Roughing end mills cut costs; cost-effective tooling and machining techniques are the name of the game in modern metalworking.
Too many manufacturing engineers blindly specify standard two-flute end mills for milling jobs. Metal-removal rates can be increased only by driving these cutters at excessive rates, which usually precipitates shank deflection, chatter, and premature cutting-edge wear.
In contrast, a roughing end mill is designed to take metal off fast, and it is built to resist deflection and chatter. After a final hogging pass, a workpiece can be brought to size by a close-tolerance pass with a standard tool. joint venture between CTD and MultiArc can remove a great amount of metal while reguiring minimal torsional force. Depending on the work material, tolerances within 0.001' can be achieved easily by a "short/fat' roughing end mill using this design. Long, smaller diameter tools can meet tolerances within 0.005'.
Rough way to go
Roughing end mills vary in the number of flutes and in tooth profile. Generally, the tools are designed with four, five, or more flutes, depending on the properties of the work material and the tool diameter. An example is the five-flute L-576 mill (shown by the title photo) from Cleveland Twist Drill, an Acme Cleveland Co, Cleveland, OH.
When taking a roughing pass, a good rule is to select a mill with the highest practical number of flutes. The reason is simple: At a given chip load, twice as many teeth remove twice as much metal.
No less important is the tooth design. A true roughing end mill features a wavy (sinusoidal) tooth form (see line drawing), while a hybrid design has a truncated (flat-topped) tooth form. Of the two, the former is a better choice. Here's why.
The sinusoidal cutting edge--with a full sinusoidal radius as a chipbreaker--requires much less horsepower and can run at increased feeds, with a deeper cut at greatly reduced side pressure and vibration. Most of the chip cross section is on the side of the chipbreaker, not the top. The cutting force, therefore, is transmitted axially through the tool and into the spindle--greatly diminishing deflection and tool chatter, which often plague hybrid roughing mills when cutting deep slots and pockets.*
* On a hybrid tool, the flute helix tends to pull the tool away from the chuck, while the cutting force on the sides of the teeth pushes the cutter into the spindle. As a result, the two forces offset one another.
Initial cost of a 1/2-dia roughing end mill is about 40 percent higher than that of a standard tool of the same size. This premium diminishes as diameter increases, and is far outweighed by improved tool performance. (Refer to the table for a conservative comparison of roughing mills and standard end mills in terms of initial cost and feed rate.)
Remember that increased metal removal can't be achieved by increasing feed rate alone. Higher feeds should be balanced against increased width and depth of cut to deliver optimum results with maximum tool life. Specific metalremoval rates, of course, are determined by the machine tool, work material, tool material, machine setup etc.
In applications where depth of cut is more than three times tool diameter, feed rate is limited severely because of the increased possibility of shank deflection. Obviously, a stubby large-diameter mill--close to a rigid spindle--is more desirable than a longer, smaller-diameter tool held by a weaker spindle.
A roughing end mill of less than 1/2 dia usually offers little or no advantage over standard tools, however. The key is to use the largest diameter rougher practical, running it to maximum depth, at maximum feed, for maximum chip load per tooth.
It's important to also consider the horsepower required for a work material before setting up and running a milling job. This can be calculated easily by the formula: HP = in3/min material power factor. The answer is crucial when using machines smaller than 5 hp. An overload could ruin a workpiece, or even the machine.
Some materials (e.g., aluminum and magnesium) are troublesome when hogging because the generated chips weld to the tool's flutes. Cleveland Twist Drill, and other tool manufacturers, have solved this problem by offering hard, wear-resistent titanium nitride (TiN) coatings applied by physical vapor deposition (PVD) on roughing end mills. In CTD's case, the coatings are applied by Scientific Coatings Inc, St. Paul, MN--a joint venture between CTD and MultiArc Vacuum Systems Inc, also of St Paul.
PVD doesn't alter the metallurgical properties of a coated tool, and no heat treating or other secondary operations are required. The coating provides unusually high lubricity--an advantage in moving chips up the flutes of mills and drills. For more about PVD, see the box "Tool-coating primer.'
After a roughing end mill wears enough to affect performance, it can be resharpened by common cutter-grinding procedures. Because the sinusoidal edge is produced by eccentric relieving, the end mills are resharpened by only grinding the flute face. Original tool geometry is maintained during regrinding, allowing the mills to retain cutting efficiency throughout their life.
For best results, use a grinding machine with a spiral-generating workhead--with automatic indexing or manual indexing of flutes--to produce equal flute spacing, uniform stock removal, and correct lead on all flutes.
Properly ground, a roughing end mill is equally efficient for conventional or climb cutting. On work-hardening materials (e.g., Inconel), however, climb milling extends tool life and should be used whenever possible.
Just how long a rougher can be used before regrinding is a matter of conjecture. Traditionally, an acceptable wear point is 0.020. There are reasons to doubt the general-purpose validity of this number. One concerns the great difference among tool designs--the varying number of flutes and the tooth form, for example. Also, the wavy OD of an end mill with a sinusoidal tooth form tends to reach 0.020 of wear more quickly than tools with trucated teeth, or other standard forms. A more precise method of gaging the impact of wear on a roughing operation is to constantly monitor cutting force.
A successful application of roughing end mills comes from Southern Devices Div, Leviton Corp, West Jefferson, NC. The plant is a tool-and-die job shop that manufactures and reconditions molds for injection and compression molding of plastic components for electrical devices.
Molds are made from S-7 and L-6 wrought tool steel, and then heat treated. Tolerances are 0.001 or less on all surfaces. Typically, the surfaces are produced by making multiple roughing cuts, followed by a single finishing pass.
When producing a wall-plate mold, maching is done on S-7 shock-resisting tool steel. The mold is machined on a Bridgeport vertical spindle Series I CNC machining center (smaller work is done on a BostoMatic vertical-spindle maching center).
Full cavities, consisting of eight molds each, are machined in a flood of water-soluble coolant. A 3/4**-dia Cleveland L-576 roughing mill makes 1 5/8 long passes.
The tooling is run conservatively in an environment more oriented to quality finishes than to production quotas. Depth of cut is 1, with a 0.100 wide cut per pass. Feed is 45 sfm vis-a-vis a recommended maximum of 55 sfm. Chip load per tooth is maintained at 0.002 rather than the allowable 0.003 max.
The CTD cutters perform well at these speeds/feeds. According to tool engineers at Southern Devices, a competitive roughing mill, operating under the same conditions, broke down prematurely.
For more information from Cleveland Twist Drill about roughing end mills, circle E55.
Table: HSS roughing end mills vs 2-flute HSS end mills (slotting out 1040 carbon steel, BHN 200)
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|Publication:||Tooling & Production|
|Date:||Oct 1, 1984|
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