To work in a shop, is to have people walk in with a very worn key to their house or car. It still works but they want a copy. With one look, I know it is not that simple. Sometimes I know a copy of the key will work fine and sometimes I do not know that.
Let's start with what I like to see. For most keys, you should see small flat areas on the key and these are where the pins or wafers are to rest on. Between each flat is some kind of ramp or peak to allow for the key to insert and withdraw. The steep parts should be even and smooth. This is a well cut house key.
You can see the 5 flat areas and the peaks between have very clear starts and stops with nice 45 degree angle slopes. If you look down at the cuts, you should see smooth cut surfaces. This photo shows the lines the cutting wheel made going up a steep. You should see these or very lightly. The key on the left is worse in that the lines are more visible and deeper. This makes your key into a small file which grinds the bottom of the pins every time you use it. Cut marks are visible if one or more of several things happen:
-- the cutting wheel was dull, or
-- the carriage was moving horizontally too fast, or
-- the cutting wheel is dull.
This is a worn key which has had time for any cut marks to be worn down. Even as a pro, I can not tell where one cut starts and the next begins. It may work fine. It may work good. Often you ask the client if it works and they will say 'yes'. The problem is it works for that person since it has worn out with use. The person has learned the sweet spot to hold the key or puts up with a bit of jiggle to get it to turn. I can feel the same lock and know the fit is just not right but the worn key does work ... if you wanna call it that.
However, if you duplicate this key, the copy may work or it could be worse. Basic key duplication is like a photocopy machine for a page of text. The machine can be better or worse but if you serially copy the copies enough, the errors build up. I can tell the key above is NOT a factory original since it is not on a Nissan blank. (Given the age of this lock series, the car is a Datsun and the original key would say that in work or logo.)
However, in the trade you do much like the hardware stores and usually just cut the copy and give a warranty that should it now work, the customer can bring it back. Unlike the hardware stores, I often know the most likely key copies to have problems and I have alternate methods to solve the problem should the client return.
A comment on key machines. The machines I see in hardware stores are semi-automatics worth $800 to $1000 but sometimes less. They can be re-calibrated if they are not cutting correctly but the staff lack the tools or skills to do that. A good lock shop will spend $1500 or more on the duplicator and since we know we have to cut keys for high precision locks we keep the machine checked for calibration. (Some locks demand highly faithful copies and the hardware stores do not even stock the blanks for them. Why should they since I doubt they would usually get a key precise enough to work?) Calibration requires tools and time and using up a few key blanks.
Don't worry too much. If your house key says WR5, WR3, KW1 or KW10, a cheap duplicator will work fine. The machining inside the lock will forgive all kinds of mistakes in cutting copies of your key. The sloppy workmanship making the lock also keeps the cost down and makes it so I can pick the lock easy should you be locked out. Have a nice day.