New Concepts in Milling Handbook
A practical approach and illustrated guide to milling cutter selection and use
<Table of Contents © 1973 Niagara Cutter Inc.
Selection of Speeds and Feeds and Cutting Tool Materials -
A definition of the formation of the chip, chemical composition of cutting tool materials and engineering charts for setting of speed and feed for a variety of materials
Cutting Speeds and Feeds
Machinability Ratings of Materials
Selecting Speed in (SFM) Surface Feet Per Minute
Selecting Feed in Chip Load per Tooth
Selection of High Speed Steels
Selection of Carbide Grades
Cutting Tool Geometry
Cutting Tool Failures
Cutting Speeds and Feeds
Probably the most important factor affecting economical milling is the determination of cutting speeds and feeds. The proper selection of cutting speed is, first of all, dependent upon the tool material. Chart 1 lists recommended speeds in surface feet per minute for representative materials being cut.
For most steel cutting applications, the range when using high speed steel cutting tools varies from 30 to 200 surface feet per minute, By comparison, carbide cutting tools when machining steel are generally used in the range of 300 to 500 surface feet per minute.
Research has established that when using high speed steel tools, tool tire is approximately inversely proportionate to cutting speed, thus increasing cutting speed from 60 to 120 sfm may reduce tool life by as much as 50 percent. In the case of carbide tools, a similar relationship exists although not quite as pronounced. Cutting feeds are also listed in these same tables. Feeds are normally specified as chip load per tooth on the cutter.
In most cases, chip loads for high speed steel cutting tools vary from .001 to .006 when cutting steel. Carbide cutting tools, on the other hand, have chip loads per tooth of from .003 to .015 for most steel cutting applications.
Tool life is not greatly affected by change of feed within recommended limits. Thus tremendous production advantages can be obtained by increasing feeds to the maximum limit while still obtaining good tool life. Surface finish and machine conditions often prevent the selection of the highest possible feed for maximum production and economical tool life.
Good common sense is important when using the speed and feed charts in practical applications. All the variables that influence metal removal must be considered (condition of the machine, shape of the part being machined, rigidity of the fixtures, cutter geometry, coolant flow, arbor size).
If a heavier depth of cut than is listed on the speed and feed chart is required, an adjustment in the speed or feed may be necessary.
If speed is doubled there is a good chance cutter life will be cut in half. By doubling the chip load, a much less significant reduction in cutter life will occur, assuming the chip load is within allowable limits.
The normal tendency is to run high speed steel milling cutters at speeds higher than recommended with chip loads too light for efficient metal removal.
Combination of Machinability and Speed and Feed Charts
The data presented in Charts 1 and 2 is extremely useful to the cutting tool specialist. The speed and feed charts list representative materials. If the material is not listed on these charts, the tool engineer should determine whether the material to be milled is listed on Machinability Rating Chart 1.
Once the machinability rating of a specific material has been determined the proper feeds and speeds can be selected from Chart 1 on the basis of the comparable machinability rating. Thus 316 wrought stainless with a BHN of 135-185 has a machinability rating of .36. It can be milled at the same feeds and speeds as Maraging Steel Wrought listed on the Machinability Guide which also has a .36 rating.
Cross-reference between the machinability chart and the feed and speed charts can be used as a guide to determine starting conditions. After trial cuts have been made, further refinements are possible in speeds and feeds, dependent upon surface finish, chip formation, coolant flow, machine rigidity, type of cutter and the many other variables encountered in metal cutting.