the prior grit. A typical grit sequence for an 8" mirror that did rough grinding with #60 grit would be:
Silicone Carbide: #80, #120, #220
Aluminum Oxide: 32, 20, 12, 5 micron
Often, #320 Silicon Carbide is used instead of 32 micron Aluminum Oxide, and #500 is used instead of 20 micron; any of these combinations will work well. See our Guide to Grits for more information.
TOOL: Use your hard tool (Glass or Tile). You will alternate Tool-on-Top and Mirror-on-Top to maintain your depth of curve (sagitta).
STROKES: Use the Normal Stroke (1/3 Center-over-Center) for all of fine grinding.
TIME: A 6" f/8 or 8" f/6 mirror will typically be fine ground in 10 to 14 hours or work, depending on mirror size, grit size used, pressure applied (more pressure means short wets but more glass removed) and personal work style. This is very approximately 1½ to 3 hours per grit.
PROCEDURE:
For each grit in your sequence:
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Use the Normal Stroke, which is a one-third center-over-center stroke. Most beginners tend to make the stroke too long. Remember, a 1/3 total stroke is 1/6 overhang on each edge -- one inch for a 6" mirror, 1.3 inches for an 8" mirror.
You can start with either the tool or mirror on top. You should aim to do equal amounts of grinding in each position to maintain your sagitta. There is no set schedule for flipping the stack over, but most people do this when they dunk their mirror and tool to clean them up, perhaps every 5 to 10 wets.
Be sure to rotate you tool and mirror (or rotate you tool and walk around your grinding stand) about every 5 to 10 strokes. This is always critical to prevent astigmatism and to make sure you get closer and closer to the ideal spherical surface shape.
At least every half hour, you should stop and measure. Clean your mirror by dunking it in a bucket of water. Measure your sagitta to see if it has changed. Then, use the following table to adjust the ratio of wets of Mirror or Tool on Top:
| Mirror Position | Sagitta | Focal Length | Focal Ratio |
|---|---|---|---|
| on Top | Increases | Shorter | Lower / Faster |
| on Bottom | Decreases | Longer | Higher / Slower |
If your sagitta is too deep, then do more work with the Tool on Top; If it is too shallow, then do more work with the Mirror on Top. When you get it back to the desired value, then go back to doing equal work in both positions. As the grit you use gets finer, it will take longer to change the sagitta and you can measure less frequently.
Check for good contact on your first and second fine grinding grit (normally #80 and #120) by using the Pencil Test. Correct contact issues before moving to finer grits. Once you have achieved good contact, you normally won't lose it, so further checking on finer grits is usually not recommended.
Depending on your grinding style, it will take between one and three hours of work on each grit. To determine if you are ready to move on to the next finer grit, Inspect your Pits for uniformity.
Plastic Squeeze Bottles used for
dispensing a slurry of grit and waterNotes:
As you use finer and finer grits, you will notice that wets last longer. This is good news, it means you do more grinding and spend less time recharging the mirror with grit and water.
When you started rough grinding, you sprinkled dry abrasive on the mirror and added water with a spay bottle. This works well for coarser grits. Many people find it easier to apply the finer grits (usually those measured in microns and not grit number) in a pre-mixed water-grit slurry held in a plastic bottle with a spout. Be sure to shake the bottle before application, otherwise the grit will settle to the bottom and you will just be squirting on water with very little abrasive (Notice the bottle on the right in the adjacent photo - all the 12 Micron powder has settled to the bottom). Also, don't use a spray bottle (a bottle with a pump), the grit might not pass through the pump mechanism or might damage it.
CHANGING TO A FINER GRIT:
When you have determined you are ready to move on to a finer grit (When to Switch Grits), you should do the following:
Check that you are making progress towards "good contact", which is ATM-speak for getting really spherical. Use the Pencil Test to see how you are doing on your first two fine grinding grits, and don't move on until you have achieved good contact.
Check your sagitta, which you should be doing about every half hour on the coarser grits, and about every hour on the finer grits, as noted above. Instructions here.
Bevel: Have you reduced or eliminated your bevel? If so, and you are at #220 grit or larger, you should put it back to where it needs to be. See How to Bevel. Caution: Beveling when using finer grits carries a very high risk of scratching your mirror surface. It is best to leave #220 with a good bevel, which should last thorough the rest of grinding and polishing. In all cases, thoroughly clean the mirror and work area to prevent the coarse grit particles from the carborundum stone from scratching your mirror.
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Clean, Clean and Clean Again: Now is the time to clean everything, and make sure none of your coarse grit is lurking about to cause a scratch in the next stages. Start by cleaning the mirror and tool to clear away the "mud" that is made while grinding. But now, you must clean them especially well. Use an old toothbrush or other stiff bristled brush to clean out every nook and cranny in you tile tool. Make sure your mirror is sparkling clean, and no grit is hiding in any surface pits on the edge or back. Then put them safely aside in a clean place.
Next, clean you work area, including your grinding stand or turntable, water buckets, spray bottle, and anything else that may be hiding that odd piece of grit. Finally, clean yourself. Change your clothes and take a shower. Clean under your fingernails also, a favorite grit hiding place. You must clean thoroughly between each grit - The penalty for missing a piece of coarse grit is a potential scratch on your mirror, and perhaps a need to go back to coarser grits to grind the scratch out.
CONGRATULATIONS: You have finished fine grinding! On to Polishing.
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Criteria: You want to grind with your current grit until you have eliminated all the larger pits from the previous grit. For example, after hogging out with #60, you want to fine grind with #80 until all the #60 pits are gone (replaced by smaller #80 pits).
General Advice: You must examine a large part of the surface, and you must check both center, middle and edges. Most, but not all, mirrors are done at the center before the edge, so pay particular attention to the outer one inch.
Magnifier Technique: The classic technique is to use a low power magnifier to examine the mirror surface in bright light. The larger pits will shine brighter and appear larger. You can use a hand magnifying lens, or if you have a lower power telescope eyepiece (approximately 25mm), you can look in it backwards to use it as a magnifier. Many people find it easiest to shine a light through the mirror blank when examining the surface for pits.
Laser Pointer Technique: Holding a laser pointer at a 45º angle to the mirror surface, scan the laser over the surface. Big pits will light up much brighter. No magnifier needed.
Sample Plates: If you are having trouble judging pit sizes, make up some sample plates. Using scrap window glass cut into small squares, or microscope slides, grind your sample plate with a single known grit and then label it. Your sample plates will have only pits of the given grit size, and you can use them as a reference when examining your mirror (which will possibly have two or more pit sizes present). This has helped a number of students in the Stellafane mirror class gain confidence in judging pit sizes; the author uses the set shown below, neatly mounted, for use in class (yours can be loose, and labeled with a pencil on one edge).
A set of sample plates made from half
microscope slides used as a reference for pit sizes.
Causes: Unfortunately, scratches happen often when fine grinding if you are the least bit careless, or in some cases just plain unlucky. A scratch is caused by a hard particle larger than your current abrasive size getting between your mirror and your tool. Often, you can hear or feel a sickening screech as you push the tool and realize you've just made a scratch. Others seem to sneak in without a hint. Things that cause scratches:
- Coarse Grit that did not get cleaned up in the work area.
- Coarse Grit due to contamination of your grit supply.
- Chips of Glass or Tile from your mirror or tool (Place your tool on your mirror gently and flat, and remove it carefully as well)
- Dust or Dirt that gets blown or dropped into the work area (beware of dust from ceilings, especially in basements)
- Damage in Storage or Transport: Wrap your mirror in a grit free, clean, soft cloth when moving it or storing it, or keep it in a clean, plastic food storage container. Make sure you mirror and tool storage devices are sealed and away from your work area before you open any grit.
I'm happy to say that both beard hair (of which I have a lot), head hair (I have very little of that left!) and cat hair is very soft, and has never cause a scratch on my mirrors. This is fortunate, since when I grind there seems to be far to much of all of it floating about.
Prevention is simple - be meticulously clean. Study the list above, and eliminate sources. If you are working in an unfinished basement, staple a plastic sheet to the ceiling above your work area. Never have more than one grit container open at once, and don't be tempted to look at all your grits when they arrive - keep them all sealed until needed, and until all prior grits sealed and cleaned up.
Cures: If you get a scratch, what should you do? Don't Panic! You have two choices:
- Ignore It That's right, ignore it. We know you wanted to make the perfect optic, and this scratch mars you handwork. But in optical terms, the defective area from a single scratch is minute -- and you will never see it in the telescope you are making. If you have one or a few scratches, you can safely ignore them.
- Fix It The other choice is grind it out. If you are lucky, the particle that made the scratch is close to the current grit size, and you can use the current grit to grind it out, or perhaps backup a size or two to work it out. Use the same technique you used to examine the surface for pits to judge the scratch size (width and depth, not length), and that will give you an idea about what grit you will have to use to remove it (usually one size finer will work). If you are unlucky and a really big particle got you, you will have have to back up a long way -- it takes coarse grit to grind out a coarse scratch.
No one like scratches, and with reasonable care they can be avoided. If you get one, take some time to think about whether it worth the time, effort and money (if you have to buy additional supplies) to repair.
 PT1: Drawing the Grid on a 10" Mirror |
 PT2: Grind a single Wet or at least go "once around the barrel" |
 PT3: Examine the remaining Grid This Mirror has "Good Contact" Click to expand, and you should see a uniformly dimmed grid. |
The Pencil Test, also called the "Sharpie™" test (after the name brand permanent marker), is used to test for "Good Contact". This is ATM talk for having a good, closely mating, spherical surface on your mirror and tool. Only if you have a good sphere will you have good contact -- other surface shapes will have hills and valleys, and will not have good contact.
When to use: It is best to check this at your first or second fine grit, usually #80 or #120. If you wait until you are using finer grits, it will be difficult, if not impossible, to correct any bad contact problems you may find.
Procedure: The procedure is simple. Use a #2 pencil or permanent marker to to freehand draw an approximately 1 inch grid on your mirror (See Photo PT1). Then grind one wet, or at least grind "once around the barrel", with the normal stroke (See Photo PT2). Dunk the mirror, pat (don't wipe) it dry with paper towels, and examine the remains of the grid (See Photo PT3 - Click to expand, you can't see very much in the thumbnail).
Analysis: If the grid you drew is uniformly ground off, or uniformly dimmed (as in photo PT3), then you have good contact, a well mated mirror and tool that is a good approximation to a section of a sphere. However, if the grid id not uniformly ground off, you probably have "bad contact" and need to correct it.
Cures for Bad Contact:
After achieving good contact, check you bevel, and re-bevel if necessary at this time. See How to Bevel. Then return to the normal fine grinding work.
Page last edited 2007-Sep-23
