reducing OD of a glass lens on a cylindrical grinder

[eKretz]

CBN for me. Hands down.

 

[specfab]

Though I'm a major adherent to the philosophy of "Use what you got, make it work", grinding of glass is far better with diamond. I haven't had any real experience using CBN, as diamond is always the automatic go-to for glass in optics fab. The size of wheel required for the cylindrical grinder obviously gives pause for thought, at least for the budget. As a sort of side note, I have had good success using a 6" wheel from McMaster-Carr that has a polymer body and circumferential metal band which has diamond "spots" applied; this is for grinding the face of the glass to create wedge on 100mm dia flats. It works nicely for that as a pre-lapping shaping step, using a glass saw/grinder with water coolant. P/N 8725A82
For waxing parts to fixturing, Universal Photonics in NY carries waxes in various temperature ranges.

 

[eKretz]

All I know is that I wouldn't try it with the SiC wheel. A light fuzz, maybe, but not stock removal. But I'd buy the CBN wheel at a good price in a second. Even if it doesn't work well for the glass you'll have it for when you want to grind some hardened steel, and you know that will eventually happen. Worst case, you have a nice CBN wheel for peanuts and then you find a diamond wheel for the glass later.

 

[ballen]

I just bought the CBN wheel, as you say, it will at least be useful for other things. Also, a CBN wheel can't blow up in your face, ought to be safer than vitreous. Is that right?

 

[michiganbuck]

Bruce, if you haven't made any big moves yet...
I suggest that you get some glass at the hardware store (perhaps a beer bottle, not BUD) and if you have a green wheel, a CBN wheel, and a diamond wheel take a wet grind in that glass.
Good to pad your chuck with a couple of layers of card paper because glass is harder than a chuck and will mar it ( and consider a beer bottle might explode and go flying).
I was thinking about how you might run it on your surface grinder but haven't come up with a decent fixturing device.
I was thinking about a wherlley at one end and a bearing spindle at the other end, set cross-ways on your chuck..with each having a face plate and two rubber O-rings pressing the part with about 25 to 35 pounds applied pressure, to be hand cranked or motorized, turning under a wheel in line with the long travel.

Sharp edges on the part will induce handling fracture so after being OD to size the edges should be honed /beveled IMHO.

QT: (You could give it a shot,) I would not take a chance on a customer's part or buy an expensive wheel not knowing/thinking it might work....with not making a few tests.
Making a circular lens grinder jig I would make it such to grind a 6" part so you might have a new talent to market (1/2 to 6").

 

[ballen]

Hi Buck, that seems like a good idea. I'll first experiment on a piece of scrap glass with the CBN wheel. I've also got a small (1 inch) resin/diamond wheel that I could try on the internal grinder setup, but I have a hard time seeing that taking off 15mm of diameter. Just seems too small. Cheers, Bruce

 

[michiganbuck]

Fixture design sent.
Agree that wheel is small..but it should show how a wet diamond wheel can grind glass. One can jury rig a tool post grinder to an Od grinder for a one -up part.

You should buy a surface grinder diamond wheel (7" or 8") 120 grit ..used is fine..but a real wheel res or vilified..not a coated wheel that only has .015 of diamond.

You may be surprised that wet grinding glass with a diamond wheel takes almost no wheel usage.
Buck

 

[eKretz]

QT: (You could give it a shot,) I would not take a chance on a customer's part or buy an expensive wheel not knowing/thinking it might work....with not making a few tests.

That's the beauty here, he says the wheel is available for a very good price. Pretty much no risk; a CBN wheel will certainly find an eventual use on hardened steel in the shop at some point anyway. Personally, those are the kinds of buys I actively seek, even if I don't need the item right now... if one waits until an item is needed *right now,* one will almost certainly pay out the nose.

 

[ballen]

While I was traveling last month, two things arrived:

(1) The lens: a cemented doublet, 52mm diameter, 282mm focal length. I needed to reduce the diameter to 36.9mm. (This was the second lens, the first time around a different vendor sent me a 220mm FL lens, probably hoping that I wouldn't notice that it was wrong.)

(2) A CBN grinding wheel, purchased used for 60 Euro including shipping.

I needed to reduce the diameter of the lens from 52mm to 36.9mm, keeping it centered to 10 or 20 microns. I wasn't sure if CBN would work, but I did look up its Koop hardness, which was about an order of magnitude harder than glass, so I thought it was worth a try.

This morning I cut the bottom off a glass bottle, mounted it on the cylindrical grinder, and reduced the diameter by a few mm. This went smoothly, and there were no signs of the wheel clogging. So then I mounted the lens and went at it:



The lens is centered and attached on the left with double sticky tape to an aluminium post, which is held in the 3-jaw chuck. On the right is a foam rubber pad, then a brass plug, then a live center.

The grinding was very easy. Apart from my initial light dress with a norbide stick, I never needed to dress the wheel. It might be that I could have just plunged in, but since I didn't want to endure the wait for another lens if I broke it, so I went slow. I set the grinder to traverse about 2/3 off on either side, with a 7 micron infeed at the left end only. A couple of hours later, it was done:



and here's in the mount:



I have read that one way to dress a CNB wheel is with a broken piece of grinding stone. I think the glass lens, being a friable material very similar to the vitreous binder of a grinding wheel, is constantly dressing the wheel. I didn't see any visible wheel wear, my diameter settings didn't change as I removed 15mm from the diameter (lens is 12mm thick, so this was about 600 cubic mm of glass).

Here's a question for the group: can someone fill in the missing markings of the CNB wheel and tell me what they mean? This is a 300 x 20 x 127mm wheel, with a 2mm CBN resin binder strip on the OD.



Thanks again for all of the advice!

Cheers,
Bruce

 

[eKretz]

Glad it worked out for you. Figured it would probably be fine. 1A1 is the wheel type, B is for Borazon, just means CBN. 126 is average grit size in microns. The last bit is probably concentration of grit and perhaps bond info. 187551 is a part number. Indimant is probably the brand name.

Probably you knew all of this already. Can't help with the exact concentration and bond info. I can't imagine that it's 18% concentration. A quick web search shows that Indimant is still in business but that they appear to no longer make grinding wheels. Instead it looks like they make diamond turning tooling for the record-making industry.

 

[ballen]

Glad it worked out for you. Figured it would probably be fine. 1A1 is the wheel type, B is for Borazon, just means CBN. 126 is average grit size in microns. The last bit is probably concentration of grit and perhaps bond info. 187551 is a part number. Indimant is probably the brand name. Can't help with the exact concentration and bond info. I can't imagine that it's 18% concentration.

Pretty sure that the obscured letters say:
B126 V18 KR ??? possibly KR191. Can anyone here decode those?

Perhaps a better question: what kind of work should I use this wheel for? And what materials should it NOT be used for?

Cheers,
Bruce

 

[eKretz]

Use it chiefly for hardened steel or iron. As you've discovered, use on glass can be okay too.

 

[ballen]

I'm curious, if I use it on unhardened steel, what will happen? Will it clog quickly?

 

[eKretz]

I'm curious, if I use it on unhardened steel, what will happen? Will it clog quickly?

Likely it will grab the abrasive particles and pull them out of the bond. So accelerated wheel wear.

 

[michiganbuck]

Creep-feed-plunge would have been ok..but you did a fine job of it.
Did it make any noise when grinding?
Nice set-up.
*Now you have a new hat. Lens grinding 50 bucks an hour.
Oh..did you mention part RPM ? (the rest of the story)

( if I use it on unhardened steel,) likely to get noisy crying for a dress, drag-down RPM, and start eating up dollars.

 

[ballen]

Creep-feed-plunge would have been ok..but you did a fine job of it.
Did it make any noise when grinding?
Nice set-up.
*Now you have a new hat. Lens grinding 50 bucks an hour.
Oh..did you mention part RPM ? (the rest of the story)

( if I use it on unhardened steel,) likely to get noisy crying for a dress, drag-down RPM, and start eating up dollars.

Buck, you were partly responsible for my trying this in the first place, because a long time ago you wrote something like "a good grinder can make do with almost any grinding wheel, it just might not be as effective or efficient", or something like that. So even though most people said, "you should use diamond for glass" I tried the CNB anyway.

Noise: a good grinding hiss, with a bit of a buzz when feeding across the wheel. After I set the machine for autoinfeed, I just sat and listened. There were a couple of times when the I heard a bit of vibration and got concerned that the wheel was clogging, but it was just resonance and tweaking the workhead rpms fixed it. Anyway, here's a video from Saturday, you can hear it for yourself. Any tips for the next time around?

For perspective, the head of the aluminium mushroom is 35mm diameter. You can see the infeed on the DRO, which is 10 microns in diameter the first time, then 7 microns the second time. I think the average was about 8 or 9 microns per pass. So it took about 1500-2000 strokes to remove 15mm of diameter.

RPM, I think it was around 200 or 250. Have a look at the movie and tell me if I got it right.

So, I didn't want to break this lens and didn't push it. But how fast do you think I could have fed in? Could I have plunged?

Cheers,
Bruce

PS: I bet the chuck is full of coolant. Short of taking it apart, is there any decent want to remove the water from behind the scroll plate?? Should I put it back on the grinder and spin it at top speed without the jaws in place and with no coolant running? Perhaps this will 'centrifuge' the water out of it.

 

[michiganbuck]

Could I have plunged?
Plunge on glass should be slow travel, same for long traveling.
Plunge allows all the wheel surface taking load-up rather than just the edges..but I have never seen loading when grinding glass.
Wheel wear can allow bonding to get high, so sticking the wheel for some materials can be good..

Glass seems to wear down bonding and so keep grits high, sticking is less needed..
I have never ground glass with a CBN wheel, your doing so has taught me a good lesson.
Buck

 

[michiganbuck]

Bruce,
I think your setup and process were excellent...and a part well done.
Thinking about other possible methods, I thought that two thin balsam wood washers made to the diameter of the lens would keep grits off the lens face, compress so as to exert even pressure, and allow indicating adjustment to the original OD. They could be OD ground in place when grinding the lens OD, and be Elmer's glue stuck to two face plates
Yes, wood loading the wheel is a possibility...but I think grinding glass unclogs a wheel.
Buck
(Note: I have used Elmer,s glue for one ups and it holds fine and makes an easy cleanup with being water soluble...the surface tension of the balsam would hold well)..I would make a simple hole punch for the balsam washer ID and cut the OD with scissors to +1 or 2 MM..
One advantage might be that with not using the double-back, adjusting OD run out might be easier..Faceplates might be made with a small concave hollow to ensure an intersection near the lens OD..

 

[michiganbuck]

PS: I bet the chuck is full of coolant.
Next time use a collet...? make a shield on that end. Use anti-rust coolant..Forget grinding and take up fishing..

 

[ballen]

I think your setup and process were excellent...and a part well done.

Thank you.

Thinking about other possible methods, I thought that two thin balsam wood washers made to the diameter of the lens would keep grits off the lens face, compress so as to exert even pressure, and allow indicating adjustment to the original OD. They could be OD ground in place when grinding the lens OD, and be Elmer's glue stuck to two face plates

Holding the lens was not a problem. I held the lens on the concave side. The aluminium mushroom was slightly convex, to more-or-less fit that. Then a piece of double-sticky tape. The mushroom was 35mm in diameter, and the tape was about 38mm in diameter. Final ground size was 36.9, and the tape protected the surface very well on that side.

On the other side, I had a piece of 6mm (1/4") closed-cell neoprene foam rubber about 30mm diameter held on with double sided tape, again about 38mm diameter. When I pressed this with the brass "foot" it swelled up to 34 or 35mm.

The only "damage" to the glass lens was some very slight chipping along the edges. That is obscured by the lens holder and affects 0.1mm of diameter. No other lens scratching or damage, in spite of the sandstorm effect.

Yes, work loading the wheel is a possibility...but I think grinding glass unclogs a wheel.

That was also my impression: the friable glass is lightly but constantly dressing the wheel.

(Note: I have used Elmer,s glue for one ups and it holds fine and makes an easy cleanup with being water soluble...the surface tension of the balsam would hold well)..I would make a simple hole punch for the balsam washer ID and cut the OD with scissors to +1 or 2 MM.

Yes, I think balsam might have worked. Not sure I want to put PVA glue (Elmers) onto a coated lens.

One advantage might be that with not using the double-back, adjusting OD run out might be easier..Faceplates might be made with a small concave hollow to ensure an intersection near the lens OD..

I first put the aluminium mushroom in a collet, but the lens was not running close enough to center. So then I stuck it into my ancient Grip-Tru chuck and centered it to 5 microns. Then I ground.

(Yes, my coolant is anti-rust, it is from Wunsch Öl GmbH and called KS-AB.)

Cheers,
Bruce

PS: I removed the jaws and spun the chuck at top speed 600rpm. Nothing came out but a drop of oil. No water!