Pouring The Pitch Lap
Making a 4.25 Inch Dobsonian Reflector Telescope
Re-Using the Tool
The traditional way to make the pitch lap is to pour pitch over the tool. This has a number of disadvantages. For one, if you change your mind as I did, and want to go back to grinding, you have a rather big job on your hands to remove the pitch. Once that is done you have to pour the lap all over again. If you want to reuse the tool and the lap on a future project, you have to decide which, you can't keep both.
Instead I prefer to use a tile tool built on plaster, with a second plaster disk for the tool.
In the class where I made the lap for my doomed 12.5 inch mirror, we used a traditional method. The pitch was melted, poured onto the tool, and then pressed down with the mirror to shape it. Then a wet dowel rod was used to squeeze channels into the lap, followed by another pressing. It was easy to make a mistake doing this. Along the way I came across a much easier way that produced a much nicer result. Kevin McCarthy makes a product called the LM-4 "Perfect Pitch" Lap Mold for $45.00 plus shipping and handling. It consists of a mold that you pour the pitch onto. Before pouring you place the mode on top of your mirror blank to shape it, and you then place the tool on top. After a little cooling, you cut off the excess and have a perfect looking pitch lap. It is highly reusable and I recommend it.