A Faster Way to Grind Mirrors


Introduction:

Many years ago, long before I ground my first mirror, I read many books on the subject. One thing that I had read was that professional opticians (who were usually mass producing some design) ground their mirrors or lenses against a cast iron tool. This way they were not only guaranteed the correct curvature, but much time was saved during the grinding operation as one didn't have to grind out a tool as well as a mirror; all the effort went into grinding the mirror.

The technique has never made it into the average ATMs repertoire because, it was said, amateurs had plenty of time to spend in grinding their mirrors nor had they the facilities to make cast iron tools. The technique was mentioned only in passing in discussions for the advanced ATM.

But being a revolutionary schoolboy I didn't believe all that I read in books and thought of another way. Unfortunately I was not in a position to try it out, and when I mentioned it to experienced ATMs they dismissed it as being

a) not worth the extra effort involved, and
b) it probably wouldn't work anyway.

I was primarily worried about not getting the right curve, or perhaps not getting it spherical. I was told this was very easy (which once you've actually done it, you discover that it is). Anyway, I kept the idea in the back of my mind for the next 20 years waiting for the time I could try it out.

The idea is quite simple; just glue some steel washers on to an existing tool of the desired curvature and hay-presto, a steel tool. Washers are mass produced from sheets of steel and are all essentially the same thickness. If ones of small enough size are chosen then their individual flatness won't matter (cast iron tools are only coarsely machined and have many hollows - at least the ones I've used). My problem was that I didn't have a tool of the right curvature, nor did I ever want to make the same focal length mirror again after I'd made one.

If the idea was so simple, why hadn't anybody else done it? Well somebody finally did, but it took them almost 20 years - and to make it even more annoying for me I was just gearing up at last to try it myself (I'd been asked to make a mirror of the same focus as one which I already had a tool). It was at the 13th NACAA (National Australian Convention of Amateur Astronomers) in Sydney in April 1988 at the TM workshop that South Australian amateur Joe Grida reported a "fantastic new method of quickly grinding mirrors" developed by fellow SA amateur Charles "Chas" Franks. He subsequently published his method in the (then) new Australian astronomy magazine Southern Astronomy (later to become Sky & Space) in the Jan/Feb 1989 issue.

The story for me would have ended there except that nobody in the wider TM community seems to have read the article and so a great idea is going to waste. Hence this page to spread the word.

Making it:

The technique as I originally intended was to use an existing tool with the desired curvature as the base. Small, perhaps ¼ to ½-inch washers (depending on the curvature of the tool), would then be glued to its face. Chas found that when he did this, small scratches were left on the mirror's surface. He put this down to grinding particles being trapped in the washers. I can't say that I understand this effect, but his fix is better than the original idea; use the "holes" from the washers (called dumps) instead.

I have used 20mm diameter by 1mm thick dumps for mirrors with curvatures down to 20cm f/4·5. I think that this tool should have had smaller dumps as they didn't seem to properly conform to the curve, however it didn't produce any problems at all. For longer radii, this size is excellent. The dumps should be low quality mild steel rather than expensive stainless steel. The better quality are harder and wouldn't allow the abrasives to bed-in as well. They may even cause scratches (this is one possible explanation for Joe's problem when he first tried it with washers).

I use 24-hour Araldite (a 2-part epoxy) adhesive to fix the dumps to the tool. While I have had the odd one or two come adrift, I don't see any reason to use any other glue. The ability to remove the dumps should one want the tool again is an advantage. Dumps can be obtained from any washer manufacturer. Check out the yellow pages of the phone book. A kilo of assorted sizes shouldn't set you back more than a few dollars.

Clean the dumps in a strong detergent solution and hot water prior to use to remove the protective grease. Wear rubber gloves and take care as they can have sharp burrs. Only clean about as many as you will need as the others will want to stay protected from rusting until you use them. You could either cut the odd shapes necessary to fill in around the edges now, or leave it until you know exactly how many and of what shape will be needed.

The pressing process used when creating the washers means that the dumps have an up and a down side; one side has a slightly raised edge, the other a slightly depressed edge. When fixing the dumps to the tool make sure you put them all the same way round. If pushed to make a recommendation, I would say that they should go raised edge up and let the initial grinding wear the edges down. This way places the flatter, and hence better datum against the tool surface and should result in an overall better surface. I've also tried it the other way and it doesn't seem to make any difference. But if you mix them around, then you may find some end up not touching the mirror at all.

Mix the glue thoroughly, but not too much at a time as it sets quite quickly. Apply a small spot to the centre of the dump then place it on the tool, pressing down firmly to ensure the glue spreads over the surface of the dump. Traditional advice for polishing laps says that you should not place a dump exactly in the centre as this will cause zones. I personally don't believe this, but have never bothered to prove it wrong. So how you position the dumps is up to you. As the glue takes time to set, and the tool is curved, the dumps will very slowly slide away from the centre of the tool. You will need a strip of cardboard or some other form of restraining band around the edge of the tool to stop them slipping completely off.

The photograph below shows a 20cm f/7 glass tool covered with dumps. The original tool had already been used for making many mirrors and was near the end of its useful life, being now too thin to be used for another mirror. The dumps have given it a new lease of life, albeit stuck as a 2800mm radius curve generator.

20cm f/7 glass tool covered with dumps
A 20cm f/7 glass tool resurected as a steel tool.

Using it:

You may have spent somewhere around one hour preparing and covering a tool with the dumps (assuming you were able to obtain a tool of the desired curvature). You will easily recover that time, plus a lot more, as you grind. I found that my grinding times were more than cut in half compared with the normal grinding procedures. For example, the time I spent roughing out a 31cm f/6·5 mirror using conventional methods was some 8 hours; with the dump covered tool this was reduced to less than 3 hours. It took less than 30 minutes to reach 20cm diameter! Other powders show similar speed gain.

There is nothing greatly different in using the dump covered tool compared with conventional methods. Some of the methods that are used for tile covered tools may apply, but as I've never used one I can't be sure. (TM6 had an article on tile covered tools.) My comments here refer to hand grinding methods; I've not tried machine grinding although I can't see any reason for any major differences.

One of the recommendations in the TM article was to fill in the gaps between the tiles with grinding powder and let the adhesive forces of the water circulate the used and fresh grains. I haven't tried this (and have doubts as to its efficiency) but one thing I did find as the wet progressed was the mud was washed down into the gaps between the dumps (with the mirror on top) and so I could do a second wet without washing down the mirror and tool. Clearly, this does circulate the mud and fresh abrasive to some extent but after this second wet, more tries without washing out the old abrasive produces shorter wets. I have found that using a conventional tool one can't use this method at all and so I usually wash the mirror and tool after each wet.

Another related difference is that with the mirror on the top, the grinding powder is not pushed off the tool as usually happens with conventional tools. The wet will continue for considerably longer and also uses less abrasive. So here are 2 obvious reasons why the grinding process is quicker and more efficient.

Some minor points are that the grinding noise is slightly different, it becoming slightly metallic and there is a less obvious change when the abrasive is finished. Also, there is no air bubble trapped between the mirror and tool, the air easily escaping between the gaps in the dumps. This means that there is no tendency for the mirror and tool to 'grab' or seize up as can happen.

The steel dumps do wear down, as can be seen from the changing sheen on their surface. Raised edges are quickly flattened and the dumps rapidly acquire a uniform spherical curve. This is also evident in the colour of the mud of the finer powders. Mine finer powders are aluminium oxide and are white, but grinding on the steel tool produces black mud which I assume is finely ground steel. The radius on the tool can change slightly as the dumps wear, although only when using the coarser powders; so during initial roughing out it might be sensible to interchange the positions of the mirror and tool. The effect is nowhere near as severe as with a glass tool, but some minor change in radius can just be measured. After grinding 3, 31cm diameter mirrors almost exclusively MOT, the finished focal lengths of the mirrors are 2032, 2027 & 2022mm.

Extensive cleaning of the tool after the end of each abrasive stage doesn't seem to be necessary. This may vary if there is excessive glue squeezed out between the dumps that may hold some grains near to the top. If the glue is only around the base of the dumps, then grains of abrasive that adhere will remain well out of the way. I clean up with a quick scrub of the tool with a firm bristled brush and have found no problems.

Also, note that the dumps will rust - and very quickly at that. So if you are leaving the grinding for a while, dry the freshly ground surface of the tool or you will return in half an hour to find it covered in small patches of rust. (Mild steel rusts much slower than cast iron. The first time I used a cast iron tool I was amazed - you could see rust marks in just 5 minutes. I was told this is perfectly normal behaviour for cast iron.)

There has been suggested one problem with this technique. I mentioned earlier that the initial tests done be Charles Franks showed a tendancy to produce fine scratches in the final stages of grinding. This was put down to the inner edges of the washers catching and holding grains of grinding powder that could not be cleaned out, but later releasing them causing scratches. Charles has subsequently told me that he often sees scratches even with the dumps, so the initial explanation may have been wrong.

I have never seen scratches in any of the mirrors I have done using this technique, but another person who has tried it has. My friend complained of continually getting scratches while using grade 1600. We brought his equipment to my place and I tried - no scratches, therefore his powder wasn't contaminated. He tried - no scratches, therefore not technique. Our explanation was that his water supply is contaminated. I live out of town and have my own water supply, which goes into a large tank which allows dust to settle out; it is also a new house with new plumbing. His house is older, and connected to the normal town supply. He switched to using water that had been allowed to settle for a long time before using it and that cured his problems. I suspect that this is the problem that Charles encountered. My theory is that the grinding action is so efficient when using the dump-covered tool that tiny dust particles now cause problems whereas they weren't noticeable when grinding normally. Further experiments are called for, but I've not done them.

Improving it:

My initial idea for this technique was based on doubt of my ability to make a mirror. I wanted a method that would ensure I ended up with a spherical curve of the correct radius. By utilising a tool that somebody else had previously used to make a mirror, I reasoned that I too would end up at the polishing stage with the right curve. I later discovered that mirror making was not the difficult task that Texereau makes it out to be; in fact it was remarkably easy.

31cm glass tool covered with dumps
A 31cm f/5 glass tool used to make a 24cm f/6·5 mirror.

Next I thought that the steel tool would be great for making binocular telescopes, enabling one to easily generate two mirrors of the same curvature. I've never made a binocular telescope, so I've never tried out this technique, but I'm sure that it would be ideal.

In both instances I assumed that the only speed gains would be due to the tool already having the right curvature. However, Joe Grida claimed that the grinding action was so swift he only needed to use 4 grades of abrasive (80, 220, 500, 1600) and finished the job in amazingly short time. While I didn't skip any grades in my mirrors, I can certainly vouch that grinding time is about one-half to one-third normal, with a corresponding saving in abrasive. Apart from my earlier observations, I believe the explanation for the great speed is due the abrasive bedding into the 'soft' mild steel. This stops it tumbling about, enabling it to work directly on the glass blank instead of against both the mirror and tool (plus against other grains it bumps into).

So, with such great savings in time to be made, the next mirror I made (a 20cm f/4·5) I rough ground in the normal manner and then coated the tool with dumps. A quick couple of wets with grade 80 to ensure the dumps and mirror were in good contact and then finish the fine grinding. I spent about 45 minutes setting up the tool, which was more than compensated by the time saved in grade 120 alone.

20cm f/4.5 glass tool covered with dumps
A 20cm f/4·5 tool. Note many areas of poor contact (now rusty) - it didn't produce any problems.

Other variations can be employed, too. If a tool of almost the right curvature is available, start rough grind with it and then strip off the dumps. Continue as normal until the desired curvature is attained and then re-apply the dumps. The time saved is well worth it. (Araldite may be stripped chemically or just by heating until it softens.)

One could "master" tools from a mirror by some variation on casting, although the method would need to be worked out. I say this as a friend of mine was given a partially finished mirror but no tool. He has been able to locate a tool a little smaller and of nearly the right curvature. The plan is to place the mirror face up, protected by a few layers of paper and then place dumps on it with blobs of glue on their exposed side. The tool will then be placed onto the dumps until the glue sets. With any luck, he will be left with a tool suitable for finishing the mirror.

Finally:

This method has many advantages and, as far as I can see, no problems. Time and effort are saved, grinding powders are saved as is wear on the tool. The tools can be re-used. Clubs can have libraries of tools and hire them out to members or use them in classes time after time with every student ending up with the correct focal length mirror in half the time.

I hope this labour saving technique encourages some new telescope makers and perhaps some experimentation by some older TNs.


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Page last updated 1996/06/29
Steven Lee