A failed lap...

[FraserClarke]

In the spirit of learning from failure, I'm showing the sorry results of this afternoon's polishing-lap making

First though, a few caveats;

1) I haven't made a lap in near 15 years

2) The pitch (from vacuum coatings, I think) is about 4 years old, and really really hard! I've softened it with some boiled linseed oil, but I think I need more.

Prep went OK for the lap, following the process GlassPusher set out in his "Making a 12-inch mirror" thread. I got the pitch melted OK, and added some boiled linseed oil. I'm melting the pitch on a gas camping stove rather than a (much prefered) electric hot plate, so didn't have fine control over the temperature. Probably heated the pitch too quickly.

Once melted, I poured the pitch onto the plaster tool I'd made earlier, with a masking tape dam to stop the over-flow. Got some bubbles in the pitch, because, I think, it was too hot. Then I struggled to judge when to do the first press with the mirror, and by the time I'd decided it was ready -- it was WAAAYYY too late and the pitch too hard. So, I poured a bit more pitch (still hot in the pan) into the middle of the lap and tried again. Much better results this time, and could form the facets. Still think I waited bit too long before pressing though. Unfortunately I didn't pour enough pitch on, so I didn't form the complete lap (see picture 1).

Pressed ahead, mainly to see how the pitch hardness was doing. After an hour or so of pressing, I put CeO slurry on the mirror and took a few strokes to test contact. Polishing action was nice and smooth, with a consistent amount of resistance. Picture through the back of the mirror (picture 2) shows the contact is OK -- but not great. Could do with more pressing and/or softening the pitch further. Thumbnail test shows the pitch is on the hard side (only a small indentation after 10s).

My lessons learned from this attempt have been;

1) I tried to pour the lap with the pitch too hot, and ended up with bubbles

2) I waited too long before doing the initial press with the mirror, and the pitch was too hard (hence the re-pour)

Lap is now in the freezer thinking about what it's done, and preparing to be broken off and remelted...

[Glasspusher]

Yes Fraser, looks like the pitch is way too hard. The very non-scientific thumb nail test is called for I think. Pressing the thumb nail firmly onto the pitch lap should leave an indentation after 10-15 secs...if not it is way to hard. Boiled linseed oil is the way to soften it, might need a few tablespoons though. Really good to see people getting involved with practical optics....good luck with the next pour.

John

[barkis]

I hope John doesn't mind me butting into his thread, but I just want to suggest a way to make your lap.

-----------------------------------

Making a lap ain't easy, so don't worry too much. My carelessness in my early attempts almost lost me my shed. I had left the pitch meting in a pan on a hot plate in the shed. Went to attend something else, and after a while, my daughter came in to my den and asked what the red glow was in the shed. Not a lot of damage, but loads of black sooty stuff covered most of the interior.

I was lucky I didn't lose the lot.

Anyway, the pitch must be heated slowly, and certainly not brought to the boil, as the fumes can be very volatile, and will ignite very easily. If the pitch is hard, you can use a few drops of Turpentine to soften it, but do this a little at a time. Test by taking a little from the pan, and drop it in cold water, take it out and let it come to ambient temperature, then do the thumbnail test that John explained to you.

Bring the pitch to a nice consistency, but not runny.

Warm the the disc that the pitch will be poured on to, by placing it in warm water for a while, but make sure it is completely dry before pouring on the pitch. Make sure the disc is level too, and don't pour too much to cause an overflow.

Allow the pitch to cool over night with a cover over it to prevent any thing falling on to the surface.

When you are ready to facet the lap, mix a little Cerium Oxide with water to a paste like consistency, not too much, it doesn't take a lot. Then paint it over the pitch surface and let it dry.

You then have a nice pink or white surface on which you can mark out the channels to be cut, making sure the the centre point of the proposed lap, is exactly where the corner of one the squares will lie.

There as been some debate about the need for this, but I always did it that way., If John suggested something else, then follow his advice.

You can then score the lines for the channels with a Stanley knife and a straight edge, taking great care whilst doing so, as you don't want the knife to slip and take your finger off.. Proceed carefully, there is no hurry. You may need a few blades as the dull quite quickly, although they can be reversed in the holder.

It helps to keep the blade of the knife well lubricated with washing up liquid to prevent the pitch from sticking to the blade.

It is sufficient to mark the channels, not cut right down to the discs surface. Once you have the channels marked out in one direction, you can do the others at 90 degrees to them.

You then have the squares already to be completed.

Warm the lap in some warm water not hot, after it has heated for a while, take it out and squirting some washing up liquid along the score marks, proceed to cut each channel right down to the disc surface, making sure the cuts are complete, and keep some liquid in there to prevent the pitch from sticking.

When you have completed one channel, you can proceed to cut it out. Run the blade down each line cut to make sure it is deep right down to the disc surface, then using a thin bladed screwdriver, you can start to prise up the pitch from the channel, taking care to follow between the lines. Do not stray underneath any of the pitch that will form a square.

Remember washing up liquid will be your friend in this operation, because it keeps pitch at bay.

Work slowly and methodically and you will succeed in making a nice lap. It is a tedious procedure, but it's the way I preferred to do it, although it might not appeal to others.

Once you have all the channels cleared out, you can proceed to press the mirror on to the lap to get the squares to conform to the mirrors surface. Warming the mirror, and keeping it covered in a soapy mixture of Cerium Oxide an Washing up liquid will soon get the lap surface conforming, although the squares will deform and channels will tend to close up, so you must keep trimming to keep them clear.

Lap making can be a horrid job, I found it to be at first, but it is a crucial and necessary part of making your mirror, and the means to turn it into a superbly polished, and figured surface. So persevere, it will be worth it.

Ron.

[FraserClarke]

Thanks for the input guys. Ron, your method is very much like what I did 15 years ago, though I think I used a soldering iron to cut the channels?? (don't have any good notes, hence having to try and learn it all again now!)

The cold (frozen) pitch came off the base very easily, and it's not back in the pan waiting to be melted again. Will post the results of attempt two, when I find some spare time during daylight...

[Glasspusher]

Fraser, I meant to ask, what are the details of the mirror you are making?

Ron, that is a nice summary of how to make a pitch lap.

John

[barkis]

Thanks John I didn't want to impose, but I when I saw the post I suddenly got the urge again . It's surprising how mirror making can get into your psyche. Although I have no reason to make a lap, describing how I did mine was a substitute best for doing it.

I can well understand how the love of optics was ingrained into David Sindens life.

Ron.

[FraserClarke]

It's a 6 inch f/6 that I started about 15 years ago. It was almost polished out (at f/5.6), but then things like degrees and careers intervened and it sat in a box for a long time.

I picked it up again a few months ago (partly thanks to threads on here). It's currently reground to f/6 (hit the focal length to within 1cm ), and awaiting polishing. Will write up a description of the whole process when it's finished...

[Glasspusher]

Looking forward to seeing your write up on the finished mirror.

John

[FraserClarke]

Pictures of failed lap #3. Much better this time, but a massive overcompensation on the boiled linseed oil has made the pitch super soft! (I see this as a positive, because I now know I can control the hardness of the pitch). Getting there slowly (only get a chance to experiment once a week at most).

In the background you can just see the edge of Texereau style pitch hardness tester (will post a thread on this at some point). For indication, lap #2 tested with a hardness of 0.6mm/5min (right at the hard end of Texereau's acceptable range). This lap went through 6mm in 45 seconds, and then hit the base of the tool!! You can see a thumbnail mark at the bottom-middle of the lap.

[Glasspusher]

That is a nice looking lap, pitch sounds like it is a little soft though. I think you will be renewing the channels on a regular basis! How much linseed oil did you add? I like the Texereau style pitch hardness tester, I really must get round to making one of these myself. Looking forward to following your progress.

John

[FraserClarke]

Added 12 teaspoons of boiled linseed oil, which it seems was far too much! I'd added 12 also between lap #1 and #2, and it only made a smidge of difference. Seems now though that I'm close to the recommended hardness region, so presumably now a lot more sensitive to the amount of oil added. I'm going to add a bit more hard pitch to mix, and let it cook for a little while to harden it up again.

[FraserClarke]

Lap #4 has turned out a lot better! Mixed in some harder pitch to compensate from the previous too soft version, and came up with a much better mix than previously -- about 1.6mm/5min on the Texereau scale; on the hard side, but probably usable.

Pictures showing the lap after a warm press with paper between mirror and lap (note matted surface on facets showing good contact), and mirror on top with CeO, again showing good contact I think...

Now polishing the mirror with this lap, to see how it behaves. Polish is coming up nicely, probably needs another few hours to polish out.