Tool unbalance vs. cutting forces.
In the July 2005 "Tooling Around" column, we looked at the calculation of balance as applied to rotating tool assemblies using the balance standard ISO (1) See ISO speed.
(2) (International Organization for Standardization, Geneva, Switzerland, www.iso.ch) An organization that sets international standards, founded in 1946. The U.S. member body is ANSI. 1940-1, and the amount of force that results from just a small amount of unbalance. (See the Archives link at Tooling & Production's website: www.toolingandproduction.com.) This month we'll look more closely at how the force from unbalance compares to what happens during the cutting process.
If we review our tool assembly: A balance quality grade of ISO G2.5 was selected. Our tool was a 1/4" diameter TiA1N-coated solid carbide carbide, any one of a group of compounds that contain carbon and one other element that is either a metal, boron, or silicon. Generally, a carbide is prepared by heating a metal, metal oxide, or metal hydride with carbon or a carbon compound. endmill, held in a short length 40 Taper taper verb To gradually ↓ a dose, usually of a therapeutic agent–eg, corticosteroids, with potentially significant adverse effects, which cannot be abruptly halted, often due to rebound effects V-Flange hydraulic chuck--with a tool assembly weight of 0.85 kilos (1.87 lb). We plan to run this tool at a maximum of 14,000 RPM. Using ISO 1940-1, we calculate that the tool assembly must be balanced within 1.45 gram-millimeters. The unbalance of 1.45 g-mm can be calculated to result in 3.11 Newtons of unbalance force.
In most machine shops, trying to relate 3.11 Newtons of force into the workday can be a bit of a challenge. But you could think of that as roughly translated to be about the same amount of force you could exert with three sticks of butter. How does that small a force compare to the force generated when our 1/4" endmill goes into cut?
If we take one of the reliable shortcut (1) In Windows, a shortcut is an icon that points to a program or data file. Shortcuts can be placed on the desktop or stored in other folders, and double clicking a shortcut is the same as double clicking the original file. methods of calculating cutting force, the formula is:
F = (Hp x 26,400) / SFM SFM Sustainable Forest Management
SFM Science Fiction Museum (Seattle)
SFM Switch Fabric Module (Cisco Systems)
SFM Scanning Force Microscope
SFM Société Française de Microbiologie
Where: SFM = Cutting Speed in Surface Feet per Minute Surface feet per minute (SFM or SFPM) is a unit of velocity used in machining to identify the machinability ratings of a material.
SFM is the velocity around the tool or the material measured in feet-per-minute as the spindle of a lathe turns. . Hp = Horsepower horsepower, unit of power in the English system of units. It is equal to 33,000 foot-pounds per minute or 550 foot-pounds per second or approximately 746 watts. . 26,400 is a constant. F = Force in pounds.
In medium alloy steel, a four flute 1/4" endmill running at 915 SFM, edging at a 0.040" width of cut, cutting 3/4" deep, with a feed per tooth of 0.0016" results in running our endmill at 14,000 RPM and 90 inches per minute. This cut would consume right around 2.7 hp. Therefore, inserting 2.7 hp into our force calculation would look about like this: F = (2.7 x 26,400) / 915 or a Force of 77.9 pounds or 346 Newtons.
So, our small 0.040" wide cut with a 1/4" diameter endmill results in a cutting force over 100 times greater than the 3.11 Newtons of force from our unbalance of 1.45 g-mm. This means that as soon as our tool gets engaged in cut, the forces inherent in the cutting process so greatly exceed the precision balancing--that quite possibly we could have wasted quite a bit of time going to such an extreme grade of balance.
Does this mean that one should never balance their tools? In a word, no--there are many situations where tools must be balanced, and it is difficult to envision how balancing could ever be detrimental to the performance of a tool assembly. But it is very possible to spend considerable time, (and therefore money) balancing tool assemblies to overly restrictive grades of balance. Good principles to always follow would be:
a) When machining at elevated spindle spindle: see spinning.
A rotating shaft in a disk drive. In a fixed disk, the platters are attached to the spindle. In a removable disk, the spindle remains in the drive. Laptops use spindle designations to indicate the number of built-in drives. speeds (>10,000 RPM), buy high quality tools and tool holders, preferably balanced by design and factory certified See certification. to a balance grade standard.
b) Keep tool assemblies as short as possible.
c) The heavier the tool assembly, the more likely the need for it to be balanced.
d) Tools that are inherently out of balance (e.g., hollow mills) should almost always be balanced.
e) Precision finishing tools (e.g., single point fine boring tools (Metal Working) a cutting tool placed in a cutter head to dress round holes.
See also: Boring ) will perform better if balanced.
By considering the above guidelines, investing a certain amount of time understanding the standards applied to balancing, and recognizing the actual forces that result during the cutting process and the amount of unbalance, most shops can more easily determine the right economics to apply for rotating tool balance situations.
All photos courtesy of Sandvik Coromant
Mark Stover stover
stalks of maize plants from which mature corn cobs have been harvested as grain, or grain sorghum plants from which heads have also been removed. The stover is usually fed by turning the cattle into the field and is subject to fungal infection, sometimes causing mycotoxicosis. provides sales and marketing services to manufacturers and distributors in the metalworking industry. He's had chips in the soles of his shoes for the last 25 years. This column appears in T&P every other month. Mark can be reached at email@example.com