SKS sells code keys at £9.60 a set and code sheets at £6.80 a series. Which makes it not cost effective to cut your own out of metal. To cut a key by code from the keys first determine the highest cut and cut the whole key to this depth. Change the key to the lower cuts as necessary and cut the remainder of the cuts. This is also handy to make a key for an existing cylinder that you do ont have the key for. Pick the cylinder or open by shimming, insert the code keys one by one to determine the code. Cut all cuts to the highest cut (as above) and then cut the remainder. Then finish off with a file as necessary. This is a cheap way of cutting keys to code, but you will need to buy a set of keys for each series of locks. Some manufactor's have more than one series make sure you're ordering the correct series! Also beware of numbers on keys as some locksmiths make up their own code.
E.g adding a number to the code 5 to 1st 4 to 2nd and so on, which is a good way of disguising numbers on your own master system.
Hope this has been some help to you.
Eddie