Hi again ceefna, that's so great to see the design come together!
I would suggest using a ANALOG moving-coil panel meter for the current as well as the digital meter. Sometimes there's no substitute for watchign that needle jump up/down while you turn the dial. Then you can use the digital panel meter to trim the current more exactly and slowly.
For higher current jobs you could just get one of those little 8" mains powered desk fans and point it right at the fins of the heatsink, you'll soon find out if it goes into thermal shutdown.
The current control mechanism is based on 2 factors;
1. The LM117 is constant current drain so it always draws the same current through the pot. That makes a set voltage across the pot (end to end) regardless of the output voltage.
2. The pot adjustment makes a reference voltage which is subtracted from the current measured voltage on the current sense resistors.
So the LM338 always regulate to 1.2v, like this;
Pot = 1v, current sense R = 0.2v
Pot = 0.6v, current sense R = 0.6v
Pot = 0.2v, current sense R = 1v
So pot=0v = full current
pot >1.2v = no current
So to fully turn the anodising current off you need to set the pot voltage at >1.2v (from pot top to wiper). Just decrease the value of the LM117 resistor a little, say from 100 ohm to 82 ohm. That gives more current through the pot and a little higher pot voltage.
Now if you are doing small parts a lot and need fine adjustment, you could get a 10-turn wirewound pot. I have one on my main bench PSU and I love it.
Or, you can put a range switch in. That's a bit tricky, it involves changing LM117 R to 200 ohms, and adding another 100 ohm resistor above the pot. That would give you a range of zero current to half current instead of zero current to full current.
You can do range adjustment by switching out one or two of the LM338, but that leaves just one doing all the work, the other system is better.
If it was me I would definitely add a 10 amp analog panel meter, and I would probably change the 0.22 current resistors to 0.33 ohm, that will give you about 10.8A max instead of 15 which increases reliability and makes current adjustment finer. Later, if you really think you need 15A then you can add one or more LM338 which will be pretty easy job.