24 replies to this topic

[duck]

How do you pull a blank which weighs 115lbs off the grinding machine?  I've seen a couple videos of professional shops making large, thick flats.  The have suction cups and a hoist.

 

What I've learned from blanks up to about 50 lbs is to avoid metal, concrete, etc.  I guess that's why you see historical photos where wood is used as much as possible.

[Oregon-raybender]

We would have a small cart near by. Slide the glass, off

on to the cart that has a shelf at the same level. The

trick is to balance the weight over the front wheel.

Yes, wood is preferred. 

 

We use suction cups, that had vacuum

pump, large cup.  The pump was powered by electrical (110 volt)

hooked to a over head hoist or engine hoist.

 

Starry Nights

 

https://www.amazon.c...r/dp/B0D2DRWR78

[TOMDEY]

Yeah, they used suction cups where I worked... Entertaining and kind of spooky to watch. Big $$$ mirrors (eight feet typical). Right tools for the job! "Old School"  Tom

[duck]

The back surface of the blank can be left Blanchard flat and the suction cups won't leak?  Or must the surface be polished?

Edited by duck, 08 December 2024 - 04:38 PM.

[Oregon-raybender]

Well then is only one way to find out, run a test.

 

Why are would you lift the back surface if the

blank is facing up?

 

Most mirrors were lifted by the optical surface.

The cups should not scratch the surface.

This is how we moved the 20 inch refractor lens

about. I know it sounds scary, but it worked.

 

Starry Nights

[starspangled]

The back surface of the blank can be left Blanchard flat and the suction cups won't leak?  Or must the surface be polished?

Grind the back of the blank with at least  220 then to 400 grit SiC  or 25 micron AO  until all the generator marks are gone . .  You will find out straight away that that the back is  saddle shaped and then it will all come in to contact when you grind out the generator marks  . A great mirror starts with a good figure of revolution on the back  free of astigmatism .

 

I have only ever worked with 24"  mirrors around 1.6 inch thick which I can just lift . The best way to move the mirror off the machine if it is heavier than you can lift safely  , is to remove  2 side clips if you use 6 , and slide it on to another trolley of exactly the same height  with some carpet strips on it . This could be integrated with your test stand so tilted up for testing  after sliding off the machine .

Edited by starspangled, 09 December 2024 - 05:04 PM.

[duck]

It will take some trials as RayBender says.  Not all work will be TOT, I imagine.  

[PrestonE]

Everything can be done TOT...

 

With a Large Heavy Mirror Blank, we would not do it any other way...

 

Mike Lockwood and Carl Z do everything TOT, to yes it can be done that way.

 

Regards,

 

Preston

[duck]

I've never been constrained to TOT.  Might be a first.

[Mike Lockwood]

Suction cups work on ground surfaces, but I generally only use those to pick up one side of a blank so that I can get a block under it to clean, etc.  Note the load limit, the inexpensive ones are ~100-125 lbs.

 

For up to ~22""-24" blanks, I can slide them off the machine and carry them because they are 50 lbs or less.  For ~24"-36" blanks, I typically slide those off of my medium-sized machine (up to 36") and onto a rolling test stand that is the same height as the turntable.

 

I have adjustable metal lifting fixtures with three arms for ~28" to 45" mirrors that are less than 3" thick.

 

For 36" and larger mirrors, I use lifting fixtures where possible, or I slide straps under them to lift with a Sky Hook or rolling gantry crane when the Sky Hook isn't capable of the lift.

 

TOT is the way.

[duck]

Thank you Mike Lockwood.  I'm begging to think I might be over my head with this one.

[Steve Dodds]

For moving 25-30" optics I bought this hydraulic table from harbor freight https://www.harborfr...cart-61405.html

I jack it up to the height of the polishing machine, slide the mirror off, then wheel it over to the test stand that can tip horizontal, slide it onto the test stand, put the pegs in and off you go. 

[Dick Jacobson]

Here's how I moved my 29-inch mirror between mirror box and storage box. The mirror is supported by four clips that I made by bending aluminum bars. The lifting device is also homemade, a parallelogram frame that can move in three dimensions with the load balanced by barbell plates. Capacity 500 pounds.

[duck]

that's scary, isn't it?  I'm not used to moving optics except with my two hands.  Nevertheless, I've chipped some.

 

Thanks.  I'll need something similar.

[MeridianStarGazer]

Here's how I moved my 29-inch mirror between mirror box and storage box. The mirror is supported by four clips that I made by bending aluminum bars. The lifting device is also homemade, a parallelogram frame that can move in three dimensions with the load balanced by barbell plates. Capacity 500 pounds.
IMG_0637_small.JPG

You are trusting those clips a lot.

I see the 4 green tabs inside the storage box for getting under it. Nice.

Edited by MeridianStarGazer, 10 December 2024 - 07:24 PM.

[Oregon-raybender]

I would add something to protect the surface. Maybe cover the clips

( rubber tubing or foam) and make a square to hold the clips in place, spaced correctly

as not to slip or move. Maybe make the clips out of Delrin

 

Starry Nights

[duck]

I'm getting the idea...now have to adapt the concept to my polishing machine's footprint.  Would like a design for which the mirror stays over the wheels without counterbalancing.  May not be entirely possible.

[Dale Eason]

I'm not sure but I think nothing of those clips touch the mirror's surface.    I can't quite envision how they work but I think the lever arm created by the line attachment point and the bottom of the U shape against the bottom edge of the mirror create an upward force on the bottom of the mirror.  Is that bracket more or less nothing but a side ways U?  Or is there more going on underneath?

[Dale Eason]

Ah,  Now I see.  Magic.  

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[Dick Jacobson]

Ah,  Now I see.  Magic.  

Yes Dale, that's about right. Plus, the force vector at the top is towards the center of the mirror, making it nearly impossible for the clips to slip off if my intuition is right. I carefully tested them before lifting the mirror.

[italic]

Yes Dale, that's about right. Plus, the force vector at the top is towards the center of the mirror, making it nearly impossible for the clips to slip off if my intuition is right. I carefully tested them before lifting the mirror.

How do you prevent the clips from slipping around the circumference? With my luck, those things would fly off as soon as it was over the concrete floor and the mirror would slip out.

[duck]

I'll test out the procedure using a cast iron lap.  It's 1" thick and 25" diameter about 120 lbs.  I've shuffled it off the polishing onto a bar stool.  Now it makes a good table top next to the machine.

[MeridianStarGazer]

How do you prevent the clips from slipping around the circumference? With my luck, those things would fly off as soon as it was over the concrete floor and the mirror would slip out.

I'm afraid of them bending. Nice thing about metal though is it only fatigues from vibration, not constant force like wood.

So if it holds up during the test, you will likely be fine later too.


...

But the lesson of the thread is, if you want to be an optician for NASA, you need huge muscles.

Edited by MeridianStarGazer, 12 December 2024 - 12:40 AM.

[Dick Jacobson]

How do you prevent the clips from slipping around the circumference? With my luck, those things would fly off as soon as it was over the concrete floor and the mirror would slip out.

There is very little force sideways along the circumference, and a lot of friction, so I don't think this is an issue.

[MeridianStarGazer]

I just rolled a 12"x10"x6" concrete block along the ground. Assuming density 2.4, it weighs 59 pounds. I could have lifted it had there been handles. My wrist injuries were complaining. I had to remember to keep them straight.

A 24" weighs close to that much. Big scopes mean cranes, wheels, ramps, etc.