Slugger CNC PCD Turning Tools Inserts-Various Angle in Diamond cutting tools
Various Angle in Diamond cutting tools
Since the birth of the indexable insert, all turning tool holders were manufactured with 5º negative rake angles in both directions. Later this became the ANSI and ISO industry standard. It would be interesting to find out what the reasons were and why that was done.
In the 1980’s several research labs ran tool life tests comparing various degrees of negative rake angles. They all came to a same conclusion that, an increase of the negative rake angles increases tool life.
As we all know, negative turning inserts have 90º sides. This also means that an increase of the top rake will increase the side clearance angle. An increase of the side clearance will absolutely increase tool life. The following sketch uses a .010” wear land criterion and compares a 5º top rake angle to a 10º. The theoretical result is that, the 5º rake has .0009” wear life compared to the 10º rake with .0018”. The 10º rake therefore produces double the tool life. In reality of course it will vary but still in favor of the 10º.
The Rake Angle: The basic tool geometry is determined by the rake angle of the tool. The rake angle is always at the top side of the tool. With the tool tip at the center line of the workpiece, the rake angle is determined by the angle of the tool as it goes away from the workpiece center line location. The neutral, positive, and negative rakes are seen below. The angle for these geometries is set by the position of the insert pocket in the toolholder. The positive/negative (d) and double positive (e) rake angles are set by a combination of the insert pocket in the tool holder and the insert shape itself.
There are two rake angles: back rake and side rake. In most turning and boring operations, it is the side rake that is the most influential. This is because the side rake is in the direction of the cut.
Rake angle has two major effects during the metalcutting process. One major effect of rake angle is its influence on tool strength. An insert with negative rake will withstand far more loading than an insert with positive rake. The cutting force and heat are absorbed by a greater mass of tool material, and the compressive strength of carbide is about two and one half times greater than its transverse rupture strength.
The other major effect of rake angle is its influence on cutting pressure. An insert with a positive rake angle reduces cutting forces by allowing the chips to flow more freely across the rake surface.
Negative Rake: Negative rake tools should be selected whenever workpiece and machine tool stiffness and rigidity allow. Negative rake, because of its strength, offers greater advantage during roughing, interrupted, scaly and hard-- spot cuts. Negative rake also offers more cutting edges for economy and often eliminates the need for a chipbreaker. Negative rakes are recommended on insert grades which do not possess good toughness (low transverse rupture strength).
Negative rake is not, however, without some disadvantages. Negative rake requires more horsepower and maximum machine rigidity. It is more difficult to achieve good surface finishes with negative rake. Negative rake forces the chip into the workpiece, generates more heat into the tool and workpiece, and is generally limited to boring on larger diameters because of chip jamming.
Positive Rake: Positive rake tools should be selected only when negative rake tools can't get the job done. Some areas of cutting where positive rake may prove more effective are, when cutting tough, alloyed materials that tend to "work-harden," such as certain stainless steels, when cutting soft or gummy metals, or when low rigidity of workpiece, tooling, machine tool, or fixture allows chatter to occur. The shearing action and free cutting of positive rake tools will often eliminate problems in these areas.
One exception that should be noted when experiencing chatter with a positive rake is, that at times the preload effect of the higher cutting forces of a negative rake tool will often dampen out chatter in a marginal situation. This may be especially true during lighter cuts when tooling is extended or when the machine tool has excessive backlash.
Neutral Rake: Neutral rake tools are seldom used or encountered. When a negative rake insert is used in a neutral rake position, the end relief (between tool and workpiece) is usually inadequate. On the other hand, when a positive insert is used at a neutral rake, the tip of the insert is less supported, making the insert extremely vulnerable to breakage.
Positive/Negative Rake: The positive/negative rake is generally applied using the same guidelines as a positive rake. The major advantages of a positive/negative insert are that it can be used in a negative holder, it offers greater strength than a positive rake, and it doubles the number of cutting edges when using a two-sided insert.
The positive/negative insert has a ten-- degree positive rake. It is mounted in the normal five-degree negative pocket which gives it an effective five-degree positive rake when cutting. The positive/negative rake still maintains a cutting attitude which keeps the carbide under compression and offers more mass for heat dissipation. The positive/negative insert also aids in chip breaking on many occasions, as it tends to curl the chip.
Double Positive Rake: The double positive insert is the weakest of all inserts. It is free cutting, and generally used only when delicate, light cuts are required which exert minimum force against the workpiece, as in the case of thin wall tubing, for example. Other uses of double positive inserts are for very soft or gummy work materials, such as low carbon steel and for boring small diameter holes when maximum clearance is needed.
Side Rake Angles: In addition to the back rake angles there are side rake angles. These angles are normally determined by the tool manufacturers. Each manufacturer's tools may vary slightly, but usually an insert from one manufacturer can be used in the tool holder from another. The same advantage of positive and negative geometry that was discussed for back rake, applies to side rake. When back rake is positive so is side rake and when back rake is negative so is side rake.
Side and End Relief Angles: Relief angles are for the purpose of helping to eliminate tool breakage and to increase tool life. The included angle under the cutting edge must be made as large as practical. If the relief angle is too large, the cutting tool may chip or break. If the angle is too small, the tool will rub against the workpiece and generate excessive heat, and this will in turn, cause premature dulling of the cutting tool.
Small relief angles are essential when machining hard and strong materials, and they should be increased for the weaker and softer materials. A smaller angle should be used for interrupted cuts or heavy feeds, and a larger angle for semi-finish and finish cuts.
Lead Angle: Lead angle is determined by the toolholder, which must be chosen for each particular job. The insert itself can be used in any appropriate holder, for that particular insert shape, regardless of lead angle.
Lead angle is an important consideration when choosing a tool holder. A positive lead angle is the most commonly used and should be the choice for the majority of applications. Positive lead angle performs two main functions:
• It thins the chip
• It protects the insert.
The undeformed chip thickness decreases when using a positive lead angle.
Positive lead angles vary, but the most common lead angles available on standard holders are 10, 15, 30 and 45 degrees.
Positive lead angle also reduces the longitudinal force (direction of feed) on the workpiece. But positive lead angle increases the radial force because the cutting force is always approximately perpendicular to the cutting edge. This may become a problem when machining a workpiece that is not well supported. Care must be taken in cases where an end support, such as a tail stock center is not used.
A heavy positive lead angle also has a tendency to induce chatter because of a greater tool contact area. This chatter is an amplification of tool or workpiece deflection resulting from the increased contact. In this situation it is appropriate to decrease the positive lead angle.
A positive lead angle protects the tool and promotes longer tool life. The tool comes in contact with the workpiece well away from the tip, which is the weakest point of the tool. As the tool progresses into the cut, the load against the tool gradually increases, rather than occurring as a sudden shock to the cutting edge. The positive lead angle also reduces the wear on the cutting edge caused by a layer of hardened material or scale, by thinning the layer and spreading it over a greater area. These advantages are extremely beneficial during interrupted cuts. Another way that positive lead angle helps to extend tool life is by allowing intense heat buildup to dissipate more rapidly, since more of the tool is in contact with the workpiece.
Neutral and negative lead angle tools also have some benefits. A neutral angle offers the least amount of tool contact, which will sometimes reduce the tendency to chatter, and lowers longitudinal forces. This is important on less stable workpieces or set-ups. Negative lead angles permit machining to a shoulder or a corner and are useful for facing. Cutting forces tend to pull the insert out of the seat, leading to erratic size control. Therefore, negative lead angles should be avoided if at all possible.
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