Edge Treatment in Cold Working

Frequently people who are grinding the edges of bowls, aperture drops and other vessels that need to have a smooth rim find that they are getting small chips of glass coming from the edge of the ground part of the glass.

There is a way to prevent theses unwanted chips  

The long established practice of glass workers has been to give the glass an arris at the end of each grinding stage before they change to a finer grit.  This small area of angled glass, allows the continued smoothing of the glass without creating such a sharp edge that the glass there is not strong enough to resist the grinding action.  

You will notice on a bowl or other rounded vessel, that the chips are almost always on the outside. The inside of the rim normally has an oblique angle to the rim, and the outside an acute angle.  The explanation is held in the angle.  As the rim is ground down, the outer acute angle becomes very thin as well as sharp.  At some point the glass is thinner than the grit used to grind the surface.  This causes little chips of glass to break off the edge.

By creating an oblique angle at the edge of the grinding surface, the glass will remain thicker than the grit being used to grind the glass.  If you feel you are taking off a lot of glass, it is advisable to check that the arris is still in place.  If not, give it a light grind to maintain the arris while using that grit. 

At the end of each stage of grinding, you need to add an arris for the next stage.  The reason for doing it with the coarser grit rather than the one you are about to proceed to, is that it maintains all the grinding at the same stage, enabling the whole piece to be finished to the same level of polish.

Wipe the surface dry and add marks with a paint marker.  Allow this to dry while you change grits.  The purpose of the marker is to assist you in determining when you have ground out all the previous marks, by the elimination of the paint.

Finger protection

Grinding lots of glass pieces often leads to a number of small cuts on the tips of your fingers. There are several things that can be done to reduce these cuts and the tenderness that comes from lots of grinding.

The first thing is to take the sharp edges off the glass. You can do a simple, light grind all the way around the piece. This removes the extra sharp edges that often remain after breaking the glass.

You can go a step further and do a light arris around the piece. This is just lightly holding the glass at about 45degrees to the grinding bit and going all the way around on all sides. This does not take off the shape of the piece, but gives a more rounded feel to the piece.

Illustration of the effect of holding the glass at an angle to the grinding head - not so much needs to be taken away as in the illustration to get the effect

It is not generally recommended that you wear gloves around rotating machinery. There is too much risk of injury, even on a small grinding machine. Some of the alternatives to gloves include plasters (band aids), masking tape, electrical tape.

Other purpose-made things you can buy include rubber finger protectors, finger tip pads, finger caps (as used in counting money).

Other tools are made to hold the glass such as the grinder cookie

and Nick's Grinder's Mate