Mirror Grinding Machine
by zakass, published
The Gordon Waite series of videos on Youtube demonstrates the process used for making mirrors using this machine and begins here:
Please see my other items for the additional tools used in this series such as the spherometer.
Additional information on telescope mirror making can be found on the stellafane webpage which is an excellent source of information:
If you require any help putting this together, please contact me in the comments box and I will do my best to help you.
THE TEXT BELOW IS FROM THE WORK IN PROGRESS STAGE OF THIS ITEM
This is a work in process for building a mirror grinding machine. I have been looking at building 3d printable newtonian reflector telescopes and while the lenses cannot be 3d printed, they can be made at home using some basic techniques and a simple grinding device i am working on here.
This is very much a work in progress that uses some 3d printed parts, copper tubing and various bolts to put together a turntable to spin the mirror while grinding out the shape using a home made grinding tool.
I intend to be able to grind a mirror up to 200mm in diameter, while still being able to print this out on a printrbot LC v2.
if you are interested in helping me develop this or have some experience with doing this, please do post comments. I am always interested in opportunities to collaborate and would very much benefit from some more experienced stargazers.
Small update: I have been sketching out various options and looking for suitable motors for the tool. I am still not sure on what type of crossbar I will incorporate into the design, but i at the moment my preference lies with a simpler design.
I have also started printing some parts and I am quite pleased with the outcome. I have done some prints of the rotary table at 40% infill using PLA and I have printed them face down. The surface finish is of a very high quality. The piece is structurally very stiff and has a good weighty feel too it for a hollow piece of plastic. the sides of the print however are not too amazing and will require some refinishing.
A few more updates. I have ordered a small motor and designed a mount for it to fit onto one of the frame blocks. i have also designed the pulley system and am currently trying to figure out some things relating to automation of the grinding process. All pieces of the rotating table have been printed and look like they will go together nicely.
Another small update: Got my hands on some decent bolts and an el cheapo 60 RPM geared motor from ebay which I am hoping is sufficient to drive this thing. I have test fitted the parts for the turntable and have realised that one side of my pulley is badly warped. I am leaving it on for now, but I may have to swap it out in the future. The parts all fit together well and the table is sturdy and has a good bit of weight to it.
There was a small problem with bolting the pulley to the bottom of the turntable as the fit was very tight. putting a temporary big bolt as a spindle kept the whole thing properly centered and it all went together nicely.
I am currently printing parts of the frame and the motor mount to see if all the mechanical parts go together well. After this I will be getting the copper tubing elements that form the main structure of the machine.
Some more work has been done on the machine. I have updated the table piece to incorporate recesses for the bolts. I have also sketched up a jig for cutting the 15mm tubes, which is extremely helpful in getting straight clean cuts.
Construction continues now that I have managed to source the metal tubes. I estimated that approximately 4-5m of copper or aluminium tubing will be needed in total.
I need to source a long enough bolt to act as a spindle next. Meanwhile the printing continues.
Another update today. I have finally managed to print off the remaining parts for the grinding machine and test fit them. This was the first time where I was able to test the mechanisms and see how sturdy the structure is. There have been 4 major issues.
1) The structure is not stiff enough in flexure. The construction is rigid, but it slpays out in a few areas.
2) The pulley mechanism to drive the system is a very poor design.
3) The cross bar is not rigid enough
4) The spindle is a bit too stiff and not rigid enough.
To overcome the first problem I have redesigned the corner blocks into an L-shaped bracket type of design. I have also added holes for fixing screws to help support the whole thing. This should help significantly reduce flexure in the structure.
To overcome the second problem, I have opted for a spur gear driven system. The main body has been re-designed to accomodate the motor and the turntable will be driven by 2x 60 mm gears. I will be testing the system at 1 to 1 gearing ratio and if more torque is needed.
Addressing the third problem, I have redesigned the cross carriage to use two rails instead of 1, significantly stiffening the cross carriage. I have also designed the brackets to go with this modification.
For problem number 4, I have re-worked the main body to have deeper holes for the main spindle. This will accomodate a marger number of bearings, which will help stiffen up the turntable a bit more.
All in all some good progress. I have fully assembled the old design now and have redesigned the new parts. I now need to do alot of printing to incorporate all the latest revisions of the design.
A dimmer switch has been ordered to adjust the speed of the motor while grinding. This will be attached to one of the frame blocks.
Some more progress today. I have been able to print out the new body and corner blocks. Everything went together pretty smoothly and the majority of the problems have been resolved. The geared system is definitely the way forward, but the current 1:1 gearing ratio is too low. This should be increased to 1:2 to improve the torque of the turntable. I have redesigned and updated the gears and reduced the size of the teeth while I was at it. I hope this will do the trick.
A few more updates here. I have finally finished printing a full set of pieces and the whole thing goes together quite nicely. There are a few smaller items that need to be redesigned for strength, but by and large the structural side of this design is close to being completed.
On the mechanical side, there are still some issues. I have set up the machine to run using the modified gears and they are still hopelessly undersized. I have sourced a new more powerful motor which I will try out with a new set of gears running at around 7:1, but I suspect that will also be too small. Still, not bad for a first iteration of the design. lets see how the new motor performs.
Big update since last time. I have been having some serious trouble with warping and adhesion of this new PLA I have had from 3d2print. The material is very different from what I am accustomed to. With the problems resolved however I had an opportunity to get this little machine to a working stage.
I have upgraded the motor to a much bigger motor salvaged from an old food chopper which is mains driven. This caused the problem that it completely overpowered the machine. To fix this I have had to design a small gearing assembly which connects onto the top of the motor housing to drive the turntable. This however still spins too fast and destroys the gears after continued operation. To take care of this and control the speed I have ordered a dimmer switch.
I have reduced the volume of the main body to make it easier to print.
I have created some jaws to go on the turntable and hold the mirror in place with 2 stages of adjustment. 1 coarse sliding stage and one fine bolted stage, which should fix the mirror nicely.
I am currently waiting on my mirror blanks to be delivered and they should hopefully be arriving very soon.
For completeness, I have added a bill of materials (which includes a printing list and a short description) and the gear calculation spreadsheet, which can be used to re-calculate the gearing characteristics for your own gears to suit your motor.
I have updated some of the smaller parts such as covers for the motors etc. I have also enlarged the jaws of the rotary table to accept the slightly larger than expected mirror blank. I have also shown some photos on how to cast a gypsum disk to make a tool. For all intents and purposes this item is now complete and will only undergo minor changes from now on.
Recent Commentsview all
So Zakass, have you ground your mirror yet? I would like to know how it went.
Someone on CloudyNights.com printed some laps(?) to make a mold from, for mirror grinding.
I haven't seen the 3d model yet.
aaah i see what you mean. I have read about these types of telescopes a while ago.
I honestly am not sure what is involved in making something like that. Im sure you can get the lens selectively aluminised somewhere to give you your mirror element, but from what i recall its not all that simple. There is quite a bit of work involved in making the lenses in the first place!
that said however, whats stopping you right?
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