Pitch Lap Part Eight

[callmeshane303]

I was cutting some glass today - to make a lapping base to clip my "wet and dry" papers onto, so that I would have a nice flat plate to lap my wood cutting tools on; and

It occurred to me that glass will grind it's self, as when grinding a rounded edge, I find the glass residue in the abrasive compound adds to the cutting "free abrasive", as the primary abrasive wears down.

So have you considered using the grinding "waste powder" as the grinding compound, in a soft matrix?

[foreilly1958]

@callmeshane303 No Shane, I have not. Grit is prettty inexpensive. I have lots of it. I really don't need to improvise. Glass is quite abrasive however. I urge caution when using precision tools around glass, it will wear them down quickly.

[callmeshane303]

@foreilly1958

Yeah - in any area where there is silica / quartz sands, I note that the aluminium sprockets on a bicycle, being relatively soft, will embed the grit into the surface, and grind away the hard steel chains at a ratio of about 5:1 - or 5 chains for every 1 sprocket.

I want to cast up a 1 meter parabolic mirror, from a borosilicate glass.

Pretty easy I think.

[foreilly1958]

@callmeshane303 I wish you all the best in your endeavor.

[carluchoparis]

Hey Francis... I've been wondering, I have not used beeswax yet on my pitch lap. As you may remember, I've gone through 3 pitch laps. The first one was too hard, second one I had added some turpentine, and didn't mix it well enough, so I had soft pitch bubbles within the lap and it failed. The third pitch lap has been working very well, the surface of the mirror is invisible to the naked eye, but the surface still reflects the light from a flashlight and a laser. Will the beeswax help?

[foreilly1958]

@carluchoparis Forget the laser. It shows the underlying structure of the glass an is not a proper test for polish. Use a maglighht instead.

Beeswax will not help. It may reduce sleeks. That's about all.

I count on about an hour per inch of aperture when polishing using Cerium Oxide. Less than half that time when using Universal Photonics Number 85 Rare Earth Polishing Compound.

[carluchoparis]

@foreilly1958 Francis, correct me if I'm wrong... With my second and third pitch laps I have been polishing a 6" f/8 mirror close to 22 hours . I think the reason it's taking so long is because I had a really bad case of dog biscuit and I may have not fine ground the surface enough before starting to polish (I still see pits mostly close to the edge)...

What do you think?

[foreilly1958]

@carluchoparis 22 hours on a six inch? Three would be more like an approrpiate amount of time. I think that fine grinding time may be the problem. I would go back and spend an hour with each grade of fine abrasive. Contact me offline for a schedule of grits.

[carluchoparis]

@foreilly1958 I was afraid of that! If anything, I've worked a lot on my strokes, learned to set up and use the Foucault Tester, and have experience in making pitch laps!

I'm going to fine grind again, I will be much more careful with the surface pits, and see if this kills the problem...

What do you mean by a schedule of grits?

[foreilly1958]

@carluchoparis I use the following Silicon carbide abrasives in this order; 80, 120, 220, 320 and sometimes 500. Aluminum Oxide 20 micron (Unless I used 500 Silicon Carbide. They are the same size.) then 15, 9 5 and then 3 for twice as long as the other grades. Using 3 is controversial as it can cause scratches if not used with lots of water. Each grade of abrasive is used for an hour on a six inch once you have the curve roughed out with 80. I generally use W strokes to avoid astigmatism.

[carluchoparis]

@foreilly1958 Hey Francis... I fine ground yesterday using 220 grit for about 1 hour and 15 mins. Then I ground for about the same time using 25 micron and carefully checked the surface for pits. I noticed hundreds of pits all over the place, this is probably part of the culprit... I will do another hour tonight with 25 micron to see what happens to the surface... This time around I think will work! Thanks for the help!

[foreilly1958]

@carluchoparis That's a very large jump from 220 to 500 micron. Do you have any 320?

[carluchoparis]

@foreilly1958 Actually I believe 25 micron is about 380 to 400 grit, however, I've been fine grinding using the 25 micron for about 3 hours and there are still hundreds of pits on the surface... Something is very wrong here... I presume I really should get 320 grit before using 25 micron... This sets me back a bit... will let you know the outcome once I use the 320 grit. I can't believe I was polishing with such a badly ground surface...

[carluchoparis]

I've been reading up on mirror making, and watching your videos has been very educational... I'm going to make an 6" f/8 telescope, and your videos are awesome for learning how to "move" the mirror around... Your videos really sink in... I hope I can do as well as you...

Thank you so much for the effort in teaching this trade!!

[Motorcoach1]

I've watched all your Vids and one thing I'm confused with . Being a newbe I was wondering if you could post a vid on how a telescope works and in another part why you grind and polish the mirrors to use in the telescope . I been working in glass all my life and spent 11 years in photo optics. I would like to build one just for fun. Mike

[foreilly1958]

The one thing that will beat my visuals is when you start pushing some glass yourself. Please keep us posted.

Best regards,

Francis J. O'Reilly

[PhnxCaller]

Thank you for sharing your knowledge, this is an awesome series of videos, really helping me out, I've read all about this, but nothing can beat your visuals! Thanks! Also looking foward to the next one!

[postcolonial1]

Looking forward to the next one.