How to drill gems?

[Sliding Head]

I drilled ruby balls with an NSK Planet air-spindle mounted in a machining centre (Cincinatti Dart). The holes were 1.6 mm diameter and 2.5 mm deep. I can't remember the feed rate downwards but the wear rate was awful. I picked up the project when one tool bit made one hole and slowly got it to 80 holes with a totally different setup. The NSK Planet was supposedly running at 60,000 rpm.

All holes were cut by circular interpolation.

The tool bits were dental drill bits that were plated with diamonds - I don't know what the plated matrix metal was. Compared to the picture in the link in a post above the diamonds are not plated in layers. The two variables were the depth of location of the diamonds ( which are not uniform in size or shape) and the friability of the diamond itself. In simple terms that means the ability of the diamond to fracture consistently. I tried to loosen the poorly plated diamonds to stop larger diamond fragments from deflecting the tool bits.

I tried one core drill once. It broke as soon as I got the air spindle running. It was too flimsy to support its own slightly non-uniform weight at those rpm - the tool stem was a tube.

To improve cutting I tried mist coolant directly into the NSK air turbine. It lowered the rpm but increased the torque. It was better but the bearings rusted over one weekend and at about £1000 each spindle that wasn't good. I also tested a spindle speed increaser for the Dart spindle and got better results. The whole setup was more rigid. In the end I got down to about 8,000 rpm, which although counter intuitive gave better results. Finally I ran a diamond cup wheel mounted on a milling arbour across the balls first to flatten the balls. This allowed the improvement to 80 holes.

Be careful about using coolant. The ruby produced a white powder in the cut which was so fine it became colloidal when wet and choked the tool bits. You have to cut/grind at the rate you can clear the debris from the hole, and that may not be at the rate indicated by the tooling suppliers.

This is from my memory of a project I did nearly 20 years ago. The machine I was trying to replace was slow but had four cutting spindles - I could cut only one at a time. Nowadays this type of hole is drilled with ultrasonic methods if the hole is blind, and lasers if it is a through hole. Watch jewels are through holes with separate end-stones if needed. I did trepann a blind hole in a 5mm ruby ball with a 65 watt DMG laser but had no idea of the depth or how to remove the central core cleanly every time. A previous attempt at a leading University resulted initially in the ruby being burnt to ash. Years later I got a hole produced by them with ultrasonics that was acceptable in finish and shape.

The lessons were:
1) spindle/tool rigidity is everything
2) the feeds and speeds that work reliably were not indicated by the tool suppliers at the outset
3) the most common advice was 'give it a try'

BTW that project took me about 4 months and was canned because of the tool wear and the taper in the hole which used all of the hole tolerance in about twenty holes. Even though I got tool life (it was still cutting up to 80 holes) the hole geometry was poor. The ultrasonics and laser methods weren't acceptable at the time.

Good luck
D