 | New
Concepts in Milling Handbook A practical approach and illustrated guide to milling cutter selection and use <Table of Contents © 1973 Niagara Cutter Inc. |
| 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 TOOL FAILURES
Cratering of HSS Cutters
Cratering of high speed steel cutters results from intense pressure and heat developed as the chip passes across the face of the tooth. The heat and friction wear and deform the face of the tooth forming a crater (see Figure 77 ). This crater works its way toward the cutting edge and will weaken the edge, eventually causing chipping or breakage of the tooth. A reduction in chip load, spindle speed or both may be necessary to reduce the cratering action. High hardness, high speed steels with higher wear and red hardness qualities may be needed. High rake angles reducing pressure may also be necessary to reduce the cratering and maintain good cutter life. Ground and polished flute faces reduce heat and pressure significantly.Cratering of Carbide Cutters
On the chart showing the chemical composition of carbide the carbide grades were categorized by the properties that resist specific wear patterns. The Group II and III carbides which contain titanium carbide and/or tantalum carbide additives resist cratering. One of these grades should be selected to reduce cratering. Transverse rupture strength and wear qualities should be considered also and can be determined from the same chart.
Abrasion on High Speed Steel Cutters
The friction and rubbing at the periphery of the cutter on the kind (which has clearance - see Figure 75 ) is abrasion. The more abrasive the material being machined or the higher the spindle speed, the more rapidly this wear will develop.
Too small a clearance angle will cause premature wear and corrective action can be taken by increasing this angle. As the "wear land" increases, a flat surface without clearance develops and the friction increases, promoting increased abrasion and high temperatures. Permanent damage to the high speed steel tool can develop.
Subsequent regrinds will not restore the cutting edge with its original high temperature qualities if the cutter is permitted to run under these conditions too long.
Determine by finish and size requirements how many pieces can be cut. Measure the wear and set a maximum on the number of pieces to be machined per cutter change. Wear in excess of .020 inch on cutters below 1/2 inch wide and .030 inch on cutters over 1/2 inch wide can be damaging.
Machines equipped with power indicators are good guides for recognizing when it is time to sharpen the cutter. When power requirements increase, it is time to take the cutter off the machine. In milling with high speed steels, abrasion is the primary wear factor to consider.
Where excessive wear occurs and proper clearance angles, cutting speeds and cutter geometry are established, super high speed steels (T-15) with tungsten, carbon, and vanadium content may be required. The high speed steel chart (page 28) indicates the chemical composition of the H.S.S. materials and those possessing high wear qualities.Abrasion on Carbide Cutters
The straight tungsten carbides listed in Group I on the carbide chemical composition chart possess the highest resistance to abrasion. The application often dictates the cutting tool material. Most slotting applications with carbide or high speed steel induce a rubbing action on the flanks of the tool causing abrasive breakdown of the cutter or saw. This frequently occurs whether the material being machined is cast iron or steel. Where very light chip loads are used due to finish requirements, part configuration or poor rigidity, abrasion is usually the cause of cutter breakdown. Carbide grades should be selected to resist a specific wear pattern by the chemical composition and physical properties they possess. Transverse rupture strength and red hardness qualities should also be considered and can be determined from the chart.Chipping on High Speed Steel Cutters
Chipping of the cutter occurs from lack of rigidity in the workpiece, loose spindle bearings or worn ways. Poor alignment of the cutter, a cutter driven by friction rather than a key, or chatter, can also cause chipping. High positive rake cutters have a smaller included angle and less strength than those with standard positive rake angles and are, therefore, subject to chipping if rigidity is not maintained. Too much relief can also weaken the tooth and cause chipping. Minute chipping is often mistaken for abrasion. The chipped surface no longer cuts and rubbing or abrasion occurs, creating the impression that the cutter is breaking down due to abrasion. An examination of the wear on all the teeth can aid in detecting the cause of the cutter breakdown. Chipping can be minimized or eliminated by reducing the chip load per tooth, increasing the speed, reducing the rake angles, reducing the relief angles, using a tougher high speed steel and checking to be certain the machine tool and holding device are rigid.Chipping on Carbide Cutters
The cobalt content plays the biggest role in resisting, chipping, or fracturing of carbide. Where other types of wear patterns also occur, grades that possess properties to resist cratering, seizing, galling, etc., should be chosen by the category they fall into. The high transverse rupture strength grades (influenced by cobalt content) in that category are selected to resist chipping and fracturing. Grain size and manufacturing techniques are important in maintaining good strength in carbide grades and account for variations in physical -properties. Some carbide grades are available that possess high strength and good abrasion qualities due to grain size and manufacturing techniques. The first C-1 straight tungsten carbide grade listed in Chart E has the same hardness (91.8 Rc) but a higher transverse rupture strength than the more commonly used C-2 grade, and is used in cases where this added edge strength is needed. Note that the first C-1 grade listed in this chart has 8.0% cobalt content and 92.0% tungsten carbide and Rockwell at 91.8A, but has very high transverse rupture strength. In comparison, the standard C-2 grade has 6.0% cobalt content, 94.0% tungsten carbide at 91.8 Rockwell A and has a much lower transverse rupture strength. (Most C-2 carbides are between 225,000 psi and 250,000 psi transverse rupture strength.)Cutter Breakage
Most breakage can be traced to the lack of rigidity in the machine tool, workpiece, or fixturing, or to the geometry of the cutter. When corrections are not made, excessive wear, cratering, chipping or chatter will cause breakage. Too frequently the tool engineer must live with poor machine conditions. A search for solutions must then be found in cutting tool material, coolants, cutter geometry, support for the cutter or workpiece, sacrifice in speed and feed rates, or a combination of these factors. The Comparative Strength Value chart (Chart 7) indicates "average" transverse rupture strength of various cutting tool materials used in milling. Grain particle size, heat treat, manufacturing techniques and chemical additives change the hardness, abrasion resistance, cratering resistance and strength values of a cutting tool material. Milling cutter selection should be based on the properties needed to resist a specific reaction created by the material being machined.Summary
The "Value Analysis Approach" provides a direct logical path to all the factors involved in the day-to-day application of milling cutters. The basic fundamentals or "tools" in chart form provide starting points for the selection of cutting tool materials, speed and feed rates, and to trouble shooting. These are the fundamentals with which the new tool engineer must begin, using the charts as guides, and bearing in mind that all the factors involved in the metal removal process are vital to and affect one another.
BASIC GOOD CUTTER PRACTICE
1. Key all cutters - don't depend on friction to drive them.
2. Arbors should be clear and burr free - clean and inspect the shank to prevent runout and spindle damage.
3. Clean the arbor collars, bearings, arbor and cutters and make sure they are free from burrs. Dirt or burrs can cause the arbor to spring out of line when tightened. Use good flat shims for cutter spacing. Ragged shims make cutter alignment impossible.
4. Direct the cutting forces into the solid part of the set-up against the workpiece support - into the table and frame of the machine if possible.
5. Keep your workpiece as close to the spindle as possible to prevent arbor deflection. Support odd shape workpieces to prevent vibration and chatter. Use double arbor supports or arm braces for heavy cuts.6. Climb mill where possible -if conventional milling, utilize sturdy work holding supports and clamps for maximum rigidity.
7. Do not put flow of coolant on after the cutter is working; put it on first and make certain a good flow reaches the teeth in the cut.
8. Determine first by finish and size requirements how many pieces you can cut - measure the wear and set a standard for changing cutters. Do not run the cutters too long - excessive wear causes damage to the high speed steel that may not be taken out with one regrind.
9. Use flywheel on heavy roughing cuts to drive the cutter through the work.
