AgX, I think we agree that Mustafa's idea isn't great.
Unluckily the chips (material taken off by the cuter) is GRIT and it is abrasive, Silicon Oxide one of the many choices of sand paper. The final polishing of the lens used very fine abrasive. The lathe used in the lens cutting is not a standard Machinist machine tool.
Originally Posted by Dan Fromm
It's not the camera......
Originally Posted by Dan Fromm
To be fair on Mustafa, I have to admit that it was me who raised that idea of selfgrinding a lens element three months ago, and Mustafa liked it. It was just an idea out of the blue as I have no experience in glass grinding (except household DIY...), and soon raised (amongst others) the issue of tolerances.
I also raised the question of spoiling the lathe, but the same time stated that on a sort of customized lathe one could encapsule part of it. Still I think this would be the least problem. As long as one does not go to a cnc-shop around the corner.
Mustafa's idea of using a custom made cam would in principle enable one to use a manual lathe at home, where all those special precautions involved in grinding/turning glass could be cared for.
Whether the basic idea of turning/grinding a glass lens on a lathe is a sound one is still open...
Well, at least one lens manufacturer does turn some aspheric elements on a lathe. But I don't think the tolerances needed can be achieved with the typically wobbly (not very) really good lathe.
If the bit follows a cam, where's the need for CNC?
Mustafa's idea, as far as I understand, was to calculate the lens and, most important here, the lens profile. And with that data have a sort of cam machined corresponding to that profile on some cnc machine .
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Many years ago (about 40) I worked for a company that among other things made various products that used both custon lenses and mirrors. The machines that they were made on were refered to as lathes. They did not look at all like the lathes seen in a machine or woodworking shop. They all had a turntable upon which the blanks to be ground were placed. Above this was placed another turntable with the grinding tools. The lower one (containing the lens or mirror blank rotated on a fixed center. The upper one(containing the tools) also rotated about its center, it also swept from side to side becoming off center as required and was allowed to roll as necessary. The tool and finished product blanks coud be flats or molded blanks as required. Lots of water flew around the room where the machines were located. A keen eye was needed to insure that the grits and water were placed to keep the grinding proper. For a simple description of the process and how to make simple lenses and mirrors look at the instructions give in telescope making books. The easily made shapes are made by "walking around a barrel". The machines duplicate this process.
Many of the lenses start with molded blanks which only need to be finish ground and polished.
While these older machines were rather crude I am sure that the newer ones are very much more technical in design.
The turn tables for flat products were flat and those for rounded products had a radius shape. Depending on the lens being ground the blanks could be on either a raised or concave table with the tool on the oppsite.
Last edited by jmcclure; 07-28-2007 at 06:41 PM. Click to view previous post history.
Reason: Add additionl info.
To my understanding these machines you describe are the typical or common lens grinding devices in the optical industry.
However they employ another principle than we here understand as a lathe.
If you search google as lens grinding subject , you find there are many machines to cut the lens with spy satellite precision with lathes and diamond tip. AgX s idea is great. And other story , archeologists found at europe some aspherical lenses which turned 1000 years ago by vikings. Rodenstock says they are good as 1950s aspherical lenses.
They didnt have electric motors , cnc created lathe parts but they succeed it. May be we can learn something from the past.
One of my friends is a cnc repairman and i asked the price of 15000 rpm electric motor , it costs 15000 dollars. He said i can do the same with 6000 rpm machine with using gear train and it costs a 6000 dollars.
I got 15000 rpm specification from a diamond turning lathe seller for lens grinding. This is a thing worldwide used , accepted technology , i removed the precision moving sensors , control electronics and piezo motors and changed them with basic spacer - or cam -.If you really dive in to this business and search for yourself , you find that if you havent got 20000 dollars , you come to my port.
The best question is this :
What is the lowest rpm for to diamond tip turning lathe a glass lens ?
As we know , human power can shape a 0.7 meter diameter glass telescope mirror in few hours. I am talking about coarse spherical shape.
We are lazy , we have no waiting customers and we dont want too much motor noise in home.
What can be the slowest turning lathe speed without loosing precision or increase low frequency vibration.
Think you are viking and your head is in danger , you have to make the lens with a turning lathe - it is believed viking used one - .What do you do ? Escape ?
Mustafa Umut Sarac
By using a high speed die grinder with a suitable burr held in the lathe holder one could grind any necessary shape on a cnc lathe. Glass worked in a lathe can be held in place with a suitable adhesive. Different burrs would be required for rough and finish grinding and also for polishing. Large amounts of coolant would help flush away the grindings to prevent machine damage.
This way only the burr would have to operate at very high speeds. Some die grinders easily work at 15,000 RPM.
By the way high speed grinders a sometimes used this way in machine shops even on manual control lathes.
The clasical method of lens making is to use two pieces of material. One for the finished piece and the other for the tool. If you place them together with a suitable grit and water you only have to use the proper motions to produce a lens. ie walk around a barrel and move the tool piece in the proper way while in contact with the lens blank. This technique has been used for hundreds of years to grind various shapes. It has been suggested the it was possible that at least simple lenses were used in ancient Rome and Greece.
same question : what is the lowest rpm possible for to turn the glass with burr ? and why slow speeds are not the subject ? what happens if the speed lowers very much for example 200 rpm where the motors come less than 100 dollars ?
Originally Posted by jmcclure
I have doubts about the use of female mold at ancient times , if they used it , which technology produce it ?
Originally Posted by jmcclure
lets come to recent times , what can be the material used at female turning mold ? tool steel ? i heard cast iron but i dont know why they use it ? can you explain ?
and i saw at biggest military and esa telescope mirror producer for chile , at moscow based company , they were very coarse steel burr turning the satellite optics before liquid based abbrasive work .do we need to do the same at our lenses with female mold +liquid technology ?
And what about the chaotic vibrations of abbrasive liquid between glass and female mold ? Are they less than diamond turning ?
it is even difficult to know when to stop liquid abbrasive work ?
Where will we find liquid abbrasive ? is it expensive ? is it possible to use the abbrasive liquid after contaminated with glass particles ?
I think higher the rpm , higher the frequency of vibration and smaller the wavelenght of wobble ? May be we need to lower the force in to the glass by diamond turning for to get more peaceful vibration with hyper low rpms ?
Mustafa Umut Sarac