Burr busch skids when cutting a seat (tension settings)

Hello,

I am trying to perfect my tension-set rings which are essentially quite simple and taken from Cogswell's book: 2.5mm silver wire, twisted and hammered to become hard, formed around a mandrel.

However, I am having trouble cutting a seat to hold the stones. On the last one I made, I cheated and cut across the silver using a needle file, but this obviously means that the stone has plenty of room to move.

I have a bush burr but it "skids" across the silver when I try to cut into it. Does anyone have any tips about where I'm going wrong, please?

Keep meaning to try tension setting... How fast are you trying to run the burr?

When I'm cutting seats, I usually run the pendant motor about as slow as I can, with one finger bracing the burr shank. Gives me a solid position to work from and lots of control on where the burr is going.

You could always use a graver to start the seat?

I have the pendant motor as slow as I can (it's only a basic one from Cooksons (Rotacraft RC230) but it's quite good). Will try to brace my finger.

Since posting, I think I've worked out that the burr was just too big for the wire gauge I was using. My jewellery teacher advised a 4mm burr as the stone is 5mm, but the wire is only 2.5mm. I've also got a couple of much smaller burrs, a 2.1mm seems to be a much better size.

Sorry, should have said - the finger both rests on the work (or something solid) and braces the shank as well. I've seen a picture of what I mean around somewhere, I'll see if I can find it.

Found it - http://www.flickr.com/photos/28855176@N04/4185296691/ - it's one of Lucy's.
And a video version : http://www.youtube.com/watch?v=Q2rLpeQKu8Q

Thank you very very much, Peter, that video is a great help. Just given it a try and it gives you a lot more control. With the smaller burr and your tip, I've just this second created the perfect seat. Much appreciated.

At the risk of flogging this subject to death I should like to make a few more pionts.
Steel burrs are run slowly to preserve their sharpness, but if they wander or skip they either need more lubricant, or they need to be speeded up, i.e. given more revs.
A second point to note is that the speed at which the blades travel at a given RPM is proportional to the burr diameter. So with a 2mm burr the blades will only be travelling at half the speed of a 4mm burr.
Next, the the stone setting burr can be more easily positioned, as Peter said, by first making a shallow groove with a graver, but also by using a small (say 0.7 mm) round burr.
Lastly, a lens shaped diamond burr is much more easily controlled on metal
and if the cut is not very sharp it can be refined with a graver, or a steel setting burr later.Unfortunately the only ones I can find are from Fisher-Pfortzheim, order no3656/33, or Rio Grande 343-102.
Lots of practice on scrap metal will be helpful. Does this help? Dennis.