Instructions on using the LM-4 "Perfect Pitch" Lap Mold

It's pretty easy! Lay your mirror on a horizontal surface, and place a sheet of paper towel on its polished face. The purpose of the paper towel is to allow the mold to separate easily from the mirror - without this, the soft polymer mold may be in such good contact with the mirror that it is hard to introduce air between the mold and the mirror to separate them. Place the pitch lap mold on the paper towel/mirror, open squares facing up. You can, of course, place the mold on a flat surface instead of your mirror, but you then miss the nice benefit of having the pitch squares conform to the curve of your mirror - this will require more time hot pressing for full contact.

Note that this pitch lap mold is unlike cheap rubber open-mesh types (I bought one of those from Edmund many years ago - it was a disaster). My mold has a continuous polymer surface on one side, so you can place it right on your mirror - this gives it the right curvature - and pour the pitch freely, with no need to worry about getting pitch on the mirror.

Now melt your pitch in a pot, and pour the molten pitch into the mold. The level of heating should be adjusted to render the pitch easily pourable, but without needless overheating or boiling. There's no need to carefully pour into each individual square - just pour the pitch over the mold until the squares are full, and then some. The pitch should cover the ribs with a thin (a millimeter is about right) continuous layer. Obviously, if you're making a small diameter lap, you don't need to fill the entire mold, but should instead pour only for the diameter you need - a little extra is handy, though, just to be on the safe side. If the lap you are making is close to the size of the lap mold, then filling most or all of the squares may lead to some spill-over off the edge of the lap mold. Anticipate this, and provide protection (a layer of plastic tarp or newspaper works great) for your worksurface. Note that for small to moderate sized mirrors (especially slow ones), the base for your polishing tool need not even be convex. If you wish to hang on to your convex grinding tool, a flat circular disc (plywood even works) can be used as the base of the polishing tool, as long as you pour deeply enough over the lap mold ribs to account for the sagitta (central depth) of your mirror's curve.

Now take your tool, and "Sphlut!" set it down on the molten pitch. If you want to decenter the squares to reduce the possibility of ring zones, set it down a few millimeters off-center. If you overpoured, there may be some side spill-over when you place the tool. It doesn't matter as far as the quality of the lap goes, but it may contribute to a mess. Go have a glass of claret while the pitch cools. To ensure proper adhesion to the tool (especially if it's made of aluminum or steel), the tool should be at room temperature. It can also help to give metal tools a wipe with a turpentine rag beforehand.

When the pitch has fully cooled (about twenty minutes), lift the tool - mold combo from the mirror and set it down, mold facing up. In hot weather, and/or with very soft pitches, 30 minutes may be in order. If the mold appears to be stuck to the mirror, don't worry - the fit is just too good. Use a rounded knife to slip some air between the two surfaces, or, better yet, apply vaseline to the mirror before you place the lap mold on it. Now simply grab a corner of the lap mold and slowly strip it from the lap and tool. It seems a bit easier to go diagonally rather than a row at a time. Voila! A perfect pitch lap in about a minute more than it took to melt and then cool the pitch! Do make sure that you complete this step, and don't simply head off to bed, leaving the stripping of the lap mold for the morning. For best results, the mold should be stripped off the pitch lap shortly after it has cooled. Also, make sure you remove any pitch squares from the mold that lie beyond your mirror's diameter. If you flex the mold, they will pop right out.

In the step above, there will inevitably be those pitch squares which extend out beyond the edge of the mirror. Since you probably poured a continuous layer over the ribs, there may also be groups of squares from the outer parts of the mold that remain connected. For any square with 25% or more contact with the tool, simply trim them as described below. Squares which only have 10% to 25% of their area on the tool may stay with the mold as you peel it off. That's not a problem; you can either ignore the small missing fraction of a square, or pop each such square out of the mold afterwords, and use a lighter to stick a corner of that square in its place on the tool. You will then need to trim off most of the remaining overhanging squares. The best tool for this is a one inch or 1.5 inch putty knife, together with a small hammer. After positioning the putty knife above the square to be trimmed, a light tap on the hammer will cleave the pitch with precision. This works extremely well, although it helps if the pitch is either on the hard side or a little chilled. If you have used a soft pitch, and/or the room temperature is a bit on the warm side, consider chilling the lap before trimming - this ensures a crisp cleave as you tap.

If you are working on a large mirror, the "Perfect Pitch" lap mold can be used to make seamless laps of any diameter. You may have noticed that the outer edges of the LM-4 lap mold are tilted inward at 10 degrees. This is an intentional design feature that assures that the distance between rows and columns of pitch squares will be maintained at its exact value when you place additional laps adjacent to prior sections, in what we call "tiling" mode. Since the sagitta (central depth) on large mirrors can be substantial, your tool will need to be convex (for a concave mirror), but it need not have a very accurate radius of curvature. Attach the pitch lap mold, open squares facing up, on a slightly larger flat board, using double-sided tape. Pour the molten pitch into the mold, as before, and wait about 60 seconds for it to begin to cool and thicken. Then lift the mold using the board, and smoothly invert the combo onto the tool surface, placing it adjacent to the last pitch pattern if one exists. The inversion step is a bit scary, but works pretty well. Press it lightly to ensure full contact (but don't push hard locally, denting the squares), and wait about twenty minutes until it has fully set. Strip off the mold as previously described, and the process can now be repeated until the lap is complete. A 24 inch lap takes a little over an hour, but most of this is waiting for the pitch to set - pouring, casting, and striping off the mold take only minutes.

Although the lap will now roughly conform with the shape of the mirror, full contact should be acheived before polishing commences. Hot pressing is called for, which amounts to gently heating the tool and lap in warm water, (or the microwave oven, if you've got the guts) and using weights to slowly achieve full contact between the mirror and the lap. If, as suggested above, you poured the pitch lap with the LM-4 laying on the primary mirror, then the lap curvature should be nearly perfect to begin with, and polishing can commence within a few minutes of the warm press. Polishing compound is used between the lap and the mirror as a release agent. I often skip the formal hot pressing step, and simply run a spray of warm water over the lap for a few minutes, and then begin machine polishing. The lap then flows into shape pretty quickly.

As polishing proceeds, the individual squares in the pitch lap will slowly begin to flow laterally and enlarge. Although the gap between the squares is ample to begin with, and the new LM-4 lap mold features a 10 degree facet side-wall angle, it's critical that the squares be trimmed well before they begin to close together. Just as you trimmed the overhanging squares when the pitch lap was new, make sure to examine the lap during polishing, and use a putty knife and small hammer to trim all the squares to clean, square edges as they slowly flow inwards. Should the squares ever flow together to the point of contact, it will be VERY difficult to chip clear the channels.

Since it's so easy to make laps with this method, you don't have to hang on to a tired old lap until your mirror is finished - just chip off the old pitch and pour a brand new lap. Or, try the trick described below. You can easily create specialized laps (such as petal laps) to aspherize or correct defects. Making the lap is no longer the stumbling block.

Here's a neat trick - while I've only tried this technique once, it worked perfectly, albeit with a metal tool. I placed my mirror face-up on a table, and laid the pitch lap mold on top of it. I then placed an ageing, closed-in steel tool and pitch lap, pitch face down, on top of the mold. I then laid a variable hot plate, face down, on the back of the steel tool. Over the next hour, the hot plate slowly warmed the tool, which softened the lap, which then flowed via gravity downward into the new channels imposed by the pitch lap mold. After the two were thoroughly commingled, the hot plate was removed, the pitch "froze", and the pitch lap was new again, with square edges, and ready for another few weeks of use. Isn't that cool?!? If you try this with a glass tool, you will need to take into account the low thermal conductivity of glass, and the possibility of fracture if the tool is unevenly heated. A better approach might be to put the tool-lap-mirror combo in an oven and very slowly raise the temperature. If you stink up the oven in the process, though, you're on your own!

Your pitch lap mold is ready to make many more laps, and does not require cleaning. Do be sure to store it in a clean place, however - any dirt (let alone abrasive!!) on the floor of the squares will end up in contact with your mirror. The LM-4 ships with a clean, heavy-gauge, zip-lock poly storage bag - use it! As you make laps with the mold, (especially with the darker pitches,such as Gugolz) it will darken and acquire pitch stains. It will still work just fine, but if you want to "bring it back to new" appearance, an immersion and agitation in a non-polar solvent (gasoline or, better yet, Coleman fuel) will work wonders, although the odor may be an issue.

Some pitch crumbs may adhere to the lap mold, usually just outside the diameter of your pitch lap. Flexing the lap mold will dislodge most pieces, especially if you chill it beforehand. A disposable plastic knife can also be used to remove pitch chips. Finally, a soak in gasoline or other non-polar solvent will remove all the pitch.

Hey, be sure to email a .jpg image of your lap to me. Great images will be added to the site.

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