Okay I'll be serious (spoilsport).
To me it seems quite fundamental. Cutting is done on a per tool, or per cutting edge, basis.
On a lathe there is one cutting edge and it is advancing at a certain distance per revolution so the simplest way to express feed is in inches (mm) per revolution of the workpiece; feed per revolution (fpr)
On a milling machine there can be more than one cutting edge advancing into the work by a certain distance per revolution so the simplest way to express the feed is to multiply the inches per revolution by the number of teeth and the revolutions per minuteto get inches per minute (ipm).
I will agree with mrainey that you can get into exotic calculations regarding horesepower and metal removal rates but when you are using a particular machine you are dealing with a fixed horsepower and you can use the spindle load meter to see if you are loading the machine too much.
I will also agree that if you have used the recommended feet per minute from a reference book you come up with numbers that are a nuisance to handle with mental arithmetic. Speeds and feeds always have a range so make things easy; round the recommended surface feet per minute to the next ten down and round the spindle speed to the nearest 100 or 1000 rpm depending whether you are dealing with steel or aluminum, etc. A two flute cutter taking 0.005" per tooth spinning at 10,000rpm needs a feed of 100ipm. This is not a difficult calculation.  |