TheSkySearchers

Update summary: Lots of work to get to this point.

Things I need to probably change or fix: My gearing of the DC motor. I will need a slow speed for polishing (5 rpm), and faster speeds for hogging out and fine grinding (15- 30 rpm), so likely need to think about adding a gear reduction.

Fixing the aluminum turntable to the wooden turntable base.

Building wedges to locate the mirror on the turntable.

Update: I located the centre of the aluminum turntable this morning.

In the bottom image, you can see the intersection of drawn lines marking centre. Not too exciting I guess, but hey.... progress!

In this image I have put together a gear reduction stage for my DC motor.

On the bottom is a 6 inch pulley bolted to a 5/8" shaft, held by two pillow blocks. The keyed shaft was kind of pricey... 25 bucks, and needed a lot of polishing with coarse sandpaper to get to fit the pillow blocks. This took me a couple of hours to get everything square and turning properly.

Edit: I can't get this photo to load, so I will show an image of the final product, when I put it together this evening!

Update August 19, 2019

This evening I wrestled with the gear reduction drive. The idea is to get to a final reduction of somewhere near 8:1 - which would be best suited to the small DC motor that I'm running.

I built this, and took it apart in different iterations about 6 times before finally getting it to work properly.


Its not terribly attractive, but that's OK.

The idea is to use two small 5/8" pillow blocks holding a common drive shaft with differential pulleys on either end. I wanted the final drive to be adjustable, so I stacked a couple of pulleys, including a 1 and 1/2" as well as a 3' pulley. Seems simple, but to make everything work together (including the placement of the motor to achieve decent belt tension was not trivial.

Anyway - here's what it looks like.

Penultimate assembly:

Almost ready to use this thing. I put it all together and adjusted belt tension so nothing was under too much lateral stress when I tightened down all the bolts.

The best part is I think its going to actually work!!!

I'm going to order a 10" blank and some grinding grit plus tool-making materials from "got grit" in Florida.

Here are some pics of the penultimate version. (I still have to screw down the aluminum top to the wooden base, and determine whether I need to add casters for stability under the turntable. Also I will need to fabricate some wedges to hold the mirror blank in place).

And a nice drum roll please?

<<crickets chirping, a sparrow yawns>>

The penultimate product, a nearly functional mirror grinding machine.

Here is a youtube link:






Enjoy!

It kind of reminds me of the old Chaplin film Modern Times.



I still need to finish the turntable, and get serious about my Foucault test rig.

Thanks for reading to the bottom.

Ian

Any updates on the machine, are you planning on doing mirror this winter?

big glass!

EAGLE 5 wrote: ↑Sun Oct 13, 2019 3:51 am Any updates on the machine, are you planning on doing mirror this winter?

I am.

I will "skill up" by first grinding one or two 8 " mirrors. These will likely be in the range of f5 or f6, and in so doing will learn how to hog out the glass, then rough grind and figure it with Foucault tests and bath interferometer data.

Thanks for asking.

Ian

yobbo89 wrote: ↑Sun Oct 13, 2019 6:29 ambig glass!

The goal is to make a 20" mirror. Ambitious, but possible I think. I would hog it to roughly f3.5 or f4, and figuring it from a sphere to an accurate parabola will be the difficult part.

The eyepieces currently made now allow for the use of very fast mirrors. Such a large, fast mirror would limit FOV, unless you bring in a very wide angle EP (100* 21 mm ethos or equivalent).

How has your mirror grinding been going? Great machine I am thinking of trying to build one. I am hand grinding a 16" F6 right now but it is going slow with a lot of hiccups along the way.