Making the Pitch Lap, and Polishing

I’m waist deep in the polishing process of the telescope making project, for me this is the final stage.  As I stated at the beginning of this project, I would omit the Foucault test as well as figuring of the telescope glass.

In truth, I had to make 2 pitch laps.  The first one was okay, but now very good.  The “channels” that were cut were far too shallow, and the lap came out lopsided.  Besides, I got a little anxious to finish this mirror and polished before fine grinding was done.  So I had to scrape off the lap with a knife and return to fine grinding.  How did I know? Well, the surface was more frosted than it should have been for starters.  Also, I found that there was a scratch that went unnoticed.

So another hour and a half of grinding is what it took to correct these few problems.  The new pitch lap is much better and it’s polishing!  Below is the setup of the area for the pitch lap making:

Lap making setup

Now, before I get into the whole process, because it is an involved one, let me state that this only one way of making a pitch lap.  There are probably hundreds of ways (no I’m not kidding) to make a successful pitch/wax lap.

All of the books I read on this stage were advising the beginner not to make a lap of pitch, but I didn’t listen, because of extensive research and tips of the actual process.  You may wonder why I said “wax” in the preceding paragraph.  That’s because a lot of people have made wax laps over the years, mainly of beeswax.  I’m not sure if you can purchase beeswax sheets anymore, but maybe some are still around.  The beeswax lap serves as an economic alternative to the traditional pitch and is easier to make (not anymore unfortunately).

On to my lap making process!

The first thing to do is to open up the pitch package, put it in an old pan, and heat it very slowly.  Because I am trying to be as economic as possible, I decided to purchase a synthetic lap, Acculap Pitch.  I bought the intermediate grade as it is best for laps of this sort, which can be purchased here.

While the pitch is heating, the surrounding area should be covered well in newspaper (see above photo).  Also, I got a large bowl and put hot water in it, then the two glass disks.  This is probabilistic one of the tensest parts of making the lap, for if even a single drop of cool water gets on a glass disk, it will crack it and it’s over.  Luckily, that did not happen.

Be sure to watch the pitch lap closely, and I meant it when I said to heat it slowly, because I became a little impatient and heated it too fast, and it started to bubble, which hardens the pitch.  Fortunately, the addition of turpentine can bring it back to it’s normal hardness.  But be careful if you do this to not get any in the flame, because of the explosive properties of turpentine.

The reddish hue is due to the Rouge from the previous lap.

The reddish hue is due to the Rouge from the previous lap.

While all of this is going on, I took a small bowl of water and filled it with warm water and a soap solution of dishwashing detergent, this will be lubricant to keep the mirror and lap from sticking while pressing is in progress.  Also, when the pitch is poured, a cardboard damn will need to be made to retain the pitch on top of the tool.  This was made out of a cereal box, and the glossy, printed side was facing in as its able to retain the moisture of the pitch.

When all of this is finished, the pitch should be very liquidy, though still thick as pitch is.  Then I removed the tool from the water bath, dried it very thoroughly to promote good adhesion of pitch and slowly poured the pitch onto the tool.  I decided to have this lap much thinner than the previous lap, because if I have to scrape it off again, it will be much easier to accomplish.

Freshly poured pitch

Freshly poured pitch

Now I let it sit for 5 or six minutes, for the pitch to harden enough to peel off the cardboard dam.  This is my favorite part.  There’s just something about the lap that really looks sharp, or as sharp as a slab of pitch on glass can look.  I did not get a photograph of it, but it still remains in my mind.  Now one of the most critical parts- pressing.

Now the soapy water comes into play.  I poured a liberal amount of the stuff onto the warm lap, then slowly set down the mirror.  Now I move the mirror slowly around a half an inch to 1 inch in different directions, also rotating the mirror.  I thought I made good contact and it feels smooth when I polish, but the edge is polishing before the center, which means the dreaded turned edge.  This can be fixed, but I need to cold press, which is a process where a rouge lubricant is put between the mirror and lap and a weight is put on top of the tool to sit for 30 minutes or more.  The only problem is that the lap dries rather quickly and I don’t want to get them stuck together.

Getting back to the lap making, I slid the mirror off after I though good contact was made, and let it cool off for a couple of minutes, and then proceeded to cut the channels.  In Amateur Telescope Making, they recommend using a razor blade to cut these large channels.  They even have pictures of  someone cutting a perfect, flawless lap from a razor blade.  This is however, not the case.  What follows is jagged, uneven impressions, and almost every time chipping off large pieces out of the face of the lap.  Me and my Dad learned this making the previous lap, which by the way, my Dad is a huge aid to me during this process.

So just to see, we tried a couple of lines, and it came just as described.  So my Dad came up with a genius way to make good channels, a small handheld grinder.  This served perfect for making neat, and good looking channels.  What was most amazing to me during cutting these channels is the effect of the ground up pitch.  I grinds it into this very fine fuzz, the closest thing being cotton candy.

A finished pitch lap

A finished pitch lap

It is not the best lap I’ve ever seen, but it does it’s job like it should.  For polishing, I decided to use Red Optical Rouge.  I would prefer to use the faster polishing Cerium Oxide, but it’s more than triple the price for the stuff and like I said, this is a budget project.  I have heard that Rouge gives a better polish that Cerium Oxide though.

The main difference between grinding and polishing is that in polishing, you have to be very careful when you have the lap on the stand.  It should be dust free, and the work is so delicate that dust will scratch the polished surface.  You wouldn’t think there would be any chance to get dust between the disks, but there are actually many.  For example, you expose the lap when you go to renew the charge of rouge.  So to stop this, I put a piece of plastic or a sheet of paper over the exposed lap to keep dust to a minimum.

One thing about using Rouge is that is is abominably messy.  The stuff gets everywhere! And it’s all over your hands most of the time, so when you go to wash your hands with a bar of soap, the bar will be covered with rouge.  It gets on clothes, doors, doorknobs, towels, even in your hair!

My new goal is to be finished with the entire project by the end of the year, which is not a whole lot of time.  I probably won’t get the mirror aluminized, but everything else (I’m hoping) will be finished.

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One thought on “Making the Pitch Lap, and Polishing

  1. HI JUSTIN;
    I’VE BEEN INJOYING YOUR LATEST BLOG UP-DATE ON THE POLISHING STAGE OF YOUR PROJECT. IT LOOKS LIKE YOUR LEARNING THE MIRROR MAKING TECHNIQUES THROUGH TRIAL AND ERROR METHODS. THIS IS A GOOD THING FOR THE BEGINNERS FIRST ATTEMPT AT TELESCOPE CONSTRUCTION. DURING THE FINAL STAGES OF ASSEMBLY YOU WILL LEARN THE IMPORTANCE OF A STURDY MOUNTING, PROPER MIRROR ALLIGNMENT, TUBE BALANCE, VIBRATIONAL PROBLEMS.

    I COULDENT HELP BUT LAUGH WHEN YOU WERE DESCRIBING THE MESS THAT ROUGE MAKES ON EVERYTHING IT COMES IN CONTACT WITH. NO MATTER HOW CAREFUL YOU ARE THAT STUFF PLAYS HAVOC ON ALL SURFACES. ONE TIME MY MOM PUT
    A ROUGE STAINED TEE-SHIRT IN THE WASHING MACHINE AND IT
    TURNED THE INTIRE LOAD ORANGE. NEEDLESS TO SAY I WAS NOT TO POPULAR. IT EVEN STAINED THE INSIDE METAL OF THE WASHER.

    THE PHOTO’S OF YOUR PITCH LAP WERE MOST INTERESTING. FROM WHAT I COULD SEE IT LOOKS LIKE YOUR GETTING THE HANG OF IT. THEIR NOT THE MOST ATTRACTIVE LOOKING THINGS BUT THEY WORK QUITE WELL. THE LAST ONE I DID WAS CALLED A MOLDED LAP. IF YOU CHECK MY BOOK YOU’LL SEE HOW I DID IT.THE PITCH WAS POURED ONTO A RUBBER MASK LAYED OVER THE MIRROR INSTEAD OF THE TOOL. THE RUBBER MASK HAD HOLES IN IT AND FORMED PERFECTLY ON THE MIRROR AND THEN I PLACED THE TOOL ONTO THE PITCH ,LET IT COOL, AND THEN RE-MOVED THE MASK AFTER THE PITCH COOLED. WAS ONE OF THE BEST LAPS I EVER MADE.

    IN CLOSING I REMEMBER YOU MENTIONED PATRICK MOORE’S BOOKS. THE ONE MR CARLE GAVE ME AT THE ASTRONOMY MEETING AS A AWARD WAS CALLED “A GUIDE TO THE MOON” IT MIGHT STILL BE AVAILABLE…

    TAKE CARE JUSTIN
    LOVE GRANDPA NEIL
    CLEAR SKYS
    .
    .

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