Flat Lap Discs

Selection

Diamond discs for flat laps are expensive and the temptation is to buy as cheaply as possible. There are a number of relatively inexpensive sintered and bonded diamond steel base discs. These are acceptable up to about 220 grit, but the finer grits can leave deeper scratches. It seems to be the grit size is not closely controlled, allowing coarser grits into the bonding process. My experience is that the scratches left by the coarser grits can be worked out with more expensive, but higher quality discs of 400 grit. This allows the finer smoothing and polishing grits to produce unblemished surfaces.

Maintenance

The lap wheel needs to be free from any grit. The disc can be visually inspected for any large particles, but this will not be sufficient for smaller particles. The cleanliness of the disc can be tested by turning on the water supply to the slowly spinning disc and placing the flat of your hand onto the surface. Any grit discovered needs to be cleaned from the surface. This is especially important when using flexible smoothing and polishing discs. If you do not, you will wear away the surface of the disc, leaving bare spots.

Similarly, when grinding/polishing with a lapping disc is finished, you must ensure the disc is clean and free from any rough spots. This can be tested with your hand on a slowly turning disc. If there are grains of glass that are not cleaned at this stage they will become imbedded in the disc and reduce its useful life. Flush the surface of the disc while slowly spinning until no rough spots, especially on the outer rim, can be felt.

Then the disc can be lifted off the wheel and the bottom surface cleaned of any debris before putting aside to dry. The storage of he discs should be dry land keep dust and other contaminants off the discs.

It is not good practice to leave a disc on the wheel for longer than it is being actively used. Rust can form on the wheel and it allows debris to collect on the disc. Anyway multiple disc changes are required to go to finer grits and polishing discs, after the shaping is completed.

There are a number of good videos on HIS Glassworks which discuss the use and maintenance of grinding and polishing discs.

Is it Possible to Make Flat Laps Yourself?

A flat lap is a horizontal spinning disk to grind and polish flat surfaces onto the fired glass piece.

Many desire one of these but are put off by the expense and sometimes the space they occupy.

There is a do-it-yourself alternative that I have used.

This is to use potter’s wheels as flat laps. Table top versions are useful as they are moveable to a storage shelf when not in use. Mine was kept on a shelf until it was taken outdoors to avoid water spray indoors. They do require some adaptations and have limitations. But the great advantage is lower cost.

There are new table top ones available from £135 with a 25cm/8” turntable. This is the maximum size. They often come up second hand on ceramic and local buy and sell sites for even less.

Adaptations are required. These include:

The wheel is surface is aluminium, so a magnetic surface must be applied, as the metal discs rely on magnetic attraction to stay in place. Magnetised sheets with self adhesive backing are available to be cut and stuck to the wheel.

A water supply needs to be fixed. This can be a removable reservoir with an adjustable flow valve, or a hose from the standard water supply with a controlled flow.

The water catchment basin around the wheel does not have a drain. You can live with that and interrupt the work to empty the basin as required. Alternatively, a hole can be drilled in the basin and a Loc Line or similar system can be fixed to drain into a bucket. The waste water should not go into a drain, because the sediment will eventually block it solid. A recirculating pump is also a bad idea, because it will distribute glass grit along with the water onto the disc, and cause scratches when using a finer grit disc.

Discs must be acquired. Consider metal discs with a progressive range of grits from around 50, and doubling the grit number (halving the grit size) to around 400. I normally start with a 100 grit disc, as the coarser grits are really only for removing large amounts of glass. 100 grit can do the same job as 50 grit, but requires longer.

Inexpensive steel disks are available. However the quality of grit sizing is not always accurate, making the use of the cheaper discs with grits above 220 inadvisable. The finer grits and smoothing pads need to be of a higher quality and their expense will be justified by the lack of gouges in the later stages of fine grinding and polishing.

If you use larger or smaller discs than the wheel, you need to mark the centre on the magnetic pad, to be able to easily centre those smaller or larger discs. Of course, the smoothing and polishing pads are on flexible backings and cannot be larger than the wheel. Only the steel backed discs can be larger.

This picture is an example of my potters wheel adapted as a flat lap. The magnetic pad has been attached to the wheel, and the water supply hose and flow valve are also attached.

Rear view with grinding disc in place 

Limitations

There are limitations to this make over of course.

• The wheel surface is 25cm/8” dia. A steel plate could be attached to make the surface 30cm/12”, although centring it may be difficult.

• The speed is easily adjustable, but the top speed is around 300rpm

• The the basin is 32cm/13” diameter and its edges rise above the wheel, limiting the size of items that can be worked.

• There is no drain from the surrounding waste water basin, so drain holes may need to be added.

In spite of the limitations, this worked well for me for several years, until I had the need to flat lap large numbers of items. For those with moderate lapping needs, this is a good, low cost piece of equipment.