2013 - January 19
Stephanie White & Robert Simmons
FLAT LAP GRINDING - REVEALING THE BEAUTY WITHIN
Window Beads on a Flat Lap
Once your beads are finished and removed from the mandrels you can open Ďwindowsí in the sides to expose features in the interior. If you have a very large bead and need to remove a lot of glass to make the window large enough you might want to consider starting by using a wet saw to cut off a slice of glass, then going to the polishing steps. Finer saw blades will produce a smoother surface for you to start polishing.
Flat lap grinders use abrasive disks to remove glass from the window surface, starting with a coarse grit and moving up to finer grit as you proceed to a polished finish. Grits are numbered, lower numbers being coarse as they are with graded sandpaper. Disks come in many different levels of abrasive and using more abrasives in ever finer textures makes it easier to achieve a very smooth surface. Some laps come with sets of four or five disks, plus a final polishing disk; for example 170, 325, 600 and 1200 grit diamond laps, plus a felt pad. Also, some grinders include an acrylic master lap which provides the backing for the diamond disks.
Start off with the lowest number grit to remove bulk glass (usually 100 to 180 grit). Make sure that you have a continuous flow of clean water across the surface of the disk as you work. Donít run the speed too high as you donít want to build up a lot of heat at the surface, I usually run at about 25% of maximum RPM for my lap. Donít press down hard on your bead when you are running it on the lap, let it float on the water film and have the machine do the work for you. Your disks will last longer this way. Hold on to your bead as it may get snatched away and flung across the room if you donít.
Grind away glass with the coarse disk until you have reached the approximate size of the window that you want. After this point you are mainly polishing that surface and wonít remove much more glass. Once you have reached this point change to the next higher number disk and proceed as before. Polish until you have a uniform surface with no large scratch marks evident. Move the piece around on the disk surface as you work and rotate it so that you donít develop a highly defined scratch pattern.
Once you have a pretty uniform surface move on to the next disk, continuing through each level of abrasive until you reach the highest that you have. Rinse your bead periodically, especially after each grit, dry it carefully and inspect the surface for larger scratch marks. You may have to go back to a more coarse disk and repeat the process if you find that you have large residual scratches.
Final polishing is generally done with a metal oxide powder mixed with water to a slurry consistency. Cerium oxide or tin oxide are most commonly used and for glass polishing are pretty much equal. I use a neoprene foam disk with the slurry of cerium oxide rubbed into the surface. The slurry also can be drizzled over the grinding surface with a small sponge. There are several types of fine polishing disks that can be used with this step (leather, felt, etc.) and the choice is often determined by what is available for your lap grinder model. You wonít be running a flow of water across the surface during this step as you have in all of the previous steps but you want it to remain wet so a spray bottle of water can be handy to spritz it as you work. Run your lap at low speed and gently float your polished surface on the disk for a few seconds at a time. Donít press down and donít leave it in place for very long. You want to avoid heat buildup. A few runs on this disk should bring up a nicely polished surface that opens up the inside of your bead. Wash it thoroughly when you are finished to remove any residual polishing material.