The rigidity of the machine.Speeds and Feeds Cerritos College
The following is for Class MTT 100 "Machine Tool Introduction"
Instructor Robert Meeker E-Mail: rmeeker@Cerritos.edu
Subject: Speeds and Feeds for Milling, Turing and Drilling
This analysis of speeds and feeds are based on the class text "Machine Tool and Manufacturing Technology," Steve F.Krar, Mario Rapisarda, Albert F. Check.
This review is not complete for it covers Speeds and Feeds for Milling only, Drilling and Turning will be added later.
Milling calculation of Speeds and Feeds are the most complex. A full understanding how to develop these speeds and feed should help when reading the text and working Drilling and Turing computations (Ref. Pg. 193-196 Drilling, 234-237 Turning).
Whether the machine operation is Milling, Turing (Lathe), Drilling or Grinding the selection of the proper speed and feed is probably the most difficult thing for the machinist due to so many variables. An experienced machinist can often set speed and feed that will work and get the job done but this may not always be the most efficient. To be efficient the cutting speeds and feeds should be coordinated, using mathematic calculation as a "bench mark." For example during the machining process heat is generated. In proper machine practice the heat should be transfer to the chip away from the workpiece and the cutting tool. This can be accomplished by having the workpiece held rigid, a sharp tool, the speeds and feeds coordinated and coolant if needed.
Also by calculating the feed "time estimates" can be made prior to scheduling and releasing the work to the shop.
Before predetermine speed and feed are calculated the following items should be known.
The rigidity of the machine.
The horse power available at the cutter.
The RPM and Feed of the machine.
The material type of the cutter, high speed steel (HSS) or carbide for
example.
The type of material and the condition of the material to be machined. (Note:
Raw ferrous material will often be harder on the outside and softer toward the
center and the feeds and speeds may need to be adjusted).
The size of material to be machine (if the workpiece is thin or small the
cutting speeds and feeds may need to be modified).
The text does not include the above items in the mathematical calculations of speed and feed. They are theoretical and show computation that maybe greater than the machine scheduled to perform the work is capable of. For example the milling machines use in Cerritos College Machine Shop Lab maximum feeds is 9 to 12 inches per minute. Here the calculations need to be adjusted to match the designated machine feeds and speeds. If the calculated speed and feed are substantially more than the scheduled machine the possibly of a machine with faster speeds and feeds should be considered.
It is machine shop practice that two types of cuts should be made; rough and finish cuts. Rough cuts should use maximum speeds, feeds and depth of cut. Finish cuts the RPM can remain the same but the feed slowed and the depth of cut set at minimum of material removal. Also smaller depth of cuts and additional passes may be needed to hold the required tolerances or surface finish.
Often rough cuts are made by "Conventional milling." Two disadvantages to this type of machining, the tooth enters the cut at zero chip thickness in an upward direction and can lift the workpiece if not held securely. Also the conventional milling finish normally is not a clean cut.
Finish cuts on the other hand are made by "Climb milling." Climb milling can also be used to take rough cuts but the machine must be in good condition and rigid to keep the table from jumping forward. For finish cuts the RPM can remain the same but the feed slowed and the depth of cut set at minimum of material removal.
The cutting speed or surface speed is based on the peripheral linear speed of the mill cutter. This should not be confused with RPM. The cutting speed is a factor used to find the RPM. The larger the diameter of the cutter the greater (peripheral) surface speed of the tool. For example if two persons are at each end of a 6 foot pole and one starts to run in a circle, the other stays in the center the speed of the runner out side will be much faster speed than the one in the center.
Review
of the text formulas.
Formula
to Find r/min (revolutions Per Minute (RPM) of the mill cutter)
Formula to find the mill table feed in inches per minute.
Walking
through the text example; Ref. Pg. 327.
2. Cutter
1” wide 4” diameter high-speed steel (HSS). Note:
the authors do not note the exact type of cutter used to form the keyway and
no illustration
is given. A cutter 1” wide 4” diameter is more than likely to big to be a Woodruff
Key cutter and maybe a “Staggered-tooth or a “Plain Side Milling”
cutter on a “Stub” arbor.
Cutting Speed 80. Here the authors used a cutting speed of 80 which falls within the center of the Machine Steel 70-100.
Ref.
Text Example, to find the r/min do the following: 80 (CS)
X 4 (Constant) = 320 ÷ 4” (Dia.)
= 80 RPM
2nd
example:
2. Cutting
Speed 600. Here the authors
used a cutting speed of 600 which falls within the lower part of the CS of the
aluminum 500-1000.
Ref. To find the r/min do the following:
600
(CS)
X 4 (Constant) = 2400 ÷ .750
(Dia.
3/4) = 3200 RPM.
To calculate the feed (Note: the authors only give one example for feed and this illustration may not be for the key cutter example).
The
authors note that the cutter is a 4”, 12-tooth helical HSS tool.
Ref. To find the feed do the following:
12 (N) X 0.010 X 80
(RPM) = 9.6 in/min. Here the feed should be rounded to the machine feeds.
Click Here to Take the Milling Speeds and Feeds Quiz
