Cutting the inlet hole

Cutting water entry holes is an important factor in achieving a balanced water flow across all dishes. If the dish is only a few days old, it will still be soft. Proceed carefully because once cement is removed, it can not be put back! Start with a lead hole that follows the line of symmetry.

Then widen that 10mm hole out with a reaming drill bit. Open the hole wide enough to get a rats tail rasp into the hole...

Carefully open it to a symmetrical, upright oval. Look into the tunnel way often to monitor your progress. Handle any uneven spots that arises from the rasping process. Your rasp will become clogged so have a wire brush handy and use often to clean your rasp. Also remember to clean it when finished so that concrete does not harden on your rasp.

The areas around the entry hole (both inside and out) can be softened off with sandpaper. Fresh 80 grit will be OK, just go easy with it, remember one can not put back that which has been taken away!

The speed of the incoming water has everything to do with how vigorous the whirling is and thus the appearance of the sculpture.

Notice how tidy and well centred the tunnel way is. The primary idea you need to have clear is that the water is delivered into the dish on the plane of symmetry and pointing slightly upwards.

Here we see the finished water delivery system i.e. all is hidden within the plinth and dishes and our only showing piece is the volume tap. This tap will be facing away from the observer when it is finally positioned.
At this point it is wise to give it a test run on it's plinth. It most probably require more filing. The important thing is that the inlet hole is your primary means of tuning the dish for optimum inlet speed.
If the water flow shots out too quickly and spills off the far side of the dish, then you'll need to make the inlet size significantly larger. Opening the inlet will also have a slowing / calming effect on the overall appearance the whirling takes. This is desirable for an optimum aesthetic effect.

I had two more filing sessions on the entry holes for this twin dish. It was test run on its plinth to observe its behavior and the oval grew by about twice its original size before I was happy with a brimming full, oscillating whirlpools. Keep in mind that this entry was widened after its plinth was finished and available for use!..

The drain holes.

The size of these holes is determined when the profile cutters are made. Dishes with a 30cm dia. will generally never exceed 3cms, so the general rule then is, this hole is no more than 1/10th of the dish diameter! This hole can also be too small if for example a 30cm dish only has a 1cm drain. For a 30cm dish I aim for about 2.2cms as a starting point. The sure sign that the drain is too small is that the dish will brim fill and refuse to oscillate It will then overflow and all whirlpools will run smoothly. Note: you may actually prefer this "quiet mode" and there is nothing wrong with your choice...
So, if your dish will not flip flop and you want it to do so, carefully enlarge all drain holes evenly and by small increments. This task is best done with the water flowing through the filing out task. Remember to keep all drains evenly sized!

Link to next tutorial Building a plinth.