ATM - Martin Cibulski - 2004-01-10
Start > Mirror Curve Generation with a Tile Cutter

Mirror Curve Generation with a Tile Cutter

To shorten the first step of grinding on my second mirror (14" F5) I tried pregenerating the curve with a diamond-studded cut-off wheel (a cheap one, 5 Euros, unsegmented). The cut-off wheel has 115mm diameter and is running on a small agle grinder at 4800 rpm. I attached the angle grinder via a plywood plank adjustable to my (rotatable) bench vise. The mirror blank lays on a simple turntable taken from my grinding machine.

It took 2-3 hours to come to a curve about 0.5 mm before the exact depth. Here I stopped my work to avoid cutting to deep. With enough water cooling the cut-off wheel was worn down only a little. I had to remove about 200 cm3 of glass (Pyrex) and I think grinding of two more mirrors of same size should be possible with this wheel.

I measured the depth with a caliper put on a straight metal bar across the mirror blank.
Then I adjusted the running wheel to the required cutting depth. I did not more than 1-2 mm per turn which is the thickness of the wheel. When turning the mirror table (only then!) I pressed my hand on the machine to flex the plywood arm a little more downwards. This results in a smooth cut around the mirror without a deeper mark at the start.

This is the assembly I built.




The surface after cutting.


Curve after cutting.


Warning !!!

It must be avoided at all costs to cut to deep into the mirror blank !
If this happens the whole surface has to be ground down to the holes depth.

To avoid this there are some measures in my assembly:

1. The counterweight assures that the grinder goes up in case of opening the bench vise to much.
2. A bolt through the long plank prevents it from slipping down through the vise.
3. The second small plank beneath the grinder allows finer adjustment by putting it on the glass and carefully pressing its back end down. Therefor it is fixed by a screw on the long plank as a pivot. (The small wheels on its fore end are in fact not necessary.)
4. The angle between wheel and glass surface has to be as small as possible to get a smooth surface instead of deep grooves.

Also important:
5. The pipe at the back end of the grinder shall prevent it from sucking glass dust and water.
6. Without cooling water the diamonds of the cut-off wheel will burn away very quickly.
7. The grinder wheel should cut towards the center of the mirror blank. Cutting to the edge will cause chips breaking off.
8. For cooling a watering pot is sufficient (I needed about 20 liters in sum) because very soon water will remain in the becoming hollow if cutting starts from the center.

Best results I got by the following procedure:
1. Measure the depth at one radius and calculate the required cutting depth there.
2. Turn the whole bench vise with the angle cutter to that radius and fix it.
3. Turn the small (!) plank down.
4. Loosen the bench vise a little (!) and turn the machine down until the small plank (not the cutter !) touches the glass.
5. Start the grinder. 6. Adjust the cutting depth carefully by grasping both planks like tongs and pressing down.
7. Tighten the vise to fix the position.
8. Turn the mirror blank slowly. Put some pressure on the grinder (while turning the table !) to cut deeper and the start mark will disappear.

Working with it I thought of some improvements of this assembly:

Perhaps a screw or threaded rod mounted on the fore end of the arm instead of the second plank would allow better adjusting.

To make some mirrors of same radius it should be possible to mount the grinder at the bottom of a sled made of two wooden templates having the same radius as the mirror will have. The sled could be put on four bearings and move across the mirror.