RPM CALCULATIONS
RPM is the abbreviated form for Revolutions Per Minute (the amount of times the
tool or the work rotates in one minute). There are two factors that must be considered
when determining the correct RPM.
1)
The dimeter of work or tool being used. (If a drill bit is being used then it is the
size of the drill bit, if a round shaft is being machined in the lathe then it is the
diameter of the shaft).
2)
The type of material that is being machined. (Easy to machine material or difficult
to machine material, refer to chart on reverse).
Using an RPM that is too slow will cause unnecessary pressure and wear on the tool.
This slow RPM will mean that the drilling pressure will be too much and the drill bit will
probably break.
Using an RPM that is too fast will cause a great deal of chatter and vibration on both
the work and the tool. The heat generated from this fast RPM will damage the cutting
edges of the tool.
RPM FORMULA
4 X C.S.
Diameter
4 = A given value for the formula (does not change)
C.S. = Cutting Speed of the specific material (changes when the material changes)
Diameter = The diameter of tool or work (size of the drill when drilling)
The cutting speed is defined as the rate at which a point on the work circumference
travels past the cutting tool in one minute. Cutting speed is expressed in either feet per
minute (ft./min.) or metres per minute (m/min.). For our purposes and because most of
the industry shops work in imperial we will be using feet per minute. Please refer to the
chart on the reverse when determining the correct cutting speed for each material.
Material
Feet per minute
(ft./min.)
Machine Steel
100
Tool Steel
70
Cast Iron
60
Bronze
90
Aluminum
200
These cutting speeds are based on cutting tools made from a material called “High
Speed Steel” (HSS). If the tools are made from carbide, a much harder and more
durable material then the final answer should be multiplied by four.
Sample Question:
Calculate the RPM when parallel turning a 2.5" diameter piece of tool steel.
RPM = 4 x C.S. = 4 x 70 = 280 = 112 RPM
Diameter
2.5
2.5
If the question stated that you were using carbide tooling then multiply your final
answer of 112 x 4 = 448 RPM
Questions: (Show all work)
1.
Define cutting speed.
2.
List the formula used to calculate RPM. Explain each part of the formula.
3.
Calculate the RPM when drilling a ½" diameter hole in a piece of machine steel.
4.
Calculate the RPM when drilling a 1-1/4" diameter hole in a piece of cast iron.
5.
Calculate the RPM when drilling a 1/16" diameter hole in a piece of bronze.
6.
Calculate the RPM when drilling a 1/4" diameter hole in a 3" diameter piece of
aluminum using the lathe.
7.
Calculate the RPM when centre drilling (1/4" diameter)in a piece of 4" diameter
machine steel using the lathe.
8.
Calculate the RPM to turn a 2-1/2" diameter piece of machine steel using high
speed tooling.
9.
Calculate the RPM to end-face a 3" diameter piece of machine steel using
carbide tooling.