Pitch Lap Part Eight

Part eight on how to make a pitch lap. Thank you for watching my video. Please post your comments!

Category:

Science & Technology

Tags:

• Amateur
• Telescope
• Making
• ATM
• Pitch
• Lap
• pitchlap

[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...

[foreilly1958]

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

[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]

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

[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]

@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.

[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]

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?

[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 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.