Boring machining helps enlarge the inner diameter of the holes that are pre-drilled on the workpieces, making them suitable for a wider range of applications.
Specifications of Boring Machines
The dimensions between the element and the tool bit can be changed around two axes to cut both vertically and horizontally on the inner surface. The cutting tool is usually single-point, made of M2 and M3 high speed steel or P10 and P01 carbide. A conical hole can also be made by turning the head.
Boring machines are available in many different sizes and styles. Boring small objects can be done on a lathe, while larger objects are machined on boring machine
. Workpieces typically have a diameter of 1 to 4 meters (3 feet 3 to 13 feet 1 inches), but can be as large as 20 meters (66 feet). The power demand can be up to 200 horsepower (150 kW).
The cooling of the holes takes place through a hollow passage through the boring bar, in which the coolant can flow freely. Tungsten alloy discs are sealed on the strap to counteract vibration and vibration when boring. Control systems can be computer-based, allowing for automation and greater consistency.
Why Is Boring Machining Required?
Since drilling is intended to reduce the product's tolerance to pre-existing holes, there are a few design considerations to consider. First, large diameter lengths for the hole are not preferred due to the deflection of the cutting tool. Then, through holes are preferred over blind holes (holes that do not pass through the workpiece thickness).
Due to these factors, drilling and deep-hole drilling are by their nature difficult areas of practice that require special tools and techniques. Nevertheless, technologies have been developed that produce deep holes with impressive accuracy. In most cases they relate to many cutting points, diametrically opposed, whose deflection forces cancel each other out.
Typically, they also include supplying cutting fluid pumped under pressure through the tool into holes near the cutting edges. Drilling pistols and drilling cannons are classic examples. These machining techniques, developed for the first time for the production of firearms and artillery barrels, are now widely used in production in many industries.
How Does Boring Machining Work? Various constant boring cycles are available on CNC controllers. These are programmed subprograms that move the tool through successive cuts of cut, retract, feed, cut, retract, return to the starting position and so on.
Most turning operations that occur with external turning can also be found in boring ones. For external turning, the length of the workpiece does not affect the tool overhang, and the size of the tool holder can be selected to withstand the forces and stresses arising during the operation. However, for internal turning or boring, the choice of tool is very limited by the diameter and length of the workpiece hole.
The general rule that applies to all machining is to minimize tool overhang in order to get the best possible stability and thus accuracy. When drilling, the depth of the hole is determined by the overhang. Stability increases when a larger tool diameter is used, but even then the possibilities are limited because the space required by the diameter of the hole in the workpiece must be taken into account when removing chips and radial movements.
Restrictions on boring stability are set because special care should be taken when planning and preparing production. Understanding the impact of tool geometry and selected cutting data on cutting forces, as well as understanding how different types of boring bars and tool clamping will affect stability, and vibration can be kept to a minimum.
What’s the Importance of Cutting forces?
During coupling, the tangential force and the radial cutting force will try to push the tool away from the workpiece, which will cause deflections.
The tangential force will try to push the tool down and away from the centerline. Due to the curvature of the inner diameter of the hole, the clearance angle will also be reduced. Therefore, for small diameter holes, it is especially important that the insert clearance angle is sufficient to avoid contact between the tool and the hole wall.
Radial deflection will reduce the cutting depth. In addition to affecting diameter accuracy, chip thickness changes as the cutting forces change. This causes vibration transmitted from the blade to the tool holder. Tool stability and clamping will determine the amount of vibration and whether it is reinforced or suppressed.
What Are the Factors that Influence the Cutting forces?
● Insert geometry:
Insert geometry has a decisive influence on the cutting process. The positive insert has a positive rake angle. Plate edge angle and clearance angle together will be less than 90 degrees. A positive rake angle means a lower tangential cutting force. However, a positive rake angle is obtained at the expense of a clearance angle or edge angle.
If the clearance angle is small, there is a risk of tool and workpiece abrasion, and friction may cause vibrations. In cases where the rake angle is large and the edge angle is small, a sharper cutting edge is obtained. A sharp cutting edge penetrates the material more easily, but it can also be more easily changed or damaged by an edge or other uneven wear.
Edge wear means a change in the insert geometry, which reduces the clearance angle. Therefore, for finishing, the required surface finish of the workpiece determines when to replace the insert. In general, edge wear should be between 0.004 and 0.012 inches for finishing and 0.012 to 0.040 inches for roughing.
● Rake angle:
The rake angle affects the axial and radial directions of the cutting forces. A small rake angle produces a large component of the axial cutting force, while a large rake angle results in greater radial cutting force.
The axial cutting force has a minimal negative effect on the operation because the force is directed along the boring bar. To avoid vibration, it is therefore preferable to choose a small lead angle, but because the lead angle also affects other factors such as chip thickness and chip flow direction, you often have to compromise.
The main disadvantage of a small angle of attack is that the cutting forces are distributed over a shorter section of the cutting edge than at a large angle of attack. In addition, the cutting edge is subjected to rapid loading and unloading as the edge enters and exits the workpiece.
Since boring is usually performed in a pre-machined hole and is marked as light machining, small rake angles generally do not cause a problem. Lead angles of 15 degrees or less are usually recommended. However, with an angle of attack of 15 degrees, the radial cutting force will be almost twice as high as the cutting force with an angle of attack of 0 degrees. A typical boring bar with an indexable insert with a rake angle of 0 degrees is shown on the previous page.
● Nose radius:
The nose radius of the insert also affects the distribution of cutting forces. The larger the corner radius is, the greater the radial and tangential cutting forces and the onset of vibration are. However, this does not apply to radial cutting forces. The tool's deflection in the radial direction is influenced by the relationship between the cutting depth and the tip radius size.
If the depth of cut is less than the radius of the blade, the radial cutting forces will increase as the depth of cut increases. If the cutting depth is equal to or greater than the apex radius, the radial deflection will be determined by the angle of attack.
Therefore, it is a good idea to choose a vertex radius slightly smaller than the cutting depth. In this way, radial cutting forces can be kept to a minimum by utilizing the benefits of the largest possible corner radius, which leads to a stronger cutting edge, better surface finish and more even pressure on the cutting edge.
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