Calculating Chip Load

Feed Rate Calculations-The feed rate in inches per tooth must be converted into feed rate in inches per minute (IPM) before you can make the feed rate setting on the machine. The formula for converting feed rate in inches per tooth into inches per minute is as follows:

Feed Rate (in./min.) = RPM x Chip load x # Teeth

This simplified version of the feed rate formula is used in most machine shops. Again, it should be pointed out that these feed rates setting can only be used under ideal circumstances. You will typically be given a range of chip load factors to use. A good rule of thumb is to start out at the low range or average feed per tooth factor and increase the feed rate to the capacity of the machine tool, the setup, and the desired surface finish. It must also be mentioned that using a chip load that is too small will cause excessive tool wear so don’t just set the feed rate low and think this is correct.

Let’s try some feed rate calculations.
A four flute 0.500 inch high speed steel (HSS) end milling cutter is to be used on a piece of 1018 steel with a brinnel hardness of 200. The closest RPM setting to perform this cut is 750 rpm. Look up the feed per tooth in the charts and calculate the feed rate in inches per minute.

RPM = 750
Feed in inches per tooth (chip load) = 0.004 avg.
Number of teeth (flutes) = 4

Feed (in. / min.) = RPM x Chip Load (CL) x # Teeth (flutes)
Feed (in. / min.) = 750 x 0.004 x 4
Feed = 12 in. / min.

Since the available feed rate settings are generally not infinitely variable, the machine cannot be set precisely to the calculated feed rate setting. Some judgment must be made in selecting the feed rate to use. Try to get to the feed rate that is nearest to the calculated amount, but if you can’t, consider the following conditions. What are your surface finish requirements? A larger feed rate will leave a rougher finish. What is your depth of cut? If it is a deep cut, go to the slower feed setting. Is the setup very rigid? Go slower for setups that lack a great deal of rigidity. Are you using coolant? You may be able to go to the faster of the two settings if you are using coolant.