This article is about making a large grinding tool whose working surface (grinding face) is made to match an already existing curve for a mirror that has already been curve generated, or for any blank already completely or partly roughed in, but for which one needs a new grinding tool.The tool described herein is particularly taylored for this kind of work, i.e., re-grinding or finishing grinding a mirror already curve generated or roughed in, or even fine ground, but needs to be re-ground for whatever reason.

While it is possible to build this kind of tool this way to start from flat with a flat blank, to rough it in from flat, there are certain risks attending making a tool this way and using it for a flat blank that require taking extra precautions. So for those of you interested in starting from flat with a channeled Winburn hex tool, which is the only kind of tool I ever use, whether starting from flat, or working an existing curve, I can describe a safer and easier to build tool to start working a flat blank that has all of the advantages of the channelled Winburn hex tile grinding face, but which is safer to use for starting from flat. That having been made clear, here, right up front, let's make a big tool to curve with Winburn hex tiles-

The mirror is a full thickness 16" diameter pyrex, cast by Corning in the thirties. The mirror and tool are still in existence, right here in Enid, and are available for about $300, I think. The mirror is ground to about f/4.71. The tool could be easily resurfaced and the f/ratio changed. The mirror is very heavy.

(1): Several squares of Winburn 1" cushion edge hex tiles. Winburn cushion edge hex tiles are ideal design for grinding, are the ones that Willman Bell says are especially made for mirror grinding tools (not true: they are floor tiles). They are manufactured in Little Rock, Arkansas. Ask your local tile dealer if he can get some for you. Do not order the two inch diameter tiles, but the one inch diameter tiles. There is a very good reason for this, which I will tell you later.

Make the two plywood "O" rings that will serve as structural supports for the illustration board coffer dam first. For mirrors below 14.25" aperture, we are going to build a tool with a one sixth thickness ratio. Actually, for tools for 12.5" and 13.1" in diameter, I think I made them just a little bit thicker. If you are making a full sized tool, you will need to make the inside of these two rings of plywood the same diameter as the mirror. The coffer dam will be a sealed band of illustration board secured inside of this double ring of plywood. The rings can be about two and a half to three or so inches wide all along their circumference. Space them apart by a little less than the intended thickness of the tool, with little struts- make the little struts out of little squares of wood, each with another little square of wood glued to it at a right angle, so that it will stand up when you glue it into place on one of the rings. Make about six or eight of these and glue them in place on one of the plywood rings, and then glue the other plywood ring on top of this one. Use something resembling a try-square when you glue the top ring onto the little struts, holding it inside and moving it around to make sure that the two rings are truly one above and one below each other, i.e., are well aligned, before the glue begins to take a tack, and then weight the top ring down.You will have a double ring of plywood ready to hold the coffer dam perfectly circular that you are going to fix in place inside this double hoop.

Make the coffer dam next by calculating the circumference of your mirror (diameter of mirror times 3.1415927). If you are building a full sized tool, the width (heighth, actually) will be the thickness of the tool plus the thickness of the mirror, as when everything is in place for the pour, the coffer dam will extend down around the mirror to the table's surface. If you are pouring a subdiameter tool onto the front of a larger mirror, the heighth of the coffer damn need only be the intended thickness of the tool. Cut out the strip of illustration board to form into the coffer dam with a number 11 hobby knife. Use a good straight edge.

Now you need to remove a lot of the little one inch diameter Winburn hex tiles from their paper backing. Do not under any circumstances use fire to burn them off. One of my friends could not understand why his tiles kept flaking off of his tool, ruining his work. I couldn't figure it out either until he told me that he burned the paper backing off with a torch! Throw the squares into a water soak, and then scrape the remaining paper off of them with a knife.

Next, prepare the match stick spacers for the tiles. Prepare them this way: measure one of the sides of one of the hex tiles. Then, mark a whole bunch of your kitchen matches to this length, and then, using the toenail clippers, cut a bunch of kitchen matches to length. Dispose of the igniteable heads properly. I know this is tedious work. That is why we are mirror makers: we thrive on tedious perfection seeking!

Open up two large plastic trash bags. They must be black (just kidding: for some reason I always used black ones!) and lay them out on the work surface, and place the mirror, face up, on the trash bags. These trash bags will be tied up over the poured tool/mirror sandwich to keep moisture in for the cure for the cement after the tool has been poured. Okay!!! Pay very close attention to instructions from here on out to avoid unnecessary disaster!

You have the mirror, face up, on the double or triple layer of heavy duty trash bags. You must not neglect to put these trash bags under the mirror. Now, cut out another piece of trash bag into a circular shape, about ten or fifteen percent larger than the mirror, in a circular shape. Wet the mirror's face thoroughly, and press this piece of trash bag onto the wet face of the mirror, and squeegie, to the best of your ability, as many of the air bubbles and creases out from between the mirror's wet face and this skin of plastic on it- squeegie them all the way to the edge and out. You will not be able to get a perfect fit of plastic against the mirror's face- this is okay. Secure the edges of the plastic that is now covering the mirror's face down around the sides of the mirror.

Now, if you are working a large mirror and are building a subdiameter tool, just place the double O ring hoops structure on the mirror's plastic covered face, centered, with its sealed illustration board coffer dam in place. If you are working a mirror that you are making a full sized tool for, make sure to fix the coffer dam inside in place in the double hoops, so that they will not slide down around the coffer dam.

Start laying tiles on the face of the mirror, placing your little pre-cut to length matchstick spacers between every side of each adjacent tile. You will finally fill in the area up to the edges; try to nip partial tiles down to as nearly the proper shape to fill in these partial spaces as you can. It is not necessary to slavishly carry this to extremes- look at the pictures, see where I left some gaps near the rim. Is not critical.

The matchstick spacers are of the ideal dimensions for both holding the tiles in place, but leaving enough room above the cement/sand mix for channels, and for providing the ideal width for the channels- about three thirty secondths of an inch (more than a sixteenth, but less than an eighth of an inch wide). Your tool will have a silky smooth, "dreamy", ball bearing action, I guarantee you! And it will eat glass like crazy! It will never seize to the mirror, either! You will never make and use another kind of tool for large glass, I promise. Now, finish the edge gaps that you did not fill in with a partial tile with little wads of aluminum foil about the thickness of the tiles. This way the cement mix will come out to the same heighth all over, even if there is not a tile in place where you have left some small partial areas without nipped down tiles, near the edge. Later, these areas will be filled in with wax. Look at pictures, above.

Follow instructions to the letter from here on out! Mix your Portland Cement and sand in about one to two, or one to three ratio (one part Portland to about two, or three parts sand)- try to get "mason's sand"- but any kind of sand will do- if you have irregular sand, screen out the large grains, they will be trouble. Sift the sand beforehand through a screen. Mason's sand is usually pre-sifted and more uniform in size. Don't mix your "mud" too wet; use a gardener's hoe and use a variety of motions to get it throroughly mixed, adding water a little at a time, incrementally. Do the "shake test" to see if it is beginning to be the right consistency; don't make it too wet, but do not mix it too dry, either- it needs to pack down into the shallow spaces left by the matchstick spacers between the tiles. Now, your "mud" (Portland/sand mix) is ready to fill into your assembled mold with the tiles spaced out across your plastic liner on the mirror's surface. This next instruction is critical; keep it clearly in mind!!

You are ready to ladle in, very carefully and gently with your small ladle, trowell, whatever, the first layer of mix across the tiles all spaced out across the mirror. You have to be gentle, and do it cautiously and slowly, or a slug of mud, just thrown in, will tilt some tiles up and out of place into the mix!! Don't let this happen!

After your first layer is ladled in, fill in more rapidly to the top of the rim of the coffer dam, and then "screed off" the top, so that this surface, which will become the bottom of the tool, will come out pretty flat. Now, this next is important: fold up a small kitchen towel, and get it soaking wet, and only partially wring it out. It should be pretty wet, but not dripping, "wringing" wet. Lay this folded towel gently across the top of the mix. Then, fold up the sides of the large plastic wrap the mirror is lying on up around the whole filled up tool/mold/mirror assembly, and carefully secure it up around the top, gently, so as not to push the towel down into the back of the mix. The wet towel is to keep the air inside very humid for the cure.

After the cure, unwrap the whole mess. You will have a nicely made tool ready to slide off. Only once did I have a little trouble getting the sandwich to come apart, and that was with my very largest tool, a full sized 14.25" If you have trouble it getting apart (you shouldn't) call me or email me. Don't take drastic, dangerous action. I got mine apart, I'll tell you how I did it, if you have trouble (I made a lot of these- I only had trouble once).

After getting the tool free from the mirror, peel off the plastic and pry out the little spacers and aluminum stuffings near the edges. Your #11 hobby knife's pointed blade will neatly spear the little matchstick spacers to lift them out. Next, take a little matchstick or other small tool, and gently scrape out little pieces of sand and mix that are standing alone, like in the vertices of where the hexes meet, where there will be little "peaks" of sand where the matchsticks did not quite meet. Please, if some of you would, take close-up pictures before you pry out the little matchstick spacers, and right after, showing the appearances of the tool up close at these stages. I lost my pictures of these steps.

Get your roll of UPS style packing, shipping tape, the clear wide kind, and wrap the mirror's back with a minimum of three layers, preferably four, of this tape, bringing the tape up along the sides of the tool as far as it will go. Alternate the parallel overlapping rows of layers tape at about thirty or sixty degrees to each other. Then, seal the sides of the tool with three or four layers, wrapped around circumferentially, on top of the other ones that came around from the back side. This side seal should come up to the level of the tiles, making an approximately one fifth of one inch high dam at the edge.

Melt a bunch of wax, pour it all across the face of the tool, filling in the channels and the empty spaces near the perimeter completely. I know- you want to know, what is the use of channels, if we are just going to fill them in? Just bear with me and you will see. The wax in the channels, which will be slightly excavated later to just slightly below tile level, serves four vitally important, separate purposes.

Now, we are going to excavate, with a small screwdriver, a little bit of the wax out of the channels- not very much; just enough to form shallow channels. Scrape off a layer about one thirty secondth of an inch deep, to just below the level of the edge of the tiles on either side of the wax filled channel. If the channels are too deep, they will just swallow and wash away grit when the mirror is on top. And even if you are working subdiameter only, with the tool only on top, you still need to wax and excavate just like those working with the mirror on top sometimes, too.

Lastly, before breaking in your new tool, you must bevel its edge. Do this exactly as you would bevel the edge of your mirror. Many amateurs use a wrong technique when holding the whetstone- many published atm books show this wrong technique. While it is true that you want to bevel the edge of both tool and mirror at a forty five degree angle, you do not want the stroke of the tool to cross the edge at a 45 deg. angle. That is to say, although you will hold the surface of the whetstone at a 45 degree angle to the mirror's edge (and the tool when you do it), you will want to make the direction of the stroke along the edge nearly tangent to the edge, instead of straight down across it. Stroking the stone straight down with direction of travel at a right angle to the edge of the mirror or tool can take a large chip or chips off of the edge of it. Don't do that. Now your newly made, sealed and waxed tool is ready to break in. I am going to make the instructions for breaking in your new tool and using it another little treatise.

Feel free to contact me via email with any questions about this.